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post #1 of 4 Old 11-23-2011 Thread Starter
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Technical propeller question

We have:

Irwin Citation 34, LOA: 34' 3", LWL: 27' 4", Beam: 11' 3" Displacement: 11500 but weight at launch is about 1300. Yanmar 2QM20H Rated RPM: 3800 / Rated HP: 20 / Gear Ratio: 2.14

Propeller is a cracked 14x10 RH three blade on a 7/8 inch shaft. With it we get:
2100 RPM - about 5.2 KN
2600 RPM - about 6.0 KN
Hull speed "should be" about 7 KN

One propeller manufacturer is recommending a 14x9. Would not a 14x9 give me less speed and perhaps a 14x11 give me a bit more????

Also - I will also happily take any recommendations for places to purchase a new or used appropriate prop.

Rik

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post #2 of 4 Old 11-23-2011
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From what I have read, if you can get somewhat close to your maximum rated RPM with a fairly clean bottom & prop you are probably pitched about right. If not, you may be over pitched. If you can pull the 14X11 to that RPM you may gain some speed, but I would think the Manufacturer has a pretty good data base to form his recommendation.

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post #3 of 4 Old 11-23-2011
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Quote:
Originally Posted by rikhall View Post
We have:

Irwin Citation 34, LOA: 34' 3", LWL: 27' 4", Beam: 11' 3" Displacement: 11500 but weight at launch is about 1300. Yanmar 2QM20H Rated RPM: 3800 / Rated HP: 20 / Gear Ratio: 2.14

Propeller is a cracked 14x10 RH three blade on a 7/8 inch shaft. With it we get:
2100 RPM - about 5.2 KN
2600 RPM - about 6.0 KN
Hull speed "should be" about 7 KN
One propeller manufacturer is recommending a 14x9. Would not a 14x9 give me less speed and perhaps a 14x11 give me a bit more????

Also - I will also happily take any recommendations for places to purchase a new or used appropriate prop.
Rik
Don't get insulted, but I believe that your boat's shaft is undersized... For that HP and displ. a one inch shaft would seem more appropriate. But, that's not what you asked...

Our boat is a 34' that weighs in at about 11K, and has a 23 hp diesel, and we use a three blade 15 X 9.5. This is still a tad over-propped and a 9" pitch would be better. I can motor at 7.2 in smooth water with a clean bottom.
My current prop is a Michigan "Sailor" and was out of pitch on one blade when delivered. If you go with that brand, have the prop precision-checked when you receive it. (Had I known that their QC was so hit-n-miss I would have gone with another brand.)
Having said that, the three blade fixed can now drive our boat at full speed thru a moderate chop.

Check your motor specs and see what rpm that Yanmar recommends for sustained cruising - I bet that it's more like 2900 or so. Pitch your new prop for that rpm. At the present 2600 rpm, you are likely only achieving about 15 hp, and that's not much help to get you thru a chop.

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post #4 of 4 Old 11-23-2011
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Ask the company that recommended a 14x9 prop what they will charge you to re-pitch the prop if you find it necessary. If you can't hit hull speed using their recommendation they may re-pitch it for a nominal fee (I doubt that they'll do it for free, but who knows). I would be careful not to over-pitch the prop.

It's often stated that you should be able to hit hull speed at about 85-90% of the engine's max continuous rpm rating, which a quick Google search indicates is 2600 rpm for (rather than 3800 rpm) for a 2QM20H. I suspect that a 2QM20H may be pretty close to the lower limit of engine size for a 13000# boat. Ted Brewer recommends 3 to 4 b.h.p. per ton, which in the case of you boat would would be just about right. However, Brewer doesn't seem to consider it necessary for an auxiliary sailboat to maintain hull speed under power. And, there's nothing magical about hull speed anyway; 6 kts is probably plenty fast.

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