Laser 140101 Tynemouth |
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Laser 28 - Excellent example of this great design Hamble le rice |
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Laser 161752 Tynemouth |
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List classes of boat for sale |
dinghy design questions |
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Rupert ![]() Really should get out more ![]() Joined: 11 Aug 04 Location: Whitefriars sc Online Status: Offline Posts: 8956 |
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Thinking more about long thin hulls v's hydrofoils, my mind was taken to Icarus, the foiling Tornado in the 70's. Now, the speeds were faster than the standard boat, but not by a huge amount, and it has really taken another 40 years to get the concept to a point where there are genuine gains to be made, especially upwind, where I'd imagine Icarus was actually slower.
So, a 20 foot long dinghy, built out of the highest tech materials possible to keep the costs down, plenty of righting moment to support a fairly big rig, but sailable by 2 people, firstly to keep it more like the boats we sail, and second to keep the weight down. So, what would that rig be, assuming you could magically change it for upwind, reaching and down wind? How different would each mainsail shape be? Would you need to change aspect ratio? |
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Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686
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JimC ![]() Really should get out more ![]() ![]() Joined: 17 May 04 Location: United Kingdom Online Status: Offline Posts: 6662 |
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The faster a boat is the less the rig needs to change upwind to down, because either way you want low drag with the apparent well forward.
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Medway Maniac ![]() Really should get out more ![]() ![]() Joined: 13 May 05 Location: United Kingdom Online Status: Offline Posts: 2788 |
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I'd go for a una rig upwind, with a sail that could be flattened considerably, enabling it to be pretty big. Boom down on deck level - at 20ft the hull is big enough that the guys can run around the back/front of the rig.
I'd talk to Dan about the advisability of using a wing mast.
Unfurl a very high aspect fractional jib coming offwind to keep the main unstalled until far enough offwind to unfurl a much bigger masthead gennaker. And good luck to anyone trying to sail it. |
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iiitick ![]() Far too distracted from work ![]() Joined: 04 Jun 14 Location: gb Online Status: Offline Posts: 478 |
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RS 300 with an extra 2' on the front. 1' wider wings and a CII rig but much taller! How about that?
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zippyRN ![]() Far too distracted from work ![]() Joined: 14 Sep 06 Online Status: Offline Posts: 437 |
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longer IS faster in displacement sailing
hull speed in knots = fudge factor * root of the waterline length in feet it stops working at a Length to beam ratio of about 1:10 - which is why multihulls don;t need to plane i nthe same way a monohull does as for hull speed purposes the multihull is 2 or 3 seperate vessels sailing in formation all with very very fine hulls |
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JimC ![]() Really should get out more ![]() ![]() Joined: 17 May 04 Location: United Kingdom Online Status: Offline Posts: 6662 |
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It doesn't actually stop working, but given a fine enough hull its less significant. There are different sorts of drag, and different hull shapes cause greater or lesser amounts of them. Given a long fine hull then wavemaking drag becomes less significant, because, to put it crudely, it doesn't push much water out of the way to make waves. On the other hand the wetted surface area drag is very much greater. |
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davidyacht ![]() Really should get out more ![]() Joined: 29 Mar 05 Online Status: Offline Posts: 1345 |
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Long slender hulls are the exception, hence rowing eights, multihulls etc. Basically a displacement boat at speed forms a wave at the bow, a wave at the stern and a trough mid ships, it requires a great amount of energy to break out of this hump, which is why Displacement Powerboats can rarely plane. Froude documented this from which we get v (knots) = 1.34*lwl^0.5
I find this to be a pretty reliable way of estimating how long it will take most boats to get around the race course
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Medway Maniac ![]() Really should get out more ![]() ![]() Joined: 13 May 05 Location: United Kingdom Online Status: Offline Posts: 2788 |
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Only very approximately - there are plenty of other factors, and once the wind drops towards light airs wetted area becomes as much or more of an issue. Likewise when boats start planing the differences can be quite large - a Cherub will get downwind quicker than a 505, for example, and probably not much slower (if at all?) upwind provided the water is flat enough for planing.
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zippyRN ![]() Far too distracted from work ![]() Joined: 14 Sep 06 Online Status: Offline Posts: 437 |
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indisputably in termso fthe drag , but this is Y+Y not AYRS or RINA ...
my point was fudge factor * root lwl = hull speed works for most normal shaped small craft and provides a limiting factor if you either can';t plane due to hull form ( like a displacement launch or a leadmine long keel yacht with little volume carried aft) or don't (yet)have the excess power to 'pop up' onto your bow wave and plane ... for dinghy shaped boats in very light airs wetted area is the main drag source - hence the tendancy for most boats to perform well when trimmed down at the bows to lift the flat rear sections out of the water even if you reduce Lwl with this trim in light to medium airs ( until you generate enough excess power to 'climb' your bow wave and begin to plane ) it;s the wave making drag and 'hull speed' which are the limiting factors - hence the perpendicular bows on medium powered development classes like the N12 and the MR to make LWL as close to LOA ( as they don;t restrict the LWL vs the LOA - much asdignhy rules used to have minimum beams as peopel thought espeically in pre-planing days of yore that fineness and therefore lightness through less material to build was more important) and the near perpendicular bows on One designs drawing fro mthe contemporary to their original time 12s and merlins ( e.g. Lark, RS200, RS400) once you are planing it's a whole different kettle of fish ... |
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JimC ![]() Really should get out more ![]() ![]() Joined: 17 May 04 Location: United Kingdom Online Status: Offline Posts: 6662 |
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Mmm, but there are no firm boundaries between planing/not planing. Its a transition. And very few sailing dinghies really get out of a semi displacement mode, especially upwind - other than Moths - and that's why waterline length is quite a powerful predictor of performance.
There are some funny things that go on with some shapes too. Some fine bowed boats hardly seem to create a bow wave at all - the 49er is one, and my PlusPlus was another. |
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