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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Permalink Reply by Rick Newman on February 4, 2015 at 4:55pm

Bob, if you look back through the search engine above you might be able to find the guy that offers UHMW sheets that have a fabric layer bonded to them. He reports that he can successfully be bonded to a plywood bottom. His firm is called: http://www.stealthcraftboats.com/

Of course the freight might be expensive as he is located in Minnesota.

Rick N

Permalink Reply by Mike Baker on February 1, 2015 at 7:17pm

Here is yet another option for the bottom that I do on quite a few boats.

1/4" ply bottom with 1/2" Plascore or Nidacore cold bonded to that then a layer of 20 oz. triax.

It makes for a stiff bottom with 1/2" of protection before you scar any wood.

Mike

Permalink Reply by Ants Uiga on February 1, 2015 at 8:20pm
Let me add my comments in response to the request for engineers to chime in.

Standardized engineering tests are conducted to cover the strength of materials over a very broad range of conditions. The consequences of a wooden drift boat hitting a rock are not included. For example, if the wooden chine hit two inches of rock it would have a different impact if two feet of rock were hit. Metals have predictable deformation until the elastic limit is reached. Composite materials of wood don't necessarily have an elastic limit. As an aside, truly composite materials have no predictable strength characteristics. My friend who designed submersibles for 2-3,000 meter depth, had no way of knowing when failure would occur, so the best he could is listen to the noises that the composites made with depth and abort if the noises seemed out of character.

Another example of applied research was when the US Army was investigating the effectives of igloos or snow constructed walls to stop bullets. This investigation into bullet stopping was more predictable than hitting a rock with a wooden dory.

So, where is the useful information. Any construction that allows flex or resists impact is a bonus. The ability to quantitify the benefit is not known unless someone wants to spend bucks.

Take advantage of any emperical or intuitive advice. The cost is cheap and the results may exceed your expectations.

Ants
Permalink Reply by Phillip Westendorf on February 2, 2015 at 3:32pm

Ants, 

I think you hit the "Nail right on the Head".  I can't imagine the variables there may be in writing a program to do the math for the "answer".  It's certainly beyond my capabilities.

Good Job explaining it, above.

Dorf.

Permalink Reply by tungsten on February 4, 2015 at 8:13pm

Mike,you could probably save yourself a few bucks on the more expensive triax and just stick with the cheaper biax 18 oz,your plywood is already giving you layers of 90 and 0 so why pay for an additional one.biax gives you 2 tows of 9 oz 45/45 giving you strength for twisting,the triax is only 6.5oz ish each.On top of that the biax is so much easier to handel and wet out.

What you think?

Edit to add I don't like the feature of posts showing up in the middle of  a thread, its like butting in front of someone.

Permalink Reply by Chris on February 17, 2016 at 5:30pm
Pardon my interruption, where to you find the plastic panels around here?
Permalink Reply by lhedrick on February 2, 2015 at 8:12am

I wouldn't get too caught in the strength of materials issue.  You options are limited by the total weight you can live with.

My friend with the decked Tatman really over built his boat.  Bottom is 1/2 inch plywood. Plywood for all the bulk head and it's a framed boat.   Plywood deck and hatch lids.  His boat could be 600 pounds.  Now he is a big strong rower, in addition he is very skilled and just doesn't hit rock or run low water.  I am 6 foot 4 and weigh 230 and that boat is way to heavy for me.  My boat is all honeycomb composite and it could be 275 pounds lighter then his all wood boat might even be less.

There is a practical limit to all this stuff.

I did a lot of impact testing over the years and when you put bulk heads or frames in a dory and then hit a rock right next to a solid frame the plywood will fold in.  The only option is to move up to 1/2 inch plywood and pay the price with a heavy slow moving boat.  What's next 3/4 ply wood, 1 inch plywood.  I'm not going to want to row it.

Your only option is 1/2 in plywood and glass or 3/8 plywood with more glass.  And by the time you add more glass the difference in weight isn't gong to be enough to make that much of a difference.  You can move up to kevlar at 5 times the cost.  You could add carbon at 20 times the cost of glass and it't too brittle away.  You can move to composite core but, this is a wooden boat forum so this is not the place for that discussion.  As Rick mentioned you can also read past posts on montana-riverboats.com as that is not just a wood forum.

Today I build with 3/4 inch composite bottoms and leave space under the bulk head to allow the bottom to flex if impact occurs.  The space can be sealed with neoprene, raft rubber etc to make each compartment water tight.  This adds to the fabrication  time so I now think it's easier to just increase the thickness of the honey comb core material and add another layer of fabric outside.  The added weight is just simpler and composites are already light.

There comes a point where saving 5 percent takes 10 times the cost and 10 times the fabrication.  At that point the effort yields little to make the invest somewhat pointless. 

Permalink Reply by Mike Baker on February 5, 2015 at 8:10am

Good advice. I have use the biax. before, been a while and don't remember if it was any easier to wet out.

Never thought about the weight of the individual fibers. The triax is actually a few cents less than the biax at Fiberglass supply (lotta plugs for these guys lately :)).

Mike

Permalink Reply by lhedrick on February 5, 2015 at 2:11pm

I have used 16 ounce biaxial on bottoms.  It's about all I can handle alone.  With 2 people it goes well.  If it's summer the temps are up around 95 wait till the next morning.  It gets hard to wet out as the epoxy kicks.

Permalink Reply by tungsten on February 5, 2015 at 5:02pm

Funny here in Canada triax 22 oz is twice the price of 18 ozDB.This is why I mentioned it.

The triax always distorted on me, each tow would move too easy and make  a loop every time you pulled or moved it around.The 18 DB is much easier to cut roll up,unroll move around with out fraying.You want a headache try 12 oz 0 -90 cloth.

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