Conversion of plans from plywood to Honeycomb or foam

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cptpaco1
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Conversion of plans from plywood to Honeycomb or foam

Post by cptpaco1 »

Hello,

This might be a dumb newbie question but... Is there a conversion or rule of thumb that allows you to use honeycomb or foam materials when taking plans that call for plywood? thanks for the advice.



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Re: Conversion of plans from plywood to Honeycomb or foam

Post by fallguy1000 »

cptpaco1 wrote: Mon Sep 26, 2022 6:11 pm Hello,

This might be a dumb newbie question but... Is there a conversion or rule of thumb that allows you to use honeycomb or foam materials when taking plans that call for plywood? thanks for the advice.
Nope. Every boat and every structure requires redesign of the laminate for core changes.
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viewtopic.php?f=12&t=62495

cptpaco1
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Re: Conversion of plans from plywood to Honeycomb or foam

Post by cptpaco1 »

Well that is a let down but i appreciate the reply

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Re: Conversion of plans from plywood to Honeycomb or foam

Post by fallguy1000 »

cptpaco1 wrote: Tue Sep 27, 2022 2:00 pm Well that is a let down but i appreciate the reply
Well, you must consider all the potential variables.

Suppose a boat is made with 6mm ply and you decide between 10mm and 12mm foam, or 10mm ply and 12mm or 18mm foam. Everything changes and so do the skins.

To make things even more complicated, and this happened here once where a fellow didn't realize the complexities, the hullsides to bottom and hullsides to deck interfaces require major modification as well, more if the boat is going to run high hp engines. If nothing is done, when the boat launches off a wave; the vertical foam will crush the horizontal bottom and deck foam. In a very short period of time; the boat will be destroyed.

If you want a cheap learning on the subject; you can buy Dave Gerr's book, "The Elements of Boat Strength".
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viewtopic.php?f=12&t=62495

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Re: Conversion of plans from plywood to Honeycomb or foam

Post by OneWayTraffic »

That book will give you an idea of scantlings that work for all sorts of different materials and some guidance on building details but it is not a boat building school. I extracted the scantlings on a spreadsheet once. I think I posted a link to it on one of my posts once. I’m on a tablet and don’t have it now. Basically there is no weight savings in boats that most build here, unless the thought of putting a hole in your boat on impact doesn’t concern you. You need a thick enough skin to resist rocks. Once you meet that you are almost at single skin weights, and about twice the weight of plywood glass epoxy.

Foam can be used for cabins, seats, roofs, coolers and the like. Not much engineering required for that. If it’s stiff enough it will probably be ok.

It’s never cheaper to use marine foam.

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Re: Conversion of plans from plywood to Honeycomb or foam

Post by rick berrey »

There is a standard math equation ( rule of thumb ) to convert ply to foam , rule of thumb is also 25' or longer to be worth while , many of the larger boats here already have a foam option

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Re: Conversion of plans from plywood to Honeycomb or foam

Post by TomW1 »

If you build one of the boat plans on here many of the plans 19' and up have been converted to foam. Just check the plans to see if it has. Just remember a foam boat will cost 21/2-3x a plywood boat due to the cost of the foam and extra fiberglass add epoxy.
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Re: Conversion of plans from plywood to Honeycomb or foam

Post by OneWayTraffic »

From Gerr, very short version:

Multiply Beam, length and height from keel to sheer. If in feet divide by 1000. Metric divide by 28.3 This is the scantling number Sn.

Now for single skin glass on the hull bottom for a non planing vessel the thickness would be 0.29xSn^0.333 inches. Or 7.3xSn^0.333 mm.

Increase by 1% for every knot over 10 knots.

Now for a cored build Gerr recommends 40% of the single skin outside the core and 30% inside. But both must be at least 2.5mm (0.1") regardless for impacts and the like. The core would be twice the regular skin thickness and fairly dense (8lb). This is for a polyester roving mat layup. Epoxy and biaxal glass would probably save weight here. You could also lower the safety margin and use thinner skins.

But what if you used a much heavier, stronger core, one with structural properties?

Then you wouldn't need nearly as much glass on the outside for impacts, and the core itself would add to the strength and stiffness, not just keep the skins apart. Thats where plywood comes in. It's stiffer than glass on a weight for weight basis, indeed stiffer than aluminium and even carbon fibre on a weight for weight basis. It can also resist impacts. So one layer of glass each side of a ply core is stronger than a similar weight of glass. Considerably so.

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Re: Conversion of plans from plywood to Honeycomb or foam

Post by Evan_Gatehouse »

I like Dave Gerr but rule of thumb conversions are pretty inexact.

I built the bridgedeckcabin of my 40' catamaran with a roof that had 666 gm/m2 stitched triaxial outside skin. That was <1mm thick. Not mat and roving but you're an idiot if you spend the money for foam core and use mat and roving on it.
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Re: Conversion of plans from plywood to Honeycomb or foam

Post by fallguy1000 »

Evan_Gatehouse wrote: Tue Oct 18, 2022 12:03 am I like Dave Gerr but rule of thumb conversions are pretty inexact.

I built the bridgedeckcabin of my 40' catamaran with a roof that had 666 gm/m2 stitched triaxial outside skin. That was <1mm thick. Not mat and roving but you're an idiot if you spend the money for foam core and use mat and roving on it.
600/225 was used on my boat and I regret using it for tabbing,,calculate about 150# more glass and 150# more resin...it was one of a couple places I lost advantages of foam core

The roof of the Skoota I built was built with hexcel about 600g woven, a really neat fabric, the bottom was 6mm ply because I wanted it to support walking loads...the savings for weight would have been maybe only a little if vac bagged.

Not to get too lost in the weeds, rule of thumb is kinda like using 600/225 for all the tabbing in a lightweight build. Wrong.
My boat build is here -------->

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