Prop size rules of thumb ?

Ross · Jun 1, 2009

Charlie, It has to do with the disk area of the prop versus the pitch. If you change the pitch from 10 inch to 11 inch you have made a 10 percent change. But if you change the diameter from 10 to 11 you have made a 21 percent change. I think the rough rule was two inches of pitch or one inch of diameter. Area of circle is : pi radius squared. So 5x5=25 and 5.5x5.5=30.25

sailingdog · Jun 1, 2009

IIRC, the rough rule was 1" of diameter or 2" of pitch equals about 400 RPM.

So a 10x12 is about the same as an 11x10 and going from a 10x12 to a 10x14 or 11x12 would reduce the WOT by about 400 RPM. and going from a 10x12 to a 10x10 or 9x12 would increase WOT by about 400 RPM.

Ross · Jun 1, 2009

cbinfl said:
Thanks for the info, and the math Ross. Logical. If a 18x10 is recommended, then a 16x14 should work ?

charlie

Close but probably more slip. One inch at 1000 rpm equals a theoretical one knot. 50 per cent slip is normal for sailboats. so you can do the math. I went from a 13x15 repitched to 13x13 on a 3:1 reduction gear for my boat and was pleased. If you could get 16x12 or 13 it might be better. Speed is not very important to me and on our last trip we motored in flat water for about 35 hours at 3.5 to 4+ knots. More than a hundred miles and used less than 5 gallons of fuel.

sailingdog · Jun 1, 2009

if you can fit the 18x10, use it instead, since the pitch is far more likely to be affected by slippage.

cbinfl said:
Thanks for the info, and the math Ross. Logical. If a 18x10 is recommended, then a 16x14 should work ?

charlie

Steve Dion · Jun 1, 2009

Charlie: If you contact a reputable prop shop with your engine/trans & boat specs they can size a correct prop for your boat.

Maine Sail · Jun 2, 2009

Steve Dion said:
Charlie: If you contact a reputable prop shop with your engine/trans & boat specs they can size a correct prop for your boat.

What Steve said. They might not always be spot on but the software they use (Michigan Wheel software usually) is about the best we have.

The best rule of thumb is that you get a prop that will allow your engine to turn it's max rated RPM at WOT.

On our engine that is 3000 RPM and Westerbeke is fairly rigid on this point when it comes to warranties being covered. If you can only get to 2750 on a 3k engine, and you know the tach is correct, you'll need to re-pitch.

Ross · Jun 2, 2009

You will get much better efficiency with the 18 inch prop.

Mike Holibar · Jun 3, 2009

Props on sailboats really need to be a compromise according to the owners requirments. Ros is right when he says the bigger prop will be more efficient. The general rule when looking at it soley from the point of view of transmitting the maximum amount of thrust from the engine is to have the biggest slowest prop you can fit. However, if you do this there will be a huge drag penalty, reducing straight line speed and pointing ability. So smaller props and "sailor" type props with reduced blade area are often used as a compromise and a certain amount of slippage is accepted. It is important, however, to ensure the prop is sized and pitched in such a way as to ensure the engine can reach its full rated rpm (with clean hull) and therefore deliver its full rated power when you need it. Having been in the engine business, I have seen many a poor propeller selection wreck an engine. Overpropped engines can't develop their power, burn more fuel and wear out more quickly than correctly matched engine propeller combinations do. Another option is the feathering type of propeller. I fittted one of these last summer and was amazed with the results. I lost a little bit of speed, maybe 1/2 knot under power, but picked up almost a knot in speed and about 10 degrees in pointing ability. The boat sails so much better that we use the engine a lot less. Have a look at this site. http://www.kiwiprops.co.nz/bench-p2.html
I am not very computer literate so I am not sure if it will work as a link.

druid · Jun 4, 2009

What Mike said!

When I put the new engine in my Catalina 36, I used every resource available to tell me what prop to use. I got every answer from 14x10 to 16x14. Robert Hess from Atomic Engine Service recommended 14x8, so that's what I tried. I was STILL a bit overpropped.

Take the calculator results with a grain of salt! Best option is to find a similar boat/engine and see what they use (and confirm they're not overpropped).

I have a 14in prop on my Crown 28, and I think I'll go smaller. There's very little clearance at the top, and I think that's causing a LOT of prop-walk, especially in reverse (I'm overpropped there too, due to the mfr telling me the wrong ratio for the transmission. But I think I'll go less diameter rather than less pitch for the reason above).

druid

Gary_H · Jun 5, 2009

How about going from a two to a three bladed prop. Is there an advantage there? I believe my two blade is a 13X10 would I keep the same size with a different pitch if I went 3 blade or should the three blade be smaller? Is it even worth worrying about. I'm think more about power than speed.

druid · Jun 5, 2009

Gary_H said:
How about going from a two to a three bladed prop. Is there an advantage there?

In a word, yes. First, you'll get less slip, so more "power" (although you'll probably need to go down in pitch and/or diameter - the calculation programs can tell you this). But the Campbell Sailor Prop website pointed out another advantage: The keel (or even the skeg) puts out a vertical disturbance. A 2-blade prop will have both blades in the disturbance at the same time, causing vibration. A 3-blade will always have at least 2 blades in clear water.

The 3-blade will cause more drag while sailing, but unless you're racing I doubt you'd notice it.

druid