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#1
12-07-2009, 03:31 AM
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| Gust Response To recap: The rig flexes in a gust to spill some pressure. Then, it gives a little pump when it returns to its original shape. That's a little bit of free & legal kinetics. So can the same be said when a boat heels in response to a gust? When the boat returns to its normal heel, is it also providing a little "pump"? Do sailors know this and take advantage of it? Can a boat accelerate out of a puff?  |
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#2
12-07-2009, 06:45 AM
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| Puffs usually come from a slightly different direction, either upwind or offwind. You can absolutely make more progress to windward if the gust is from upwind. Otherwise, going the otrher way, you lose progress. I don't see an extra "kick" when relaxing back into the "groove". The boat's been slowed slightly, and it's almost like coming out of a tack. |
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#3
12-07-2009, 11:49 AM
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| If we are talking about small boats and racing, then yes, it is possible to hide a violation of Rule 42.2 in a gust. No Warranty Implied, Your Mileage May Vary, Do Not Try This At Home, No Stunt Pixels Were Injured In This Demonstration. In a larger sense, energy is stored when the hull heels by lifting the CG. In some small way, that energy is realeased when the hull returns to "normal". Some hulls are better at releasing that energy to forward motion than others (think strong stern quarters), and indeed you could design a mechanical propulsion system based upon wave roll of the hull. However, as Alan said, it is very unlikely that a puff can always be harnessed to improve boat speed.
__________________ A vessel is nothing but a bunch of opinions and compromises held together by the faith of the builders and engineers that they did it correctly. Therefor the only thing a Naval Architect has to sell is his opinion. Last edited by jehardiman : 12-07-2009 at 11:49 AM. Reason: Obvious Typo |
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#4
12-08-2009, 04:09 AM
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| Quote:
Jehardiman, cool reply. You're probably right. The hull and appendages may not deliver increased forward drive as the boat is rolling back up. But maybe the sails do. I suspect that they are providing one long pump as the boat comes back up onto her previous trim. Maybe we should schedule a race in gusty conditions. A conventional yacht against that unusual boat with the canting mast. Pick your favourite. |
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#5
12-08-2009, 04:44 AM
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| If you're sailing a dinghy well, you don't really heel much to a puff. Sure, you can accelerate when you roll back to the correct angle of heel - but you normally lose more by heeling to leeward in the first place. Good sailors certainly know that you can 'pump' a boat when you rock it back down to an even keel - that's why they roll tack - but they know you normally lose too much early on to make it really worthwhile. |
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#6
12-09-2009, 03:47 AM
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| SuperPiper: a gust always changes the apparent angle of attack of the wind... thats the reason why you heel over in the first place when going to windward... since the trim of the sails do not fit the apparent wind in a gust... you have two options then: - change course into the apparent wind so that the trim (and the heel) suits this 'new' wind... - keep the course, ease sheets and trim the sails accordingly... easy to do on a dinghy... the latter gets more difficult the bigger the sails... with a good trim the boat will shoot into the wind of its own accord depending on the responsiveness of the yacht... but in both cases you will feel an acceleration because the windspeed is higher and therefore the forces the sails produce increases... it is not the heeling over and restoring to the original heel which gives you acceleration... and not every yacht is responsive enough to use this gust to increase speed... it is a question of displacement to length ratio - ULDBs accelerate really fast while heavy boats just heel with almost no noticable acceleration at all... on the run it is not that noticeable because a gust will tip down the bow and thus increasing drag... there might be no acceleration worth to mention... |
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#7
12-09-2009, 05:06 AM
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| So I'm going to chase this a little more. On my planet, sailboats heel. As Jehardiman mentioned, a heeled boat has stored some potential energy by raising its CG. Does that potential energy get converted into forward velocity when the boat returns to its original trim? Do some hull shapes do a better job of this? A flat hull has more initial stability than a hull with round sections. The flat hull may return to its original trim with a snap. What if a boat doesn't heel? Like in the case of a catamaran? There is less conversion of the inital gust to potential energy. It's sails are not trimmed for the new apparent wind. Does it just side slip? Bury the leeward hull? If there is some speed potential available and we realize it, then we can capture it. Bear away, roll up and get on surf . . . |
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#8
12-09-2009, 09:54 AM
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| Yes, a heeled boat converts into forward velocity when the boat returns to its original trim... but it doesn't accelerate. Just doesn't slow down so quickly. A "nonheeler", on the other hand, has an instant response to wind velocity, nothing else.. And it's same thing with rigg flexing.. |
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#9
12-09-2009, 11:09 AM
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| Quote:
Quote:
gust = stronger wind = more force from the sails = acceleration - provided the trim is correct... initially it will not speed up because the sails have the wrong trim = more heel = increased drag... but i have been on ULDBs which - proper sailtrim imperative - shot to windward in a gust and almost immediately accelerated with just a very short time where the heel got increased a fraction... you actaully had to control the boats tendency to go further to windward by steering against it... once trimmed correctly to the apparent wind - runing neutral on the helm again... Quote:
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#10
12-09-2009, 12:32 PM
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#11
12-09-2009, 12:51 PM
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| vimes.. You got to ask SuperPiper what he means with it, but reckon "original balance" in this case the original heeling angle in steady wind.. Quote:
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#13
12-10-2009, 03:07 AM
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any acceleration which might occur in a gust comes from the gust = stronger wind... i do not think that restoring to original heel will give any forward velocity... heel comes from 2 forces - the transversal force from the sails and its counterpart the righting moment - both are always in equlibrium... no matter the actual angle... there is no 'kinetic energy' stored... do a test - tip a keelboat over at the mooring and release it again... it will not move a single cm foreard...  what might happen underway is, that due to the weatherhelm the keelfin experiences a waterflow which comes at a certain angle of attack... thus creating a force (lift) mainly directed transversal but with a proper findesign it also has a small forward directed part... when restoring to the original heel the angle of attack gets increased resulting in increased lift, increased forward force from the keel... but this force is not that big... it is just a very minor part of the whole lift... drag gets decreased when heel-angle gets less which might be 'felt' like acceleration but i do not think that from this rolling movement any significant forward velocity is produced... |
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#14
12-10-2009, 04:07 AM
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| I know a J24 crew that would sail into the harbour, drop their sails and then rock the boat side-to-side to skate it into the slip 100 meters away. A keel is intended to create lift to windward to counter the push to leeward of the sails. We always think of the keel at some kind of steady state. But if we acknowledge that it will also be rolling then its design has some other considerations. The angle of attack at the root is less than the angle of attack at the tip during a change in heel. So maybe an 8% T/C near the hull but a 12% T/C at the bottom? That's handy because it can accommodate more ballast down low. If we can harvest energy from the boat's change in heel, then perhaps the keel should be design to NOT resist these angular accelerations: Long chord at the root but short chord at the tip. So instead of calling "ease the traveler, ease the traveler" we might start shouting "store that energy, store that energy". Kinda makes you think, eh? |
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#15
12-10-2009, 04:21 AM
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| if you sail a dinghy into the harbour and drop the sails to early leaving you stone-dead a distance away from the mooring - you can propel the boat to the mooring (if there is no wind pushing you back ) by flapping the rudder from side to side...but you would not consider that as a proper method for increasing speed under way - would you? what you described was exactly what i wrote earlier... and keels are designed in a way to maximise their lift under sails by minimizing the drag... but there are a lot of other considerations to take into account as well... wetted surface, draft, aspect ratio, load and weight distribution, impact resistance, flow of vortices at the tips etc bb... this is the keel of the famous Australia II winning the americas cup beeing the first non-US boat for centuries... http://upload.wikimedia.org/wikipedi...inged_keel.JPG looks very much like what you proposed... hmmm? |
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