View Full Version : Prop size

10-06-2009, 03:11 PM
"I installed a 2YM15 recently, with the 3.2 to 1 reduction gear. In calm water, in gear, my full-throttle maximum rpm is 2900, which is 76% of the maximum (intermittent, not continuous) rpm for this motor (3830 on my tach).

"John" the diesel guru tells me that I should get a maximum rpm of about 3450 under load, under way, which is about 90% of the no-load max of 3830. He says I need to get my prop re-pitched so that my engine is turning about 550 rpm faster at full-throttle (3450 - 2900 = 550).

When the prop is adjusted, John says I should cruise at about 2800 rpm which is about 80% of 3450. He says until my prop is fixed, I should cruise at 2300 rpm which is about 80% of my current maximum loaded rpm of 2900.


1. Does John know what he is talking about? If not, what are reasonable numbers?

2. At full throttle now, I can run for hours without overheating, at somewhat more than my theoretical hull-speed. Is the engine really working too hard if it is not overheating?

3. As John mentioned, until I get my prop fixed, I should cruise at about 80% of my current max. rpm, which is 2300. If I can reach my hull-speed and not over-work the motor, why should I worry about changing my prop pitch?


Notes: My sailboat is 28' and 9500 lbs, with a full-length keel. Hull is clean. Prop is a 14x11 3-blade "Campbell Sailor". I sized the prop with the prop calculator at boatdiesel.com and with the manufacturer's input."

10-06-2009, 05:18 PM
"Hello Jack, I may be able to help. Rule of thumb is your engine is rated at 3600 rpm. You would size your prop for 3600 rpm and run at 3000 rpm.
This is what they call the continuous curve
You can run at 3600 rpm for 1 hour out of a 8 hour shift. This is what they call the intermittent curve. To apply Horse Power for a sail boat to get to hull speed is 3 hp per ton.
I find the 3.22:1 ratio odd as the more popular is the 2.62:1 The information you need is the Make of vessel, Waterline length, Displacement weight, Engine make and rpm and horse power.
Check out this site, and I know Norms will answer all your questions, he has been applying and selling various propellers for years now.
http://www.westbynorth.com ("") Best regards, Ron"

10-06-2009, 06:02 PM
"Thanks Ron.

Norm at West-by-Northwest sold me the prop....he gave me the input in addition to the boatdiesel.com prop calculator. I'll be calling him also, to get his input now that the motor is operational.

I'm still looking for answers to my 3 questions."

10-14-2009, 06:11 PM
"Hello Jack,
Sorry I am not more of a prop sizing spe******t. let me know what you find,