Sounding a hull
- Thread starter Brian
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A moisture meter is not a substitute for sounding a hull, and if used it should be in addition to sounding.
What sort of hammer depends a lot on the construction material of the hull in question, since the idea is to perform non-destructive testing. For GRP and wooden hulls some surveyors use a rubber mallet, some use the handle of a screwdriver or bradawl, and I'm sure there are plenty who use something completely different. For steel hulls a ball-pein hammer is often the tool of choice.
Most of the time, what the surveyor is listening for is anything that sounds different to the rest of the boat, especially if there isn't an obvious cause for a difference (such as a bulkhead). Imagine tapping a mallet on a solid plank of wood, and then tapping it on a rotten plank, the sound from the rotten plank won't resonate as much and will have a distinctly different quality (terms like 'dull' and 'flat' spring to mind), the same goes for good GRP and delaminated GRP.
What sort of hammer depends a lot on the construction material of the hull in question, since the idea is to perform non-destructive testing. For GRP and wooden hulls some surveyors use a rubber mallet, some use the handle of a screwdriver or bradawl, and I'm sure there are plenty who use something completely different. For steel hulls a ball-pein hammer is often the tool of choice.
Most of the time, what the surveyor is listening for is anything that sounds different to the rest of the boat, especially if there isn't an obvious cause for a difference (such as a bulkhead). Imagine tapping a mallet on a solid plank of wood, and then tapping it on a rotten plank, the sound from the rotten plank won't resonate as much and will have a distinctly different quality (terms like 'dull' and 'flat' spring to mind), the same goes for good GRP and delaminated GRP.
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They are called phenolic hammers. Usually, one end of the double-headed hammer is rubber and the other is yellow plastic. The plastic head will allow a sharp rap to the hull without any damage-like teadaemon said, the surveyor is listening for differences in sounds.
Moisture meters are often mis-used. Any metal will cause a conductive moisture meter to read high. Bottom paints can cause high readings. To be valid a boat has to dry out. A good article on moisture meters is at: http://www.yachtsurvey.com/moisture_meters.htm
When sounding the hull with a hammer, coin or what ever, they are looking for voids, blistering or delamination. There are other ways of examining a boat bottom--including ultrasonics. Unfortunately this is not frequently used, or well understood. Also thermal imaging can be used to determine the condition of hulls.
When sounding the hull with a hammer, coin or what ever, they are looking for voids, blistering or delamination. There are other ways of examining a boat bottom--including ultrasonics. Unfortunately this is not frequently used, or well understood. Also thermal imaging can be used to determine the condition of hulls.
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Have you ever used your knuckle to sound a wall to find a stud? Same thing. When you hit the stud, you can hear the difference.
The first boat I bought I had surveyed. I was watching the guy sound out the boat and my heart sank when I heard his hammer go "thud". There was some deck delamination but we bought the boat anyway.
The first boat I bought I had surveyed. I was watching the guy sound out the boat and my heart sank when I heard his hammer go "thud". There was some deck delamination but we bought the boat anyway.
I've come across ultrasonics for thickness-testing of steel (or indeed aluminium) hulls, but as far as I was aware it's virtually useless on GRP/FRP hulls as glass/kevlar/etc and resin have different sound-propagation characteristics, which makes interpretation of the results very difficult indeed.
I've never come across thermal imaging as a way of determining the condition of a hull, and I'm somewhat confused as to what you'd be looking for, and how easy it would be to interpret data from thermal imaging in a real-world situation.
teadaemon, you are probably not aware that I have been working with one of the top non destructive testing engineers on ultrasonics for boat laminate condition determination. How do you think that the military and aerospace people test their laminates? Ultrasound is one of the primary modalities. Part of our delays are related to high demand for the instruments by military.
We have the instruments designed and we cut up 20 hurricane damaged boats to prove the technology. We started with smaller sections, such as thru hull and deck cut outs. The problem is that the company we are working with (which used to be owned by my friend) is selling similar instruments to the military and aerospace companies for from $10,000 up to $60,000 an instrument. The instrument can be commercially built for less than $1,000. But then if these same "cheap" instruments were available to the high end (government) buyers, the sales of the expensive instruments would dissapear. We are still working on this aspect. It is our goal to get instruments in the hands of surveyors for $1000 to $1500. Ultra sound works very well in solid laminates. There are a number of other ways that ultrasound can be used--such as having a sending transducer on one side, and a recieving transducer on the other side(difficult to do on a built out boat--but easy on an airplane wing)--or doing what is called a Pitch and catch--where the beams are shot off an interface, which allows the echo pattern to be visualized from the "front side". Also available is mapping technology--where there are A/B plots, and all of the data is stored in the memory and then down loaded onto a computer, where a 3 D model of the laminate can be constructed--along with its defects. The methodology is extremely accurate and will find defects smaller than a grain of sand in a laminate. It is just the price at this point.
Incidently there are one dimensional ultra sound devices which are excellent in steel and glass for about $1000. It is the two and three dimensional devices which get expensive. Yet very few surveyors own these for guaging tanks, steel boats, laminate thickness, pipe wall thickness etc.
Thermal imaging is relitatively rarely used because of the expense of the cameras. We have done some experimentation with modification of digital cameras--but probably this will be best addressed as the cost of purpose built color IR cameras decreases. (Again--if the number of produced units are high, the cost per unit will be lower). Thermal imaging is perticularly useful in wet cores, (moisture in the core gives a lower radiated temperature), delaminated cores, in determining if and where repairs have been made on a hull. It is also very useful in looking at wiring, electrical installations, heating and cooling installations and engines--such as heat exchangers and manufolds--as well as engine blocks. A couple of years ago, the cheapest digital color thermal imaging unit was about $15,000. Today there are some for less than $6,000. (these are lower resolution than the more expensive cameras) For some examples of marine use go to: http://www.goinfrared.com/industries/marinesurveyor/
Hopefully at some point marine survey will catch up with current technology--but at this time the costs of these technologies, plus the unfamilarity of the value of the technologies have discouraged the use. I personally would have been willing to pay twice the fee if these technologies had been used in boats I had purchased in the past. Even in new boats, these technologies would find many manufacturing defects. As we were looking at the market for the ultrasonic devices, the high end manufacturers were very interested in purchasing an instrument--even at a higher price. The high end customer wants assurance of a quality hull. The mass producers were not at all interested--and they are the ones who need to do quality control!
Perhaps at some time, the boat buyer will demand that this modern technology be used--because it is at the buyer's expense that the repairs to the boat must be made. A few hundred dollars more in the cost of a survey, to avoid thousands in repair costs makes good economic sense.
We have the instruments designed and we cut up 20 hurricane damaged boats to prove the technology. We started with smaller sections, such as thru hull and deck cut outs. The problem is that the company we are working with (which used to be owned by my friend) is selling similar instruments to the military and aerospace companies for from $10,000 up to $60,000 an instrument. The instrument can be commercially built for less than $1,000. But then if these same "cheap" instruments were available to the high end (government) buyers, the sales of the expensive instruments would dissapear. We are still working on this aspect. It is our goal to get instruments in the hands of surveyors for $1000 to $1500. Ultra sound works very well in solid laminates. There are a number of other ways that ultrasound can be used--such as having a sending transducer on one side, and a recieving transducer on the other side(difficult to do on a built out boat--but easy on an airplane wing)--or doing what is called a Pitch and catch--where the beams are shot off an interface, which allows the echo pattern to be visualized from the "front side". Also available is mapping technology--where there are A/B plots, and all of the data is stored in the memory and then down loaded onto a computer, where a 3 D model of the laminate can be constructed--along with its defects. The methodology is extremely accurate and will find defects smaller than a grain of sand in a laminate. It is just the price at this point.
Incidently there are one dimensional ultra sound devices which are excellent in steel and glass for about $1000. It is the two and three dimensional devices which get expensive. Yet very few surveyors own these for guaging tanks, steel boats, laminate thickness, pipe wall thickness etc.
Thermal imaging is relitatively rarely used because of the expense of the cameras. We have done some experimentation with modification of digital cameras--but probably this will be best addressed as the cost of purpose built color IR cameras decreases. (Again--if the number of produced units are high, the cost per unit will be lower). Thermal imaging is perticularly useful in wet cores, (moisture in the core gives a lower radiated temperature), delaminated cores, in determining if and where repairs have been made on a hull. It is also very useful in looking at wiring, electrical installations, heating and cooling installations and engines--such as heat exchangers and manufolds--as well as engine blocks. A couple of years ago, the cheapest digital color thermal imaging unit was about $15,000. Today there are some for less than $6,000. (these are lower resolution than the more expensive cameras) For some examples of marine use go to: http://www.goinfrared.com/industries/marinesurveyor/
Hopefully at some point marine survey will catch up with current technology--but at this time the costs of these technologies, plus the unfamilarity of the value of the technologies have discouraged the use. I personally would have been willing to pay twice the fee if these technologies had been used in boats I had purchased in the past. Even in new boats, these technologies would find many manufacturing defects. As we were looking at the market for the ultrasonic devices, the high end manufacturers were very interested in purchasing an instrument--even at a higher price. The high end customer wants assurance of a quality hull. The mass producers were not at all interested--and they are the ones who need to do quality control!
Perhaps at some time, the boat buyer will demand that this modern technology be used--because it is at the buyer's expense that the repairs to the boat must be made. A few hundred dollars more in the cost of a survey, to avoid thousands in repair costs makes good economic sense.
The guy who sounded my hull used a small stainless steel ball-pein hammer. It made sharp clear sounds on the the stringers and interior of the hull. He hit they hull very softly but every once in a while there was a thud. He got the moisture meter out every time he got a thud. Hull passed with flying colors. He used a rubber mallet on the deck. All was solid. You don't want to hear snare drum sounds.
I can assure you that these techniques have "main stream" acceptance in the aircraft industry! There is a well qualified marine surveyor (BS in Naval Architecture and MS in naval structural Engineering from MIT) who wrote two feature articles in "Composites Manufacturing" magazine this month. One was on the technology of hull survey and the other was on methodology of composite hull repair. Mentioned in this article are some "instrumented hammers"--these actually analyize the sound wave and vibration patterns. I can also assure you that the Navy and some of the high tech, mega yacht survyor use these modern technologies.
I had a house guest last night who builds the interiors of megayachts--and there is a demand for proofing the hulls of large yachts before delivary using this technology.
I had a house guest last night who builds the interiors of megayachts--and there is a demand for proofing the hulls of large yachts before delivary using this technology.
Fair enough, by 'main stream', I was thinking of most ordinary marine surveyors at least offering this kind of investigation as an option, even if it wasn't routine. For that to happen, the costs involved in using this technology (both monetary and in time taken for training) have to be low enough that the surveyor's clients are prepared to pay for them. I don't think this is the case at the moment, but it could be in 3-5 years, it really depends how it's marketed to the boat-buying public.