|Topic Review (Newest First)|
|01-12-2012 09:11 PM|
The other postings very well describe the lift that is developed from a keel. the P30 keel is similar to the famous NACA010 keel shape and does develop lift ... if the correct angle of attack is held by very slight ~3° of rudder angle
Originally Posted by zedboy View Post
1. the jib is operating at near 'design' shape ... a loose/slack forestay will cause the draft to go aft, draft will increase (powered-up and slow), and the leech will 'hook up' to weather and no longer be parallel to the boat's centerline.
2. most important, the (curved or 'hollowed') luff shape (luff 'entry' shape) with correct forestay tension will be aerodynamically operating at optimum as close as possible to the boats centerline. If the luff sags off from of its 'expected' shape and position, the jib's angle of attack will not match the boat designers 10-12° tack to jib fairlead angle ... and the boat will no longer be optimized for sailing 'forward' but will increasingly want to sail at a lower angle ... and the keel increasingly 'slips' to leeward (and no longer provides optimum lift as the keel begins to 'stall' along the 'windward' side) as a result. Simple speak: (totally Ignoring the poor shape induced by a too slack forestay) ... Operating a jib with a loose forestay, when beating, is like repositioning the tack of the sail to leeward from the boats centerline.
The 'article' I referred to (in an earlier post) about matching forestay sag to luff shape should better explain the effect of this 'matching' of sail shape and forestay tension: MatchingLuffHollow.gif picture by svAquila - Photobucket ... this is not a 'sail trim' issue, this is a sail SHAPE issue - controlled by forestay tension. If you cant read the URL because of the magnification, PM me and I'll send a .pdf of this article.
|01-12-2012 05:24 PM|
|nickmerc||Correct, the leeway is what generates the lift with the fin keel on a P30. It is a symmetrical foil so the angle of attack (AOA) is zero if you are moving directly forward with no leeway.|
|01-11-2012 04:36 PM|
As I understand it, the initial 'sideslipping' creates the angle of attack... as the boat picks up speed this translates into a partially compsensating lift force to windward, reducing ultimate leeway.
Excessive pinching/trying to point too high can defeat this effect (the keel can 'stall').. which is one reason why rearranging the deck plan of some boats to improve sheeting angles won't necessarily improve VMG to windward.
From discussions with my son (racing a Melges 32) failure to attach that flow to the foil really hurts because these keels have minimal lateral plane to resist leeway otherwise...
Any true experts should feel free to correct this if necessary....
|01-11-2012 03:43 PM|
Originally Posted by zz4gta:815566
Or do we look at the beam-on component of apparent wind as if the boat is moving laterally and say the keel is "generating lift" by resisting skid, even though of course relative to the water the boat's motion is only forward? But that seems counterintuitive as a description of what's going on...
|01-11-2012 02:31 PM|
The keel works just like any other foil. It generates lift from differences in "pressure". Even though the keel is the same profile on bother sides the angle of attack is enough cause a change in pressure. There is lots of info online about this.
Angle of attack is how airplanes can fly upside down.
|01-11-2012 12:37 PM|
Originally Posted by RichH View Post
Can you elaborate on what you mean by the keel "lifting to weather" and affecting whether the boat tracks or skids off to lee? The wake not trailing straight back means you are already skidding off, right? Is all you mean that if the jib shape isn't good (because the forestay is sagging) the jib will cause excessive heel which makes the keel less effective at keeping you on track?
One of the big goals for this coming season is improving windward performance
Thanks for the Arvel Gentry link, amazing stuff. Started on it a little last night, will go further during lunch...
|01-11-2012 10:30 AM|
Thanks for the comments. I was kind of struggling with your conclusion when I started this thread. It just didn't seem to want to bend in spite of many other things I had read. Seems to be a pretty unanimous conclusion at least for the P30 masts.
Thanks to all for the comments.
|01-11-2012 08:17 AM|
|sailingfool||The masts on these old '70s boats are built like telephone poles, and are not designed to bend, so dont put too much effort into bending it. Sounds like you've got as much bend as it wants to permit. Definitely do not tension the topping lift to try to add bend, you may break something, especially if you have a mid-boom mainsheet. The mainsheet needs to be tensioning the mainsail, not restricted by the topping lift.|
|01-10-2012 09:49 PM|
Thanks for the kudos, Faster. You do bring up a very important point.
I race ILYA scow boats with extreme on the fly mast rake and radical mast bending ... and you are absolutely right with extreme bowing of the mast the cap shrouds can become soooo loose as a result, that the only way such boats can keep the mast 'straight up' is to use bodacious mast wedges in the the mast's 'deck hole'.
Something else along the same line of Faster's post/info is to consider is that to accomplish more than an inch or two of mast prebend on a single spreader non-tapered mast one is definitely going to have to massively 'crank up' some of the rig tension. Anytime that the tension in any shroud or stay goes much beyond 30% of ultimate strength .... you become *very* vulnerable to accelerate premature 'fatigue failure' .... and I have lost a few masts because of this ..... very unpleasant, and I now take it easy and only 'normal' tension a rig; its less expensive than replacing a mast, etc. :-o
|01-10-2012 07:46 PM|
Brett, Rich H is one of our better advisers on things sailtrim and esp Pearson boats... You're unlikely to get better information - even if it's what you want to hear.
Keep in mind, too, that even if you're able to 'induce' such bend in a masthead rig through backstay tension, for example, that means the rig is also becoming shorter.. ie the masthead is closer to the deck.. that slackens your shrouds. Given that the Pearson hulls are known for flexibility, all of this seems a bad idea.
Get the right sails for your boat, esp if you're serious about fleet/club racing.
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