That's not a quantification it's a trivial definition. Of course sum (integral) of the perpendicular forces over the sail tells you the lift. That doesn't help quantify the strength of this force relative to any other. It just defines how we'd find it.
Right, so, having found it, and having found the others, now we know their relative strengths? I don't get your complaint.
The force is not infinite but the mechanics of how the force is generated allow speed to theoretically hit infinity.
I was worried for a minute that I might be misunderstanding you; I'm glad this isn't the case.
A friction-less boat could reach infinite speed. Are you saying it can't?
Nope! I'm saying that the drive-generating mechanism will never generate infinite force. A frictionless boat under the impetus of a constant force would indeed accelerate forever.
To go back to "the mechanics of how the force is generated allow speed to theoretically hit infinity". To me this means you are saying, "some configuration of the system will produce an infinite force", which is obviously wrong. A frictionless boat will never experience infinite force due to interaction with the wind.
Lift remains for most an imprecise term that doesn't contain the boundaries for what a sailboat can do.
For example why can't a sailboat sail into the wind?
Because there would be no way to produce a lift which has a forward component.
Why can't the sail generate lift when the wind is flowing straight over it when a boat is in irons someone might ask.
Sails can generate lift when the boat is in irons. Don't you know how to get out of irons? The lift won't have a forward component, so the boat will not go forward, but it will turn.
Even you had trouble allowing that a sail will luff in this situation./quote]
I have no trouble allowing that there are situations when the sail will luff. I agree that the main will luff if the wind is parallel to the boom.
I don't deny this but I don't think it's an effective way of explaining it. Once you allow for the fact that you're sailing off aparent wind, "A sailboat makes it own wind" is a phrase I've seen, you open yourself up to the kinds of mistakes that brehm62 points out. It becomes all to easy to get confused.