feedback on hull design

[mattias]

I would really appriciate if you could give some feelback on this hull design. It is my first design and I want to know if I’m way off line here! The hull is 6.4 x 2.2 meters and I plan to put an 115 hp outboard on it. I will use epoxi for this sandwich contruction. The core will be made of Polyurethane 50 mm. The outer and inner layer will be made of one layer of Powder Bound Chopped Strand Mat 300 gm and one layer of Kevlar Fabric 170 gm.

Thanks!

[ErikG]

AFAIK you can't use powder bound CSM with Epoxi. To use any CSM with Epoxi it needs to be stitched to your reinforcement.
I think a better way to get a nice outer surface with Epoxi would be to use a layer of thin satin woven glass. but hey I'm still learning so don't read this like the bible.

Just using a CSM and one layer of Kevlar 170g/m^2 sounds like it might not be impact tolerant enough. As I'm not up to speed on Kevlar as a reinforcment I probably should just stay quiet, but hey fellow swedes :-)

Which is most important money or strength/speed?

ErikG

[mattias]

Hej ErikG

The Powder bound CSM should be all right with epoxi according to CFS fibreglass Supplies

http://www.cfsnet.co.uk/acatalog/CFS_Catalogue_GLASSFIBRE_MAT___FABRICS_10.html

I try to keep the hull as light as possible and I'm not sure if using a CSM and one layer of Kevlar is sufficient? Maybe someone out there knows. Strength/ speed is more important than money.
thanks,

Mattias

[Tim B]

It may sound archaic, but for a boat this size, wood (well chosen wood) is a good material. It also saves making a plug+mould (though if you could pioneer a mould-less process...). Anyway, having built the boat from plywood, with apropriate internal structure, then a layer of kevlar, then a nice bit of finishing should give you a boat that is light, stiff and will absorb shock (impact tolerance question). Since the wood itself can be relatively thin, you will find that there is considerably more room inside. This may be advantageous for engine mounting/access etc. Since you've used CAD, you can calculate the weight of plywood needed. Add in a few bulkheads, and the Kevlar skin, and find out what the difference in weight is. Give it a shot.

Cheers,

Tim B.

[mattias]

Tim,

What kind of plywood and how thick would it have to be? How much does plywood weight per square meter? Is just one layer of kevlar as an outer skin (and some bulkheads) all that is needed to get a stiff hull?

Thanks,

Mattias

[ErikG]

It's not the kevlar that makes a wooden boat (as described by Tim) stiff, its the wood, stiffeners and longitudinals.
The outer Kevlar is great to have because it is so scratch tolerant. No mormal wear and tear would go into the wood and destry the "watertight" seal

[Willallison]

Tim:
It also saves making a plug+mould (though if you could pioneer a mould-less process...

Many one-off foam core boats are built without the use of molds. It just means that there is a considerable amount of fairing to be done to get an acceptable finish. Which is not to say that ply should be ruled out as a core material, of course.

Mattias: To begin with, I'd read the article on core materials posted elsewhere on this site ( http://boatdesign.net/articles/foam-core/index.htm ). There's a great deal of info in it.
I wrote to Trevor Grundberg (the author of the article) and he was very helpful in suggesting core and skin weight / types for the Option One project ( http://boatdesign.net/forums/forumdisplay.php?s=&forumid=14 ) I'm sure that any of the reputable suppliers would be equally happy to assist you with layup weights etc. You will need to be able to supply them with details of your design - things like maximum unsupported length and the like.

As far as the design goes, what's your thinking behind the 'tunnel' shape in the bottom of the aft sections?
It's difficult to tell from the posted images, but you will need to have the transom at an angle (12 degrees I think from memory) for the outboard.

[gonzo]

Why are you combining Kevlar with chopped strand? One saves weight and the other is a cheap but heavy bulk material. If you use foam, there is no need for an extra amount of core material. Remember that the problem with very thin laminates is buckling. Chopped mat would take care of that, but then it makes no sense to use an expensive and difficult to use fiber like Kevlar.

[mattias]

Thanks Will and gonzo for the input!

Great links!

I will change the transom angle to 12 degrees and skip the kevlar in the hull. I got the idea of the "tunnel" shape from a concave waterski. Maybe it will give more "bite" when turning and make the boat plan easier.

Mattias

[Willallison]

The concave won't make the boat plane easier - to do that a flat section would be best and you often see a flat section in very fast v-hulls - often called a pad. It can also help avoid chine walking.
Having said all that, I'm not sure how the tunnel will effect the handing etc - there are far more knowledgeable people on these boards than me who may be able to tell you....

[Tim B]

You're possibly better off questioning in the 'boatbuilding' part, but here goes...

The thickness (and thus weight per sheet) of plywood changes dependant on two factors, firstly, the curvature of the hull, and secondly what's available and practicle (don't try to laminate a 12mm hull with 1mm ply). The serious strength in the wooden boat comes from the frames/bulkheads and stringers, chines,decks and joints.

Remember that a good building method is to aim for complete tubes or circles (that is to say that a frame supporting the hull will be a lot stronger if it has a deck-beam across the top (thus createing a'circle', the hull skin and deck create a sort of 'tube')).

A quick sum will determine the loading on a panel of any given size, so the structure can be designed to that. It is, of course, preferable for the structure not to flex (noticably) under the designed load conditions, it will reduce the life if it does.

A mixture of frames and bulkheads is usual, bulkheads where the strength is needed (eg in the bow) and frames where more space is needed, or the loading is lower, or where there is a considerable change in geometry (ie. the step, where two frames should meet a bulkhead between them. as much structure as possible should continue through this bulkhead, though not to expose end-grain)

Atop the concoction of wood and resin, I would put a finishing of a two layers of very thin epoxy, followed by a layer of composite (standard weave (material dependant on design weight - Kevlar=light - Glass=Heavy) and then a finishing coat of epoxy. The finish should stop most water ingress. Remember, also use a few very thin layers of epoxy (matt if possible) on the inside of the hull. once it has really dried, the lifespan of the boat should be comparable to GRP.

Good Luck,

Tim B.

[Tim B]

By 'standard weave' I mean cloth

[mattias]

Thanks Tim,
I really appreciate you taking time helping me.
mattias

[Tim B]

You're welcome mattias, it is, after all, what we're here for. I would advise checking the boatbuilding section of the forum though...

Cheers,

Tim B.

[JPigott]

Mattias,

On Kevlar, my limited understanding is it's effective use requires a good understanding of it's characteristics. It stretches very little before reaching it's maximum strength while fiberglass stretches considerably more. In it's simplest sense it would tend to put under load first and fail or conversely not add anything to the strength of the boat. Remember something about it being useful on the inner hull (tension) but as the primary lamination.

Problem with dissimilar materials engineering qualities:

Going further out on a limb you would use kevlar with materials of similar characteristics carbon fiber, post cured low elongation epoxies etc.

Biaxial fiberglass might be worth considering.