Bonito

an area to discuss dinghy developments
JimC
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Re: Bonito

Post by JimC » Thu Jun 18, 2009 10:10 am

Nessa wrote:, but how can I get something that must be about 10mm thick to be strong enough?
carbon fibre?

Sometimes its not hard to understand how come the Laser and Topper did so well is it!

Nigel
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Re: Bonito

Post by Nigel » Thu Jun 18, 2009 4:19 pm

Everything flexes a bit. Just find something that flexes more along the front edge than the back :wink: .

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Nessa
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Re: Bonito

Post by Nessa » Thu Jun 18, 2009 7:16 pm

Does anyone know how long the new dagger board should be? Is there a formula for such a thing?
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Nigel
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Re: Bonito

Post by Nigel » Thu Jun 18, 2009 8:24 pm

Hi Nessa,

you may find something on a US site but failing that, a quick scaling up of a line drawing in Burret give 22-23 inches from the bottom of the hull to the tip (you will have to measure how much is in the hull and sticking out the top). The shape shown is parallel sides with the front corner removed in a quarter circle.

Hope that helps,

Nigel

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Nessa
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Re: Bonito

Post by Nessa » Thu Jun 18, 2009 8:37 pm

Thank you Nigel - we still have the top bit; it stayed in the hull while the other bit disappeared!
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Re: Bonito

Post by Nigel » Thu Jun 18, 2009 8:41 pm

Hi Nessa,

You are welcome. I should have added that the radius of the circle seems to be about the same as the width of the board.

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Re: Bonito

Post by JimC » Thu Jun 18, 2009 10:16 pm

Nessa wrote:Does anyone know how long the new dagger board should be? Is there a formula for such a thing?
It should have an area in proportion to the rig, so if you go for a tad bigger surface area than a Laser you won't be far out.

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Nessa
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Re: Bonito

Post by Nessa » Tue Aug 25, 2009 3:06 pm

Having made the following modifications:

doing away with the sleeved boom so the enormous sail can be reefed
fitting a centre jammer and ratchet
a decent functioning kicker
changing the sheeting to laser style sheeting
fitting righting lines
putting on a basic outhaul that still needs work
creating a new daggerboard

Her indoors took the tuna tub for an outing today during my Women on the Water session and begrudgingly admitted that she a) enjoyed herself and b) found the boat very comfortable. The bonuses for her are the high boom, lots of room in the cockpit, and the fact that when hit by a gust the boat sits up on its ear, but the sharply squared off sides seem to stop it going right over. She came very close to admitting I may have done the right thing in 'forcing her to buy it'.

:D
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Ed
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Re: Bonito

Post by Ed » Tue Aug 25, 2009 4:13 pm

Nessa wrote:
, but how can I get something that must be about 10mm thick to be strong enough?
Ask me after Roadford (hopefully....if it stops raining long enough to put it all together)

My jollyboat has a pretty big board.....well I say board....but being the 3rd ever built, it was built in the times when the presumption was that you would have the steel plate.....which it did have till about 7 or 8 years ago. Then I used an old ali albacore plate (same shape), which was so bendy it was a joke. It was home-built and underspecced thin ali. I have tried for years to find another decent ali plate with no luck.

So....this year I decided to build a new 'thin' carbon c-board.

Problem is that not only was it only about 20mm to start with, but it has then warped and shrunk in so that most of the case is 11-12mm wide at best.

Well I had some carbon left over from IC build plans....so....

I ran an old mahogany board through a thicknesser/planer till it fitted and then started to play.

I routed out a 2ft x 6 (in centre of board thru waterline) in depth to 3mm and filled out with a double layer of 300g carbon uni and a couple layers of 200g twill, oh and a layer of kevlar for good luck.
Sanded that all down flush and covered the whole board with a layer of 280g carbon and some kevlar under the top quarter (to just under the water line).

Has it worked?

Well maybe....but I have to admit that I talked to Milanes a few years ago about this plan and he said it just wouldn't work as it was just too thin and I would never get it stiff enough to be worth the money and effort.

My guess is he is right.

It ain't light....about the same as ali plate, but no lighter than a 25mm laminated mahogany board. It 'may' be strong (we shall see) but it is no way as stiff as a proper 25mm board.

Interesting to see how it works.

I am sure putting carbon on a 10mm ply board would help....certainly make it stronger, but I doubt it makes it any stiffer really.

Of course the Jollyboat is a pretty big old boat....so we shall see.

cheers

eib
Ed Bremner
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Re: Bonito

Post by JimC » Tue Aug 25, 2009 9:24 pm

Ed wrote:It ain't light....about the same as ali plate, but no lighter than a 25mm laminated mahogany board. It 'may' be strong (we shall see) but it is no way as stiff as a proper 25mm board.
Your layup seems odd to me... I don't think the kevlar will do anything significant in this job, and all that weave means that it will have excess fore and aft strength and be low in the main direction... A conventional layup might be
one layer 200 g/m² unidirectional carbon over all of the foil
a second layer of 200 g/m² unidirectional carbon over the top half
a third layer of unidirectional carbon, approx.150mm wide, over top part of the foil, extending approx.100mm past the bottom of hull when the foil is right down.
one layer of 200 g/m² woven carbon cloth,
finally a layer of 86 g/m² glass cloth (some/much of this will disappear fairing)
Now that is a conventional thickness board, but note the ratio of unis to cloth. - 600gm2 unis to 200 cloth, and that the long axis has effectively up to 700gm2 fibres

You've got:-
600gm2 unis,
680gm2 various cloth which is~ 350gm2 on the long axis,
so that's 950gm2 total on the long axis
so you've got about 50% more fibres on the long axis for about half the thickness, so flexibility is unsuprising.
I'm no expert, and haven't done the sums, but I would guess you'd need between two and three times the fibres on the long axis for the thin board, so
I think I would probably have gone for (tapered up to) 5 layers of the unis, plus one of cloth, giving 1600gm2 on axis for probably little more thickness than you have now...

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Ed
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Re: Bonito

Post by Ed » Tue Aug 25, 2009 10:44 pm

Cor, Jim....

you lost me there....

sure your'e right.

there wasn't a ton of thought went into it...just more a case of looking to see what carbon I had in the workshop and using that.

It certainly wasn't a normal type lay-up and I didn't have the carbon to do that anyway.

What I had was a couple of metres of 300mm wide 300gm uni....and some 280g woven and some other odds'n'ends.

I didn't have the carbon to cover the whole thing with too much stuff. Would of loved to do 5 layers of uni! I am sure that would of made it plenty stiffer!! But jollyboat boards are big (maybe twice the size of an IC one) and I was pinching from IC building stock.

Hence routing out a 1m length of 6in wide, from above bolt to maybe 2ft 6in below on each side, doubling over the carbon uni and filling out with some other carbon...and that kevlar.

the kevlar ain't gonna be here or there, but somehow I do like its ability to slowly fall apart rather than go with a bang.

I would of layed the carbon on top.....but couldn't because of the thinnness of board.

Once I had the two carbon skin-beams on either side of board I just put a layer of the 280g cloth over the whole thing.

So as you say.....the beams are about 600gms of uni and maybe 400gms or so of cloth both sides, but faired down to size of the wood, then the whole thing had another 280gms on top.

The beams must be around 2-3mm thick before the carbon skin.....but were not vacumed.

Do explain more what you mean by 'on the long axis'? of the board? or of the cloth?

I was running the carbon uni down the length of the board....rather than across of course, although there must be about the same weight of cloth as uni overall including the skin.

Anyway, as I said, I don't claim it to be much more than a bodge.

but my point still stands, at this width.....the amount of carbon you would need to make a difference seems to be rather prohibitive...and using less possibly pointless.

Will try and find some images to post if I find the time.

cheers

eib
Ed Bremner
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Firefly F2942
IC GBR314 ex S51 - 1970 Slurp
MR 638 - Please come and take it away
Phelps Scull
Bathurst Whiff - looking for someone to love it

JimC
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Re: Bonito

Post by JimC » Wed Aug 26, 2009 9:37 am

Ed wrote:Do explain more what you mean by 'on the long axis'? of the board? or of the cloth?
Of the board...
Caveat - I'm not an expert on this stuff, but I've listened to people who are and sometimes may have understood them...

the thing about composite layups is that in each given direction only the fibres that are aligned in that direction are contributing to the stiffness of the final result on that direction. So if the main worry is longways bend then only the longways fibres in the cloth are contributing to reducing that bend. So in the case of a daggerboard or a mast you build it mainly out of longways unidirectional fibres, and finish it with a layer of cloth to keep everything aligned. Just like wood really - that's mainly unidirectional fibres down the long axis too [grin]. Imagine building a wood mast with the grain running acros the mast, not along!

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Ed
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Re: Bonito

Post by Ed » Wed Aug 26, 2009 10:15 am

OK
well that is pretty much what I thought.

Hence the 2 x 300g uni being the basic part of the layup

couldn't find the build photos but will look again soon

cheers

eib
Ed Bremner
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Jollyboat J3
Firefly F2942
IC GBR314 ex S51 - 1970 Slurp
MR 638 - Please come and take it away
Phelps Scull
Bathurst Whiff - looking for someone to love it

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