Designing DC's

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colin brown
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Designing DC's

Post by colin brown » Thu Sep 04, 2008 12:36 pm

I am about the attempt to design my first DC. Before I start this process can some of you very learned people out there advise me of te following:
1. Does the canoe actually plane? Or does it just go very fast through the water?
2. Does a modern non foiling moth plane?
3. An 'A' class cat?
4. What do you think the minimum distance is between the shroud plates?
Using your responses I can start to draw out the sort of shape I need, be it pure displacement / planing or something down the middle. Also I will have a good idea of the minimum beam at deck level.
Finally, can anyone see a 10.6m una rig with our mast height restriction ever being as fast as the universal 2 sail sloop rig. This will have a bearing on beam width.
colin brown

jimc
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Re: Designing DC's

Post by jimc » Thu Sep 04, 2008 2:17 pm

> 1. Does the canoe actually plane?
Probably

> 2. Does a modern non foiling moth plane?
Possibly

> 3. An 'A' class cat?
Probably not

I'm not convinced that there's really a hard and fast division between planing and not planing when it comes to long thin boats...

> 5. Can anyone see a 10.6m una rig with our mast height restriction ever being as fast as the universal 2 sail sloop rig.
Jury very much out on that I think. My own gut feeling is that its not beyond the bounds of possibility. Its going to be all about the tradeoffs between upwind and downwind performance

Chris Maas
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Re: Designing DC's

Post by Chris Maas » Thu Sep 04, 2008 4:45 pm

I would say Jim's got it right. The IC slips from one mode to the other with, to me, no noticeable transition. When you are moving fast I would guess that the hull is mainly supported by hydraulic pressure and that is what I think of as the definition of planing. So yes, we are a planing displacement hull. Somewhere between an I14 and an A-cat and it's up to the designer where to place the emphasis.

I wouldn't be comfortable going narrower than say 700mm at the chain plates. That will take some pretty beefy and well thought out structure and probably a high modulus carbon mast. I went with 760mm on my new boat. String Theory is 800mm. A number that I think is just as important is the distance from the line of the shrouds to the center of the mast. I put it at 230mm on my boats. As you make the shroud base narrower it limits how far the boom can go out unless you move the shrouds forward, which I would be reluctant to do. Keep in mind my boats are designed to support my 90kgs.

I think the Una rig has promise. Phil Stevenson wasn't too far out upwind at McCrae and he had some refinement that could be done. I understand he's now fit some diamonds so maybe we'll know more after the Aus nats. I'm not sure I'd like to deal with all that area swinging across the boat in a gybe on the skinny boats though. You would be hard pressed to come up with a better upwind rig for the IC than the sloop.

Phil Stevenson
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Re: Designing DC's

Post by Phil Stevenson » Fri Sep 05, 2008 7:51 am

My call on your questions Colin:

1. I think the DCs at McCrae were two types, Chris' and Mine were pointy enough to go fast without planing, they could knife through the water like a cat. They did not lift the bow by themselves but if you moved aft they would certainly plane, not necessarily faster than knifing but would certainly feel more re-assuring, particularly Chris' with the low bow. I have never nose dived the Log due to the deep bow, and I do not move the carriage aft often.
The other boats I sailed, Josie, Mayhem, Donkey, were more like typical planing boats, The bow came up early and they were always trying to plane even at displacement speeds.

2. I have been sailing narrow moths for 7 years and I do not think I have ever felt like planing. They push the bow down at speed like a cat and so have a speed limit (which used to be about 17kts before foils, still pretty quick for 11ft LWL)

3. I sailed ACats 30 years ago when they were heavier and slower and never thought they planed. I am not sure about the new light boats because they do tend to sit a lot more on top of the water.

4. The obvious answer for the shroud base minimum is zero. My boat does not have stays.
But if you have a narrow stay base problem one remedy would be to have the mast plug into the hull, and raise the spreaders so that the shroud angle to the mast is the same as what is normal, No extra compression above the spreaders and the socketted mast will stay in column below. Its really going midway from my una rig to Chris' rig as per his answer.

The una rig at McCrae was not a success upwind, it was fast downwind despite being measured area of 9.46 sq M and me weighing 90kg. I now consider that the mast was too soft and have fitted diamonds and a forestay. I have sailed it only last weekend with these fitted (first time out for me since McCrae) and I think it goes better. The sail does not now look like a Laser rag when out on the seat and now at least and I raced effectivly in up to 15kts. But we will not know for another 4 weeks when we meet again for the Aust Nationals.
The diamonds, three separate luff pocket zips (to meet rules only) and all the complication take a lot of the simplicity away from the una rig, But I was still the first unrigged and packed up last weekend amoungst a mixed fleet of skiffs, cats and dinghies.

Hope that is helpful,
Phil S
Design perfection is achieved, not when there is nothing else to add but when there is nothing else which can be taken away.
http://philscanoes.blogspot.com/

Steve Clark
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Re: Designing DC's

Post by Steve Clark » Fri Sep 05, 2008 11:25 pm

I don't know if any sailboat really planes in the classic powerboat use of the term.
Certainly there is a regime where the hull raises out of the water on dynamic pressure instead of being sucked down as true displacement boats operating above their hull speed are. But unless you have some fancy flow code, who cares?
In other news, I don't know what the minimum shroud base is, but I walked around the yard with a tape measure and got the following numbers:
720, 740, 780. The josies have the narrowest at 720 and these seem to be OK.
GER 78, has 740 and John Kells' boat (here for a brief vacation while he takes up all the space in the Little Shop of Horrors building a new seat) has 780 between pins.
It is also worth noting that these rigs are all about 300mm further aft than the Nethercott standard that I used to use. That is to say the mast step is about 2000mm from the bow. This puts the rig back into the fatter part of the hull, but also puts your jib 300mm behind the others on the starting line. It's all in the trade offs...
SHC
Beatings will continue until morale improves

colin brown
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Re: Designing DC's

Post by colin brown » Sat Sep 06, 2008 8:49 am

Mnay thanks to you guys and the excellent info. One of my early design problems is the position of the BMS . If its towards the rear of the canoe then it leads to a very deep canoe , needing to be 300mm deep to measure. I with Steve on this one , I would like to keep the free board as low as possible here.
Still think that the single sail low cost option is the way to go. Lets see how Phil goes in the Aus champs . Any designs for sale or available for a small royality ?
colin brown

Phil Stevenson
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Re: Designing DC's

Post by Phil Stevenson » Sat Sep 06, 2008 11:01 am

Complete how to build article on Hollow Log is still available on Aust IC web site. (links from Blog site below). Or use Steve's cad file ply cut out and follow the method only.
Design perfection is achieved, not when there is nothing else to add but when there is nothing else which can be taken away.
http://philscanoes.blogspot.com/

Chris Maas
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Re: Designing DC's

Post by Chris Maas » Sat Sep 06, 2008 4:01 pm

I believe the height at BMS is 275mm. If it's not I'm in trouble.

Have you been following DC thread on Dinghy Anarchy?

H Virtue
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Re: Designing DC's

Post by H Virtue » Sun Sep 07, 2008 7:14 am

Chris Maas wrote:I believe the height at BMS is 275mm. If it's not I'm in trouble.

Have you been following DC thread on Dinghy Anarchy?
I beleive it is. Again if its not Im in trouble also. Early on it was 300 someone pointed out the 300 didn't allow a Nethercott to measure to the new rule so Steve fixed it.

colin brown
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Re: Designing DC's

Post by colin brown » Mon Sep 08, 2008 6:53 am

Yes, your both correct. Its 275mm I checked last night. I was 300mm , but as pointed out the 'one design ' would not measure so it was reduced to 275mm.
While on line, can anyone tell me why modern 'A class' have a small kink in the rocker line and so far back in the hull line. It looks like 2 straight runs connected by a tight curve to change direction.
colin brown

jimc
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Re: Designing DC's

Post by jimc » Mon Sep 08, 2008 7:14 am

colin brown wrote:small kink in the rocker line and so far back in the hull line.
It seems to be found in a lot of multihull classes... I don't know the design reason, being comletely ignorant of multihull design (Steve(US)C, your cue) but when tried in planing dinghies (eg Laser EPS, some Howlett 80s dinghy designs) my opinion, FWeverIW, is that its death to speed but seems to make the boat a bit easier to sail and less nosey downhill.

Phil Stevenson
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Re: Designing DC's

Post by Phil Stevenson » Mon Sep 08, 2008 7:26 am

Agree with Jim, I do not like spring aft in dinghies/canoes.
I was sailing cats 40 years ago and my take is that when the Tornado appeared people noted the kink in the spring line just behind the fin case, observed that it tacked more easilly than older straight designs, especially when the crew weight moved aft of the kink, lifting the bow, and consequently a lot of designers followed the idea.
I have been out of touch with cat design for a long while now but it looks like the same philosophy continuing. I watched the modern As last weekend and they can certainly lift the bow out and tack very effectively.
It would also help in reducing nose dive tendencies, as it does in dinghies, but maybe the narrow, non planing hulls do not suffer the same slowing impact as observed by Jim and myself in wider boats.
Steve, your ideas?
Design perfection is achieved, not when there is nothing else to add but when there is nothing else which can be taken away.
http://philscanoes.blogspot.com/

jkells
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Re: Designing DC's

Post by jkells » Mon Sep 08, 2008 12:33 pm

Colin,

On Mayhem, I certanly feel like the boat is planing. I am getting a lot of dynamic lift in the forward sections. The volume distribution is very different from what Chris & Phil have created. I am hoping to be able to sail against SHC on GER 78 before delivery!

Now to finish that new seat.......

Best

JK
USA-244

Steve Clark
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Re: Designing DC's

Post by Steve Clark » Mon Sep 08, 2008 7:14 pm

This is another one of those areas where our little bit of the sport has eluded the benefits of research. Quite a lot of work has been done on planing motorboats and been paid for by the navies of the world. Not much has been done on double curvature shapes that are pushed from a center more than 6' above the center of gravity.
There are many competing needs to be met. One wants to have the center of gravity fairly far aft to make the bow light and lively in a seaway. As a result center of buoyancy also needs to be pretty far aft. This requires submerged volume. You don't want to drag around a big submerged transom at low speed, so you want the load waterline to come out pretty close to the surface at the stern. So you play rocker or spring off against prism drag and against wetted surface and hope you get it the lowest drag combination.
The second problem is that if you believe you want to generate dynamic lift, you need surfaces that at a positive angle of attack, which means that you also want to push the deepest part of the hull aft. This also drives curvature into the back half of the boat.
Third there is trim. You need to balance the driving force that is trying push the bow down either bu moving the CG aft or by managing the dynamic forces caused buy water flowing by the hull. When the hull surface turns negative relative to the flow, the result is suction not lift. Thus the stern sections of a rockered hull will help trim the bow up. In a catamaran, where there is always the problem of pitching the thing over the bow, this is a good thing. The Nethercott is generally accepted to have poor characteristic in this regard.
So if you look at Josie and Wonk you see boats which have lower freeboard and narrower waterlines forward, but flatter cross section shapes. The flatter sections and aft rocker provide lots area for the hull to generate lift. The rocker has been pushed aft to get adequate buoyancy under the dance floor ( the rigs are also 300mm aft from the classic Greymatter position) to accommodate the aft CG. This all means that there has to be some pretty reasonable fore and aft curvature in the hull if I'm not going to drag around a lot of excess surface area and or a submerged transom.
That is the rationale at least. It is entirely possible that I overcooked it a bit, But none of us who sail the Josie design are convinced the damn things are slow.
SHC
Beatings will continue until morale improves

Alistair
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Re: Designing DC's

Post by Alistair » Tue Sep 09, 2008 9:08 pm

looking at the different hull shapes that we had in oz I am not sure you can draw any hard and fast conclusions. I think rig had a large part to play in the results and of course the way people sailed. I think that there are a few very general rules that you can follow to taylor the boat to your requirements, rocker aft will stop you nose diving, a rounded hull section will be unstable, having volume aft means you will need to get the seat back. I am not sure that there is much more to add, except that a unirig means relearning how to sail a canoe, and many hours developing things, its all about time, and I am not sure if its slower or quicker....
Alistair
Alistair

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