Designing DC's

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wee mcp
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Re: Designing DC's

Post by wee mcp » Thu Oct 23, 2008 4:47 pm

" uncanny resemblance" -
If you make allowances for the very different rocker, multi- chine versus arced bottom and vertical sides, deeper forefoot -Yes an uncanny resemblance.
Sorry, Alan couldn’t resist that! :D

Ian McP
"Sometimes its a boat, sometimes more of an accident .. It all depends" - Pooh Bear
IC GBR305

alanppowell
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Re: Designing DC's

Post by alanppowell » Sat Oct 25, 2008 10:17 pm

Cheers mate! But at least you've got me off the plagiarism hook.

colin brown
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Re: Designing DC's

Post by colin brown » Mon Jan 05, 2009 3:22 pm

I am proposing to build a 1/5 scale model of my new DC. Assuming a design displacement of 125kgs will the scale model need to displace 1kg? Advice please............
colin brown

Steve Clark
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Re: Designing DC's

Post by Steve Clark » Mon Jan 05, 2009 4:24 pm

Correct. For 1/5 scale: divide linear dimensions by 5, area dimensions by 25, volume dimensions by 125. Or stated more generally: by the scale, the square of the scale and cube of the scale.
Looking forward to seeing the design!
SHC
Beatings will continue until morale improves

colin brown
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Re: Designing DC's

Post by colin brown » Tue Jan 06, 2009 8:08 am

The design will follow the existing designs already sailing, must be inexpensive to build and will be a version of 'second string', a ply development canoe. At the moment I cannot get enough displacement with 50mm rocker, it seems to sink its ends rather more than I would like. However I have a great deal to learn as I'm not sure what happens when it starts to move as dynamics take over, how important is displacement then?Moth designs do not seem to be over concerned by lack of displacement when at rest ( pre foiling )
colin brown

Alistair
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Re: Designing DC's

Post by Alistair » Tue Jan 06, 2009 8:49 am

Hi Colin
Monkey had 100mm rocker and sat with 150 kg load (on the cad) with the tip of the bow touching and tip of the stern touching. However the boat was built 12 kg overweight and I am about 90 kg in the buff so I recon the sailing weight is about 70 kg boat +sails+wet rope and 100kg of me with gear on, making 170 kg. She still picks up, not as well as I would like but I suspect that is due to no rocker aft, its all up the front under the mast, which I think was wrong
Alistair
Alistair

Steve Clark
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Re: Designing DC's

Post by Steve Clark » Tue Jan 06, 2009 2:34 pm

The Second String design definitely has both bottom of the stem and the bottom of the stern underwater when at rest. You have to get well forward in light air to stop the stern from gurgling. As soon as the wake starts to break away cleanly from the stern, you can move aft, the bow comes up and she seems to run very nicely. It is all too similar to the 29er and 49er.... Bethwaite under displaces his boats by about 25% so that they are the "right size" when transitioning to planing, and aren't oversize when at full speed. Think semi sinker sailboard. GER 78 hasn't had enough ailing with other canoes to know if this is a real problem or not, and I'm more than a few Kg heavier than you are. My best guess is that you "may" want to consider putting a tuck in the aft end to add a bit of rocker aft of the CB trunk and thus add volume.
There are several cheesy ways to do this
SHC
Beatings will continue until morale improves

colin brown
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Re: Designing DC's

Post by colin brown » Tue Jan 06, 2009 4:16 pm

Thanks Steve and Al. for your valued advice. We did think of doing both of the suggestions, increase rocker by 20mm or so and move it aft , and take the sharp corners off the rear chines. Model no. 2 is on its way. Interesting point re. 29 and 49ers, low displacement.
colin brown

Alistair
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Re: Designing DC's

Post by Alistair » Tue Jan 06, 2009 5:22 pm

here's one I have been working on
Attachments
model14.PNG
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Alistair

Chris Maas
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Re: Designing DC's

Post by Chris Maas » Tue Jan 06, 2009 6:54 pm

Both String Theory and my new boat have about 300mm of rocker. With a total displacement of 140kg the bow and stern knuckles are 25mm under water. Like Steve says for GER 78 you need to sit right up at the mast in very light air to keep the stern from gurgling.
With the pintail designs, where the chine aft is submerged, the water seems to slip along the hull nicely with little turbulence. It seems like there might be a draggy transition as the water breaks away from the chine as speed increases but I have very little time in the pintail boats so I can't say. Phil S or Andy P could tell us what happens back there through the speed range.

colin brown
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Re: Designing DC's

Post by colin brown » Wed Jan 07, 2009 8:51 am

Hi Chris, again thanks for all the advice, that the really good deal about the class, everyone shares their advice freely. Chris I presume you mean 30mm of rocker rather that 300mm! Interesting point re vee tails / pintails, I was looking at reducing the width of the 'tail' aft , making the plan shape more canoe and less skiff, now as ever not so sure. I think the answer is ' just do it, now'
Always grateful for ideas.
colin brown

wee mcp
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Re: Designing DC's

Post by wee mcp » Wed Jan 07, 2009 12:13 pm

Steve Clark wrote: My best guess is that you "may" want to consider putting a tuck in the aft end to add a bit of rocker aft of the CB trunk and thus add volume.
There are several cheesy ways to do this
SHC
Steve,
Could you expand on the cheesyness a bit.
Thanks,
Ian McP
"Sometimes its a boat, sometimes more of an accident .. It all depends" - Pooh Bear
IC GBR305

Steve Clark
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Re: Designing DC's

Post by Steve Clark » Wed Jan 07, 2009 2:22 pm

Chris Maas wrote:Both String Theory and my new boat have about 300mm of rocker...
You are kidding.... right? If I read your table of offsets right the mighty String Theory has 72mm of rocker ( just under 3" for imperialists) not 300mm ( commonly known as a foot.)
Ian:
You can put some saw kerfs runnung forward from the aft chine, I find it is hard to keep the surface fair as you dice it into smaller and smaller pieces. Alternatively you can roll the aft end of the hull in tighter than the designed beam. Tabb it there and then spread it back out. Spreading the hull should always increase the rocker.
Alternatively: I put a stringer on center line as well as the gunwales before I start you roll the hull up. The center line stringer is useful as a base for all triangulated measurements and braces to make sure the beast comes out fairly straight and untwisted. Because I am a hack, I cut a slot in the top of this stringer with a table saw so that I can just "drop in" the center line web without having to tap some of my non existent craftsmanship skills. IF after rolling the hull up you decide you need to add volume aft, you can extend this slot down through the ply skin. At this point you can shim the tow halves of the bottom apart, which increases the rocker and volume.
All part of the make it up on the fly aspect of water boarding plywood.

SHC
Beatings will continue until morale improves

wee mcp
Posts: 90
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Location: United Kingdom

Re: Designing DC's

Post by wee mcp » Wed Jan 07, 2009 3:53 pm

Steve thanks for the expansion on cheesyness.

"All part of the make it up on the fly aspect of water boarding plywood."
SHC[/quote]
As you've said in the past - "We don't torture plywood, but we do ask it hard questions."

Ian McP
"Sometimes its a boat, sometimes more of an accident .. It all depends" - Pooh Bear
IC GBR305

Chris Maas
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Re: Designing DC's

Post by Chris Maas » Wed Jan 07, 2009 5:48 pm

Steve Clark wrote:
Chris Maas wrote:Both String Theory and my new boat have about 300mm of rocker...
You are kidding.... right? If I read your table of offsets right the mighty String Theory has 72mm of rocker ( just under 3" for imperialists) not 300mm ( commonly known as a foot.)
SHC
I wasn't kidding - but I was wrong. I meant about 75mm or 3 inches.

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