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gbrspratt View Drop Down
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Post Options Post Options   Quote gbrspratt Quote  Post ReplyReply Direct Link To This Post Topic: dinghy design questions
    Posted: 08 Oct 14 at 8:27am
I've been reading this website and its led me to a few thoughts and questions

http://www.devboats.co.uk/plusplus/index.htm

If we take a single hander una rig.

If you were to make the boat purely for upwind performance and pointing (no compromise just purly upwind?) what would the hull shape be? Why?

Or for downwind? What would change on it to optimise it for this?

And I guess it has to be asked for reaching too?

Just curious.
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Rupert View Drop Down
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Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 08 Oct 14 at 8:29am
Upwind, long, long, long, min wetted surface (so semi circular) narrow with lots of leverage.

Hmmm, like a catamaran.
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Medway Maniac View Drop Down
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Post Options Post Options   Quote Medway Maniac Quote  Post ReplyReply Direct Link To This Post Posted: 08 Oct 14 at 9:03am
Yep, as long and narrow on the waterline as you can make it, with the widest racks imaginable plus a trapeze, until only one man in the world is capable, with practice, of sailing it.

If you are allowing foils like an Int 14 or Nat 12, it can be shorter, but then why not go fully foiling and go quicker still?

And there's the rub. If you want a boat that is nice to sail, reasonably forgiving so that you can look around (e.g. for tactical fun), then you need to be prepared to make compromises on the absolute top speed. No big deal, given that the quickest sailboat you can make is slower than your car. How much and what sort of compromise is down to the individual, which helps explain the plethora of classes out there racing.
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Post Options Post Options   Quote iiitick Quote  Post ReplyReply Direct Link To This Post Posted: 08 Oct 14 at 9:54am
Rupert....what would the fastest current upwind non-foiling mono-hull be then.....RS 600? or Contender cos' its longer?
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Post Options Post Options   Quote Null Quote  Post ReplyReply Direct Link To This Post Posted: 08 Oct 14 at 9:59am
International canoe!
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Post Options Post Options   Quote boatshed Quote  Post ReplyReply Direct Link To This Post Posted: 08 Oct 14 at 10:23am
I was going to say the RS Aero, as it planes upwind.    But probably a 49er, 18 foot skiff or even a windsurfer.   Int 14s are effing quick upwind as well.   All of these are dog slow in a flat calm.
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Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 08 Oct 14 at 10:44am
The question is, which is better, a shorter hullform that will plane upwind, or a longer one that doesn't, but is thin enough that the wave form doesn't affect top speed as it does in a fatter boat?

I have a feeling that the Contender is quicker upwind than the 600, and the IC faster than both, which might partly answer the question. But the weight of the 600 must have an effect.

Trouble is, we have only focused on the 1st part of the question. Once we bear away, do we still want the long, narrow hull? Again, catamarans seem to go fast enough on a reach, so it can't be all bad, but they don't plane. So, do we still want a very long boat, but compromise the shape from pure upwind work to allow the boat to plane? Or does the extra weight involved offset the advantages?
No point in suggesting a windsurfer, as like the catamarans they can only be used as a guide to what might happen, as the query was about dinghies, but from my understanding, a long board is much better upwind. What happens when you bear away - does the short board take over as the faster? I have no idea how long the boards used in the speed trenches are. I assume short, as once planing the extra size would be a waste or weight. Does this transfer to dinghies?
The FD was the fastest boat around in a straight line for many years, mainly because it was 2, 3 or even 4 feet longer than most of its rivals. Would a 20 foot 49er be quicker than the 16 foot version?
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Post Options Post Options   Quote Medway Maniac Quote  Post ReplyReply Direct Link To This Post Posted: 08 Oct 14 at 11:13am
Planing cats have been tried over the years but failed to show any advantage, so I think that if we're ruling out foils then long and narrow still has to be the way to go in average conditions - which typically means if there is a lot of wind there will be waves to cut through.  In lighter winds, then the long hull will easily beat the non-planing planing hull, of course. In a trench with flat water, I'd take a shorter wider planing hull.

Actually, the questioner didn't specify the conditions and we've presumed plenty of power will be available.  If, on the other hand, we're talking light airs, then we want a shorter hull to reduce wetted area, with plenty of rocker and a narrow waterline beam.  Take it to conditions where you can see the specks of dust drifting past on the water surface and a 4:1 length:waterline beam teardrop shape underwater would probably be best, but hopeless as the wind picked up.

Variable geometry, anyone?  It will come some day, for sure - just a question of working out the practicalities.
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Post Options Post Options   Quote Chris 249 Quote  Post ReplyReply Direct Link To This Post Posted: 08 Oct 14 at 11:25am
Originally posted by Medway Maniac


Variable geometry, anyone?  It will come some day, for sure - just a question of working out the practicalities.

Imagine the complaints about the PY differences between all the different variable-geometry classes! Dead
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Post Options Post Options   Quote Chris 249 Quote  Post ReplyReply Direct Link To This Post Posted: 08 Oct 14 at 11:35am
Originally posted by gbrspratt

I've been reading this website and its led me to a few thoughts and questions

http://www.devboats.co.uk/plusplus/index.htm

If we take a single hander una rig.

If you were to make the boat purely for upwind performance and pointing (no compromise just purly upwind?) what would the hull shape be? Why?

Or for downwind? What would change on it to optimise it for this?

And I guess it has to be asked for reaching too?

Just curious.

As noted by others, it's not just the point of sail but also the wind strength that will change things. An old-style International Canoe will struggle to beat a Laser on a square run in light winds but have no issues beating one downwind in a breeze.

I've been lucky enough to talk to lots of top designers about design. All of them say that longer is faster just about all of the time; as one said "length isn't everything; it's only 99%". So people like Bethwaite reckon that even an 18 Foot Skiff would be faster if it was longer (about 22' from memory).

While a short boat can really fly downwind (like a Cherub does) an unrestricted sail area design that is really short will have handling problems unless you compromise the hull design towards survivability and away from pure speed, so even something as amazing as a 12 Foot Skiff is slower downwind than an 18 or 49er. Similarly, although the 1' wide pre-foiling Moths were amazingly efficient upwind and reaching, they hit a speed limit downwind in a breeze. At the other end of the wind range, some people reckoned that in really light stuff a '60s Moth like a Duflos (sort of like a more radical Europe) was actually quicker than the last of the seahugging Moths, which may mean that they would still be faster than a modern foiling Moth in really light winds.

Overall, there isn't one fastest shape across all conditions; it's one of the beauties of dinghy design.


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