Had an idea I thought I'd run by you guys. I've been planning on building a light 14 footer to go along with a homemade short tail. I like the idea of being light enough that I can drag the thing off of anything I happen to run it up on if I'm by myself and I like being able to launch anywhere with a fourwheeler if I need to. Well I was thinking, If I fully welded in a floor about an inch and a half off the bottom of the hull and then cut some holes to pour in some of that 8lb per cubic foot foam and let it expand, I bet I could get away with not putting down any stringers. That stuff is hard and with the expansion it would make the bottom super rigid. Save money and weight and get almost all of the flotation under the floor. Any of you boat builders have any thoughts on this? Don't talk crap about the title. I just wanted to make sure you @ssholes gave it a look.

Not a boat builder, but fall into your @sshole category. The pour in foam you speam of will not fill in the way you imagine, you would need a "revolutionary" product to do that. It is however very firm when it dries, though the air trapped inside of it allows it to dent pretty easily. The hull may help reduce that with a solid foam backer, in theroy, but only testing it will let you know. If i was to do what you are purposing, i would put atleast a 1x2 stringer down the center before putting in the foam. This would give you a place to brace the center of your floor, as well as a guide to help cut the foam after you fill in the now, two sides of your boat. You could also use a few strips of luan on their edge as cutting guides, but i think atleast one brace down the center would be well worth the weight and effort. Having a guide would give you the ability to ensure you have foam all the way through, avoid dead spots that may cause problems like where you stand, and add to the rigidity of your hull. Though once again, not a boat builder, but that's how i would do it if asked.

It's just gonna get water logged.

stupid is as stupid does!!!!!!! just poking ya

I think you may be right about the one stringer. More so to keep it from bulging out the floor if it were to expand and not have anywhere to go. And no it would never even see a drop of water unless you ripped a hole in the bottom. The whole idea is that the floor is welded in with no gaps so that when the foam expands you can get it to fill every little bit of void and put a little outward pressure on everything. Somebody build one and let me know how it works ha

Just build the whole thing out of foam and use it as a couch in your den. This way you don't waste any aluminum.

Haha I can see a couple problems with that... First the 8 lb per square foot expanding foam is about as hard as pine and I like to be comfortable. Second I would get asphyxiated when workin on my motor.

theres not a whole lotta weight in the stringers, probably what the foam would weigh and the foam would hold water making it heavier after using it....

You guys aren't getting what I'm saying. The foam would never get wet. It would be sandwiched in between the bottom of the boat and the floor that would be welded up from bow to stern leaving no way for water to get between. I realize the stringers aren't that heavy, but having high density foam between those two layers would reinforce both the bottom of the boat and the floor. This might allow me to use thinner aluminum especially since its a smaller boat which would actually be a significant weight savings. I'm thinking 40 inch bottom, any wider and I'd be less comfortable with it. Just think about it this way, gotta put foam in the boat somewhere, might as well engineer it so that the foam is more than just flotation, its also structural, and out of the way! If you don't need the stringers, why waste the weight, money, and time? I know a lot of you guys don't believe in this, but lighter and simpler is better.

Problem i foresee with it is being able to pour the foam in a way that would ensure every square inch of space is filled without the expansion pressure pushing the floor and the hull apart towards the midline of the boat. like 55@28 said, maybe put one stringer down the middle would keep the floor from bulging. possibly a long hose and pumping a known amount of the foam in starting towards the transom and putting in layer after layer and letting it harden while working your way towards the bow.

Everyone gets what ur saying its just a dumb idea, move on.
The foam is sandwiched between two pieces of aluminum so it can't get wet. Listen closely, it'll still end up getting wet and soaking water like a sponge till one day u got to pressure was it out bc ur rig won't plane out
Move on

Your plan is understood, i was pointing out a way to do it, though i never would. One thing you are missing on floatation, if you weld it up air tight, there is no need for foam, as you said it will never touch water, so what magic property does it have to improve boyancy while being totally divorced from water. I understand the rigidity part, floatation does not come into play here.

The foam has no tensile strength. Yea, it's relatively strong in compression but there would be nothing to keep your boat from bowing down when you stand in it and up when you hit something. Try this, go to Home Depot and find the sheets of closed cell foam they sell. Grab a sheet and try to break it over your knee. Then take the broken sheet up front and pay for it and forget your ever brought this idea up.

And before you say it, yes the floor and sub floor would give you some strength, but not enough. Hence the reason why builders put ribs in boats.

maybe i just need you to enlighten me. Is there some magical process by which water goes through sheet aluminum. weird they build boats out of it if thats the case. You think its impossible to keep water away from the foam, but its possible to keep water from coming through the transom of your boat? How about somebody who knows what they're talking about take a crack at it.

You can do a search on here and read about the many folks who have opened their "sealed" compartments housing foam to remove what turned out to be heavily water logged foam.

And those are factory sealed compartments not something someone did themselves.

I don't think anyone really knows why or how it happens, just does.

You came on here and asked a question, and then got the answer.

Sorry if you don't like it, but please don't argue with it. Everyone is speaking from experience which is why you came here with the question in the first place.

handimans post made sense. the flexural strength (bend strength) of the foam I'm talking about is 750 psi. anybody know how that compares to aluminum?

yeah have somebody hold a small foot long of .125 aluminum tubing and another with foam, then judy chop'em and tell me which one broke your hand

plus everybody is always tryin to cheap their way into this badass way of ours life by attempting to play bob the builder. Just save up and buy you a good 6.5 LT 16x36 rig from GD or holler at uncle j or another builder on this site and you will not be happier, it'll roll behind your 4-wheeler and everything. your welcome for the advice and this thread is solved good day.

Without the aluminum bracing, welds WILL crack.
When welds crack, water gets places it is not ment to.
When foam contacts water, it absorsbs water.
When foam absorbs water, it gets very heavy.
When water logged foam gets very heavy, your boat performs like shit.
When your boat performs like shit, you get on here and ask questions that have been asked a hundred time.
When you ask these questions, people tell you to use the search feature.
When people tell you to use the search feature instead of answering your question, you get mad.
When you get mad, everyone joins in on phucking with you.
When everyone phucks with you, you get really mad.
When you get really mad, someone gives you an invitation to Academy.
When you show up at Academy, you get an ass beating and wake up in a roadside ditch.
Don't wake up in a roadside ditch.
Just phuckin listen to everyone.

If you really want to know about foam as part of a boat structure, read up on Boston Whalers. You are describing how they are built, kind of. They use a mold on the outside of the hull and deck. They inject more foam than the volume between the deck and hull requires. After the foam cures, they remove the molds. Without the molds the hull and deck would blow apart as the foam cooked off.

I still don't think aluminum with foam is the way to go. Just thought I would tell you where to learn about it.

thanks over the line. haha you think I'm going to let you guys get me mad? I will say that I'm getting quite motivated. I'm not worried about the foam getting wet. It doesn't happen to everyone who has a sealed bottom. And obviously manufacturers would offer that if it were a sure thing to get waterlogged. when that happens its just got to be an defect. The way you take advantage of something with a high compressive strength like this foam is you put it in a sealed container.

Concrete has similar tensile strength to the foam. I imagine that you would agree that you could probably take a 1 inch diameter cylinder of concrete and break it pretty easily. Now imagine putting that concrete inside an aluminum tube.

Concrete:

Density : 2240 - 2400 kg/m3 (140 - 150 lb/ft3)
Compressive strength : 20 - 40 MPa (3000 - 6000 psi)
Flexural strength : 3 - 5 MPa (400 - 700 psi)
Tensile strength : 2 - 5 MPa (300 - 700 psi)

Product Information:
Free Rise Density: 8.0 lbs per cubic ft.
Expansion Rate: Approx. 8x Liquid Volume
Buoyancy (flotation): 54 LBs per Cubic Ft.

*Physical Properties:
Parallel Compressive Strength: 250 psi
Tensile Strength: 225 psi
Shear Strength: 130 psi
Flexural Strength: 350 psi

Now i said similar. But you wouldn't want it to be as stiff as concrete anyway

I guess i can argue about it or I can go build it

if you didn't want our opinions then why ask the question? you seem pretty set in your ways. Since you know all, you should go do that to your boat.

http://www.bostonwhaler.com/Page.aspx/p ... haler.aspx

click on the unique unibond construction process vid

you guys think it will work with fiberglass but no way with aluminum?

We really don't care what you waste your money on and do to your boat.

I do want them. I wan't an educated opinion though. Not some crap about no matter what you do its impossible to stop your foam from getting waterlogged cuz its simply not true. Check the specs I posted. It's doable on a small boat. If it works then its nothing but advantages. The only welds to crack will just be rake welds and the transom welds and that will be reinforced with aluminum tubing anyway. That way I don't end up in a roadside ditch

Educated opinion is; it's a bad idea. Not only because the foam will get water logged, but also because it will not be strong enough. The boat will crown and the welds will pop. Like DDB said though, nobody really cares. If you want to waste your money and time, have at it. Post pics of it when it starts falling apart so we can all laugh and say I told you so.

What is the difference in weight? How many cubic feet of foam equals how much? How much will aluminum stringers weigh? I see no significant weight savings and alot less headaches in the near future.

The foam definitely needs reinforced. But would you agree that concrete or foam can be reinforced from the outside like the example I gave? What I'm really proposing is a piece of 2 x 40" rectangle tubing with foam under pressure. foam reinforces the aluminum and the aluminum reinforces the foam. I'm looking at removing 4 stringers that weigh 10 pounds a piece. The foam would be approximately the same weight so no weight savings there, but there would have to be flotation somewhere anyway... so it does save weight there. Not much. Like I said since the bottom is backed it may allow for a thinner hull which would offer significant weight savings. you guys might be right but I would love it if you weren't. I'm going to get some of the foam and poor it into aluminum tubing and test the strength to prove the concept. I'll let you all know how that turns out

who wants to start a pool?

i will take the split of the Oklahoma season when he pulls it out.

How much weight savings do you propose by using thinner aluminum? What gauge? Have you ever look at the weight per sqft2?

And my boat doesn't have any foam so yours is heavier than mine already

Phuck it. I'm going build a mud boat out of concrete.

I'm Putting some shoulder straps on a longtail and wearing skis.

Actually, I'm pullin for this to work as well. I'd like to see it. The boat would still need cross bracing to keep the sides rigid.


Sent using my bag phone

ha at least one person wants this to work

like a whaler, carolina skiff and others like them are build like you are attempting in alum. its been awile back but the carolina skiff post addressed the same problem, water soaked foam.

correct me if im wrong but if you were to use closed cell foam you wouldnt have to worry about water absorption. not sure who you know but i got a buddy whos a lineman and they got this closed cell foam they use to set poles that sets up harder than concrete and its relatively light from what i remember. maybe you could find something like that

I'm the one with the Carolina Skiff post and a Boston Whaler guy. Lots of Foam in my Cat 5. Closed cell foam will hold water but it has to come from somewhere. In the case of my Lawsuit Carolina Skiff, I am almost sure that there was an open area in the rigging tunnel. All the wet Boston Whalers I have known were because of holes drilled to mount stuff above and below the waterline or rotted drain tubes.

Good luck if you go this way but your R & D is gonna cost.

God forbid a man argues his point with you people. I am truly sorry if I hurt anyones feelings. You obviously can't be sensitive if you want to have a conversation on here. Like people have said, if water is going to get into the foam, it has to come from somewhere. Cracked welds, poor job sealing all the way around. Is there anybody on this forum who has foam sealed in the bottom of their boat and hasn't had this problem? Who actually has experience with the stuff I'm talking about? Thats who can back me on the fact that its strong. And nobody is spending a ton of money. It's cheaper than aluminum. There is one grade of foam that is higher density than that which is literally as stiff as concrete. Its heavier though of course so I would have to cut the gap between the floor and the hull down to 7/8 of an inch so that it wouldn't weigh more than the stringers.

Tensile Strength: 450 psi
Shear Strength: 230 psi
Flexural Strength: 750 psi

The flexural strength is actually more than really stiff concrete and the tensile is right on par with medium grade. (I just use this example so we have something to compare it to)

Now if there was a material that was light, floated, and as stiff as concrete, and it could be reinforced by pouring it into a tube. I would imagine that would be strong enough. I will also be putting a couple braces laterally across the boat like gator trax does. Those are very important. The force exerted on the hull when you come over a wave at an angle is what cracks welds. It puts the hull into torsion and thats what the cross braces prevent. The only reason mud boats have the longitudinal bracing instead of horizontal is to preventing hooking the hull. The cross braces are actually better and thats why you see them in every other boat.