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Rupert View Drop Down
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    Posted: 08 Oct 14 at 3:05pm
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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JimC View Drop Down
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Posts: 6662 		Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 08 Oct 14 at 3:09pm
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 View Drop Down
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Posts: 2788 		Post Options Post Options   Quote Medway Maniac Quote  Post ReplyReply Direct Link To This Post Posted: 08 Oct 14 at 3:17pm
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 View Drop Down
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Posts: 478 		Post Options Post Options   Quote iiitick Quote  Post ReplyReply Direct Link To This Post Posted: 08 Oct 14 at 3:32pm
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 View Drop Down
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Posts: 437 		Post Options Post Options   Quote zippyRN Quote  Post ReplyReply Direct Link To This Post Posted: 09 Oct 14 at 3:46pm
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 View Drop Down
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Posts: 6662 		Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 09 Oct 14 at 4:29pm
Originally posted by zippyRN

it stops working at  a Length to beam ratio of about 1:10 

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 View Drop Down
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Posts: 1345 		Post Options Post Options   Quote davidyacht Quote  Post ReplyReply Direct Link To This Post Posted: 09 Oct 14 at 4:49pm
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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Posts: 2788 		Post Options Post Options   Quote Medway Maniac Quote  Post ReplyReply Direct Link To This Post Posted: 09 Oct 14 at 5:05pm
Originally posted by davidyacht

I find this to be a pretty reliable way of estimating how long it will take most boats to get around the race course 
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 View Drop Down
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Posts: 437 		Post Options Post Options   Quote zippyRN Quote  Post ReplyReply Direct Link To This Post Posted: 09 Oct 14 at 5:34pm
Originally posted by JimC

Originally posted by zippyRN

it stops working at  a Length to beam ratio of about 1:10 

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.


 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 View Drop Down
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Posts: 6662 		Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 09 Oct 14 at 6:41pm
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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