Using basalt bias for dinghy

[Shelley Ornsby]

Planning a unicat dinghy substituting honeycomb core for plywood.

Although experienced with glassing with various cloths (carbon, Kevlar and fibreglass) we haven't used surfboard glass before.

Does anyone have advice on using basalt bias glass with fibreglass over it?

We would like to know if 2 layers of light glass is as strong or stronger than 1 layer of heavy glass?

Core is probably going to consist of 2x10mm honeycomb laminated together.

Dinghy has a seat down the middle and crosswise near the front giving additional strength.

Photo is basic design. We are lengthening the dinghy and changing the transom for additional buoyancy in order to counter the weight of outboard.

 

[bajansailor]

Welcome to the Forum Shelley.

That is an interesting design that you are building , especially re the inverted vee bottom. It reminds me a bit of the Sea Sled being built by @DogCavalry
I guess that the page you have attached is from the plan supplied? Can you tell us what design she is please?
And how much are you lengthening the dinghy, and what sort of changes are you making to the transom?

Another question - what is the intended usage for this dinghy? I am guessing as a yacht tender?

You mention

We would like to know if 2 layers of light glass is as strong or stronger than 1 layer of heavy glass?

It all depends on how light the light glass is, and how heavy the heavy glass is - also what type of glass.

Have you done an initial weight estimate to see how the weight of your design will compare against what I presume is the 'standard' plywood design?
If you haven't, you might well find that there is not much savings in weight to be had by building it with a honeycomb core instead of plywood.

 

[Shelley Ornsby]

Hi, the plan is from a NZ designer called Unicat 2.4
We are extending it to 3 metres. Yes it is for our yacht tender. We have had no luck with inflatables and don't like the weight/cost of hard dinghies that are on the market.
Part of the extension is at the back, sort of like a sugar scoop, split in the middle for the leg of the outboard. We have seen the benefit first hand of buoyancy added to a friends dinghy.
We will also change the seat at the front so that we have an open area for stores and dive gear. So instead of a cross it will be a T Shape.
Yes we estimated the honeycomb is approximately 1/4 of the weight.
We like the strength characteristics of Basalt Bias, we found this on Sanded Australia. It is used as the base layer on surfboards and features are strong impact protection (will not shatter), High Tensile strength and similar flex to fibreglass.

Hence our question of whether we can use only 1-2 layers of light glass as opposed to a layer of heavy glass.

 

[bajansailor]

Here is some further info on the Unicat 2.4 :
https://www.jbwatercraft.com/unicat-2-4

And some info on her slightly bigger sister, the Unicat 2.8 :
https://www.jbwatercraft.com/unicat-2-8

Shelley, did you consider the larger Unicat, and then just adding a shorter extension to make her 3 metres long?
Will you be stowing her on deck, or in davits?
Apologies for all these questions, but background info is always useful.

 

[fallguy]

How does laminating honeycomb together end up light?

Two layers light glass is fairly arbitrary. You won't get much wear layer from say 2x6oz woven; so best to add a couple epoxy neat coats to infill above the weave and then varnish.

Personally, that boat is a bit heavy for a tender. I am aware of a plan which is far lighter, but it is not available for sale just yet.

 

[wet feet]

Unless you have already got hold of the honeycomb its hard to see any advantage it offers over closed cell foam.In fact,having to laminate it together is a bit perverse as you pretty much have to use a skin of some kind to do the job of holding it together.You certainly aren't likely to get all the cells to align any other way.The material the honeycomb is made from is something to keep in mind-is it paper,Nomex or aluminium?Paper shouldn't really be around a marine environment and aluminium may not like being around any metal fastenings that pass through either of the skins.Which leaves Nomex-a fine material in it's own right but far from inexpensive.

One layer of heavy glass or two of light glass shouldn't make too much practical difference in terms of total strength.The difference comes in losing the joints where the material overlaps.The heavier the glass,the thicker the overlap and with two skins you can stagger the overlaps so that they occur at different locations.Is the plan to lay up one surface on a shiny board of some kind and then apply the core or to form the core to the hull shape and then skin both sides?

 

[Shelley Ornsby]

Shelley, did you consider the larger Unicat, and then just adding a shorter extension to make her 3 metres long?
Will you be stowing her on deck, or in davits?
Apologies for all these questions, but background info is always useful.

Hi, didn't know about the larger plan so am happy to hear about it and I am about to order it along with the full size template. That will make it a lot easier. It also means we only need to add the extra buoyancy at the rear.
We will be hanging her off the davits which is why we are using honeycomb.

In answer to:

How does laminating honeycomb together end up light?

This is relative as ply is 480kg/m3 and honeycomb is 80kg/m3. However my husband talks out loud with his ideas and then later adjusts the plan after thinking further. We have decided to go with the thicker honeycomb but only one layer (the thinking was the laminate of honeycomb would give strength, oh and it is simply glued together and does not need to align the honeycomb in answer to Wet Feet. ) Additionally we do not want to use ply for one other reason aside from its weight the risk of a nest of white ants coming back to our boat if we are ashore for a few days scares the crap out of us as our boat is cold moulded cedar!!!!

In fact, having to laminate it together is a bit perverse as you pretty much have to use a skin of some kind to do the job of holding it together. You certainly aren't likely to get all the cells to align any other way. The material the honeycomb is made from is something to keep in mind-is it paper, Nomex or aluminium? Paper shouldn't really be around a marine environment and aluminium may not like being around any metal fastenings that pass through either of the skins. Which leaves Nomex-a fine material in it's own right but far from inexpensive.

We have used both close celled foam and honeycomb whilst in the Philippines, however we are now back in Darwin in Australia and delivery is something else we have to put into the equation. Honeycomb is easier to hold in place (stitch) than foam.

 

[fallguy]

Foam can be stitched with a light laminate on one side, but thanks for clarifying...could not see how laminated honeycomb was light(er)

 

[wet feet]

I suspect we may be describing different things as honeycomb.the raw honeycomb I am used to handling for composites looks like this.

The other form that I have used a couple of times looks like this:

You can work out which is harder to laminate together.I'm a little surprised that decent foam isn't easier to find as it isn't hard to stitch to a rudimentary former.Its still a miserable job filling and fairing the outside of the laminate,whichever way you go.

 

[fallguy]

A homemade dinghy doesn't require much perfect fairing.

Built with foam, all tabbing can be recessed.

 

[wet feet]

A lumpy boat will certainly float but it may not bring much pride of ownership.I suppose it will be less likely to be stolen.