Hi,

I'm a teacher and work with teenagers and am considering building a kayak with one particular class group. I already have a mould from which one successful polyester/glass boat was pulled about two years ago. The mould is also polyester/glass with a polyester gelcoat. The mould needs some minor modifications to change aspects of the kayak design that have become apparent in the meantime. Basically, I'll need to cut six slots, remove some material and then re-glass up the mould to make it rigid again. The total area of the modifications will be 6 slots each about 1 inch by 3inches. I intend to make the new boat from resin infused epoxy with a mixture of reinforcements including carbon, diolen, glass and kevlar with soric as a core material.

I understand that a polyester mould is risky to be using with epoxy, so my first question is whether the epoxy compatible gelcoat will lessen the chances of the new component sticking to the old mould?

The second question is whether I should use other materials to modify the mould to prevent there being any fresh polyester in contact with the new component? (if so, which materials?)

Third question, what release agent should I use? (the mould has a good finish and was well prepared with release wax previously),

Thanks,

Donnacha.

It is an eight foot long fast river surf boat designed for one specific wave - think Riot Glide but much, much less rocker, about 2" at the stern. The glass version I have is very carvey and extremely loose. I know it sounds strange but then it is a strange set of requirements. I have a squirtboat myself, its an Underdawg by Murky Waters.

Construction wise, it has to be light and extremely stiff on the hull because of how flat it is and the natural tendency of flat sections to bend, part of the reason I want to go with resin-infusion is because of the use of a core. The river is deep and rock free when the wave is in, so not too concerned about impact. The deck shape is high volume and rounded so naturally strong.

Hi Donnacha,

One final question, is there a limit to the number of layers that you can safely infuse and be sure of a thorough wet out?

No, not really. The key is to ensure that all the materials you use are suitable for infusion and therefore don't restrict resin flow through them too much. To give you an example (not that you're likely to try but...) chopped strand mat will pretty much stop and infusion - the closely packed fibres which sit flat, i.e. are none-woven, will not really let resin through in any meaningful way. Unidirectional material, including UD carbon, also sits too flat and will restict or stop resin infusion.

Most woven fabrics will infuse nicely. You'll always find that looser weaves like 2/2 twill and 5HS will infusion faster than plain weaves but they'll both infuse.

I've never tried infusion the alufibre. It might infuse quite slowly because it's a pretty dense material. You might want to do a small test infusion with the Alufibre just to be sure. Another option, for total peace-of-mind would be to do a small test infusion panel of the proposed laminate. This would serve the double purpose of ensuring that it infuses properly and also it would give you the oppertunity to evaluate the stiffness, weight and characteristics of the finished laminate before doing the whole boat. If you do this, it's a good idea to time the infusion; this is useful information when it comes to planning the full boat infusion.

The boat must be light and stiff and I intend to use some fabrics that I already have. On the hull (which is extremely flat from side to side and minimal curvature from bow to stern) I was thinking of Alufibre (primarily cosmetic as light colours don't show scratches as much), then diolen, glass, soric, kevlar, kevlar,

I have no doubt that this laminate would be strong enough for what you need. My question would be whether it is perhaps stronger than it needs to be. You mention the importance of of the boat being light; if you have a weight figure in mind for the finished boat (based on other boats you've tried and what you thought of their weight) then you could work out what this boat would weight (total weight of fabrics, core plus resin uptake for the fabrics and the core) and get a feel for whether this is as light as you want it to be.

Remember that as a primarily decorative fabric the alufibre is heavier than it needs to be. It has the strength of a woven glass but, because of its aluminium coating, is actually a little heavier than woven glass would be. It's only a subltle difference but if you're striving for the lightest possible boat that I'd consider swapping it for something higher performance like carbon/twaron.

All the best, Matt

Matt Statham
Easy Composites / Carbon Mods - Technical Sales

Check out the double dutch and Vajda websites and see what spec they are laying up current slalom boats up as.  They are very rigid and very light so a similar lay up should serve you well.