Molding a stitch and glue built boat

[Daltoys]

Hello everyone,
I've just finished building my first 24-foot planing outboard hull in epoxy plywood. After several trials, I really like the design of the hull, as well as its seaworthiness. Everything exceeds my expectations, so I'd like to take the next step, build a mold and make a few more units.
It seems that the use of polyester resin is more suited to the sector to which large skiffs like mine belong. Can I simply wax my hull several times and gelcoat then plasticize in large quantities without risking damaging my current paintwork through exotherm or problems between epoxy and PE? (My hull is composed of: epoxy, plywood, epoxy, epoxy primer, PU primer and PU paint).
Will I be able to create a hull that rivals my stitch-and-glue, in terms of rigidity and weight, using only fiberglass and PE? I'm aware that my whole internal reinforcement plan has to be revised for this, and I'm thinking of using PVC foam, but I confess I haven't made much progress on this yet.

 

[gonzo]

A foam sandwich laminate can be lighter for the same stiffness as plywood/fiberglass. However, it won't be as tough. Small boat construction is often driven by stiffness and toughness rather than strength. Toughness is the amount of energy a material can absorb before breaking. For example, glass is stiffer than rubber, but rubber is tougher than glass. Boat that may run aground or a beached will need to be tougher than those that don't. It is hard to beat the weight to stiffness & toughness of wood in a smaller boat. As far as waxing your boat to make a mold, that is a common method. If you have so much exotherm that it damages the paint, the resin was excessively catalized or you got carried away and made the laminate too thick. Usually is good to wait until it cools down before adding more layers.

 

[wet feet]

Does you hull have correct geometry for moulding?If there is any tumblehome you may have to use a split mould and whatever type of mould you use,there is a risk that the polyester resin could react with the paint you used.Do you have any reason to believe that it won't?Well cured 2 part polyurethane ought to be safe but other types of paint may not work.If the shape of the hull is such that the prototype performs well,there is a good chance that a GRP derivative will work as well.What isn't true is that a novice will be able to produce such a boat.You may need to contact a composite engineer to determine a suitable laminate and internal structure and it may be best to find a professional to laminate the mould and the subsequent boats.

 

[Daltoys]

Thank you very much for your very constructive replies!

So if I understand correctly, the ideal is to obtain a very strong hull bottom, perhaps in PE multiaxial fabric assembly, with rigid transverse and longitudinal reinforcements based on foam/PE composite? So I don't get an equivalent of my glued sewn construction, but something that should come close? Thanks for the answer about the exotherm, it's OK, I was following the manufacturer's recommendations and that should do it then?

In fact, I had foreseen the possibility of molding the hull in case of success, so no shape should pose a problem.

The paint is 2-component PU. I'm quite open to the possibility of contacting a composite engineer, but will he want to work on the basis of existing plans? Do you know of anyone capable of taking my request into account? On the other hand, I'm against subcontracting the laminating work. I think I've already got a good grounding and I'd train myself properly, even if the operation would be more profitable otherwise.

 

[Daltoys]

Isn't there another moldable composite solution that matches epoxy plywood in terms of thougness and rigidity?

 

[Blueknarr]

Isn't there another moldable composite solution that matches epoxy plywood in terms of thougness and rigidity?

Like the old saying:

Same rigidity
Same toughest
Same weight

Pick two

 

[Rumars]

Toughness is overrated. This is a 24ft planing boat, we are talking about 6mm okoume ply at the most, with maybe 200g/sqm glass on the outside. You drag it on the beach much or hit a rock it won't perform significantly different from a 12mm foam core with 600g/sqm biax plus 200g/sqm plain on the outside.
It really is simple, keep the boat away from sharp objects and abrasive surfaces, it's not designed to break ice.

Weightwise it's a wash, plywood 3-3.3kg/sqm + 400g/sqm glass epoxy outside + 200g/sqm epoxy inside primer + paint = approx. 4kg/sqm. To this we add all the filleting and framing he has inside.
Foam core: 12mm foam 1kg/sqm + 2800g/sqm glass /resin + gelcoat = approx. 4kg/sqm. To this we add some framing, wich over all should not be heavier then the ply version.

This isn't to say that the above is the laminate schedule he should use, it's just a starting point.
First thing I would do is weigh the boat and calculate the existing weight/sqm in order to have a baseline so the GRP boat doesn't end up heavier.

 

[wet feet]

If you are looking for a source of composite engineering input,try entering "composite engineering consultancy service" in the search engine of your choice.While I would like to offer every encouragement to move the project ahead,there are several obstacles to overcome if you have hopes of offering boats for sale.To an established builder who is anticipating likely sales in the hundreds or thousands it won't be much of a shock.To a new entrant to the market it will cost time and money to arrive at a product that is compliant with all requirements.As an EU resident you have to comply with the Recreational Craft Directive and each boat sold will have to be accompanied by the correct paperwork.The situation is more acute as you live in a country where boats have to be registered and evidence of compliance with these requirements may have to be produced when you seek a mooring in a marina.

If you are not deterred by the bureaucracy,there are several technical decisions to be made about mould construction and the physical challenges of inverting the boat,deciding on the design of the sheer to accept a deck,Framing and support of the mould and the type of wheels to equip it with is another consideration .The last point is important because I have seen too many moulds that have inadequate wheels to support the weight of the mould plus hull as it moves around and it gets worse if the floor is uneven.After flimsy wheels collapse it is necessary to buy something strong enough and hope that the collapse didn't initiate star crazing in the mould-so you have the cost of the weak wheels added to the cost of the wheels you should have bought in the first instance.I could go on and will if you need more details.

 

[Daltoys]

Thank you very much for your answers!
In fact you're just seeing Rumars, the hull is built in 5mm Okoume with 400 biax on each side, 1 layer overall + 3 layers in each corner, reinforcement, etc..
I'll start with your method, to think about the laminate programme, thanks!

Wet feet, thank you very much too. I've already got through the paperwork and have the authorisation to build boats under 12 metres, self-certifying their conformity. Stability, deck heights, freeboards and other design details are fairly easy to comply with, the thing I'm having trouble with at the moment is mainly the structure of the boat. I've had very good feedback on the structure of the mould, its handling and so on. I'll do what's necessary and exceed the requirements so I don't have to redo the work.

Perhaps there are plans of boats of similar sizes and engines with a laminate programme, I can exceed the recommendations to be sure of my structure since I will not have the same shape of keel as the plans? Then, as the experiment progresses, I'll certainly be able to judge which elements were superfluous and unnecessary?

 

[Rumars]

Perhaps there are plans of boats of similar sizes and engines with a laminate programme, I can exceed the recommendations to be sure of my structure since I will not have the same shape of keel as the plans? Then, as the experiment progresses, I'll certainly be able to judge which elements were superfluous and unnecessary?

Sorry, but you have only two possibilities:
1. Take the shape and weight to a naval arhitect, pay him for calculating the structure. He will ask the necessary questions like speed, max. engine weight, production capabilities, etc.
2. Start reading boat design and composite engineering books until you are able to do it yourself.

In short, fiberglass is not an isotropic material (like you treated plywood) and trying to make it so will result in to much weight. The core thickness (and possibly type) as well as fiberglass weight and orientation will be different throughout the boat. You can not achieve this by looking at other boats, it has to be calculated properly.

The best thing you can do is compose a mail detailing your requirements and start sending it to different naval arhitects wih the request for a price. I would start local and gradually expand the area until you find someone willing to do it for a price you can afford.

 

[gonzo]

If you are going to market the boat, a foam sandwich is a better idea than plywood and fiberglass. I prefer a plywood core, but the common belief is that wood cores will rot is short time. Also, foam core has the appearance of a more modern structural design. The issue may be that a design made for plywood has a shape that looks antiquated or home-made. A boat sales more for aesthetics than any other reason.

 

[TANSL]

A "plywood core" structure is a fallacy. In reality it is a plywood structure, and as such it must be calculated, with a protective/waterproofing laminate on the outside and inside. The functions that the "normal" core performs in a sandwich structure can never be performed with the same efficiency by a board (strength and low weight. Plywood will never have the resistance to interlaminar stress that a sandwich core needs). In a sandwich, the fibre layers must be considered as resistant and their compliance factor must be checked with their maximum admissible tensions. These calculations are not necessary in the lamination of a plywood structure.
The shapes, in fact, are very conditioned when working with plywood but if you want, for whatever reason, to use wood, there are several other solutions.

 

[Daltoys]

Okay, thanks a lot Rumors.
I'll explore both possibilities then. Do you have any idea of an acceptable price range for this kind of project? I guess it's impossible to set a range without knowing my exact needs, but hey....

Yes indeed, it seems that wood is rather frowned upon in the boating industry, that said my small niche clientele would be easily convertible... The problem with epoxy plywood for me is mainly the fairing, it's really long... It's hard to make a profit. As for the shape, it seems that French customers, at least where I am, are very fond of traditional shapes and materials, so I think I can find my target by following this path. The marketing addition of a modern foam composite structure, on a traditional base with attractive woodwork finishes, should work well. Thanks for the comment!

 

[Daltoys]

Okay, thanks a lot TANSL, I think I understand that my dream of a rigid, strong and light hull, built in a mold, is slowly fading away. That said, if I admit to an increase in weight, I should be able to regain my rigidity and resistance then! The idea of an ultra-light hull is seductive, but I believe that the very low V of my hull, displacing very little water, will replace this advantage. I'll compensate with the motorization!

 

[wet feet]

You have identified the main objection to building in wood that has to be sheathed and then brought to a good finish .The extra cost of all that work on each and every hull will push up the price that you need to sell the boat for and will have a corresponding effect on reducing the number of potential customers.I would expect,purely as a guess, that by the time the third hull has been built that you would have been better off paying for a professional to determine the structure.Have you located a composite engineer at a convenient location for a preliminary conversation? A search for "marine composite design consultancy" brought lots of results.If your location in France is anywhere near Morbihan there must be somebody servicing the needs of CDK and Multiplast.