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
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?
What I mean by correct forestay tension is twofold:
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.