Distance from prop to skeg

[Tar Devil]

Say someone wanted to build this Ed Monk Skiff...

http://i240.photobucket.com/albums/ff298/TarDevil/MonkSkiff.jpg

In my plywood boat building Bible (Glen-L's book), it says to end the skeg forward enough to prevent cavitation.

How far is forward enough? Does it depend on HP or hull speed? Both?

Thanks!

Phil

[donald branscom]

The best book i know is C.A. Marche's book, "The Aerohydrodynamics of sailing."

I would look at some production boat manufactures have built and get an idea that way. They may have put some research into that. I am sure there are many varibles and have a feeling that the more space the better. I do know that streamlining appendages in front of a prop is helpful.

But think about this, when boat builders first built these boats i do not think they did ANY hydrodynamic tank testing at all. They just made sure there was enough space for a zink on the shaft and there was clearence to have room to work on it. Just practical concerns.
The people that design props probably know a lot about all this too.

Even on airplanes i don't remember ever seeing any discussions about how far in front of a wing the propeller should be. But I am sure some university some where has papers written on the subject.

[Tar Devil]

You're probably right, Don. As evidenced by the run aft, this hull wasn't designed to go fast. I have a couple of 7 - 8 HP motors that should work perfectly for a design like this, but was curious about the skeg.

Thanks for the reply!

Phil

[Paul Pless]

The skeg as shown is pine. I'm assuming its not glassed. You could always build it full length as shown in the plans and then easily trim it back as needed after testing.

[Paul Morris]

On a 16' Thompson lapstrake I helped restore this past year, the outer keel or skeg ended about 15" ahead of the transom edge. Obviously this is a planing hull with a common HP around 60 to 85. The outer keel size is about 1-1/2" tall by 1-1/4" at the base, sides tapered towards the top to a continuous 5/8" top (boat upside down).
On a low HP, strip planked, narrow, round chine, 16' 1948 Peterborough (Canadian) fish camp boat I am currently restoring, the distance from the outer keel is 9" to the transom edge.
It appears that there is a correlation to a boats speed and the potential for trapped air along side a skeg - especially planing hull designs.

[Ian McColgin]

Garr's propeller book gives the clearances you need, depending on prop size, running speeds, etc.

[Tar Devil]

Gheez, I forgot about Garr... I have his "Nature of Boats" book... might find something there.

Paul, this is slow boat, so your Peterborough will most likely be pretty close.

Just thinking about this boat... I kinda like it. Good rowing boat, perfect for my motors, nice fishing platform.

Phil

[donald branscom]

Garr's propeller book gives the clearances you need, depending on prop size, running speeds, etc.

Thanks for the reference Ian. That is what is great about this forum.
There is going to be someone who knows.

[paladin]

Now ya know where to find the info, if you can't find a copy, send me the info on the prop, diameter and pitch, and I'll draw up the recommendations for the prop aperture etc for you and waive my $125 an hour fee, 4 hour minimum.