Please help with cheap ama construction

[stove]

Hi
I need some advice on building two cheap amas from a plug that is almost finished. I've built the plug by reshaping and lengthening a Tornado cat hull to 9m (+- 29ft). One third of the plugs surface is Tornado hull and the other two thirds are plaster of paris over various structures including chicken wire and wood. The plug still needs around two weeks sanding and filling to get it to a respectable fairness and finish.
I need advice on the following:
1. What is the cheapest method to get two amas from this plug. I don't need hi-tec or low weight, the main center hull will be a Soling mono hull without it's keel. The Soling is constructed out of single skin GRP and weighs about 430KG. In other words this Trimaran will not be a light weight racer but more of a cruiser camper day sailer.

2. I cannot afford to make a female mold and would like to turn this plug into a male mold that will withstand two hulls, afterwards the plug will be disassembled and the Tornado hull currently in bedded in the plug will be reunited with it's partner and hopefully sail again.
I have no idea on how to strengthen the current plaster and Tornado plug to make it capable of giving me two hulls. My question is: How should I go about finishing the plug/male mold?

3. If my thinking is correct the cheapest way will be single skin Polyester and glass for the hulls with lots of sanding and fairing before painting.
I have no clue on what the layup should be if this should be the way to go.

Please help, I cannot afford costly mistakes. This project is really running on a shoestring budget.
Many thanks

 

[redreuben]

I would think the cheapest hull would be a fabric coremat layup in polyester.
I doubt anyone here will give you a specific layup as there is just not enough information, beam of the boat, size of the rig, weight of the main hull, how many bulkheads etc are all factors in deciding how strong the skin needs to be.
I assume your not going to pay for engineering so the best I can suggest is find something of similar size and loadings and copy it.
Another angle is perhaps one of the fibreglass suppliers can give you a ball park laminate.

 

[Corley]

You could also look at Dave Gerr's "elements of boat strength" or "Nature of Boats" for a scantling guide. It covers solid polyester/glass layups I believe and sandwich types. A thought I had while reading your description is why not take a female mold off your plug? It would probably be a little undersize if you have made an allowance for skin thickness but otherwise it will give you a means to turn out as many hulls as you like in future.

 

[stove]

Thanks for your reply Redreuben and Corley. The boat will only be used in protected inland waters. Can I assume that single skin polyester and glass hulls will be the cheapest way to go? What are a typical single skin layup for a 20ft beach cat? I can also use as many bulkheads as is necessary. The hulls are basically very long Tornado hulls 9m or 29 feet. Corley I only need two hulls, I would like to skip the female mold if possible. Thanks for your help

 

[redreuben]

To many unknowns, a Tornado will be foam sandwich, beyond your budget.
Choose a budget beach cat hull and reverse engineer it.
My guess for budgetary reasons will be csm and coremat but how much is your call.

 

[Corley]

I think polyester resin/csm and roving glass would be the cheapest way to do it. Use plenty of release agent and wax to try and minimize the chance of the plug failing.

 

[stove]

Thanks Redreuben, I'm having a look at Dave Gerr's "elements of boat strength" It looks promising. I have some reading to do, this book is what I was looking for.
Thanks again

 

[catsketcher]

Solid glass is hard to do over a rough male mould. Best to use a cheaper core and cover with 600gm biax. Cheap core is always hard to find but around 12mm thick plastic honeycomb with a layer of 600 stitched fabric inside and out is a standard for many cruising multis as a starting laminate. The core will take out some of the wobbles in the mould.

I built a 3m catamaran dinghy out of a mould and used CSM and polyester. The resin was okay but the CSM is terrible stuff - heavy and absorbs so much resin it makes you cry - weak as anything. I think it would have been cheaper and faster to make the thing out of 10mm Polycore with 400gm glass skins - lighter, cheaper and probably faster to build out of a mould anyway.

Most of the owner built multis are of a pretty standard laminate - almost always less than 1150gm/m on cedar is a heavy one. 600 outside and inside would be fine and with doubling under bulkheads if the shape is nice and curved.

I don't really like Dave Gerr's book. It produces very heavy laminates for owner built multis.

cheers

Phil

 

[stove]

Thanks catsketcher, I will take this into consideration, maybe start looking for an affordable core. I will make my male mold as fair and smooth as possible to save on filling and sanding later.

 

[redreuben]

Stove;
Have you considered folding up a flat sheet of glass as per the Tornados plywood method ?
Add reinforcement after the shape is obtained, just a thought.

 

[stove]

Hi redreuben, the male plug is more or less 90% complete, many hours of building sanding filling sanding etc. Too late to change now, unless it can be folded around the plug

 

[redreuben]

Stick with plan A then.

 

[Turnpoint]

Stove
I fear the plaster of Paris part of the plug will likely fail with the first part. You will most likely need to spend some time reconditioning the plug to pull the second part. If you can, you will save lots of time in the long run by pulling a female mold from the plug and polish that surface out before pulling parts from it. Otherwise you will end up sanding and fairing 4 times-- 2 x plugs and 2 x finished surface of the hulls.

 

[stove]

Thanks Turnpoint I will take your advice into consideration. Will 8-10 layers of 450 csm be enough for female mold? Is there no way I can maybe treat the plaster to make it release easier, I don't need a perfect surface on the inside of the hulls.

 

[rcnesneg]

If you could get plastic sheeting to smoothly cover the mold it could work. Epoxy peels off the plastic rather easily.