Going down the mine ...
Printed From: Yachts and Yachting Online
Category: Dinghy classes
Forum Name: Dinghy development
Forum Discription: The latest moves in the dinghy market
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=1272
Printed Date: 15 Aug 25 at 4:41am
Software Version: Web Wiz Forums 9.665y - http://www.webwizforums.com
Topic: Going down the mine ...
Posted By: Guest
Subject: Going down the mine ...
Date Posted: 07 Dec 05 at 7:25pm
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Seems to be some differences of opinion here ... I have always found that kite up going downwind in a breeze lifts the bow and reduced the chance of a pitchpole ... in all classes I have sailed. ... and of course it is much more fun ... Any one disagree? Rick |
Replies:
Posted By: Wave Rider
Date Posted: 07 Dec 05 at 7:30pm
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I have heard that having the kite up is trying to pull the top of the mast into the water therefore making it easier to pitch, but n my cherub experience, this isn't the case and we seem to have the bowflying high with the kite up. I wonder if the bow is so high becuase we get told how painful pitchpoles are so we move back a bit too far! ------------- -[Franko]- Chew Valley Lake Sailing Club RS600 933 |
Posted By: Guest
Date Posted: 07 Dec 05 at 8:01pm
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I am sure this must be covered in Bethwaites book ... anyone know? Rick
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Posted By: Stefan Lloyd
Date Posted: 07 Dec 05 at 8:30pm
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Not true in keelboats. They rarely pitchpole but bow digging in following by a broach is very common. Kite up definitely makes that far more likely. More fun with the kite up? Yes of course.
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Posted By: ssailor
Date Posted: 07 Dec 05 at 8:49pm
It depends ont eh situation and the boat - if youre sailing in constant
breeze the fairly even pressure in the kite lifts the nose, however in
really gutsy conditions bearing away fully poweder with the kite up
causes the nose to dip as the rig wants to go faster than the boat thus
pushing the nosed down! therefore if uve always sailed in vaguely ok
places u wont notice - until you come to our res  ------------- Any one in need of quality carbon fibre work (tillers etc) at decent prices! Int 14 Gbr 1244 'Nucking Futs' The New Port rule!!. |
Posted By: simsy
Date Posted: 07 Dec 05 at 8:50pm
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Well having the kite up downwind certainly makes things easier. From experience, it does feel like the kite lifts the bow when flying downwind, but also from experience, flogging the kite also seems to lift the bow (but maybe this is due to the boat slowing)....I really dont know, seems im just babbling nonsense. Maybe theres someone with some proper facts. |
Posted By: Jamie
Date Posted: 07 Dec 05 at 10:37pm
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Surely the bow only lifts because the boat is on the plane. It is easiar to get on the plane with the kite up obviously, but surely once you are on the plane it makes little difference. But given the choice fly the kite every time. ------------- www.sailfd.org/GBR - GBR Flying Dutchman |
Posted By: m_liddell
Date Posted: 07 Dec 05 at 10:49pm
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Apparently the kite does not lift the bow but lifts the whole boat up. CFD research conclusions: http://www.sailingsource.com/cherub/aero.htm - http://www.sailingsource.com/cherub/aero.htm |
Posted By: Guest
Date Posted: 07 Dec 05 at 10:51pm
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there we go then ... get the kite up at all times rick ------------- |
Posted By: JimC
Date Posted: 07 Dec 05 at 11:13pm
Since I wrote those conclusions I've also come to believe that the kite acts as a pitch damper, because as the bow pitches down I think the lift of the kite must increase and thus pitch the boat forward less, and the reverse as the bow pitches up. That's my additional theory to explain the steadiness. After all, if kites were truly pitching the bow up then twelve footer and R crews, with the exceptionally raked leading edge, would need to be trapezing in front of the shrouds to keep the bow down! The other thing is that if you hunt out the videos of Mango Jam on the Cherub web site there's one where they let the kite flog for a couple of seconds and you can see the bow lift as the pressure comes off. |
Posted By: Chris 249
Date Posted: 07 Dec 05 at 11:47pm
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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. |
Posted By: JimC
Date Posted: 08 Dec 05 at 8:54am
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... |
Posted By: Stefan Lloyd
Date Posted: 08 Dec 05 at 9:23am
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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. |
Posted By: ssailor
Date 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!
------------- Any one in need of quality carbon fibre work (tillers etc) at decent prices! Int 14 Gbr 1244 'Nucking Futs' The New Port rule!!. |
Posted By: Adds
Date Posted: 08 Dec 05 at 10:24am
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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. ------------- Cheers Dudes |
Posted By: Chris 249
Date Posted: 08 Dec 05 at 10:43am
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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! |
Posted By: Stefan Lloyd
Date Posted: 08 Dec 05 at 11:04am
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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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Posted By: Guest
Date Posted: 08 Dec 05 at 11:12am
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FW? http://www.techdictionary.com/chatsms.html - http://www.techdictionary.com/chatsms.html
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Posted By: ssailor
Date Posted: 08 Dec 05 at 1:08pm
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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  ------------- Any one in need of quality carbon fibre work (tillers etc) at decent prices! Int 14 Gbr 1244 'Nucking Futs' The New Port rule!!. |
Posted By: Guest
Date Posted: 08 Dec 05 at 1:38pm
Quite ... I think they have got FUBAR wrong ... ------------- |
Posted By: BBSCFaithfull
Date Posted: 08 Dec 05 at 1:59pm
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I would also think that the size of the pole would dictate the amount of burying of the nose. For example a boat like the 4000 would not pitch pole very easily as it's pole is so long it pulls the nose really far out when it's planing. ------------- Greatfully Sponsored By www.allgoodfun.com Int 14 GBR 1503!! |
Posted By: Chris 249
Date Posted: 08 Dec 05 at 10:47pm
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Sorry Stefan, IIRC = "If I Recall Correctly", "FW"= "Formula
Windsurfing" (or Windsurfer?) class, AIUI = "As I Understand It",
BIKFAAI= "But I know very little about it", IASIWUTMA= "I'm A Silly
Idiot Who Uses Too Many Acronyms" and OGNTRLCA = "Oh God, Not That
Raving Loony C249 Again". Stefan, because of my job at the time I got to sit down with guys like Robby Naish and a tape recorder and ask them about technique. The guys I interviewed on surf slalom were pretty specific (IIRC) that you did NOT sheet out to slow down. The whole point was that you did not want to slow down, and the best way to keep going fast was to sheet out so your inevitable jumps (we sometimes had mast-high waves in surf slalom on the pro events) were as short as possible and you could get back onto the water and start driving again. It was understood, I think, that sheeting out reduced lifting force rather than slowing you down. The sheeting out was very brief and was done IN the air; it was not long enough to significantly slow the board and if you had wanted to slow to avoid a jump you'd have slowed BEFORE hitting the wave and getting launched. In the air you are slowing anyway (no sideforce from the fin= no forward power from the rig). As SSailor said, "If you jump and bring the rig to windward you begin to create the lift hence the bigger air time" and that seems (to me) to underline that inclining the rig creates lift and hence an inclined kite luff could create lift. I'm not what I'd call a hot slalom sailor but when I was doing it I beat most of the guys at the worlds (a limited-entry event with about 30+ countries competing) in a field including Bjorn Dunkerbeck so I think I was on the right wavelength. I totally agree about keeping the rig upright normally for speed most of the time (surf slalom is a special case) although some very clever guys (Jim Drake among them) say that some classes like big-sailed Formula Windsurfing (FW) boards are happier with 15 degrees of windward rake. Our IMCO national champ (Olympic medallist, world champ) advocates keeping the rig as vertical as possible and Jim Drake wonders whether difference in rig rake may be related to the fact that the IMCO has a smaller rig and is a heavier board. But it's interesting that in another longboard class (with an even smaller rig and heavier board) the guy who beat me in most events at the nationals (a two-time world champ and Olympic team member himself) carries enormous amounts of windward rake in strong winds and he is unstoppable upwind in those conditions. Which may just show how complicated the whole aspect of rake, lift, aerodynamics etc is! |
Posted By: Chris 249
Date Posted: 08 Dec 05 at 10:50pm
Hmmmm, interesting, thanks. I just realised how old my understanding of the forces on the head of a kite are. PS agree with you about the myths of windsurfer rigs when the boards are not jumping. |
Posted By: Jack Sparrow
Date Posted: 09 Dec 05 at 12:23am
Thing is when you go up you must come down and that is when this threads topic happens.
I personally believe that the visual effect of lift from a spinnaker is an amalgam of many of the factors that have been expressed here. And I tend to side with Jim on his study that the spinnaker doesn't create upward lift. But only to a point. I think the factors that come into play are: Length of pole. mast height kite luff length kite shape water line length wave length crest height / angle rocker line hull weight mast foot pressure main sail head shape kicker tension AND whether you escape your own wave ( get over the hump ) Without talking loads of technical crap. I think that in general only a small proportion of lift ( not enogh to give phisical lift ) is generated from the kite in normal conditions. But combine a short waterline length with a long pole and a significant attitude change due to wave length and the potential for lift alters significantly. So I think Jim is right if the models are for a steady state but wrong if you add in the large variants listed above. In other words L4000 and boats like them trim slightly bow up with the kite up due to rocker-line / pole length / crew position and that they don't get over the hump often ( they sit on the hump ), so they don't leap ( and then crash ) in the same way as a 12-ft skiff / UK Cherub e.t.c because the longer waterline length doesn't allow the angle to change to such a large degree as a short water line length. Combine a short water line length with a long pole, light weight, masthead kite, large kite area and chop and a rapid change in angle of attack and the kite generates significant lift or at least the appearance of it. Other craft mostly sit on there planing wave giving the impression of lift form the kite. Because they rarely escape the hump unlike true skiffs. When you have escaped the hump and you are getting lift then maybe the factors i have metioned above do create lift from the kite. I sail a 2005 rules Cherub with a short pole, low rocker and flat sail shape and I can tell you that she is prone to sticking the beak down. She is optimised for grunt, with a fat head main. And in a sea she is very fast but we have to pick our way where other notable boats with longer poles, deeper kites and less in the head have a noticeable bow up attitude that we don't. The interesting thing about the two boats I mention is that one is very flat in the rocker whilst the other has a resonable amount. So i would say, to sum up, that the kite can lift the bows but only TRUELY in the right circumstances. And those circunstances don't happen to the majority of boats. But then others would say if you need other factors then is it true spinnaker lift?  I would say it's like hydrofoiling :- you
need other things to happen before you can get up, but those factors get you to a point where you can hydrofoil. ------------- http://www.uk3-7class.org/index.html" rel="nofollow - Farr 3.7 Class Website https://www.facebook.com/groups/1092602470772759/" rel="nofollow - Farr 3.7 Building - Facebook Group |
Posted By: aardvark_issues
Date Posted: 09 Dec 05 at 9:15am
| Longer pole on the next one then Jack?! |
Posted By: Jack Sparrow
Date Posted: 09 Dec 05 at 9:22am
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Errr... Yep! ------------- http://www.uk3-7class.org/index.html" rel="nofollow - Farr 3.7 Class Website https://www.facebook.com/groups/1092602470772759/" rel="nofollow - Farr 3.7 Building - Facebook Group |
Posted By: Rupert
Date Posted: 09 Dec 05 at 9:23am
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Lifting the bow out looks great for the pics, but is it actually quicker? ------------- Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686 |
Posted By: Bumble
Date Posted: 09 Dec 05 at 9:37am
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What your talking about is called the 'dory effect' or 'heavy soap dish'. It trims up cos its bow wave is so large, and it doesn't reach for the sky like a cherub, cos it can't go any faster and mount the bow wave. |
Posted By: aardvark_issues
Date Posted: 09 Dec 05 at 10:12am
I dont think its a matter of quickly - its when the conditions mean that you need to get the bow up to avoid stacking it - if you are right at the transom and cant get the bow up you are in trouble. You can always stand further forward with a longer pole... |
Posted By: Guest
Date Posted: 09 Dec 05 at 1:07pm
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Bow up is much faster than bow down ... Perfect trim is of course the goal; it's all about risk management when it gets "full on" Rick ------------- |
Posted By: JimC
Date Posted: 09 Dec 05 at 1:54pm
| Bow too far up is slow however. The boat starts pushing water out of the way instead of skimming over. How much is right is very dependant on boat shape and also survival - 15 knots and no pitchpoles is probably faster than 30knots and two pithpoles per downwind leg |
Posted By: ding33
Date Posted: 09 Dec 05 at 2:19pm
| Have recently started sailing an 18 ft skiff and can report bow definately lifted by the kite! Scariest move is bear away (slow speed, no kite up, so big potential for nose dive). Apparently, biggest crashes are when bow is lifted so much that boat gets fully airbourne and then remembers it can't fly and comes crashing down bow first. Trick is to flog kite over biggest waves to avoid this. |
Posted By: Guest
Date Posted: 09 Dec 05 at 3:33pm
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But which is more fun ... ------------- |
Posted By: ssailor
Date Posted: 09 Dec 05 at 7:23pm
Yep I agree with all that you say, the sheeting out may relate to slight loss in power causing you to return to the water quicker. either way it worked, and bringing the rig slightly to windward increases air time!! And one thing is for certain its very complicated!! ------------- Any one in need of quality carbon fibre work (tillers etc) at decent prices! Int 14 Gbr 1244 'Nucking Futs' The New Port rule!!. |
Posted By: Contender 541
Date Posted: 10 Dec 05 at 7:48am
Its debatable which is the most fun, but I know which in the long run is the most expensive... ------------- When you find a big kettle of crazy it's probably best not to stir it - Pointy Haired Boss Crew on 505 8780 |
Posted By: Granite
Date Posted: 11 Dec 05 at 3:48pm
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It is tricky to say if the kite lifts the bow it certainly can as the
boat jumps waves, however it definatly pushes the bow down less than
the mainsail/jib combination. If you work out the vectors for the where the kite attaches, then for a verry short mast and a verry long pole the kite would lift the bow. I think that a more significant effect comes from where fore and aft the driving force comes from, with the main/jib it comes from about hounds height above the mast step. with the kite up I think it comes from a bit lower down but much further forward possibly in front of the bow. This makes a difference in the torque link between the driving force and the drag force, which acts to push the bow down, this force is less the larger the distance fore and aft between the drag and the drive. ------------- If it doesn't break it's too heavy; if it does it wasn't built right |
Posted By: Scooby_simon
Date Posted: 11 Dec 05 at 7:00pm
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Gotta agree with Granite that the shape / vectors control this. I've a Very tall thin kite and do not get a great deal of lift (but better than nowt). ------------- Wanna learn to Ski - PM me.. |
Posted By: Chris 249
Date Posted: 11 Dec 05 at 11:37pm
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Check out http://www.sailworks.com/04-05/pages/video_gallery.cfm A minute or so in, the guy starts jumping. Check the attitude of the rig when he is going for the biggest jump, compared to the attitude of the rig when he is sailing normally. |
Posted By: Bumble
Date Posted: 12 Dec 05 at 8:09am
MMMMmmmm..... what a hunk Dale Cook is.
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Posted By: mike ellis
Date Posted: 30 Dec 05 at 11:07am
| i am but a humble feva sailor so i do not have any experience of playing with realy big kites. however when we get a SW breeze blowing up the western solent it piles up the waves and we get tremendous surfing conditions which gets our small kite easily on the plane. the trouble occurs when we chicken out because its too windy and we quickly realize that with no kite the feva goes bow down in the big waves off Hill Head very quickly. when the kites up the bow does still go into the next wave but the boat does not burry itself past the shrouds in water. therfore i have come to the conclusion that kite = lift of bow = no nosedives |
Posted By: Wave Rider
Date Posted: 30 Dec 05 at 11:10am
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Indeedy but this could be becuase the pole is quite big for the size of kite the Feva has, it is also not very far up the mast so can not use the mast like a big lever to push the bow down.... ------------- -[Franko]- Chew Valley Lake Sailing Club RS600 933 |
Posted By: mike ellis
Date Posted: 30 Dec 05 at 1:44pm
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thanks for that. now i can force my crew to ut the kite up on windy days for "scientific reasons"
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