structural foam for a floor

[grady edwards]

I am just starting to build my first skiff and the plans call for 3/8 plywood on the floor. I am planning to glass the whole floor and was wondering if I was to use the structural foam sheets if it would hold up as well as a wood sub floor.


Thanks for any input

Grady Edwards

 

[Mr Efficiency]

For recreational use, a suitable foam sandwich is perfectly adequate. A thin-skinned sandwich panel is vulnerable to damage from heavy impacts, but it ain't the end of the world if that happens, it can be repaired easily enough.

 

[PAR]

Welcome to the forum.

I'd strongly recommend you contact the designer of your boat, as you don't know how much loading this sole plate is intended to absorb. As mentioned, sandwich construction (foam core) can work, but this needs to be a real sandwich structure, with the appropriate load bearing skins (sheathing thicknesses) worked out. Most of my designs use the sole plate as a structural element, not just a cover to prevent folks from tripping over stringers. This means a substitute needs to carry the same loads and this requires a reasonable laminate schedule for your sandwich panel.

 

[TANSL]

Most of my designs use the sole plate as a structural element

I do not know how to interpret this and can be an interesting discussion. In my experience, the thickness of the bottom or side plates, as well as the resistant modules of the bottom and side reinforcements, are calculated without regard to whether there is a sole panel or not. It is true that the existence of a sole greatly increases the longitudinal strength of the hull but this study is not usually necessary in these small boats. It is necessary in the case of large openings in deck and in some cases of transverse structure. In general, it is sufficient to place some longitudinal girders with adequate resistance..
It is curious to note that the design loads proposed by the different regulations for the decks are, in small boats, very low and it is necessary to calculate these items according to the specific loads they must bear (weight of the equipment on deck, weight of people, etc.) or, as Mr Efficiency points out, by impacts on its surface. In short, local loads but almost never as a structural element of the whole.

 

[PAR]

You're essentially correct TANSL, though think of how much weight you can save if you did incorporate the sole plate into your scantlings calculations. In this case, a skiff, which I assume is fairly small, the sole is going to greatly contribute in athwart stiffness, as well as bottom rigidity. This permits smaller, wider spaced partitions and stringers, assuming taped seam construction. I guess I'm jumping to a number of assumptions and I know where this can lead, but weight in small craft is a real bugger, if you'll looking to get the most from the materials and power you provide. I'm working on a moderately deep deadrise, offshore powerboat currently, in taped seam construction. At 18' (5.49m) on deck It only requires 3 longitudinal stringers, of relatively dainty proportions (lots of depth because of deadrise), a handful of athwart partitions and a perimeter cleat for the sole plate to land on, which is also part of the structural mix. The result is a triangular shaped box beam that's considerably lighter and a fair bit stiffer, had I not incorporated the sole into the equation. Simply put, less materials, less weight, which equals less to buy, cut and install, plus the performance benefits this approuch brings to small open boats (or any boat for that matter). My design isn't anything new, though it is about 1/2 the weight of a conventionally framed boat of similar proportions. Some of the savings were placed in freeboard and other features, given her offshore fishing role and additional reinforcement in key areas.

My concern is the next, seemingly obvious questions from the OP. Such as, how many layers of 'glass do I use? Do I use the same count on both sides of the foam? How do I handle the hard points and edges? We can just make a guess based on experence, which will offer him a heavier than necessary, resultingly more costly laminate schedule or we can have a look at this design and make a more informed reply.

 

[TANSL]

, lets say LlR, Bv, D NV,The criteria that I use for the scantlings of the small boats, apparently, are totally different from yours. I do not study the ship like a beam subjected to the bending moments in sagging or hogging and, therefore, I do not add any load due to these solicitations. The calculations are made taking into account only the local loads and, therefore, the fact that it exits a sole or that does not exist, as I said before, does not influence at all in the dimensions or thicknesses of each structural element. Now, if the boat has problems of longitudinal strength, I add longitudinal elements (not necessarily a sole, which would greatly increase the weight). This does not mean that the frame gap can be increased but, as the panel width probably decreases, it may allow me, in very few cases, to decrease the thickness of the panel.
Of course, I find it hard to believe that placing a sole reduces the overall weight of the boat. Perhaps I will investigate this matter.
In small boats it is very important, as in any other, to get the minimum weight hull but I do not think that there are many builders of small boats of FRP that are able to do that, not even to know how it is done. Nobody studies the tensions in the layers to know how to deduce the weight of each layer. Few know or a few are concerned, for example, how to reduce resin%.
I have seen shipbuilders in GRPs much more concerned with getting the lower-cost structure in man-hours than the lower-weight structure. And in each of these cases, the composition of the laminate may be totally different.
And, by the way, if you want to do things correctly, you should advise the OP to put the same layers on both sides of the core. Not just the same number of layers but the same composition. That's not how many people do it either.

My design isn't anything new, though it is about 1/2 the weight of a conventionally framed boat of similar proportions.

Frankly, I do not know if that says very good things about your "designs" or very bad things about others. In any case, a 50% reduction in weight seems excessive. My experience is that the weight of a structure calculated by applying a regulation, lets say LlR, BV, DNV, can be at most 10% (and that is already a lot) heavier than calculated by direct calculation

 

[PAR]

The 50% reduction is a general observation, comparatively between traditional plank over frame to taped seam builds (which is the type I'm currently working on). I've run the numbers on a few, with obvious variances, but a well designed taped seam will be about 50% that of a traditional plank on frame. Since this is (again assumed) a production GRP boat with a need for a new sole, savings will be minimal, as these are price point built, resulting is less than desirable weight, resin/fiber ratios, etc. They just build it stiff and strong enough to fulfill its warranty period and appropriate certifications. Don't even get me started on class association requirements and guidelines.

Back to the OP's needs, yes a foam cored sole is possible, though you'll want to use the right foam, with an appropriate laminate schedule and hardpoint details. You might save a few pounds, though this will be modest at best. The biggest savings will be a sole that doesn't rot again. How big is this skiff and can you post photos? On a 17' or less skiff, modest V or flat bottom with modest power (less than 50 HP), a 1" (25mm) thick core with 2 layers of 12 ounce (400 GSM) on each side of the foam, with solid laminate tabbing around the perimeter of 4 layers of 12 ounce cloth will get it done, in most cases. I'd prefer to see the tabbing in biax (3 layers of 12 or 2 layers of 17), instead of cloth, but small, lightly powered and loaded boats can get away with cloth. These would be rough, likely overly strong schedules, for a small power skiff.

 

[TANSL]

What would you think if I said that the welded construction saves weight in relation to the riveting ?. You would say that of course, but you would add that two different constructive systems should not be compared, it is misleading.
Traditional plank over frame and taped seam builds are different constructive systems, imo.
Going back to the OP's question, structural foam sheets is not necessary. I'm closer to what Mr. Efficincy says in post # 2.

 

[Mr Efficiency]

A sandwich panel of equivalent weight, properly done, will be stiffer that 3/8 ply, need less support points to have the same solidity underfoot, and even offers good positive buoyancy, as well as what PAR mentioned, no rot. Cost will be somewhat more. But really, assuming this is a ply skinned boat, staying with ply seems the sensible option, as little doubt that is what the design calls for. I'd say in a glass boat it would be well worth it.

 

[PAR]

A sandwich panel of the same weight of 3/8" plywood, is a waste of composite materials and material costs. Getting similar physical properties from these two engineering approaches, typically costs a fair bit more (often quite a bit more) in composite. This is why plywood still is highly employed in production builds.

Yes, obviously plank over frame and tape seam are different methods, but it's an often asked request to do perform this conversion, so the results are easy enough to interpret. I'm assuming this is a GRP production boat with a rotten sole.

 

[Mr Efficiency]

No, he is building his first "skiff" he says, I'd assume ply. I suspect he has got the idea to glass over the sole to hide seams and whatever, and imagines he might as well go for all glass (+core). I would not bother doing it, far easier and cheaper to screw down some ply. Or maybe he thinks he can do away with a lot of framing by using a relatively unsupported sandwich panel, but that might have undesirable structural complications. Better to follow the plan on a first build.

 

[grady edwards]

Mr Efficiency you are correct the plans call for glass over ply inside and out. I was looking for options on the inside to reduce some weight of the total boat. I like to inshore fish and make very long runs some times so a lighter boat will help increase the fuel efficiency and allow me to pole the boat in shallow water to trout and red fish easier.

 

[PAR]

The weight savings will be modest at best, maybe 20 - 30 pounds, which is hardly enough to get worried about.