Correct prop for 25D

[Jhn Vigor]

Can anyone please tell me the correct diameter and pitch for a three-bladed prop for a 25D driven by a Yanmar 1GM (6 1/2 to 7 1/2 hp)?
The previous owner of my boat, Jabula, fit a three-bladed 12-inch diameter by 11-inch pitch prop, and I get a top speed of 5.4 knots at a max of 2,700 rpm.
Opening the throttle further merely results in lots of black smoke from the exhaust.
My feeling is that this is over-pitched, and that I should be able to get 3,600 top rpm with a 12 x 9 prop and a top speed of 5.8 knots.
But I'd like to hear from anyone with actual experience of a 12 x 9.



jvig@whidbey.net

[Lee H. Hodsdon]

John,

I had a similar condition with my 25D, though with a Westerbeke 10-2. I agree with your assumption of the incorrect prop. It seems when these higher revving engines first came out no one believed they could rev this high and still stay together. My prop change resulted in top rpm increase from 2400 to 3600 rpm, with a corresponding increase in hp, decrease in heat and no smoke. Quite a deal. My prop was purchased from Campbell in Canada. This was an expensive purchase, but in my opinion well worth the expense. I'd recommend you check out their web site at westbynorth.com and see for yourself. It may not suit your situation, but the information is worthwhile.

Sincerely,

Lee



lhodsdon@nh.ultranet.com

[John Vigor]

Thanks, Lee. I did the sums, and figured a 9-inch pitch was correct if I was going to get the revs that give the power. I'm guessing the extra revs will give me at least one more horsepower--quite a lot from an engine that's capable of 7 1/2 hp at best.
Bob Ohler, owner of 25D hull #2, says he also has a 12 x 9, so it really does seem that the previous owner of my boat got it wrong.

Sincerely,

John V.



jvig@whidbey.net

[Bruce Barber]

John,

Here's another data point - I changed from the original two-blade to a 12 by 8 three-blade. With a clean propeller, my 1GM now can turn to 3400 rpm. On that basis, I would guess you'd max out at something less than 3400 with 9 inches of pitch.

There's another variable that's seldom brought into this discussion, and that's blade area. Some propellers have narrower blades than others, and they'll allow the engine to run up higher. I'm sure this is partly explains variations in reported performance among propellers with the same diameter and pitch. Unfortunately, I don't know the blade-area ratio of mine, only that it's from Michigan Wheel.

Bruce Barber
CD25D Nancy Dawson, Hull # 77



bbarber38@aol.com

[John Vigor]

So, a 12 by 8 by golly! No wonder I'm struggling with a three-bladed 12 by 11. Thanks for the info, Bruce. Yes, if you're topping out at 3,400 rpm, I'd expect to get only 3,000 to 3,100 rpm out of a 12 by 9.
You're right. Blade area does affect things. More blade area reduces propeller slip and loads the shaft and engine more. I, too, have a fat-bladed, Michigan-type wheel, and in my calculations I estimated slip at 45 percent. I am going to speak to the prop shop and see how much pitch they can take off my existing prop without destroying it.

John,

Here's another data point - I changed from the original two-blade to a 12 by 8 three-blade. With a clean propeller, my 1GM now can turn to 3400 rpm. On that basis, I would guess you'd max out at something less than 3400 with 9 inches of pitch.

There's another variable that's seldom brought into this discussion, and that's blade area. Some propellers have narrower blades than others, and they'll allow the engine to run up higher. I'm sure this is partly explains variations in reported performance among propellers with the same diameter and pitch. Unfortunately, I don't know the blade-area ratio of mine, only that it's from Michigan Wheel.

Bruce Barber
CD25D Nancy Dawson, Hull # 77

jvig@whidbey.net