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PY / Top Speed formula?
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pondmonkey View Drop Down
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    Posted: 14 Mar 13 at 9:14am
Originally posted by Peaky

That's the efficiency bit. But, within limits, big speeds happen in big winds, so you need plenty of righting moment and/or low heeling lever. Weight is simply an approximation for drag at high speed. It doesn't matter how good the hull shape is if you can't harness the power.

novice response, not a clever dick answer....

but, if righting moment is so essential, why are the speed boards Zara Davies uses so bloody skinny?  Is that simply down to reducing drag, and in effect, she's only using stiff-as-feck fin and a tiny bit of board area under the back strap?  
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 13 at 9:26am
On a board the rig is decoupled from the hull, so the hull has no heeling moment imparted from the rig and provides no righting moment*. Therefore you just use the lowest drag hull you can.

Concepts like Speedrocket work because by angling and offsetting the rig and foils, there is no heeling moment on the boat, so it always sails upright. Therefore you can sail in super strong winds or carry a very big rig. There are pitch poling and dynamic control issues, but the risk of 'normal' capsizing is eliminated.

* Not strictly accurate!
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Posts: 118 		Post Options Post Options   Quote Wetabix Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 13 at 3:30pm
The question was: 'how do you normalise the speed using PY'? The answer is you multiply the speed by PY and divide by 1000. Thus if a Mirror manages 10 kts for the required period (30 secs was mentioned) you score 10x1385/1000 = 13.8  . If a Weta managed 20kts it would score 20x950/1000=19kts.  Nobody asked if PY can be usefully used for this purpose (I doubt if it can) however, after a few results have come in it might appear that PYx a Constant would give a usable result.  As to what is possible, the AC45s supposedly go at twice the windspeed but I have never noticed one going much more than 28kts. Perhaps that was twice the local windspeed.
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Posts: 1314 		Post Options Post Options   Quote Neptune Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 13 at 4:12pm
i suspect you'll find that low PY numbers do ok, the PY is based upon speed around a course and fast boats as a general rule seem to be slower in the corners but make up for it while moving.  Still maintain this is in normal winds though, dogs off chains I reckon you can push a slow boat relatively faster.
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Posts: 2202 		Post Options Post Options   Quote pondmonkey Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 13 at 4:21pm
Originally posted by Neptune

Still maintain this is in normal winds though, dogs off chains I reckon you can push a slow boat relatively faster.

I dunno- I reckon a (sensible) trap boat can driven best in big wind... something classic like a 470 or 505 would be pretty special, singlehanders then an IC (okay planks, not traps) or a Contender.  Or as Jim said, a 97 rules cheerub would be a pretty interesting addition to the party... 
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Posts: 4028 		Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 13 at 4:27pm
This little EPS can get a bit of a shift on off wind, it's quite staggering down the lake when you do get a puff, it tanks past just about everything, not saying it's in the twenties but I bet it'll probably record upper teens. If I remember I shall Strava it this weekend and see what it does, I think there's going to be breeze about.

The thing about just sailing for speed without the constraints of having can things to point at or wooden flotsam and jetsam cluttering up the lake getting in the way.. And it not mattering how you  get back up wind can also be a hoot.

I shall watch this competition with interest seasoned speed sailor that I now am.. Wink


Edited by G.R.F. - 14 Mar 13 at 4:28pm
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Posts: 1314 		Post Options Post Options   Quote Neptune Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 13 at 5:23pm
Originally posted by pondmonkey

Originally posted by Neptune

Still maintain this is in normal winds though, dogs off chains I reckon you can push a slow boat relatively faster.

I dunno- I reckon a (sensible) trap boat can driven best in big wind... something classic like a 470 or 505 would be pretty special, singlehanders then an IC (okay planks, not traps) or a Contender.  Or as Jim said, a 97 rules cheerub would be a pretty interesting addition to the party... 

not really 'fast boats' any more though are they, maybe 15 years ago  Wink
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Posts: 742 		Post Options Post Options   Quote rb_stretch Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 13 at 5:37pm
Originally posted by Neptune

i suspect you'll find that low PY numbers do ok, the PY is based upon speed around a course and fast boats as a general rule seem to be slower in the corners but make up for it while moving.  
 
Agreed and I would argue that fast boats are less likely to take every wind shift (especially inland) for the same reason, making their handicap relatively even worse.
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Posts: 3011 		Post Options Post Options   Quote RS400atC Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 13 at 5:50pm
Originally posted by Neptune

Originally posted by pondmonkey

Originally posted by Neptune

Still maintain this is in normal winds though, dogs off chains I reckon you can push a slow boat relatively faster.

I dunno- I reckon a (sensible) trap boat can driven best in big wind... something classic like a 470 or 505 would be pretty special, singlehanders then an IC (okay planks, not traps) or a Contender.  Or as Jim said, a 97 rules cheerub would be a pretty interesting addition to the party... 

not really 'fast boats' any more though are they, maybe 15 years ago  Wink


 A 505 is still a fast boat, and also a very seaworthy boat that will allow people to sail in more wind than most other boats.
For sure they get blown away by the likes of RS800's in middling winds, but I suspect they are still sailable when an RS800 would have me watching from the bar.
Once you are not seeking to go much faster than the true wind (because there's so much of it) it is no longer such an apparent wind game.
Likewise a Fireball in the right hands would possibly outperform its PY substantially compared to asymmetric boats that have similar PY's.
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pondmonkey View Drop Down
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Posts: 2202 		Post Options Post Options   Quote pondmonkey Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 13 at 5:58pm
Yep- that's my point. In 30 knots a 49er is pretty slow, sat in the boat park as the owners pray they tied them wings down properly.
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