how many fibreglass layers for teeny tiny pram?

[Errand]

I'm building a tiny boat for my nephew; its more or less a scow bow flat bottom dinghy for him to play with in the pool and if the design works I will build a more refined one with proper materials to use on Narrabeen lake. [secured by a cable]

its 1 meter LOA and has .8 meter beam, and not sure about the draft yet although the transom is about 25cm high. It's built more like one of those polycraft tuff tenders, rather than an open boat - it's completely enclosed with a small sunken space for sitting in.

I've made a mock up out of cardboard, styrofoam and paper mache.. the structure is quite strong, as in I can sit on it without it flexing, and very light, around 2kg.

My nephew weighs about 18 kg [he is only 3] and I was attempting to make it stable and pleasant for him to experiment with since he loves boats.

I've got a fair amount of leftover 4oz fibreglass and surfboard epoxy resin from a different project that I want to cover the entire hull and deck in so it becomes waterproof.
I'm curious as to how many layers i would need to ensure a watertight/waterproof shell? I don't need any structural strength but water/moisture inside the paper mache/cardboard would be a disaster.
I was going to just lay on 2 layers and a hot coat and see how it goes, but was wondering if anyone has any opinion/experience?

[if this design works out i intend to build it again using xps foam plug and heavier fibreglass, not much worry about water ingress in that case. And maybe thin plywood trim].

 

[jorgepease]

You don't need proper scantlings for the pool boat, just throw a layer of glass on it and make sure to follow with a nice coat of epoxy and paint. I have seen bikes made purely of cardboard and epoxy no glass.

 

[Errand]

oh thankyou, that's a relief, I'm half done and i put 2 layers of 4oz on so far into the annoyingly square deck box. It looks a bit like a miniature tugboat with a raised prow and a tiny open 'pilot house'. I will just use one layer for the hull if that's all that is needed. The cardboard/papermache is holding up well so far.

 

[rob denney]

oh thankyou, that's a relief, I'm half done and i put 2 layers of 4oz on so far into the annoyingly square deck box. It looks a bit like a miniature tugboat with a raised prow and a tiny open 'pilot house'. I will just use one layer for the hull if that's all that is needed. The cardboard/papermache is holding up well so far.

If you only use one layer, roll on a coat of neat epoxy before it cures to fill any pin holes. 2 layers of the 4 ounce would probably be just as easy and more watertight.

 

[SamSam]

My nephew weighs about 18 kg [he is only 3]

its 1 meter LOA and has .8 meter beam, and not sure about the draft yet although the transom is about 25cm high. It's built more like one of those polycraft tuff tenders, rather than an open boat - it's completely enclosed with a small sunken space for sitting in.

I'm thinking possible death trap...?

 

[Errand]

I hope not, at worse I was calculating if turned over it would just float about on the surface like a rubber dinghy. Even with the deck full of water, it wouldn't sink, would just sit lower [inside deck volume is about 70L]. I calculated the displacement was around 190L in total.
Weight after the glassing I can't say for sure yet but it will be not more than the weight of the epoxy and glass over the existing structure - probably 4 or 5kg after it's done. He also is too small to go into the pool alone, and knows how to hold his breath under water, float on his back, and turn over by himself.

Its hard to explain without a picture [i can't seem to put any up here yet]. Cardboard/paper mache is really horrible to glass over btw. There are a lot of bubbles popping up, possibly because its not completely smooth, and there may be moisture inside still somewhere from the mache. At least it didn't cost much at all.
If this design works well, I will do it again in full styrofoam and make sure it's as smooth as possible.

 

[messabout]

Errand, a "boat" with those dimensions will be extremely sensitive to even the most subtle shift of passenger weight. The simple act of merely moving ones head and shoulders will set up an unwelcome pitch or heel. Make the boat longer and wider and you will surely be more pleased with the result. Sam Sam is pretty close to right about the prospect for such a floating vessel.

A box of that sort will have some positive buoyancy. That is not a sufficient reason to jeoperdize the safety, or the fun content, of a small juvenile. At one meter by 0.8 meters the box would support about 45 pounds per inch of immersion. That is true only if it is a rectangular closed box with plumb ends. If it is shaped more or less like a conventional boat then something on the order of 27 pounds per inch of immersion.

 

[Errand]

Hmm thanks for pointing that out. I certainly don't want it to be unstable or tippy, that would ruin it. The smallest smallest tiny boat plans I could find on the internet were this 1.2m half pea, which looked easy enough to convert to styrofoam but seemed too big still. The builder can stand up in his - Halfpea | Free Boat Plans http://hvartial.kapsi.fi/dinghy44/dinghy4.htm.
Please don't laugh but here is a picture of mine when it was in cardboard - that top sunroof/pilot house thing I redid in colored plastic cardboard and is a separate piece, just sitting in the deck, [is for show, has virtually no weight to speak of].
I had thought it would be more stable than the half pea for the expected load, since the bottom is entirely flat and sides are vertical/straight, even though is 20cm shorter. The deck is right down at the bottom -2cm and the sides all around are thicker now, about 10cm all around as I was trying to force the sitting position to be as close to the center as possible.

I did also consider one of those mouse boats by Gavin Atkins, but also, i thought too big, and i wasn't sure having his legs enclosed was a good idea although he's pretty nimble for his age. I read it's a bad idea to change a boat's design which is why I just made my own, but if it looks unsafe I'll do the half pea.

 

[SamSam]

You'll notice the guy standing in his boat has his legs spread wide. A kid will lean over the side to look in the water and the dynamics are different. Get one of these, or at least get some ideas from them. inflatable kiddie boat - Google Search https://www.google.com/search?q=inflatable+kiddie+boat&source=lnms&tbm=isch&sa=X&ved=0ahUKEwisy_zS3ITXAhWX14MKHbBwB8IQ_AUICygC&biw=1366&bih=628 The kid will grow quick (hopefully) and within a short time might be too big for the teeny tiny thing you're making.
First thing is to teach him how to swim.

 

[Errand]

sorry to necro this thread, I thought I'd share what I ended up doing - the paper mache boat with fibreglass coating was a disaster as the core literally filled with water and disintegrated after the first day in the pool. So there was like a soggy shell left. It was a very bad idea, as I learned that fibreglass won't cover up bumps and mistakes.
So I made it out of ply instead, I got a sheet of 4mm marine ply, and used 2oz glass plus extra tape on the seams. I used bote cote. It turned out really nice [not perfect], and the boy loves it - its so stable it takes a lot of effort by an adult to flip it. It's a bit heavier, about 9kg i think. I also lengthened it to almost 5 feet, by about 90cm. I doubt it would win any races as its got a really fat behind and drags a wake. But its stable, very buoyant, and kind of pretty in the pool. I've been working on making a bilge pump 'outboard' motor which is the next project.
thanks for all the advice