16 Lb. urethane foam vs. plywood

[swimmin' for shore]

Fortunately for me, I was around for the "ping pong balls can float a boat" discussion, so this cannot ever be proven as the worst idea in boating history....not to mention the guy who wanted to put a 150 hp motor on a 14' boat last week....at any rate, I was building a cooler out of 8 lb. urethane foam last week, and I gave some of the overspill a beating with a hammer. The stuff was strong as could be, and hard also. I had just finished a new transom for my 78 Sea Nymph, and got to wondering if you could use the 16 lb. version of urethane foam to cast and build a transom. I looked it up on uscomposites.com and discovered the strength properties of it, and it's incredibly strong. I tried to look up the properties of plywood, as well, but have had no luck. I wanted to bring it up here, as so many of you have so many fantastic ideas. Would a transom, cast from urethane pour foam and then glassed over, be strong enough to support a motor under throttle?

 

[Kajun37]

Re: 16 Lb. urethane foam vs. plywood

how much would it expand once inside those thin sheets of fiberglass?

 

[Tail_Gunner]

Re: 16 Lb. urethane foam vs. plywood

8) abit i have been there, that is poured 9lb foam after installing stringer's and asked my self why.

The answer is simple, it only takes a day or two to build and lay wood stringer's, now if you pour that high strength foam behind it how can you go wrong?

Dont short change your rebuild, you will never have to reguess yourself when on the water with your loved ones...................8)

 

[tashasdaddy]

Re: 16 Lb. urethane foam vs. plywood

i would think there would be too much flex, thus cracking the glass.

 

[Bondo]

Re: 16 Lb. urethane foam vs. plywood

Well SfS,.......
If you were around for the Ping Pong Ball debate,....
You probably remember that I'm an advocate of Foamless Boats,......
Soda Pop Bottles maybe,.... But NO Foam.........

I can't feature a Foam Transom,........ I wouldn't even use Seacast.....
Muchless Foam.........

Most of the boats that We're all working on are 15, 20, even 30 Years Old..........
If the Wood has lasted as Long as it Has,.......
Well,........ Just How Long do you Expect your Rebuilds to Last,..??......

 

[ondarvr]

Re: 16 Lb. urethane foam vs. plywood

There have been companies that used high density foam in the transom instead of wood, not pour foam though, they use better grades of sheet foam. Cost is the main reason they don't use it more often, good, high density sheet foam can be very expensive.

 

[Mark42]

Re: 16 Lb. urethane foam vs. plywood

There are lots of hi-tech composits you can use. The problem is hi-tech = high dollar. And 50 years of plywood transoms prove that they work. Even if they rot when wet.

 

[A Fn Noob]

Re: 16 Lb. urethane foam vs. plywood


Id like to have some of that 16lb pourable foam just to test.... I was amazed at the strength of the 4lb pourable foam that I used under the deck in my boat. I cant imagine what 16lb foam would be like. It sure is expensive though!

I think omitting foam, especially when it was there before, is insane, from a safety and strength aspect.

I tabbed in 3/8 marine plywood sections running perpendicular to the stringers (Im sure there's a term for this, but I dont know what) to "compartmentalize" the area under the deck, so that if one section were to ever get saturated, the others will not.

Back to the original subject, It seems that most of the projects Ive seen with foam&glass stringers just make the stringers wider, and are rectangular in shape. It seems that if they were fashioned in the shape of a "T" or an "H", that it would add much more strength. Thats what I'd try to do anyway.

 

[Tail_Gunner]

Re: 16 Lb. urethane foam vs. plywood

On a more serious note id think you'd find laminated fiberglass strength on par with a steel beam, sheer strength is one thing tensile strength is another.

 

[swimmin' for shore]

Re: 16 Lb. urethane foam vs. plywood

Ah, you guys have been busy with answers. I want to see what I can do to answer all of this at once.
Kajun, I don't think I hadn't really thought of this in terms of a fiberglass boat, where you would re-use the old fiberglass skins of the transom. I really was thinking of it in terms of my current transom, which is laminated plywood that drops into place. I'd be more likely, in my scenario, to make a mold for the foam, pour, sand off the excess, fiberglass the whole thing, and put it in place of the plywood. It would, without doubt, be lighter than the plywood transom. The question is whether or not it would have the flexural strength of the plywood transom.
I don't disagree that plywood has worked for a long time. I totally agree with that. I just redid my transom for my big aluminum boat, and I glassed it in with epoxy and sealed all thru-hulls and screws with 5200. It's solid as a rock now, and If it had always been done that way, I wouldn't have had to replace a soggy transom. The things that we've always done certainly work, but WHAT IF someone proved to you that you could make a transom that weighed half as much but still had the same strength? Like epoxies and 3M 5200, we'd be all over it, right? I could be totally off base with this whole thing, but I was born in Illinois. You just gotta show me.
I still can't seem to find the properties of any kind of plywood, but from uscomposites website, I'll list the properties of 16 lb. expandable foam below. In the meantime, I think I'm going to email uscomposites.com and see if they'll add to the subject.

Product Information:
Free Rise Density: 16.0 lbs cubic ft.
Expansion Rate: Approx. 4x Liquid Volume
Buoyancy (flotation): 46 LBs per Cubic Ft.

*Physical Properties:
Parallel Compressive Strength: 580 psi
Tensile Strength: 450 psi
Shear Strength: 230 psi
Flexural Strength: 750 psi

 

[CodyGrizz]

Re: 16 Lb. urethane foam vs. plywood

I was watching VS the other night and they were building a 32' center drive fishing boat and guess what they used? Thats right, 16lb foam. They said it was WAY easir to cut and use. They used a blue glue/caulk to seal it in. Then they clamped it in while it dryed. Then glassed it in. And they used dual outboards. It was a Florida company..was pretty interesting to watch.

 

[swimmin' for shore]

Re: 16 Lb. urethane foam vs. plywood

Oooh. We're getting closer to proving my theory(which apparently is not mine, if it's being watched on TV).

 

[Mark42]

Re: 16 Lb. urethane foam vs. plywood

I was watching VS the other night and they were building a 32' center drive fishing boat and guess what they used? Thats right, 16lb foam. They said it was WAY easir to cut and use. They used a blue glue/caulk to seal it in. Then they clamped it in while it dryed. Then glassed it in. And they used dual outboards. It was a Florida company..was pretty interesting to watch.

Interesting. Does that program have a website? Maybe they have some info on the project there.

 

[shadetree]

Re: 16 Lb. urethane foam vs. plywood

Hey Kajun37,
How much did that catfish weigh? Not to change the subject, but that thing is mighty impressive.

 

[Bondo]

Re: 16 Lb. urethane foam vs. plywood

While I'm still watching this thread,+ Don't believe it's the Right way to go,.......

When you're looking up all these properties of the Foam,.......

I Question it's Compressive Strength,..??..??.....

If this is an Outboard boat,..... What's to Stop the Clamps,+ Bolts from Compressing INTO the foam,+ becoming Loose,..??...
If it's an Outdrive boat,... Same Question......

 

[CodyGrizz]

Re: 16 Lb. urethane foam vs. plywood

I looked for a website but dont see the name of the show. When I see the show again I will jot down the site for the show. The foam they used was stiff as heck, but way lighter they said. They use to use like 1.5-2" pressed wood then tried the foam 2" of foam was less than half the wight and just as strong. Then you glass it, it was like wook. And these are some big boats, so weight is always a premium, especially when you throw 2 or 3 big outboards on it. It was cool, they were talking about getting a special spray style to build smaller boats out of..like gel coat, spray in glass, then the foam, then glass again.

 

[swimmin' for shore]

Re: 16 Lb. urethane foam vs. plywood

Bond-O, I write this stuff from work, but I'm going to drill a bolt through a piece when I get home, and then try to work it loose in the foam, so I'll get you an answer on that. I'll try, within limited resources, to put an excessive amount of tension on the bolt with a pipe wrench or something of the kind. In answer to the other half of your comment, if you were to glass the skins, or just glass the inside on my aluminum boat, with the aluminum skin of the hull for the outside, the bolt would compress the foam between the outside and inside skins, so I believe that's where the pressure would be the greatest...which is why we use washers to back our bolts.
I would like to see the website for that show, Cody. I did a lot of research the other night, and I know that there are a LOT of different polyurethane mixtures out there. I found one in particular that advertises a much higher strength than plywood, by using glass reinforcement in the actual foam. If it's only a matter of mixing in fibres, any of us can do that, but you probably wouldn't want to pay 197.50 per 1/2" 4x8 sheet of theirs. At the same time, for one time use, there's no way that this could be as cost-effective as actually building from plywood, mostly because of the need to build a mould for the foam.
Also, to add to what I've already said in the first posts, it's always been accepted that laminated things have higher strength properties than non-laminate. Thus, we have plywood. In this case, it might be better to build up a 2" or 3" transom in 1/2" laminated sheets. For that matter, if you're willing to invest the time, you could glass with woven cloth or mat between the laminated sheets, for even greater strength. While doing that, you'd want to pay close attention to the weight gain with that much glasswork. I'd hate to get through a whole transom, just to discover that it's 4 lbs. heavier than it's wood counterpart.

 

[croSSed]

Re: 16 Lb. urethane foam vs. plywood

It's not necessary to pour this foam. You can purchase it already formed and cut. I'd use heavier foam for a transom, though. I'd go with at least 20 lb. Go here to check this out. I'm probably going to use all foam (pre-formed and cut) in my boat. See Avenger Project. Many boat manufacturers these days have completely removed wood from their boats (Skeeter, Triton, Allison hasn't used wood in their boats for many years).

www.generalplastics.com

TG