Replicate rotomolded plastic

I am building a model of a 13 foot long boat. Originally a 1:10 scale and I have been informed on this great forum that I need to cube this model's weight (.5 KG) so the finished prototype is way to heavy at over 500 KG.
Now I'm sure my model is heavier than it should be as not all components have been reduced in weight to scale.
The plan is that this boat, if it did get to the manufacturing stage, (yes I am dreaming big) would be built in a rotomolding plastic fashion like a kayak/peddle boat. By using a foam core for the first one, plywood, epoxy and glass I can make one that will look exactly like the finished product - but this weight issue keeps me from starting. Any thoughts how to make myself a prototype keeping in mind the rotomolding plastic finished product?

 

Why worry? No small boat is calculated without the rower anyway. Why not build it from foam and add weight to represent the rower's additional weight?
You know, don't you, that a roto-molded boat is incredibly expensive to tool for? I live close to Old Town Canoe and I once saw an old roto-mold they had brought to the metal yard. It was a beautifully made super heavy stainless steel canoe all by itself, perfectly curved in all dimensions. I'll bet it cost a couple hundred thousand dollars to make. I wanted to buy it from the yard for scrap metal costs but I knew they couldn't sell it because the Old Town Canoe Co. would certainly have demanded it be destroyed.

 

Thank you very much for your positive advice - I believe this boat may have interesting and sellable applications but if I only ever make one I'll be just as happy. If it was to go into production, the design, look and end user pricing I believe is well suited to rotomold production - so if I get to this stage the investment could be worthwhile. We'll see. Thank you for your "Why worry" comment. I can breathe again and I will go ahead and make it and test it out. No worries.

 

Though the mold is expensive, you can pop them out at a rate few other processes can. I've been long looking (waiting) for someone to make a nice sailing hull for something like a daysailor. And remember, plastics are not just plastics anymore, there are things like super plastics. Also additives of all sorts also change the characteristics, stiffness and strength.

 

Most of the roto moulds for 'complex' shapes and I include hulls when appendages/internal details etc in that, are made from aluminium castings post machined. If Alan saw a S/S mould that would be to make many thousands, it may even have been for a sophisticated form of 'Blow moulding' which would be mega bucks. I'm not sure Al sand casting and CNC machining is quite in that cost league so don't be put off.

If your shapes do not have undercuts, have decent split lines and around 3 degrees draft (2 at a pinch) for gloss surfaces and 5 degrees for textures you should be OK. The fewer truly sharp corners the better unless you use these deliberately for mould split lines ie unbolting a mould panel. There are a lot of 'tricks' for placing inserts ie places and parts to post bolt to etc as well. Be aware that the pattern needs to be about 4% larger to accomodate shrinkage firstly on the aluminium castings, and secondly on the actual polmer moulded. PP or PE materials with triple layup self foaming inner.

There is no reason why you cannot replicate the shape you want with pretty much any material - wood, glassed over foam etc. With the fillers and materials around these days it is more time than anything else. Go for it.

You will also need a cooling jig or jigs to place the hot moulded item into to restrain the dimensions within reason. Shrink tolerances are significant on unrestrained mouldings and you would not want distortion on a new hull.