Carbon Fibre on Plywood in mini 6.5 sailboat

[transat650]

Hi.

I'm looking at building a Dudley Dix Mini 6.5(http://www.dixdesign.com/didimini.htm) but I'm not too keen on al all-plywood boat.

The standard boat has an either 6 on 9 mil ply skinned hull (depending on the strength you want), with either doubled 3 mil or 4.5 mil on curved surfaces and 6 or 9 mil on the flatter underside.

I am thinking about building it with a single 3 mil ply hull and then skinning the inside and outside of the hull in carbon fibre/epoxide composite (ie hand laying the CF). However, I don't know that nuch about this material, so am looking for some help!

What I would like to know is-

How many layers and what thickness will I need to equal or better the strength of 9mil ply...

Whether it is possible to just 'cover over' the internal bulkheads (http://www.dixdesign.com/minikit49.jpg) by butting up the CF sheet to them and then covering them, or whether this will weaken the structure and the hull needs a continuous CF skin not interrupted by bulkheads... (ie the sheet going between the bulkheads and the skin)

Whether it will be possible to make the bulkheads narrower, giving more internal space, if they are reinforced with carbon fibre.

Many thanks in anticipation - and I hope that I don't sound like too much of an engineering ignoramus!!

[gonzo]

It will make your boat heavier and maybe more flexible. Both things are bad. Have you heard of any of those boats have a structural failure?

[transat650]

I don't think that there are too many of these boats afloat, but I havent heard of a structural failure.

My only reason for wanting to do this is that
a) I thought that carbon fibre composite would be more durable than plywood, which seems to me to be a rather short lived material - I want a boat which will last

b) I thought that it would be possible to build the boat stronger without a weight penalty

If you think that I'm barking completely up the wrong tree then please say!

[Boston]

have you considered building a mold and just making the whole thing out of CF

[transat650]

Not really - I've always assumed that building a mould would be really difficult and that only campanies would bother. Also I need the whole process to be quite cheap.

However, if there is a way of making a mould and complete CF hull in a 'shed situation' then I'd really like to know!

Thanks for your replies, by the way.

[Crag Cay]

Also I need the whole process to be quite cheap

No boat project is cheap. Add CF and it becomes even less so by several orders of magnitude.

Either build a Dudley Dix Mini 'as is' because that's what you want, or build another design of mini in carbon because you want the competitive edge. The Dudley Dix mini is not a competitive design in that class anymore.

But it sounds like you don't really want a DD Mini either as it doesn't have the room you want. Then why not buy something like a Beneteau First Class 7. You can pick one up for the same price as buying the raw materials for the hull of your 'improved' DD Mini.

Prioritise what you want and then work out how to get there within your budget. But remember, with small boats, the cost of a new rig and sails is often more than buying the National Championship winner's boat secondhand, in a class like the National Sonata.

[Herman]

And building a mould, then a boat, is very doable in a shed situation, but get the exact info you need from a supplier that is willing to take some time and give you the info.

However, you will be either building a boat (plug, model for the mould) first, then the mould, then the actual boat. Which is some work.

I did this excercise once, but I had the luck of finding someone willing to lend me his boat, so I could make a mould at once. And having the info, experience and materials that I have today, I would be able to do it in a couple of days, instead of a couple of weeks...

But indeed prioritise. If you want to sail, buy something second hand. If you take some time, and are willing to do a quick deal once the opportunity arives, you can buy a boat for way less then the cost of materials. I even once bought a boat just for the fittings that were on it. I took them off, sold the trailer and the rig, cut the boat in pieces and burnt it up, and had my fittings for not even 10% of the price of new ones.

[transat650]

Many thanks for your replies, folks.

I take your point about buying a second hand boat - that would probably be the sensible thing to do.

However, I'm looking for a challenging boat to sail big distances, so I am more interested in the Mini than the production boats of a similar size. I also actually want the 'bother' of building a boat!

You are probable right that I should just build the standard Didi mini. However, I am put off by the idea that the ply construction would be less durable than composite as I'm looking for a boat which would last.

Would it make sense to simply skin the standard boat in a single thin layer of carbon/glass fibre to prevent water from reaching the ply and making the boat last longer?

Again, I'm really grateful for your help.

[gonzo]

A good quality marine plywood with oil based paint will last at least thirty years.

[Herman]

I second that. I have had a couple of cold molded boats, made from okoume, glued with resorcinol, that were still competitive after 30 years or so.

If you glue with epoxy, and put one layer of glass on the outside, you are building something your grandchildren can enjoy as well...

[Crag Cay]

Would it make sense to simply skin the standard boat in a single thin layer of carbon/glass fibre to prevent water from reaching the ply and making the boat last longer?

Read the plans; I'm sure the hull is glass/epoxy covered.

However, I'm looking for a challenging boat to sail big distances

.
Many of the costs of sailing big distances are fixed; that is the cost of charts, food, instruments, emergency kit, etc doesn't alter just because because your boat is small. Look at the cost of a second hand Beneteau Figaro One. They represent fantastic value for money and many will come fully tricked out for long distance sailing. The other alternative is a JOD35.

Any boat, new or second hand still represents a huge amount of work before you can go 'long distances'. No one has ever complained about the lack of work required in making these projects happen.

[Steve W]

A properly built plywood/epoxy boat properly maintained will outlast you,the problem plywood boats are the ones where the builder used crap plywood,sheathed with polyester etc,etc and then were poorly maintained. I did major repairs in december to the port ama of a searunner 25 tri which had dragged its mooring across the harbor in a gale.This boat was very well built by an amature nearly 30 yrs ago and i and everyone else who saw it was very impressed with its condition,NO rot,NO delamination,absolutly NO issues whatsoever other than the damage,and this boat was not even built with marine ply but was built with epoxy and excellent workmanship and this is key, one suprise to me was that apart from the chines,the hulls were NOT glass sheathed,just epoxy sealed and it worked,not something i would recommend but it has worked for 30yrs.
Ragtime,the 64ft ULDB that started the sled movement in California is a hard chine plywood boat that had a great season of racing in 2008 winning the Tahiti race,the coastal classic race in her birth country of New Zealand and the icing on the cake,the Sydney Hobart race in her class some 40 plus years after they had changed the rules to keep her out on the pretence that she would break apart.
So, i would suggest that the determining factor as to longevity of the boat will come down to your choice of materials and your workmanship.
Steve.

[transat650]

Brilliant - thanks.

So the conclusion is that a good marine ply/epoxy boat will last. That's destroyed a personal misconception so thats really useful.

However, the Dudley Dix Mini plans don't involve any glassfibre skin. So, if the normal hull is 9mil ply (2 layers of 4.5mil) then what sort of thickness would a single layer glassfibre skin replace, in terms of strength? Or would it just be put on top of the standard 9mil hull?

[Boston]

ya having been a builder for my whole life I can assure you that used properly and with an appropriate amount of craftsmanship plywood of good quality can be a godsend

its the misapplication and sloppy craftsmanship end that will jump up and bite you

[Munter]

I'm surprised the design doesn't use a glass fibre skin. Perhaps have a look at the I550 class for typical details of ply construction in that size range.