Expandable Boat Mold Design

[Brian242]

Hello all,

I am looking into how to make an expandable mold design. I have plans to produce some vessels that have the same width, depth, and hull design but have different lengths. Ideally, I'd like to have a couple sections I can add in to make the final hull longer or take some panels out to make it shorter. The hull design is really simple so not dealing with any changing curves, etc.

If this is possible, would the locations of the seams make the hull weaker? If so, what can be done to mitigate this? The first hulls will be fiberglass with core material. I would like to explore carbon fiber/ kevlar down the line.

I am fairly new to boat building. I have built a few smaller runabout boats but this is my first step into something production worthy (and my own design).

 

[bajansailor]

Be aware that the section (or two sections even) that you add / remove will need to be 'parallel mid body' units, with a constant shape throughout the length.
Can you post some details of the plans that you have already?
How long / wide are the boats that you would like to build? And what type are they?
I presume that the extra sections (along with the original sections) will have flanges on the outside of the moulds, so that they sections of mould can be bolted together.
You could perhaps use plasticene (or play-dough or similar) as a filler in the seam in way of the bolted joints.

If this is possible, would the locations of the seams make the hull weaker?

I don't see why this should happen, if you are laying up a fibreglass hull in the mould, regardless of how long it is - a well laid up hull should be fairly homogenous.

 

[Brian242]

Thanks for the reply and all the information, Bajansailor.

The design is very simple. I'm essentially trying to build something similar to "monster" Carolina Skiff DLX-EW. So, its a very simple flat hull design and the inserts will definitely be constant shape. The "base" size I'm going to build is 30ft long 12ft beam with a 25in gunnel. I want to be able to expand it up to 40ft long with two 5ft long inserts.

I've seen molds that split down the middle length wise but have never seen any that can expand in the midsection. Does it make sense to have the inserts overlap the "parent" mold, or does that just overly complicate things? I will definitely have flanges on the outside to bolt them together. I hadn't thought about the plasticene idea. That makes a lot of sense. I was initially concerned that the gap in the seam may cause fiberglass to not layup correctly and create a weak point.


I do have an idea for a catamaran design later on. But, I want to start off with this skiff design. Especially with it being a lot simpler and to some extent, my lack of experience.

 

[Rumars]

I would make the mold 40ft long from the start and have one additional (or several if necessary) removable transom insert mold. To build a boat that is shorter then the mold you just move the transom mold to the desired position.
There is filleting wax for filling and smoothing the corners.

 

[wet feet]

Thanks for the reply and all the information, Bajansailor.

The design is very simple. I'm essentially trying to build something similar to "monster" Carolina Skiff DLX-EW. So, its a very simple flat hull design and the inserts will definitely be constant shape. The "base" size I'm going to build is 30ft long 12ft beam with a 25in gunnel. I want to be able to expand it up to 40ft long with two 5ft long inserts.

I've seen molds that split down the middle length wise but have never seen any that can expand in the midsection. Does it make sense to have the inserts overlap the "parent" mold, or does that just overly complicate things? I will definitely have flanges on the outside to bolt them together. I hadn't thought about the plasticene idea. That makes a lot of sense. I was initially concerned that the gap in the seam may cause fiberglass to not layup correctly and create a weak point.


I do have an idea for a catamaran design later on. But, I want to start off with this skiff design. Especially with it being a lot simpler and to some extent, my lack of experience.

It has been done quite often in the past and will work.I think it is a better way to go than inserting a false end for a transom further forward for two reasons,one of which is that a transom can have quite a bit of detail around radiused corners or outdrive flats which are not totally compatible with a flat surface. Secondly,the sheer will be more balanced visually in spite of the flat central section.The principal bow and stern sections really ought to separate at the low point of the sheer and not at half length.

The hull strength will not be compromised by the mould having several sections since it's strength is derived from the fibres of the layup.Core and stringers can be used as deemed necessary.

 

[kapnD]

I would make the mold 40ft long from the start and have one additional (or several if necessary) removable transom insert mold. To build a boat that is shorter then the mold you just move the transom mold to the desired position.
There is filleting wax for filling and smoothing the corners.

This is the best way, but only one transom mold needed, to be inserted into open ended hull mold at whatever length needed.
This resetting of the transom mold can really slow down production, though, so if you’re looking to produce a lot of boats, I’d build a complete mold @ the length you know will sell best, at least for starters. Custom lengths can then still be produced, but longer ones will require two separate layup sequences in the mold.

 

[wet feet]

This is the best way, but only one transom mold needed, to be inserted into open ended hull mold at whatever length needed.
This resetting of the transom mold can really slow down production, though, so if you’re looking to produce a lot of boats, I’d build a complete mold @ the length you know will sell best, at least for starters. Custom lengths can then still be produced, but longer ones will require two separate layup sequences in the mold.

Could you expand on that please?I'm a bit confused by what is proposed because I would expect the layup to be done in a single operation once the length has been determined and the mould prepared-regardless of the length of the boat.

 

[TANSL]

@Brian242, there may be a couple of solutions for what you are asking. One or the other will be more suitable depending on the shapes of the boat.
Other important topics are:
- the new distribution of weights and buoyancy can make the trim vary in an unacceptable way.
- check if by varying the length/breadth ratio the stability of the boat will not be impaired.
In summary, before seeing how to build the moulds, it would be convenient to study the seaworthiness, operations, stability, safety in general, of the resulting boats.

 

[mudsailor]

Adding mold sections in the middle of the hull will lead to ‘seams’ that will print through into the finished hull, no matter how carefully you try and fair it all. Do as above, build the longest mold and set the transom where you need it (this is what has been done many many time before) A transom corner will be much easier to radius in the mold with wax and then polish (if needed) after de molding the part.

 

[wet feet]

How big a radius?Something like this isn't easy to do and a sectional mould is an easier solution.

 

[mudsailor]

How big a radius?Something like this isn't easy to do and a sectional mould is an easier solution.

A small one, maybe 1/2”, much easier than seams

 

[wet feet]

I'm quite happy that we have a world where individuals can hold different opinions.

 

[mudsailor]

I'm quite happy that we have a world where individuals can hold different opinions.

I agree, but I’ve worked at builders where we had a moveable transom in the mold and it worked very easy and only minor rework…..we also had parts that were constructed from multiple mold parts and sometimes had to respray whole parts to get the seams to truly disappear…..
Rule 1 - one seam is always better than two.
Rule 2 - a seam at a corner is always better than a seam in a flat panel….