Do I have the right size prop? - SailNet Community
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post #1 of 11 Old 04-21-2013 Thread Starter
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Do I have the right size prop?

I've got a new-to-me 1980 Spirit 28 with original Yanmar 2QM15. PO replaced original 2 blade prop (D13, P13) with a 3 blade (D13, P11). I'm not sure why he made the change, or how he calculated the new prop size, but he was a mechanically inclined sort, so I suspect he used generally accepted calculations.

I took the boat out today to make some test runs. Here's a summary of my observations (all readings taken with original factory tach and GPS (averaged over two runs in opposite directions). Note that Yanmar service manual specifies 3000 max RPM.

WOT in neutral: 2900 RPM
WOT in gear: 2700 RPM

RPM Knots
2700 6.55
2600 6.4
2400 6.2
2200 5.95
2000 5.75
1800 5.35
1600 4.85
1400 4.45
1200 3.9
1000 3.3

No black smoke whatsoever, even at WOT. Just a trace of white smoke, which dissipates immediately.

The only thing here that makes me wonder a bit is that it won't reach factory max speed (which I assume is governor limited) at WOT.

Based on all this, should I have any concerns regarding prop sizing?
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post #2 of 11 Old 04-22-2013
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Re: Do I have the right size prop?

Looks to me like you should have a bit less pitch on that prop; maybe 13/9 or 13/8. You should be able to achieve max RPM with the throttle wide open (that your engine can't quite get there in neutral probably means that the engine governor needs a bit of an adjustment). Also, it looks like your boat gets up to hull speed quite a bit below max RPM, I would think you would want to be a little closer to max RPM when HS is achieved (maybe 85-90% of max rpm). However, you're close enough to the ideal that it may not be worth messing with it.

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post #3 of 11 Old 04-22-2013
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Re: Do I have the right size prop?

I wouldn't trust the boat's tach to be that accurate.

I've been worried about my prop for a while because my tach reads 2000-2100 at cruising speed and at best has hit 2400rpm in neutral. Today I finally remembered to bring a digital tach down to the boat with me, so I checked it with that. When my boat's tach is reading 2100 the engine is turning at just under 2800rpms. Since my engine is a Yanmar 2GM20F with a max rating of 3400rpm that puts me right in that 75-80% range.

I'm using a 3 blade Campbell Sailor 14x8 prop. It has less blade area (I measured) than my original factory 16x10 prop. It looks like they knew what they were doing when they suggested this prop for my engine.

I'm using this tool to check my tach. I've doubled checked the laser tach against a milling machine with reliable known RPMs, it is accurate:
CyberTech Digital Photo Laser Tachometer Non Contact Tach: Amazon.com: Industrial & Scientific CyberTech Digital Photo Laser Tachometer Non Contact Tach: Amazon.com: Industrial & Scientific



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post #4 of 11 Old 04-22-2013
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Re: Do I have the right size prop?

Oh, and the reason that most people select a 3 blade prop vs a 2 blade is that it runs much more smoothly when under power (with increased drag, unless the prop has smaller blades as in my example). A 2 blade prop "pulses" as it passes through turbulent water behind the keel and then cleaner water off to the sides. A 3 blade prop pretty much always has one blade behind the keel and two out in open water.

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post #5 of 11 Old 04-22-2013
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Re: Do I have the right size prop?

Rusty, Contact your local prop shop and give them the specs on the boat and tell them what prop you have now. They will tell you if it is correct or what you need to do. Chuck

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post #6 of 11 Old 04-22-2013 Thread Starter
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Re: Do I have the right size prop?

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I'm using this tool to check my tach. I've doubled checked the laser tach against a milling machine with reliable known RPMs, it is accurate:
CyberTech Digital Photo Laser Tachometer Non Contact Tach: Amazon.com: Industrial & Scientific
Thanks for the lead on the digital tach -- I ordered one today.

Regardless of the absolute accuracy of my factory tach, my readings at WOT differ (by about 200 RPM) between neutral and forward. In an ideal world, should these numbers be identical?
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Re: Do I have the right size prop?

I would expect them to be different, but I am certainly not an expert. In neutral there is no load, in forward there is a high load. A prop with very low pitch might be able to get to high RPM in forward, but you'd be underpropped and wouldn't make good headway.

My 2GM20F has two RPM ratings that seem to reflect this. There is a peak low load rating of 3600 rpm, and a continuous rating of 3400 rpm. 3400 rpm is what is written on the engine plate, but both ratings are mentioned in the service manual.

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Re: Do I have the right size prop?

I'll take another look at my manual. The peak no-load and high-load readings you describe are quite similar to what I'm observing (in relative terms, if not absolute), which is encouraging.
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Re: Do I have the right size prop?

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Originally Posted by CapnChuck View Post
Rusty, Contact your local prop shop and give them the specs on the boat and tell them what prop you have now. They will tell you if it is correct or what you need to do. Chuck
I may end up doing that eventually, but the shop has a bias towards making a sale, so I'd prefer to learn how to interpret the operational characteristics myself (with the help of my SN buds).
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Re: Do I have the right size prop?

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Originally Posted by SlowButSteady View Post
Looks to me like you should have a bit less pitch on that prop; maybe 13/9 or 13/8. You should be able to achieve max RPM with the throttle wide open (that your engine can't quite get there in neutral probably means that the engine governor needs a bit of an adjustment). Also, it looks like your boat gets up to hull speed quite a bit below max RPM, I would think you would want to be a little closer to max RPM when HS is achieved (maybe 85-90% of max rpm). However, you're close enough to the ideal that it may not be worth messing with it.
From my numbers, what would you estimate my hull speed is? When I started this experiment, I figured the graph would flatten out at hull speed, but in reality, the line is relatively straight. But 6.55 knots seems pretty fast for a 28 footer -- I doubt that my true hull speed is higher than that.

Your last sentance is where I'm targeting -- not really interested in shelling out for a new prop, unless it returned significantly better performance or engine life. 6 knots at 2200 RPM (with no black smoke) seems reasonable to me, but I'm just a novice.

Last edited by Rusty123; 04-22-2013 at 11:56 PM.
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