By David Pascoe... Marine Surveyor..... David Pascoe has performed over 5,000 marine surveys, both pleasure craft and commercial.
Click here to see his excellent website
Contrary to common belief, a great deal can be learned about the condition of a hull from examination of its interior. One just has to have experience and know what he's looking for. I have condemned dozens of wood hulls long before the survey ever got to the haul out stage and I'll explain how and why.
Opening Up Concealed areas within a hull are always a problem but, most wood vessels are constructed in such a way that enough access is available to make a fair assessment. It is usually possible to pull up floors (such as screwed in place plywood panels and the like) and remove enough panelling that one can get a fairly good glimpse of the bottom and lower sides. Carrying an electric screw gun is a must in order to do this quickly and effectively.
Yacht surveyors do carpeting and beds. That means one has to move a lot of mattresses, bedding, carpets and emptying out of lockers, etc. Once the interior is opened up as best possible, he's ready to begin. The above discussion provides a lot of clues as to what to look for.
Planks & Frames Three important tools are a slim but heavy gauge pry bar - of the sort used for pulling nails - a heavy hammer and a large standard blade screwdriver. Check the joints between planks and frames visually, looking for gaps or any sign that the plank is not tight against the frame. Then use the screw driver to test the wood for softness on both plank and frame near the mating surface. If the frame is cracked or the wood is soft, one doesn't have to go any further. Soft wood and cracked frames are dangerous conditions that mandate repair.
Try to slip the pry bar under the frame and pry gently. Does the frame move or rock slightly? If so, there is a fastener problem. This should be done at every opportunity along the keel or garboard area. If the frame ends are split or soft, repairs are necessary. If dealing with tall, sawn frames, one can hit the frame with the hammer to see if it is loose.
Weepage Weepage is a process of very slow leakage, very often involving the capillary effect in addition to just water pressure from outside the hull. Weepage is not referred to as leakage because the rate is so slow that the water evaporates nearly as fast as it enters the interior. It can occur with no sign of wetness, but inevitably leaves some trace of its existence such as stains, accumulation of salts and so on.
Certainly its not feasible to go through the entire hull testing all frames and planks in this manner, and fortunately it is not necessary except for the keel area where this needs to be done wherever possible. Further up from the keel, we can limit our physical testing by looking for signs of weepage. Anytime there is evidence of water migrating through seams, corrosion of fasteners has to be suspected. Weepage shows up in various ways, often depending on whether the wood is painted or bare, its age and so on.
In addition to water stains and evidence of corrosion such as rust or green copper oxides, accumulations of dried salts and so on, angle hair, or shredded wood fibers is a dead give away of weepage. Contrary to mythology, angle hair or fuzzing is not caused by electrolysis but rather the constant wetting and drying of salt crystals within the wood fibers which damages the wood cells. This effect was first recognized in the cypress timbers used to shore up the Morton Salt Company mines under Lake Erie. It was found that exposure to raw salt over time caused the wood to become badly shredded. This is a condition which only occurs in sea water and is often found around sea cocks, butt blocks, port holes and engine room vents, along with any other location where water is leaking from the decks above.
On the hull bottom and lower chines, it means weepage and that hull fasteners in any area which reveals this condition must be considered as suspect. This condition is not associated with fungicidal attack because the high salt levels preclude fungus. Scrape the fuzzy area hard with the screwdriver blade. If it is only superficial, this condition can be stopped by wire brushing and sealing the wood, followed by finding the cause of weepage and stopping that too. This condition only affects the inner surface that is exposed to air and evaporation, but once the surface is fuzzy, it has increased evaporation capability through wicking and must be addressed. This condition will not extend between the plank and frame, but will draw water into the fastener area because the accumulated salts are hydroscopic and attract and condense moisture out of the air.
Use the large screwdriver for testing the hardness of the inner planking, particularly in the deep bilge or any place that is wet or looks suspect. I suggest not using an awl or ice pick because this tool penetrates the wood too easily and may give you a false impression. The screwdriver blade is just right, and if it goes into the wood, you know for sure that its deteriorated. Poking around like this is quick and easy so that most vulnerable areas can be quickly covered.
Chine Areas The chines are an area not only of high stress, but an area that is also prone to leakage. And it is this leakage that endangers the fasteners. The surveyor should take every opportunity to inspect the chine areas and when evidence of weepage is found, the area should be targeted for special attention. Remember that weepage is the precursor to corroded fasteners.
Bottom Frames are often joined to side frames with knees that are through bolted. Looseness or corrosion on these bolts are a warning sign that all is not well.
Forefoot The two most common areas for sprung planks to occur is the garboard and the forefoot area. The forefoot planking is difficult to check because this is the point where the planks narrow into the stem. But, again, signs of weepage or leaking is usually present when fasteners are wasted and planks are loose. Use the screwdriver and insert the blade into the intersect of plank and stem and push hard. This should be done on both sides in every area that can be reached. Again, this does not take long if the area is accessible and will readily show up rot and looseness.
Transom The intersect of bottom and side planking to the transom is yet another area where leakage and deterioration are prevalent. This area is also often difficult or impossible to reach, being obscured by fuel tanks and exhaust pipes and whatnot. Yet the surveyor still has a few diagnostic tricks.
If reachable, probe the wishbone transom frame from both above and below. Probe from the intersect of bottom planks and frames, and transom plank and frame. If the wood is at all soft, the problem is serious and needs further opening up and investigation. Examine the intersect all the way up to deck level. Is there water leaking in from above? If so, what is it doing to the wood and fasteners? Check from the exterior: are there open seams and signs of rot on the corners? If you see it above the waterline, what's going on below? Remember that open seams are allowing rain water in, and fresh water can be disastrous.
Keel Bolts, Keelson, Clamps and Stringers Check these major structural members for signs of working. Look for unevenness of scarph joints or any other signs of movement or working. Probe the keelson with the screwdriver for evidence of softness. Check the intersects of transverse frames for signs of rubbing or chaffing that indicates movement. Check visible bolts and bolt heads for corrosion. Also check for discoloration around the bolt heads. If the wood appears white and soft, this is an indication of weepage and the same condition that produces angle hair. Be careful about diagnosing this as "electrolysis." Its probably not.
If there is water getting at the bolts, a serious corrosion must be suspected. The only conclusion to be drawn is that the bolts must be drawn and inspected. Don't rely on just tapping these bolts to see if they're loose. They may be tight now, but may go loose when the hull is working at sea. Bear in mind the forces that operate on a hull while underway.
Be wary of oily bilges and wood that can obscure this evidence. Poke around in the wood surrounding the bolt head. If its soft, you can be sure that there is weepage and the bolt is subject to corrosion.
Inaccessible Areas These are the areas that almost invariably cause the surveyor his greatest problems for he can't get at them to check. Frequently, these are the areas where structural deterioration takes place because neither interior inspection or maintenance is possible.
Outboard and under fuel tanks
Behind large exhaust pipes and mufflers
Under refrigeration and freezers
Under fish holds
Under shower pans and stalls
Under lined rope lockers
Behind hulls that have full hull side ceilings
There's one thing you'll notice about most of the above listed areas and that is the potential for condensation and lack of air flow in these obscured areas which is highly conducive to causing deterioration of wood and metals. These obscured areas should be viewed with extreme caution. The only acceptable conclusion is guilty until proven innocent. To prove soundness, fasteners or planks must be pulled.
Fuel & Water Tanks Failed or improperly designed fuel and water tank foundations are a common cause of catastrophic hull failure. Because of the extreme weight of tanks, if supports fail, or were never properly designed in the first place, the planking or individual frames could end bearing a major part of the tank load. When this happens, hull failure usually results.
Be they cylindrical or square, tanks on saddles or on decks not fully supported by hull girders must be considered as suspect and the entire load bearing structure examined and evaluated. This is usually not as difficult as it might sound, for anyone with a good knowledge of proper construction can quickly size it up if the structure is accessible. If there is any doubt at all, particularly on aging structures, then other means of evaluation must be found.
The Exterior There is one example of aging wooden structures that I can give that nearly everyone is familiar and can relate to. That is driving through the country side and seeing a very old barn that is starting to fall in upon itself - the kind with the sway-back roof and bulging sides. If you would like to understand what happens to old boats, all you have to do is look at that old barn which is subject to nothing more than wind, rain and gravity.
Because boats are subject to much greater stresses, old boats rarely ever get to that point without breaking apart first. Even so, aging boats will reveal the same signs of age. The first sign is open seams that just won't stay closed no matter how much caulking the owner does. As the wood weakens and the fasteners corrode, the entire hull structure just keeps getting looser and looser. Eventually it reaches the point where the whole thing is working every time it goes to sea and it then becomes just a question of time before something pops loose and an accident happens. Or if the owner is lucky, it just quietly sinks at the dock, as most do.
Open hull seams above the waterline that won't stay closed are what surveyors should be looking for. That and bleeding fasteners, loose guard rails, leaky decks, warped or cupped planks, butt ends standing proud, fungicidal rot and so on. If the hull sides don't look good, how much better is the bottom?
The Bottom Survey If the surveyor has done a good job with the interior, then his work on the bottom is going to be the easiest part of the job. By this time, he already knows if there are loose planks, bad frames, deterioration, weepage or leakage and where all these things are located. Long before it comes out of the water, he has a pretty good idea of whether this is a sound hull, and in many cases he'll already know that it isn't, so there's no point in hauling.
How likely is it that an old wood vessel could have bad fasteners and yet show no evidence of that fact on the interior? By my experience, that is not possible at all. Yet there are always a few cases that seem borderline and the surveyor hasn't enough evidence to say one way or another. In that case, he's got to go to a hauled survey.
Steel Fasteners Its true that wooden ships have been built with iron fasteners for several hundred years and archaeologists have found some that are still in good condition. But those iron fasteners involved huge planks and beams and were as precisely fitted as a rivet, but that degree of care was abandoned long ago. Steel fasteners, whether galvanized or not, are a very poor way to fasten a vessel if you want it to last a long time. My attitude toward them is one of fear - for myself and the passengers.
On the other hand, bronze fasteners haven't fared a whole lot better because they are very expensive and so builders have skimped on their size, and the dimensions of the framing into which they are set, thereby reducing their effectiveness as well. Chris-Craft yachts and others were a prime example of vessels with light scantlings and small fasteners that have disappeared from the scene after only twenty years or less.
Nailing hulls is fraught with all kinds of problems, not the least of which is the problem that nails tend to split the wood. When this happens, water gets at the fastener immediately, so whether a vessel is 5 or 30 years old may have nothing to do with soundness. Further, when water is getting at the fastener through the interface between plank and frame, its also going to corrode rapidly.
Ultimately, the problem with nailed boats involves so many factors and hazards that coming to any conclusion of soundness is nigh impossible. Nails cannot be pulled without causing much damage to the plank, if they can be gotten out at all. Inspecting the heads only tells one the condition of the head, not the rest of the nail. And tearing planks off means that they have to be renewed and the cost far too high. Nondestructive methods such as X-ray are both costly, difficult and not necessarily reliable.
Taking all these factors into consideration, steel fastened vessels are a hazard to everyone who gets involved with them.
Screw Fastened Vessels Utilizing all the techniques outlined above, along with removal and inspection of fasteners, can provide a reasonable degree of certainty as to a hull's soundness. Moreover, screw fasteners can be replaced if they have good holding ground, meaning that planks and frames are not split or deteriorated.
To evaluate a screw-fastened bottom, first isolate the likely problem areas, including all of the garboards, under fuel tanks, fish holds and so on. One should not resort to the practice of laying out a pattern of evenly spaced points and pulling fasteners in this method as it is strictly hit or miss. First examine for:
Cupped or warped planks
Open seams and weepage from interior after bottom is dry
Planks with split ends or splits anywhere
Loose seams
Sound planks with heavy hammer for sound of looseness.
Discoloration around screw heads
Special attention to butt ends
Under tanks and engines - stress from heavy loads
Damp environments like under fish holds, refrigeration, etc.
All of the above areas should be marked and targeted first, for it is here where the problems are likely to be. Then, if all these suspect areas prove out okay, you may not even need to pull fasteners in the non-suspect areas and this will reduce the work load greatly.
Unless a plank is cupped, go to the butt ends and edges first and pull fasteners. If there are splits and open seams and other suspicious indicators, and yet the fasteners are still in good shape, the surveyor isn't going to have to go much further. Planks with split ends must be replaced. If these fasteners prove thinned down, then the logical thing to do is to then work outward from the most highly suspect areas. The objective here is to determine whether the hull has a general condition of wasted fasteners or whether it is isolated due to the initiating factors discussed above.
To reinforce my findings, and to reduce work load, I like to find a spot on the interior where that looks very good and then to pull a few screws on the exterior. This gives me a good indication of whether I'm dealing with local or isolated conditions. If isolated, I then need only to determine how isolated.
All of these judgments are either reinforced or eased by my knowledge of particular builders - the size of fasteners, planks, overall scantlings, materials and construction methods. There's nothing like knowledge of a particular builder's methods and how well they hold up to help one along in his work.
Acceptable Degree of Wastage Essentially there is no acceptable degree of wastage in a fastener. If water is getting to the fastener, then it must be considered as subject to an accelerated rate of corrosion and will fail soon. Removed fasteners should be clean and dry. When a fastener that is tight is backed out, friction of the threads against the wood should remove any trace of corrosion and should generally the threads should appear bright. If the metal is pink, dezincification is occurring and the fastener weakening and should be replaced.
Tight fasteners should be hard or impossible to move. If it won't turn, don't bother trying to force it because its okay. Fasteners that just spin without backing out are bad and one needn't waste his time trying to get them out.
Summary of Structural Strength
The most important thing to understand about wood hulls is that they are in no way similar to any other material as far as aging is concerned. As wood hulls age, they deteriorate and weaken generally. The constant destructive action of stress, working, weakening of the wood and corrosion of the fasteners means that the hull is getting weaker and all the connections looser and looser. This process is very highly progressive, meaning that the rate of deterioration and weakening advances rapidly once the general weakening process has set in. Once it reaches this point, the whole structure is at risk. Its no longer a matter of this area or that area being bad, but a matter of the overall weakened state of the entire structure. Thus, when approaching wood boat surveys, there must be an holistic evaluation. And owing to a lack of consistency in the nature of wood and construction methods, it is not reasonable to attempt to ascribe a certain number of years as a viable life span. This simply cannot be done because of the diversity of the product. Every vessel must be evaluated on its own condition and merit.
Click here to see his excellent website
Contrary to common belief, a great deal can be learned about the condition of a hull from examination of its interior. One just has to have experience and know what he's looking for. I have condemned dozens of wood hulls long before the survey ever got to the haul out stage and I'll explain how and why.
Opening Up Concealed areas within a hull are always a problem but, most wood vessels are constructed in such a way that enough access is available to make a fair assessment. It is usually possible to pull up floors (such as screwed in place plywood panels and the like) and remove enough panelling that one can get a fairly good glimpse of the bottom and lower sides. Carrying an electric screw gun is a must in order to do this quickly and effectively.
Yacht surveyors do carpeting and beds. That means one has to move a lot of mattresses, bedding, carpets and emptying out of lockers, etc. Once the interior is opened up as best possible, he's ready to begin. The above discussion provides a lot of clues as to what to look for.
Planks & Frames Three important tools are a slim but heavy gauge pry bar - of the sort used for pulling nails - a heavy hammer and a large standard blade screwdriver. Check the joints between planks and frames visually, looking for gaps or any sign that the plank is not tight against the frame. Then use the screw driver to test the wood for softness on both plank and frame near the mating surface. If the frame is cracked or the wood is soft, one doesn't have to go any further. Soft wood and cracked frames are dangerous conditions that mandate repair.
Try to slip the pry bar under the frame and pry gently. Does the frame move or rock slightly? If so, there is a fastener problem. This should be done at every opportunity along the keel or garboard area. If the frame ends are split or soft, repairs are necessary. If dealing with tall, sawn frames, one can hit the frame with the hammer to see if it is loose.
Weepage Weepage is a process of very slow leakage, very often involving the capillary effect in addition to just water pressure from outside the hull. Weepage is not referred to as leakage because the rate is so slow that the water evaporates nearly as fast as it enters the interior. It can occur with no sign of wetness, but inevitably leaves some trace of its existence such as stains, accumulation of salts and so on.
Certainly its not feasible to go through the entire hull testing all frames and planks in this manner, and fortunately it is not necessary except for the keel area where this needs to be done wherever possible. Further up from the keel, we can limit our physical testing by looking for signs of weepage. Anytime there is evidence of water migrating through seams, corrosion of fasteners has to be suspected. Weepage shows up in various ways, often depending on whether the wood is painted or bare, its age and so on.
In addition to water stains and evidence of corrosion such as rust or green copper oxides, accumulations of dried salts and so on, angle hair, or shredded wood fibers is a dead give away of weepage. Contrary to mythology, angle hair or fuzzing is not caused by electrolysis but rather the constant wetting and drying of salt crystals within the wood fibers which damages the wood cells. This effect was first recognized in the cypress timbers used to shore up the Morton Salt Company mines under Lake Erie. It was found that exposure to raw salt over time caused the wood to become badly shredded. This is a condition which only occurs in sea water and is often found around sea cocks, butt blocks, port holes and engine room vents, along with any other location where water is leaking from the decks above.
On the hull bottom and lower chines, it means weepage and that hull fasteners in any area which reveals this condition must be considered as suspect. This condition is not associated with fungicidal attack because the high salt levels preclude fungus. Scrape the fuzzy area hard with the screwdriver blade. If it is only superficial, this condition can be stopped by wire brushing and sealing the wood, followed by finding the cause of weepage and stopping that too. This condition only affects the inner surface that is exposed to air and evaporation, but once the surface is fuzzy, it has increased evaporation capability through wicking and must be addressed. This condition will not extend between the plank and frame, but will draw water into the fastener area because the accumulated salts are hydroscopic and attract and condense moisture out of the air.
Use the large screwdriver for testing the hardness of the inner planking, particularly in the deep bilge or any place that is wet or looks suspect. I suggest not using an awl or ice pick because this tool penetrates the wood too easily and may give you a false impression. The screwdriver blade is just right, and if it goes into the wood, you know for sure that its deteriorated. Poking around like this is quick and easy so that most vulnerable areas can be quickly covered.
Chine Areas The chines are an area not only of high stress, but an area that is also prone to leakage. And it is this leakage that endangers the fasteners. The surveyor should take every opportunity to inspect the chine areas and when evidence of weepage is found, the area should be targeted for special attention. Remember that weepage is the precursor to corroded fasteners.
Bottom Frames are often joined to side frames with knees that are through bolted. Looseness or corrosion on these bolts are a warning sign that all is not well.
Forefoot The two most common areas for sprung planks to occur is the garboard and the forefoot area. The forefoot planking is difficult to check because this is the point where the planks narrow into the stem. But, again, signs of weepage or leaking is usually present when fasteners are wasted and planks are loose. Use the screwdriver and insert the blade into the intersect of plank and stem and push hard. This should be done on both sides in every area that can be reached. Again, this does not take long if the area is accessible and will readily show up rot and looseness.
Transom The intersect of bottom and side planking to the transom is yet another area where leakage and deterioration are prevalent. This area is also often difficult or impossible to reach, being obscured by fuel tanks and exhaust pipes and whatnot. Yet the surveyor still has a few diagnostic tricks.
If reachable, probe the wishbone transom frame from both above and below. Probe from the intersect of bottom planks and frames, and transom plank and frame. If the wood is at all soft, the problem is serious and needs further opening up and investigation. Examine the intersect all the way up to deck level. Is there water leaking in from above? If so, what is it doing to the wood and fasteners? Check from the exterior: are there open seams and signs of rot on the corners? If you see it above the waterline, what's going on below? Remember that open seams are allowing rain water in, and fresh water can be disastrous.
Keel Bolts, Keelson, Clamps and Stringers Check these major structural members for signs of working. Look for unevenness of scarph joints or any other signs of movement or working. Probe the keelson with the screwdriver for evidence of softness. Check the intersects of transverse frames for signs of rubbing or chaffing that indicates movement. Check visible bolts and bolt heads for corrosion. Also check for discoloration around the bolt heads. If the wood appears white and soft, this is an indication of weepage and the same condition that produces angle hair. Be careful about diagnosing this as "electrolysis." Its probably not.
If there is water getting at the bolts, a serious corrosion must be suspected. The only conclusion to be drawn is that the bolts must be drawn and inspected. Don't rely on just tapping these bolts to see if they're loose. They may be tight now, but may go loose when the hull is working at sea. Bear in mind the forces that operate on a hull while underway.
Be wary of oily bilges and wood that can obscure this evidence. Poke around in the wood surrounding the bolt head. If its soft, you can be sure that there is weepage and the bolt is subject to corrosion.
Inaccessible Areas These are the areas that almost invariably cause the surveyor his greatest problems for he can't get at them to check. Frequently, these are the areas where structural deterioration takes place because neither interior inspection or maintenance is possible.
Outboard and under fuel tanks
Behind large exhaust pipes and mufflers
Under refrigeration and freezers
Under fish holds
Under shower pans and stalls
Under lined rope lockers
Behind hulls that have full hull side ceilings
There's one thing you'll notice about most of the above listed areas and that is the potential for condensation and lack of air flow in these obscured areas which is highly conducive to causing deterioration of wood and metals. These obscured areas should be viewed with extreme caution. The only acceptable conclusion is guilty until proven innocent. To prove soundness, fasteners or planks must be pulled.
Fuel & Water Tanks Failed or improperly designed fuel and water tank foundations are a common cause of catastrophic hull failure. Because of the extreme weight of tanks, if supports fail, or were never properly designed in the first place, the planking or individual frames could end bearing a major part of the tank load. When this happens, hull failure usually results.
Be they cylindrical or square, tanks on saddles or on decks not fully supported by hull girders must be considered as suspect and the entire load bearing structure examined and evaluated. This is usually not as difficult as it might sound, for anyone with a good knowledge of proper construction can quickly size it up if the structure is accessible. If there is any doubt at all, particularly on aging structures, then other means of evaluation must be found.
The Exterior There is one example of aging wooden structures that I can give that nearly everyone is familiar and can relate to. That is driving through the country side and seeing a very old barn that is starting to fall in upon itself - the kind with the sway-back roof and bulging sides. If you would like to understand what happens to old boats, all you have to do is look at that old barn which is subject to nothing more than wind, rain and gravity.
Because boats are subject to much greater stresses, old boats rarely ever get to that point without breaking apart first. Even so, aging boats will reveal the same signs of age. The first sign is open seams that just won't stay closed no matter how much caulking the owner does. As the wood weakens and the fasteners corrode, the entire hull structure just keeps getting looser and looser. Eventually it reaches the point where the whole thing is working every time it goes to sea and it then becomes just a question of time before something pops loose and an accident happens. Or if the owner is lucky, it just quietly sinks at the dock, as most do.
Open hull seams above the waterline that won't stay closed are what surveyors should be looking for. That and bleeding fasteners, loose guard rails, leaky decks, warped or cupped planks, butt ends standing proud, fungicidal rot and so on. If the hull sides don't look good, how much better is the bottom?
The Bottom Survey If the surveyor has done a good job with the interior, then his work on the bottom is going to be the easiest part of the job. By this time, he already knows if there are loose planks, bad frames, deterioration, weepage or leakage and where all these things are located. Long before it comes out of the water, he has a pretty good idea of whether this is a sound hull, and in many cases he'll already know that it isn't, so there's no point in hauling.
How likely is it that an old wood vessel could have bad fasteners and yet show no evidence of that fact on the interior? By my experience, that is not possible at all. Yet there are always a few cases that seem borderline and the surveyor hasn't enough evidence to say one way or another. In that case, he's got to go to a hauled survey.
Steel Fasteners Its true that wooden ships have been built with iron fasteners for several hundred years and archaeologists have found some that are still in good condition. But those iron fasteners involved huge planks and beams and were as precisely fitted as a rivet, but that degree of care was abandoned long ago. Steel fasteners, whether galvanized or not, are a very poor way to fasten a vessel if you want it to last a long time. My attitude toward them is one of fear - for myself and the passengers.
On the other hand, bronze fasteners haven't fared a whole lot better because they are very expensive and so builders have skimped on their size, and the dimensions of the framing into which they are set, thereby reducing their effectiveness as well. Chris-Craft yachts and others were a prime example of vessels with light scantlings and small fasteners that have disappeared from the scene after only twenty years or less.
Nailing hulls is fraught with all kinds of problems, not the least of which is the problem that nails tend to split the wood. When this happens, water gets at the fastener immediately, so whether a vessel is 5 or 30 years old may have nothing to do with soundness. Further, when water is getting at the fastener through the interface between plank and frame, its also going to corrode rapidly.
Ultimately, the problem with nailed boats involves so many factors and hazards that coming to any conclusion of soundness is nigh impossible. Nails cannot be pulled without causing much damage to the plank, if they can be gotten out at all. Inspecting the heads only tells one the condition of the head, not the rest of the nail. And tearing planks off means that they have to be renewed and the cost far too high. Nondestructive methods such as X-ray are both costly, difficult and not necessarily reliable.
Taking all these factors into consideration, steel fastened vessels are a hazard to everyone who gets involved with them.
Screw Fastened Vessels Utilizing all the techniques outlined above, along with removal and inspection of fasteners, can provide a reasonable degree of certainty as to a hull's soundness. Moreover, screw fasteners can be replaced if they have good holding ground, meaning that planks and frames are not split or deteriorated.
To evaluate a screw-fastened bottom, first isolate the likely problem areas, including all of the garboards, under fuel tanks, fish holds and so on. One should not resort to the practice of laying out a pattern of evenly spaced points and pulling fasteners in this method as it is strictly hit or miss. First examine for:
Cupped or warped planks
Open seams and weepage from interior after bottom is dry
Planks with split ends or splits anywhere
Loose seams
Sound planks with heavy hammer for sound of looseness.
Discoloration around screw heads
Special attention to butt ends
Under tanks and engines - stress from heavy loads
Damp environments like under fish holds, refrigeration, etc.
All of the above areas should be marked and targeted first, for it is here where the problems are likely to be. Then, if all these suspect areas prove out okay, you may not even need to pull fasteners in the non-suspect areas and this will reduce the work load greatly.
Unless a plank is cupped, go to the butt ends and edges first and pull fasteners. If there are splits and open seams and other suspicious indicators, and yet the fasteners are still in good shape, the surveyor isn't going to have to go much further. Planks with split ends must be replaced. If these fasteners prove thinned down, then the logical thing to do is to then work outward from the most highly suspect areas. The objective here is to determine whether the hull has a general condition of wasted fasteners or whether it is isolated due to the initiating factors discussed above.
To reinforce my findings, and to reduce work load, I like to find a spot on the interior where that looks very good and then to pull a few screws on the exterior. This gives me a good indication of whether I'm dealing with local or isolated conditions. If isolated, I then need only to determine how isolated.
All of these judgments are either reinforced or eased by my knowledge of particular builders - the size of fasteners, planks, overall scantlings, materials and construction methods. There's nothing like knowledge of a particular builder's methods and how well they hold up to help one along in his work.
Acceptable Degree of Wastage Essentially there is no acceptable degree of wastage in a fastener. If water is getting to the fastener, then it must be considered as subject to an accelerated rate of corrosion and will fail soon. Removed fasteners should be clean and dry. When a fastener that is tight is backed out, friction of the threads against the wood should remove any trace of corrosion and should generally the threads should appear bright. If the metal is pink, dezincification is occurring and the fastener weakening and should be replaced.
Tight fasteners should be hard or impossible to move. If it won't turn, don't bother trying to force it because its okay. Fasteners that just spin without backing out are bad and one needn't waste his time trying to get them out.
Summary of Structural Strength
The most important thing to understand about wood hulls is that they are in no way similar to any other material as far as aging is concerned. As wood hulls age, they deteriorate and weaken generally. The constant destructive action of stress, working, weakening of the wood and corrosion of the fasteners means that the hull is getting weaker and all the connections looser and looser. This process is very highly progressive, meaning that the rate of deterioration and weakening advances rapidly once the general weakening process has set in. Once it reaches this point, the whole structure is at risk. Its no longer a matter of this area or that area being bad, but a matter of the overall weakened state of the entire structure. Thus, when approaching wood boat surveys, there must be an holistic evaluation. And owing to a lack of consistency in the nature of wood and construction methods, it is not reasonable to attempt to ascribe a certain number of years as a viable life span. This simply cannot be done because of the diversity of the product. Every vessel must be evaluated on its own condition and merit.
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