Propcalc by Surfbaud is a free xls spreadhseet that does a pretty fair
job of selecting prop sizes. You enter HP, RPM, gearing, displacement,
LWL and a couple of other variables. By playing around with the
variables you can get a pretty good idea of what various effects will
be. I found it at http://www.yosc.bctalk.net/navsoft.htm

Castle Marine has another free calculator on their site
http://www.castlemarine.co.uk/pitch.htm
It is a windoze app and a bit more polished than the Surfbaud
spreadsheet and more oriented to powerboats than sail but it gives the
same results.


Sylvain Sirois wrote:
Hi, first, excuse my english.
I am in the process of buying an old 44' rescue boat.
http://home.online.no/~lawford/xrl/xrl.htm, and restoring it.

It was originaly equiped with detroit diesel, serie 53. ± 200 hp

Cruising speed was 13 kn and top, 16kn.

Now, the engines are twin cummins 220hp at 220 RPM
The gear box are different, one is a twin disk 2.0 to 1.0
The other is a Borg warner 2.5 to 1.0

The prop are 24 x 20 3 blades.

I would like to restore the original performances of the boat, without
changing the engines.

What combination of gear box and prop i need ?

Thank you

Sylvain
Baie-Comeau, Québec


--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com