Fairing out the trailing edge of the keel.

tsmwebb · Jul 25, 2009

jpekas said:
Working on a 1981 Lippincott 30, and and just completing a fairing out the hull
after a restoration due to pox, many mini blisters. Anyway, my question is on the
trailing edge of the keel, should it be squared off or faired smooth to an edge
as I did on the lead egde of the keel. Does anybody have an input on this?
Thanks, JP...

You want the flow to separate from the trailing edge as cleanly as possible. Ideally you want the edge to be perfectly sharp and perfectly true. That's not attainable especially on a painted surface. Squared off as far aft as practical is usually easiest and good enough. Again you want the corners to be sharp. If you want to knock off one corner of the square and fair that side of the keel to the remaining corner so that it ends at a asymmetric edge that will be better and easier to paint. The finer the angle the better. Avoid rounded.

--Tom.

s sauer · Jul 25, 2009

Trauling edge

jpekas said:
Working on a 1981 Lippincott 30, and and just completing a fairing out the hull
after a restoration due to pox, many mini blisters. Anyway, my question is on the
trailing edge of the keel, should it be squared off or faired smooth to an edge
as I did on the lead egde of the keel. Does anybody have an input on this?
Thanks, JP...

Our trailing edge is squared off about 1/4" wide similar onthe rudder but slightly narrower. Wet sanding the Baltoplate bottom paint, fairing in the thru hulls and using a folding prop probably reduces a lot more drag also..

Ted · Jul 25, 2009

Trailing edge

The trailing edge of the keel and rudder should be squared off. Depending on your foil shape, it will probably be between 1/4" and 1/8" wide. Definitely do not shape it come to a point or round it. Square end with sharp corners is what you want.

tsmwebb · Jul 25, 2009

Ted said:
The trailing edge of the keel and rudder should be squared off. Depending on your foil shape, it will probably be between 1/4" and 1/8" wide. Definitely do not shape it come to a point or round it. Square end with sharp corners is what you want.

The amount a foil is degraded by having a thick training edge depends on lots of things -- foil geometry, velocity, cord length, angle of attack, boundary layer thickness and so on. Squared off is a decent compromise but maybe not the best we can do even in a practical world. Tests have been done on this and the results have been widely published and in the public for decades. Amazingly folks still muck it up. The original j/24 keel is a classic example. I've seen variations on the following table in numerous books published since the early '70's:

(this one from Larsson & Eliasson)

It takes square as the base line for efficiently shedding vortexes. Numbers less than 1 are better than square. Numbers greater than one are worse. Very sharp is best as in #'s 5 and 8.

--Tom.

Don12364 · Jul 26, 2009

Sailing Theory & Practice - extract

Herewith below an extract from page 284 of Prof C.A. Marchaj's book "Sailing Theory and Practice". Professor Marchaj was the renowned hydrodynamicist from the Wolfson Unit of Southampton University:-

"The trailing edge is of relatively little importance, as it lies in the area of turbulent wake. There is no scientific foundation for the popular belief that the centerboard or fin must taper as sharply as a razor. Experiments have proved that cutting the trailing edge off blunt for up to 5 per cent of the profile length does not influence resistance, and is undoubtedly more practical."

tsmwebb · Jul 26, 2009

Donalex said:
...Professor Marchaj was the renowned hydrodynamicist from the Wolfson Unit of Southampton University:-
...

And a very fine sailor, but if we believed Tony we'd all be sailing delta rigs...

--Tom.

RichH · Jul 26, 2009

The leading edge shape of a keel or foil is vastly more important than the exit shape. Usual is cut a square 1/8" flat at the aft edge ... less vulnerable to damage, damage that can propagate destructive 'upwash'. Marchaj is/was probably right in his analysis/advice.

The sharp exit shape is subject to too much vulnerability. My scows' boards and rudder aft shaped used to be 'sharp' ... 1. class rules made them illegal. 2. didnt matter all that much. (BTW/FWIW - these were flat plate foils ... also with 'sharp' leading edge / entry angles)