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Chris 249 View Drop Down
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    Posted: 07 Dec 05 at 11:47pm
Jim, is it fair to paraphrase your original conclusions as "the combination of bow up trim and mast rake enables both spinnaker and mainsail to create a lifting force" BUT that the spinnaker is most definitely pitching the bow down".

Sounds coo. But wouldn't the forces be developed along a vector that is dependent on the measurments such as luff rake/bowsprit length etc? If you had enough rake on the luff (ie enormous pole) wouldn't that create vertical lift enouogh to compensate for the pitching moment?

The Rs and 12s do sail with the bow up more than any other class I've ever seen, with a strange-looking lifting motion that makes them seem to float up over the troughs, so doesn't that indicate that the long pole does lift the boat?

Secondly, the 12 guys say that a couple of seasons ago they went to more "dangerous" kites, ie ones that lifted less - which indicates that kite shape influences bow lift. I think the change was to flatter head sections which possibly had the effect of reducing vertical lift in the head?????????????????????????

Obviously the lift is NOT enough to require guys to trap forward of the shrouds, but it may still be there. I can't think of another reason why 12s and Rs have the distinctive bow-up angle downwind (OK they are short and light compared to a 14, but is that all there is?)

In boards (where the raked rig seems similar to a kite, with the uni taking the role of kite tack, and the sailor's feet acting as mast/stays and sheet) the vertical/bow lift seems to be clearly related to the fore-and-aft rake. Obviously there IS that lifting-but-pitching effect you refer to (if I read it right) but when the rig is raked back, the lifting effect seems to become more pronounced and dominates.

I'm not questioning (I hope) the work you and Mikko did which is way above my head, but wondering whether the effect and results would change if you put a 12-length pole and massive kite into the equations.



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JimC View Drop Down
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 08 Dec 05 at 8:54am
Originally posted by Chris 249

But wouldn't the forces be developed along a vector that is
dependent on the measurments such as luff rake/bowsprit length etc?

Yes, and Mikko hasn't modelled a 12 so I have no informed opinion to offer. My gut feeling though is that there isn't enough rake even on the 12s to produce that effect if I look at a few photos and try and imagine the vectors. I also suspect tend to think of the myths surrounding windward inclined rigs on sailboards...
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Post Options Post Options   Quote Stefan Lloyd Quote  Post ReplyReply Direct Link To This Post Posted: 08 Dec 05 at 9:23am

Originally posted by Chris 249

In boards (where the raked rig seems similar to a kite, with the uni taking the role of kite tack, and the sailor's feet acting as mast/stays and sheet) the vertical/bow lift seems to be clearly related to the fore-and-aft rake.

I don't see how a board sail is anything like a kite. A kite can develop lift because of the part billowing out near the head. A board sail is more like a main and is almost invariably sheeted well in. Boards go "nose up" because they plane easily and furiously, not because the sail is lifting them. Jim's document shows that the lift a Cherub rig develops comes almost entirely from the kite, not the main.

I used to race boards so I have some direct experience here. You do not feel the rig lifting upwards. It pulls sideways and forwards.



Edited by Stefan Lloyd
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ssailor View Drop Down
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Post Options Post Options   Quote ssailor Quote  Post ReplyReply Direct Link To This Post Posted: 08 Dec 05 at 9:35am
Yeh im also a freestyle and wave windsurfer, and once planing with correct technique you rake the rig back which forces the mast into the board leveling the nose. the reason you will see boards with their noses up is either the person is learning or in most situations they arent planing flat out and thus the back of the board is sinking - like standing on the back of a laser and expecting the nose to stay down!
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Post Options Post Options   Quote Adds Quote  Post ReplyReply Direct Link To This Post Posted: 08 Dec 05 at 10:24am
The kite lifts the bow. But when there are waves the kite makes you go much faster and more likely to plow into the wave infront, so in windy wavey conditions it much harder to fly the kite, without stickin the bow in.
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Chris 249 View Drop Down
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Post Options Post Options   Quote Chris 249 Quote  Post ReplyReply Direct Link To This Post Posted: 08 Dec 05 at 10:43am
Sorry guys, I didn't make it clear about the boards. I made it sound like the rig is normally lifting the board a significant amount, which I agree it does not do when you're racing along.

What I should have made obvious is that I was talking about the way you move the rig when trying to get a long jump (not a high one) in waves, when you often "hoik" the rig down closer to horizontal, and it's accepted (by even the best) that this gives you a bit of extra flight.

You tend to do the same sort of thing when chop hopping, whereas when you are say going for a forward loop you keep the rig pretty much vertical then sheet in - or I think you do, haven't done those (or any competitions in waves) since I woke up in hospital with the surgeon tut-tutting me about getting it all mixed up!

The FW guys always seem to say (IIRC) that if you sheet out when overpowered upwind, you will automatically lift off the water and crash. Again, there seems to be a direct link between rig attitude and lift.

While Stefan's comments about sail shape sound quite convincing, wavesails are flat and  they do seem to have a fair bit of vertical lift available when used for jumping. For example, in surf slalom days everyone sheeted out briefly to avoid jumps because jumps actually slowed you down. No-one said you avoided the jump by slowing down - they say you avoided the jump by sheeting out. I used to race slalom against guys like Robby, Bjorn etc and got to ask them how they did it.

I'm no Bjorn so I may be wrong. Just food for thought. This whole rig attitude stuff has some very smart people on each side!


Edited by Chris 249
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Post Options Post Options   Quote Stefan Lloyd Quote  Post ReplyReply Direct Link To This Post Posted: 08 Dec 05 at 11:04am

I agree a board sail can generate vertical lift if pulled over to windward. In racing, you don't normally want to do that; you want to keep the rig vertical. That, anyway, is what the RYA coach taught us!

Sheeting out will evidently slow you down, unless you were oversheeted in the first place. I suspect sheeting out to avoid jumps is as simple as that.

"The FW guys always seem to say (IIRC)" - sorry but I don't know what FW or IIRC mean.

 

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Post Options Post Options   Quote Guest Quote  Post ReplyReply Direct Link To This Post Posted: 08 Dec 05 at 11:12am

FW?

http://www.techdictionary.com/chatsms.html

 



Edited by Guest#260
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Post Options Post Options   Quote ssailor Quote  Post ReplyReply Direct Link To This Post Posted: 08 Dec 05 at 1:08pm
chris 249, yep wave sails are flatter, and what you have said is true, however wave sails tend to be flatter as they are used in higher wind conditions. cammed sails have little cams (like teeth) that go round the mast these make sure the battens stay behind the mast creating the curve in the sail in any wind therefore they are quicker than rotational sails (wave/freestyle etc). however due to the cams they have larger luff tubes and are therefore a lot harder to waterstart.hence in waves you need the ability to get up and going quickly with a light sail for throwing around etc.

If you jump and bring the rig to windward you begin to create the lift hence the bigger air time, plus it is quit often done so that u are ready for the landing when the fin bites back in.!

with forward loops (i can do em - great fun!!) youre actually going forward and around so the sails CofE is goin forward and to leeward - it is only the top guys in big waves that can do end over end forwards!!

hope something made sense


Edited by ssailor
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Post Options Post Options   Quote Guest Quote  Post ReplyReply Direct Link To This Post Posted: 08 Dec 05 at 1:38pm
Originally posted by turnturtle

Originally posted by Guest#260

FW?

http://www.techdictionary.com/chatsms.html

 



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