Designing to Steve Clark's proposed new rules

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Steve Clark
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Post by Steve Clark » Thu Oct 19, 2006 8:51 pm

woops.
I think we may need to fix that.
SHC
Beatings will continue until morale improves

Mal Smith
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Post by Mal Smith » Fri Oct 20, 2006 1:43 am

I've just checked my Nethercott CAD model which is lofted to the existing rule measurements and the hull depth at 2600mm forward of stern is 286.5mm

Futhermore, the hull depth at 1300mm forward of stern is only 187mm. The Nethercott easily satisfies the minimum hull beam at 100mm above keel rule throughout the full range from 1300mm to 2600mm.

If the minimum depth rule was based on the average hull depth for the Nethercott over the range, then the figure should be about 235mm, and certainly it shoud be no more than 280mm. The DC's produced so far do look like they have noticably more freeboard than the Nethercott. So perhaps inadvertantly, the charactor of the IC has been significantly altered by the new rule?

Mal.
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Phil Stevenson
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Post by Phil Stevenson » Fri Oct 20, 2006 9:14 am

I guess it it correct to have a rule to which existing boats (nethercotts) actually measure, so maybe the 300mm requirement should be reduced slightly.

But I am happy with the freeboard of my boat and in fact I will be raising the seat higher when I build a new carriage. So maybe the new rule DC boats will end up being higher than the 300mm rule requires anyway.

SO: not a problem!! Stop discussing and just build some boats!!!
Design perfection is achieved, not when there is nothing else to add but when there is nothing else which can be taken away.
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Paul Scott
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Post by Paul Scott » Fri Oct 20, 2006 4:08 pm

Mal-

The new rule at Beam min. (i.e. the 750/100/300 rule) does typeform to a large degree esp. concerning Wetted Surface and Underwater beam cross section. WS at B min seems to come in between 29+ sq ft(ish) and 35+ sq ft(ish) pretty much no matter what design shenanigans I go through. Given how big DC's are, it's a bit ironic that the box, at least at it's inner dimensions, seems to point to modern Moth hull design.

Paul:(
"Exuberance is better than good taste" -Flaubert

Andy P
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Post by Andy P » Fri Oct 20, 2006 5:12 pm

But a degree of type form is what is required... the same sort of waterline beam as before, but freedom to change the overall shape.
My design WS is apparently 2.77m² = 29 sq ft
Next time, I would have more height / freeboard. The height now is just >300 at BMS, but might be better with more.

The measurement to allow Nethercotts to comply only needs to be ~ 275mm, but the BMS has to be at midlength / 2600 from stern.

Mal Smith
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Post by Mal Smith » Fri Oct 20, 2006 11:44 pm

Paul,

Pushing the rule to the limit by locating the BMS at 1300mm and using very rounded sections, I was able to get down to WS=2.2m^2 (23.7ft^2). The resulting hull would be pertty unstable, but still more stable than a Moth.

Mal.
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Paul Scott
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Post by Paul Scott » Sat Oct 21, 2006 5:12 pm

Mal-

I agree. I got things down to the high 22's with kind of round sectioms, some overhangs, and scant rocker. When I saw the stability numbers, though, I blanched. I did another iteration really round and a 85mmish draft and got the same ballpark. But I wonder if the resulting narrow WL beam on the first hull (around 720mm) really fits with the intent of the rule.

Just for fun, I did a 2' ish WL beam hull with fairly squareish sections and got 20.2 sq ft, and the stability numbers were better. If you're going to to do a big Moth....

I live in a light wind area, and pushing this rule in that direction is ennervating.


Paul:oops:
"Exuberance is better than good taste" -Flaubert

Phil Stevenson
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Post by Phil Stevenson » Sun Oct 22, 2006 12:24 am

Too much science. I just bent the ply until it had a nice shape which complied with the rule.

I think targetting surface area will lead boats towards getting more like a hemishere (min WSA/volume), where it would appear that wave making resistance is more critical most of the time. So bow angle and topside width should be minimised too. And this will lead to increases in WSA.

Classes of boats from Radio Marbleheads, I14s, A Cats, and Americas Cup where they are allowed to, are reducing topside beam and making the boat as fine as possible, because that is what makes boats fast.

The DC limits of 750 x 100 appear to have placed (or reinstated an old one) a reasonable limit for canoes. The boats are more wobbly than a Nethercott and we are yet to see if we can race them sucessfully against Nethercotts in tough conditions, but it is worth the challenge. Maybe in the end some people will choose to build slightly wider in the interests of handling, but we will not know if more people do not build boats.

Paul, my boat probably has more WSA than you have calculated because its ends are pointier (low PC), but in less than 5 kts it is blisteringly fast against all the big cats and skiffs, as well as all the ICs I have came against. The DC has the best Sail Area/weight/length/WSA ratios/numbers of anything. And it is a dream to sail in these conditions compared with a tippy moth.
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/

Paul Scott
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Post by Paul Scott » Mon Oct 23, 2006 3:58 pm

Phil-

Good point. It was originally my intent to put some EPU on the frame and start sanding. ...but six weeks stuck at home recovering from sugery, hearing the siren call of Vacanti Yacht Design Software, has brought me to this pointy headed state.

I think Norman Mailer ( in Tough Guys Don't Dance (I think))) said something about horror films not preparing us for the hours lost in searching after one clear thought.

Paul [:0]
"Exuberance is better than good taste" -Flaubert

Paul Scott
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Post by Paul Scott » Fri Oct 27, 2006 3:37 pm

Phil-

When you said you had a low Cp, without giving anything away that you don't want to (and given the paranoia about giving out Cp's among the TP52 crowd, I understand if you don't want to..), did you go under .53? I've been obsessing around .50(!). Which worked on some gunnish long sailboards.

Paul8)
"Exuberance is better than good taste" -Flaubert

jimc
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Post by jimc » Fri Oct 27, 2006 7:27 pm

Under .53!! Good grief. I make a Nethercott .59 and I just pressed the button on the hard chine shape I'm laying with and got 0.61... But its so dependant on trim anyway - move the cog 100cm forward on mine and its nearly .64, move it back 500 and its .57. I'm very agnostic about cp at the moment, esp after I did an interview with Julian B and he was scathing on the subject!

On Phil's point below, I am bemused that my shape has ended up with a higher cp than the Nethercott. Its far finer in the bow. Its basically a hard chine Nethercott with vertical topsides from about the static waterline up aft of daggerboard and a virtully straight waterlines bow forward of that...

Image

Phil Stevenson
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Post by Phil Stevenson » Sat Oct 28, 2006 12:29 am

I have no idea what the CP number is because I have not even plotted the final sections. The shape is what it turned out to be, based on where I started with panel shapes and where I bent them to. I said it had a low CP because it is clearly pointier in the bow and stern than a Nethercott or Wonk. The chines are as good as straight from the bow to well aft of the mast. The stern is much finer than the required 45deg each side.

A long time ago I got the idea that if I could bend plywood around a shape then water would go around that shape prety well also. Most of my more successful boats have followed that theory not the numbers gererated by computers. Along the way I learnt to get the volume correct and as low as possible, get the spring as small as possible and still be able to steer, and get it as narrow as the rules or contol allows. The stressed ply method allows all that but you never know what the sahpe will be until you are finished. Design by strategy, target and means, not by detail.
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/

Phil Stevenson
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Post by Phil Stevenson » Sat Oct 28, 2006 9:01 am

Jim,
Nothing above the WL will affect PC, so vertical topsides are irrelevant. The Blunter the waterline at each end and blunter the sections sections at the bottom, the higher the PC will be. My boat has pointy WLs which will bring the No down, but is very flat bottom U shaped forward, which will increase it. More spring will also bring it down and my boat does not have a lot.

If you started with a nethercott which has bluntish WLs and wide U/V sections, then reduce the spring because it would need less volume to carry less weight, you will get a higher PC. Bringing in the sides above the WL has no impact.

Your drawing looks a lot like my boat except it is wider in the stern. My chine is higher forward, but with more bottom curviture it virtually disapears.

Of course I do not have the flares as I have no stays.
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/

Geoff Harman
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Post by Geoff Harman » Sat Oct 28, 2006 10:52 am

Don't get hung up on Cp's. It is an old coefficient for comparing the hullforms of traditional displacement hulls and has no relevance for semi-planing and planing hulls.
By definition it is the displaced volume divided by the product of the area of maximum cross section and waterline length.
Some examples
IC Nethercot .53
ICDC 3 .53
Typical kayak .63
NS 14 (similar to a Tasar) .6
Moth - narrow .62 Design from about 5 years back
Cigarette speed boat .78
As Phil says design for good flow lines with minimum distortion and sufficient area to get some dynamic lift.
I considered wider aft sections like your are suggesting JimC but went for narrower sections as there is no spinnaker and moving the seat aft will sink the stern easier and hopefully reduce any nose diving tendency from the lack of volume in the bow.
Geoff Harman

jimc
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Post by jimc » Sun Oct 29, 2006 2:05 am

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Geoff Harman</i>
I considered wider aft sections like your are suggesting JimC but went for narrower sections as there is no spinnaker and moving the seat aft will sink the stern easier and hopefully reduce any nose diving tendency from the lack of volume in the bow.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

The aft sections is an interesting one. Assuming I get off my ass and build this one (and I've been landed another boat project this winter for the partner which is getting bigger all the time) then it will be a budget experimental and semi disposable shell to just bolt rig and accessories off my Nethercott on to. My gut feeling is that a fine stern boat like Andy's (or Phil's) will be better, but there's no point in my exploring that because they will do it better than me. Given general fair sections, waterlines etc a wider stern might be expected to be a bit more powerful with more lift off the sten wave system in moderate conditions.

On the other hand sailing the Nethercott I notice that the water is very turbulent off the stern with vortexes coming off all along the rurn in aft, and a fine immersed stern might have more attached flow and better pressure recovery and be a lot slicker in lighter conditions.

On the third hand there's some evidence from some design history work Chris Thomas has assembled that traditionally fuller ended more poweful Canoe shapes have out performed fine ended ones. I can't quite cope with the idea of a full bow, which the Nethercott does anyway, but I thought a fat stern might be worth looking at.

Nosediving is an interesting and I think complex subject. I'm not sure that anyone has really studied the dynamics, although I'm prepared to be corrected. There's a lot of things going on. My gut feeling is that just immersing the stern more and lifting the bows up won't help much, because its normally a transient phenomenum, so its what's hapening in the special events that counts. I'm not sure it matters too much whether the stem is 4 inches or 8 inches off the water when its not going down, but who knows...

One factor I'm sure is important is how rapidly the drag builds up as the nose goes down, decelerating the hull so the rig presses more and more over. The vertical topsides are an obvious mitigation point here, I am convinced from other boats that full bows that get blunter above the waterline mean that the boat trips up more. That's an argument for a raked or even clipper bow in a one design, but that's another story... But that's about surviving the event, not preventing it.

On prevention another uninformed guess is that there is a lot going on with the pressure changes under the hull between mid length and aft. Its noticeable that semi bustle rocketed boats, with rocker aft like many Howlett designs, his 14s and Cherubs and also the Topper boats are quite safe in this respect. Its almost as if they suck the stern down. However they're also dog slow at any speed, so maybe that's not a great route to explore...There's some evidence in Cherubs that fat sterned boats have a tendency to nosedive more than fine ones, but my gut feeling is that's more to do with the "whole boat in the air, help, what do I do now" bit which isn't so much a design consideration for ICs.

Oh well, to quote Paul Bieker "that's the great thing about dinghy design, its just too difficult..."

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