The Waternut Stunt hull build

[Matt_E]

Calculate the volume of the sheet.
Multiply that volume by the density of water.

That's the approximate weight it'll float.

For a 4x8 sheet of 1 inch thick Poly foam, that's 166lbs.

 

[ncbrock]

hmm im was thinking for the scupper on this thing you would need dual scuppers because once the water goes down one gunwhale chances are it wont be coming back up. did you have to like make a ramp so the water goes directly out the scupper?

 

[Waternut]

one sheet will float 60 lbs????

i used the 2 inch thick poly , and maybe half the sheet

does that mean I am only going to float 30 lbs??

I dont believe that , I am going to go float my boat and see what happens ..

One sheet of 1/2" thick 4x8 polyethylene will float 60lbs. Sorry I didn't clarify that.

hmm im was thinking for the scupper on this thing you would need dual scuppers because once the water goes down one gunwhale chances are it wont be coming back up. did you have to like make a ramp so the water goes directly out the scupper?

That's pretty much exactly why you'd need dual scupper unless you're making your own bulkhead and foaming everything in. This is my reasoning on why my later rides yesterday felt more normal instead of nose heavy because I inevitably had 10-20 lbs in the side with the plugged scupper. I did consider the whole ramp idea for the scupper but realized that was over analyzing the situation. When the ski is inclined during normal riding, the amount of water that can be trapped in the chine is virtually meaningless and whatever foam, epoxy, bondo, etc. that you might put in there to neutralize the water weight will add almost as much weight and require a lot more work.

It's a boat... It's going to have excess water at any given time. Turf absorbs water but I'm not going to run less of it.

 

[ncbrock]

One sheet of 1/2" thick 4x8 polyethylene will float 60lbs. Sorry I didn't clarify that.



That's pretty much exactly why you'd need dual scupper unless you're making your own bulkhead and foaming everything in. This is my reasoning on why my later rides yesterday felt more normal instead of nose heavy because I inevitably had 10-20 lbs in the side with the plugged scupper. I did consider the whole ramp idea for the scupper but realized that was over analyzing the situation. When the ski is inclined during normal riding, the amount of water that can be trapped in the chine is virtually meaningless and whatever foam, epoxy, bondo, etc. that you might put in there to neutralize the water weight will add almost as much weight and require a lot more work.

It's a boat... It's going to have excess water at any given time. Turf absorbs water but I'm not going to run less of it.

that is true. good point.

 

[RICHI587]

Dan said he uses 2 liter pop bottles and and puts them in there through the gunwhales. Thats what i was thinking about doing. He said you can fit enough back there to make it float in case of emergency. lol

 

[Waternut]

Ignoring the fact that 2 liter bottles would be extremely ghetto, they don't offer much buoyancy either. You might be able to squeeze 3-4 in the hull if you run front exhaust and even all of those would only float about 16-17lbs. Maybe that's enough... I don't know for sure. I do want to do something in case something bad happens but expanding foam is ABSOLUTELY OUT OF THE QUESTION!!

 

[yamanube]

Based on the area they occupy pop bottles should float more weight than foam correct?

 

[grezzmky]

I used the 2 inch thick stuff on my stck lenth SJ and didnt pack it as tight as ive seen the 1inch stuff packed. I used a LITTLE 2 part on top and left the nose foam out. Ive sunk the ski twice and it floats nose down with about 6-8 inches of the back sticking out of the water (fresh water)

 

[RICHI587]

yes in fact ghetto...but its all about occupying space...right? even if its 1 liter bottles. But i knwo what you mean...

 

[Waternut]

Based on the area they occupy pop bottles should float more weight than foam correct?

2 liters of foam and 2 liters of air inside plastic would displace the same amount of water. The only difference is weight and floatation of the material being used which I would think would be negligible in two things that only weigh a couple ounces.

 

[yamanube]

2 liters of foam and 2 liters of air inside plastic would displace the same amount of water. The only difference is weight and floatation of the material being used which I would think would be negligible in two things that only weigh a couple ounces.

Wouldn't the plastic bottle contain more air than a piece of foam cut to the exact same shape thus be more boyant?

 

[teton]

2 liters of foam and 2 liters of air inside plastic would displace the same amount of water. The only difference is weight and floatation of the material being used which I would think would be negligible in two things that only weigh a couple ounces.

you should look at canoe/kayak air bladders....maybe you can find one that would fit in the gunwales or nose of the ski, from what I remember they are made of pretty thick material too, and would expand to fit wherever you needed it...

I really think that one way valves would be better than a scupper in an application like this, especially with what you have at stake...

 

[Waternut]

Wouldn't the plastic bottle contain more air than a piece of foam cut to the exact same shape thus be more boyant?

That would assume that the foam absorbs water. The floatation is more about displacing water and not allowing the water to sink the ski instead of actually floating the ski. Think about someone sitting on the hood. That won't sink the ski and half an engine bay of water won't sink the ski but a full engine bay will. So if you could displace half of the area that's in the engine bay the ski couldn't sink.

you should look at canoe/kayak air bladders....maybe you can find one that would fit in the gunwales or nose of the ski, from what I remember they are made of pretty thick material too, and would expand to fit wherever you needed it...

I have. A playboat bladder is about 24" long and a normal river runner one is about 30" long. The ones I was looking at are made of 10 mil urethane and designed to hold air indefinitely. They also have extended tubes to blow them up so I can inflate them without taking out components and deflate and clean them occasionaly to prevent mold and rot. When I get home, I'll see which one would work better. They're around $20 each so that's not too bad. Haven't found an exact weight for them but I don't remember them being very heavy when I did kayaking years ago.

 

[yamanube]

That would assume that the foam absorbs water. The floatation is more about displacing water and not allowing the water to sink the ski instead of actually floating the ski. Think about someone sitting on the hood. That won't sink the ski and half an engine bay of water won't sink the ski but a full engine bay will. So if you could displace half of the area that's in the engine bay the ski couldn't sink.

That is assuming the area that you foam is enough to float the weight of the ski. I am just saying if you are limited on space, and the area you can fit foam is not actually enough foam to float the ski, the bottles may be more effective as they are more boyant. Because its not just about displacing the water, its about displacing the water with air.

 

[SuperJETT]

Wouldn't the plastic bottle contain more air than a piece of foam cut to the exact same shape thus be more boyant?

If an object displaces 1 cubic foot of water and the object weighs nothing, then it will have ~62lbs of floatation. If the object weighs 10lbs, then you subtract that from the 62lbs/cu.ft. of water, so you get 52lbs of net floatation. It doesn't matter if it's filled with air or not, all you need is the weight of the object and the volume of water it displaces (and the density of the water).

However, to add to that, some materials compress so as they go further down, you will lose some floatation or if they absorb water, same thing.

 

[yamanube]

If an object displaces 1 cubic foot of water and the object weighs nothing, then it will have ~62lbs of floatation. If the object weighs 10lbs, then you subtract that from the 62lbs/cu.ft. of water, so you get 52lbs of net floatation. It doesn't matter if it's filled with air or not, all you need is the weight of the object and the volume of water it displaces (and the density of the water).

However, to add to that, some materials compress so as they go further down, you will lose some floatation or if they absorb water, same thing.

I see, thanks for breaking it down. So I wonder if a bottle is heavier or lighter than the same size piece of foam (now that I see the math, I agree with Waternut that the weight difference is negligiable). Although, as Sportfish posted in his foam thread, even the pink insulation foam can absorb 30% of its weight in water (considerably less than OEM 2 part foam but not 100% waterproof)

 

[WAB]

I am pretty ghetto and I used bottles in my build; Lots of them, all different shapes & sizes.

If you put the bottles uncapped in the freezer for a few hours you will get more air volume in them. :idea:

 

[SuperJETT]

If you put the bottles uncapped in the freezer for a few hours you will get more air volume in them. :idea:

Which in theory will actually make them less buoyant, because the air will weigh more. The bottles still displace the same volume of water.

 

[crashnburn86]

Glass in 2 compartments under the gunwales and pressurize them with 10-20psi of helium.... Should be buoyant enough.

 

[ncbrock]

all this math and numbers is making my head hurt