Steve… I have your Offshore Cruising Encyclopedia and I have almost finished building this 35′ steel sail boat. I’m having a little trouble sizing the prop…Boat is 9.5 tons…engine is Farymann 32 HP…Prop is 16×13, 3 blade… Max. RPM under no load conditions is 2400. Max. RPM in forward is 2100 at max hull speed of 7.5 kn…reduction of transmission is 2:1. Should the max. R’sPM under load be closer to 2400?… Should top speed be reached before 2100?… I hope you can help with this … Thanks–Basil
Hi Basil: You are asking a very tough question, with lots of variables.
First–there is a large section as you know in Offshore Cruising Encyc. regarding props and sizing. This will give you a pretty good start on the problem–it should get you into the ballpark (which is where you sound like you are at present).
The issue to look at is the max continuous speed (rpm) of the engine. With a fixed three blade prop, with twisted (and therefore airfoiled) blades, if you want to be able to use all the power in the engine, you need to be able to reach max continuous in smooth water.
The next issue is the speed length ratio at which the boat is operating. I am going to guess here and figure you have a 27 foot waterline. If you take the square root of this–which is 5.2, and then divide this into the 7.5 knots you are achieving you get a speed length ratio of 1.44– which is very high (unless you have a very light, modern design, my guess is that your speedometer needs to be calibrated).
Most cruising under power is done at SLRs between 1.0 and 1.1 because mileage is better and noise and vibration are down. The normal top speed for moderate displacement boats is an SR of 1.3.
If you are now limited to 2100 rpm by prop size, it means the blade is a bit too big (on pitch or diameter–or both). This oversized prop will work efficiently at slower speeds with the bigger blade turning more slowly, as long as you remember not to overload (lug) the engine. Ideally, you should not run the engine at more than 75 to 80% of its usable power. So, if you max out at 2100 rpm now, you would want to operate at no more than 1680 rpm for long periods.
This will be OK in light airs, and when motorsailing–very efficient in fact. But when you are fighting headwinds and head seas, you may need more power. This is where the smaller prop comes into play. By allowing the engine to turn up to full speed, you have more continuous power available. But, at slower speeds and in more benign weather the smaller prop will be less efficient.
Just one of the many thousands of tradeoffs in sailboats! Regards–Steve
Posted by Steve Dashew (November 30, 1999)
