Wooden Boat People
By McKenzieDriftBoat.com
panel strength
I have always wondered what the relative strengths are for different bottom panels. For a drift boat this might include impact, bending ect. So does anyone know the differences on plywood versus plywood with epoxy/glass, cold molding, strip , and maybe the composit core that montana boats uses? Mostly I have wondered if a cold molded panel is better or worse than good marine ply.
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Replies to This Discussion
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Permalink Reply by Phillip Westendorf on
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Bob,
I think this is a great question that many of us here have asked ourselves. I think the guys at Gougeon Brothers in Bay City, MI who wrote the book on epoxy and do a lot of testing for boat (big ones) manufacturers for insurance reasons. Many articles have been printed in their Epoxy Works Magazine over the years. The mag. is free, just go to West Systems.com and sign up.
I think you could ask your question to them and get an good answer. We do, though, have some a real experienced and smart educated folks here and may shed some light on your question.
G'Luck
Dorf
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Permalink Reply by Rick Newman on
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Panel strength is also dependent not only upon the "core" material but also the thickness of the "core" material. What I am saying that a composite structure comprised of a lamination of epoxy and fiberglass on both sides of a core material will increase in exponentially. So an example would be a 1/2" core with laminations will be four times more resistant to specific forces, than a 1/4" core.
With that being said the most frequent drift boat repair that I am aware of be it fiberglass or wood/composite is damage to the chine area when a boat hits a rock or similar item. That is one of the reasons for chine caps that are easily removable. If broken, unscrew them and replace them.
It has been my experience in both drift boats and rafts that if I travel over a rock I either get a scratch in the graphite/epoxy mixture or a rub mark or at worst case in older style non-self-bailing rafts a tear can occur. When I stuck a raft on rocks in the Grand Canyon last year there was no damage as the floor flexed. My drift boat has both been run over rocks and run into rocks. So I have either scratches or beat up White Oak chine caps.
When I build my drift boat my primary concern was to build it so that I could withstand reasonable impacts to the floor and sides. In order to do so I laminated S-Glass to the interior of the sides prior to assembling the boat. The floor was constructed of 1/2" Meranti I believe. I laminated 18 ounce triaxial weave fiberglass to the inside of the floor before I installed it. It made it quite difficult to bend but it also made it much tougher. As fiberglass resists being bent when it is stretched it provides the most strength on the inside of the boat where it can resist intrusion from a rock or something similar. After I finally got the floor to bend to the shape of the boat I added an additional layer of 18 oz triaxial weave fiberglass and then several coats of epoxy then epoxy and graphite powder to the outside bottom of the boat.
The result has been an exceedingly stiff floor that should resist and has so far resisted any damage other than scratches and has withstood some very loud, potentially hard impacts from rocks during low water floats. My aim was to build a boat that would withstand reasonably hard hits and would be field repairable to get to the takeout. So far so good.
The "bottom" line is you can indeed build a very strong boat bottom, but at what cost and at what difficulty during the assembly of the boat. Also since most damage occurs from the chine cap on up that is where more strength is seemingly needed. A
Dorf, (Phil Westendorf) is the engineer around here, I'm sure he can provide further elucidation on the mathematics of strength. I am simply sharing what a few thousand miles of floating has taught me in the riverine environment. Your mileage may vary.
I have never experienced any drift boat with a cold molded panel, Dorf's boat is essentially a cold molded panel of sorts as he laminated strips of wood in several directions on the bottom of his boat. I don't know if he has done a cost comparison on materials. You might ask him about that and also inquire how much time it took to assemble the bottom of his boat.
A consideration about the synthetic materials, foams and honeycomb cells materials is they will initially weigh less than wooden products so you can increase your strength by increasing the thickness without a large weight penalty. The tradeoff is cost of the materials and potentially the cost of freight.
Another thing to consider is that rot and improper storage and care is probably the most common bottom repair issue that is brought up on these pages. Another thing that damages bottoms is intrusion of water into the plywood and then not being promptly repaired.
Rick Newman
Rick N
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Rick,
There have been some very interesting discussions here in the last few years that were over my head like Guy Fredrickson's spread sheet on the offsets he created for his boat. My Engineering background is more in the practical applications and processes. But I have had enough strength of materials applications a long time ago to be considered "dangerous". My son recently came over to visit and had a text book I had used in college (mid 60's) in a Strength of Materials Class with a homework assignment in it. He could't believe we had to do all the calculations by hand on paper. As I remember the first HP calculators were introduced to the public in the early 70's. For the record, we did a lot of the calculations with a slide rule but the instructors wanted the calculations done on paper so they could see the process we used to get the answers they expected. Rick, I am sure you can relate to that, eh.
Bob,
The floor variations you asked about are as varied as the people who build them. Each has advantages over the others and disadvantages also. If you read Wooden Boat Forum (and you should) there are several opinions on the discussions from all around the globe on just about every aspect of boat building. A lot can be learned from the "Old Salts" there too, I did.
As Rick has said I made a "cold molded" floor for my drift boat, i did. It's made of two layers of 1/4" Ash. The strips are 1/4 x 2 1/4 wide and bead and cove. The inside layer run fore and aft the outer layer is 60 degrees to the inside layer. It has 2 layers of 6 oz. glass, epoxy and graphite on the outside bottom. The inside bottom has a single layer of glass & epoxy. The Strength of the White Ash allowed me to cheat on the glass. I was/am concerned mostly of keeping the wood and the water from never meeting to prevent long term problems, both cosmetic and structural. Note: Rick used 18 oz. on his plywood bottom to get the strength he was looking for.
The reason I made it this way has many considerations, but the bottom line there is no plywood in my boat. I am of the philosophy that if I can make it rather than pay someone else to make it that's what i do. I found it a challenge to build a drift boat using local materials at a low cost. I am a cheap SOB when it comes down to it. I had cheap access to White Ash (200+ BF @ $ 0.50/BF for the millwork) thanks to the Emerald Ash Bore that has/is killing a bazzilion Ash trees here in the Midwest. Another consideration was, here in Michigan we don't have any white water rivers like out West. The Manistee (a typical Michigan Trout Stream) has mean flow of 1690 CFS. So that becomes another consideration. As I am retired, time wasn't an issue so I purchased a few books on boat building and read a lot and lurked on Wooden Boat Forum almost daily. The cold molded allowed me to build a strong floor at a minimum of cost, but a trade off was my time.
For the record, I don''t think I'd build another drift boat using the cedar strips with Ash flooring unless someone with lots of money can along and made me a rich man. I'd use more conventional materials and do it more like Rick and others have done.
So I hope you'll keep asking questions and I am sure we'll have plenty of opinions and advice.
G'Luck with your build.
Dorf
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Permalink Reply by Bob bryan on
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a bit off topic but I just the other day ran into your boat building saga. Wow wonderfull job, beautiful boat and as unique as you can get. kudos
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Permalink Reply by Josh Case on
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I can't speak for wood or composites, but i scarfed in a screen door into the bottom of my dory, sprayed on some Flex Seal and have been good to go for a few years now. YMMV :)
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Permalink Reply by lhedrick on
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If your core material is plywood then most of the strength will be from the plywood. If it's coved with glass it will add stiffness but the glass is not really where the strength is. Getting away from wood there are foam and honeycomb which are a bit beyond a simple answer. To get the strength of a 3/8 plywood bottom using only epoxy and fiberglass you will need a panel which will be heavier then using a plywood core.
Comparing a plywood hull to a comercial hull built from polyester resin and glass will show the difference. The wood boat will be lighter. This comment is not meant do knock commercial boats, we can't really compare the two, the materials are not the same.
You won't find any commercial fiberglass boats going down the Grand Canyon and there are reason for that. All glass and all wood boats both have their place.
Wood is still a great material but it needs to be cared for a bit.
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well thanks for the reply's the answers so far have explained why we dont see a simple chart for comparison. What I am hoping for is that someone has a qualified comparison while trying to match apples to apples as well as can be done. I have been running rivers here in washington since I was 12(sorry old 60 now) using everything from rubber rafts (1st and least favorite) to my 34 year old 17 1/2 ' tatman. in between those 2 were 12' cartopper , really not the best idea for the upper sky, 8' wood pram , used it trapping beaver one year & was very scary with me & 3-4 beavers in her, 10' wood prams, nice old steele drift ect. I Know the variables are many but I still hope someone here can maybe at least give their opinion based on either tests or personal experiance. I do know this . Back when there were as many wood boats as glass & aluminum the pattern I saw was this. Of all the wrecked boats/boat pieces I have seen on the water the most were glass boats then wood.. then well I think I remember 1 aluminum wreck. The reason I am interested is on my next boat it would be nice to know if say a 3/8 ply with heavy glass/epoxy layers would be stronger or perhaps a standard 1/2" with the same or a bit less glass would work out better. The other options might be the ticket too but who knows. Maybe someone can scam some grant money and do a study. I have even tried to figure out how to imploy a UHMW bottom such as in a boulder hull. When you read about cold moulding the claim is much better strength compared to other materials but that is always based on a boat that never hits the bottom so drift boats are in their own little department. Nothing wrong with a plywood bottom , my 34 year old tatman is tired but has taken terrible abuse as a trapping and guide boat with the only damage 2 cracked ribs.
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Bob on the montana-riverboats forum several people did some bare-bones tests. They laminated up a variety combination of materials and then did beat on them with hammers and rocks and bowling balls. Results were varied and in my opinion were only valid for the type of stresses that were imposed for example a piece of 1/2" plywood with 18 oz of E-glass laminated with System 3 laminating resin suspended between two eight inch high cinder blocks could withstand four hammer blows but not a bowling ball dropped once. I have never hit a drift boat bottom with a hammer or bowling ball. I have however hit a few rocks with the bottom of my drift boat as I moved down the river with the water supporting part of the load. My point is that the tests that were conducted while somewhat useful as they compared a few layups with static tests they didn't replicate real world conditions with measurable stresses and loads.
I would bet that a professional engineer could probably make a good living for a while testing all the possible combinations. Not only does the core material make a difference as does the thickness of the core and the type and brand of resin used. The combinations would be almost infinite. Biaxial weave glass versus triaxial weave glass versus combination of the two, brittle resins versus more flexible resins, the amount of resin used, whether or not the laminate was vacuum laminated or resin infused. Then the engineer would have to determine what types of stress would be placed upon the structure, how to place the load upon the structure, point load or torsional or bending? How would an engineer compensate for the movement and support of water that would disperse the loads placed upon the structure.
How about the speed at which the bottom actually hits an object, how would you replicate that and measure the failure point. There are differences in the the strengths of S-Glass and E-Glass because of the inherent strengths of the different glasses used in their manufacture. What happens if you were to incorporate some kevlar, would you want biaxial or triaxial fabric. Carbon fiber could also be added, then you need to test which direction and amount and type of carbon fiber would you want, tape, webbing, cloth, etc?
Then you need to figure out the weight of the floor and how that would actually affect the handling of the drift boat. One might be able to determine the strongest "composite" structure but could you then bend it to conform to your boats sides. The layup I used was almost impossible to form to the bottom of my boat. lots of straps, weights, etc were needed plus screws every 1/2" to pull it down. A stitch and glue boat won't have an interior chine to hold screws.
Then there is a big factor for me, what can you afford to spend on the bottom of your boat? I have to ask, what is the construction or layup schedule for your current Tatman boat that has withstood damage for so long?
You might consider searching the web site for Brad Dimock and Resin Research. Brad has been boating the Grand canyon for decades and has been building and repairing boats for there almost as long. Real world experiences led him to experiment with bottom construction materials and methods. In a nutshell his research led him to Resin Research. They have developed an epoxy resin that is more flexible than others, even within their line of resins. Brad and his buddies laminated a roll of toilet paper and endeavored to crush it with a hammer. They were not able to easily break or damage the toilet paper laminate because of the ability of the resin to flex rather being rigid and brittle. Brad then used Resin Research in at least several Grand Canyon style dory floor/bottom repairs. I haven't seen him post anything about the failure or success of the floors/bottoms. His website is fretwater.com.
Rick Newman
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Here's the post from Brad Dimock on his Resin Research 2040 testing.
lhttp://fretwaterlines.blogspot.com/2011/12/demo-derby.html
http://www.resinresearch.net/id2.html
Rick Newman
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Thanks Rick . That resin certainly sounds like a newer batter mouse trap.I suppose I should just look at how well my 34 year old boat has held up and not worry about it but when you start planning to build a new boat deciding on materials & construction is half the fun.
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I spent a year thinking about it!
Rick
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i didnt answer your question about my tatman. It is just a standard 17-1/2 ' highside. I bought the hull from him in 84. Originally it had a UHMW shoe, loved it though it did of course cause the normal problems with expansion. In spite of that I will probably use UHMW again as there is nothing to compare in skinny water. With two big 200#+ clients in front I could easily push & slide it over almost dry rocks and if I had to I could probably do that on a dry rock bed. I replaced it with glass/kevlar & graphite about 95 & have regretted it ever since, just no comparison. I am hoping one of the new glue it on systems will prove out. I also keep trying to think up a attachment system that can avoid the fastener problem. I spent 6-7 years running a beaver/otter line on the sauk, NF stilly and skagit in november/dec before turning to guiding once I had thinned the beaver down & the fish showed well. The upper sauk is especially tough on the bottom in low water & with 6-7 beaver, traps plus 50" sand bags of rocks the boat was worked pretty hard. I did crack the two frames under the front seats & fractured the ply on the fwd cracked rib. I replaced the ribs and just soaked epoxy into the ply and that has worked fine. I had a "tarp yurt" I kept it in for many years until the wx finally brought it down, Then I just kept a tarp over it as I was not home much & never got around to a new tarp yurt. Predictably that has caused rot problems though most seems to be on the chine cap. She is a tired old boat but once I do the needed repairs all will be fine. I am just waiting for the time at home to build a new one mostly just to build it. I fish for a living so am gone a lot out at sea. Considering how long it sat without a good cover it is pretty decent shape what with all the abuse I put it through. Knowing myself I am sure once I build a new one it will be tough to sell it and most likely I will keep it. Too many memories & miles.
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