Different types of boat mold

Hi everyone, I wanted to ask for some advice on various types of molds. I'm building an R3-class skiff and I'm having a hard time deciding on the mold. In the past, I've used wood and then filled it. This time, I was thinking about a foam mold, but I'm not sure which one to use. I'm also open to suggestions on other mold ideas.

 

Welcome to the forum.
You should provide more details about the boat you want to build, also, the type of mold you need may depend on the number of hulls you intend to build with it.

 

The boat to be built is for an Italian competition. The mold is only needed to build a single boat.

 

A sketch of the hull form would be useful.Are there chines?Is the surface developable?A foam female mould wouldn't be my first choice,but in the right circumstances it might make sense.A male mould will inevitably lead to a lot of filling and fairing,but with the advantage of a clean internal surface.The type of deck and the method of attaching it to the hull needs to be considered in detail too.

 

The boat is something similar to a 49er, and it has a chine.
The idea is to make a female mold so as to have a perfect external surface.

 

To make a female mold, first you build a male plug. If you are doing only one, it means building three boats to get one. The work involved into faring a hull is the same as fairing a plug. You need to build a jig for the plug, which can be used to build a boat.

 

Why? Since I have a female mold, I can laminate inside it or infuse directly onto it, so as to have a perfect external surface.

 

It depends entirely on the resources and budget you have.

EPS/XPS: can be shaped by a hot wire cutter, milling and sanding, then the surface is stabilized with one layer of fiberglass in epoxy, then you fill and sand to perfection.
High density (300-600 kg/m3) PU foam can only be milled/carved and sanded but it holds detail much better, and can be filled directly. It is also more expensive then EPS.
MDF can be substituted for foam in machine milling operations. It could even be done with real wood if that would be a requirement. Cardboard/ papier-mache can also be used.

CNC type: a big multi axis can not only carve the hulk shape from a big foam block, you can then cover the surface with a hard filler and remachine to a very precise tolerance cutting down on hand sanding. If the available cnc is just a 3 axis one for flat sheets you need another strategy for cutting and filling/sanding is entirely by hand.
If both cnc types are available then you can start combining their strengths and save on foam. This comes at the expense of computer working hours, so the next question is what work is your team best at? Do you have plenty of free 3d modeling resources or are they better at hand sanding?

Then there is 3D printing, it also allows the fabrication of direct female molds.

 

I see that Rumars as advanced the discussion.He is entirely correct about what can be achieved by machining directly but before anybody gets too carried away,I would caution that for a hull mould there needs to be consideration of actually having space to allow the machine to move along it's cutting path.Which may mean either a central split or making a modular mould that can be assembled for hull production.I have seen an example for a cruising boat that featured two halves that were nicely machined but where no thought had been given to actually holding the sections together even though somebody had the sense to include machined locating features.I did advise that wooden uprights bearing on the outer surfaces and ratchet straps might work and they followed my advice,not entirely happily because I laughed at the lack of thought.Not a particluarly cheap way to go and I suspect the same might be true of 3D printing but then what value does one assign to several days of filling and polishing?

 

In the USA the value would be about $140/hr for a shop to do it. The amount of labor depends on the shape and the precision of the mold or plug. Fairing a female mold is a lot harder than a male plug. As far as machining the mold directly, the cost of CADD and then a CNC shop would probably be higher. Also, that assumes that simostreca is able to provide a table of offsets to the CADD technician to generate a DXF or similar file.

 

The R3 is a class for university competitions, real world economics don't apply. Labor is free since it's done by the students, machining is free on university tools but you have to use what's available or find someone willing to sponsor machine time. The materials budget is often the most limiting factor unless you find sponsors who can donate all of what's needed.

What's possible depends on what the university offers in tools and money, as well as the skills of the participating students. You see, if you have a team of senior sculpture majors you don't need offsets or CNC's, you show them a picture and they will do it by hand and eye.

 

That assumes you have a perfect female mold which would be harder to create than a male plug.
It's just my personal opinion based of decades of involvement with one off racing canoes that the most effective simplest and efficient means of production is to just build the thing.

 

This will be the third boat we are building. As mentioned previously, for the first two boats, we used wooden female molds produced by a company based on the 3D files we provided. We then faired and laminated them to create a surface capable of holding the vacuum for infusion. For this new boat, we want to try a different material for the female mold and were considering foam. We would like to understand the potential cost of purchasing foam blocks to have them CNC-machined by a company using our 3D model.

 

For using foam, the female mold is larger than what the finished boat will be. The surface has to be built up with a hard surface. For infusion, the surface will have to be laminated with fiberglass, built with gelcoat, and then machined to a mirror finish. The structural design needs to have the forces that will be generated from the vacuum. I believe there will have to be a structure to hold the foam from deforming. Maybe a plywood box where the foam is glued into.

 

Having good 3D files with the normals facing the right way is an excellent state of affairs.Using foam that will survive the forces of the vacuum bag when infusing is perhaps the only challenge as soft foam is quite cheap and strong foam tends not to be.Presumably you would have a rough surface machined,perhaps 5mm beyond the finished shape,then add a filer paste to which the final machining could be directed.After some very fine sanding,you should be ready to seal and infuse,but you would ideally support the foam on an impermeable and perfectly flat surface.