On Oct 16, 5:07 am, Frogwatch wrote:
... tsmwebb points out that the flow is rarely directly fore and aft and I
admit this had never occurred to me, however, is he correct? If he is
correct, I would not expect a 3 bladed prop to cause much loss of
sailing speed. ...

Well, I think it's true. You can approximate the lateral
deflection of the keel wash by multiplying the distance from the
trailing edge of the keel to the prop by the tangent of you leeway
angle. My guess is that on your boat a degree of two of leeway angle
will send the wash wide of your two blader if it is locked up and
down. However, there is more to it than that. Your boat also has a
somewhat modern keel. The "standard" section on fin keels when your
S2 was designed was the NACA 63xxA. At small leeway angles this
section is very low drag. As a result, it has a very narrow and
relatively short wake. My recollection of the S2 8.5 is that the keel
is a couple of cord lengths ahead of the prop. At the speeds you are
likely to be going and if your keel is in good condition there really
shouldn't be a significant amount of keel wash left when it reaches
the prop even in the zero leeway angle case. So, I don't think your
prop will be in the keel wash in most cases regardless of leeway.
That's the bad news, because the speed of the water relative to the
prop in the keel wash is less than the free stream speed and drag is
primarily a function of that speed. But, that' still not the whole
story. The drag of the prop is proportional to its effective area
times the square of the speed of the water flowing over it.
Generally, two bladed props have less area than three blade props so
they have less drag. There are other complicating factors, but I
think it's a good bet that the two blade prop will be less draggy
under sail than the three blade one but it will not be as good as a
folding prop or a prop that is hidden behind the keel.

-- Tom.