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Old 07-02-2018, 08:56   #1
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12 HP Yanmar Diesel question

I purchased a beautiful classic O'Day 30' last summer. The 12 HP Yanmar diesel was rebuilt in 2010. In and out of the harbor, motoring to where we want to sail, or just getting anywhere with any speed is nearly impossible. She moves top speed at about 3 mph. I dont intend to ski behind her, unless under sail (lol), but would like a bit more power. Any suggestions? I am no mechanic, but have friends who can help. Thanks, Meis.
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Old 07-02-2018, 10:00   #2
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Re: 12 HP Yanmar Diesel question

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Originally Posted by jmmeis View Post
I purchased a beautiful classic O'Day 30' last summer. The 12 HP Yanmar diesel was rebuilt in 2010. In and out of the harbor, motoring to where we want to sail, or just getting anywhere with any speed is nearly impossible. She moves top speed at about 3 mph. I dont intend to ski behind her, unless under sail (lol), but would like a bit more power. Any suggestions? I am no mechanic, but have friends who can help. Thanks, Meis.
Many production boats used cheap mis-sized props. Check your to make sure is optimal for the boat and engine. Make sure hull is clean. I wouldn't think 12h.p. is going to attain hull speed but I would think 5 knt's more reasonable
to expect.
I'm on a 30' similar to your boat, with 2qm15. It was a 5 knt boat with the old michigan 3 blade. Changed it to a Campbell sailer prop, sized to their specs for boat and engine. The prop brought my cruise speed up to 6.25 knt's.
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Old 07-02-2018, 10:10   #3
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Re: 12 HP Yanmar Diesel question

I agree it may need a prop pitch change or other prop. However, it is also a very minimal engine for a 30 ft boat. If it runs well, I bet you can get more speed from a proper diameter and pitch 3 blade prop. If it has one of those old Martec folding props , that's likely your issue, they are terrible.... only good for getting in and out of harbor on racing boats.
MIT Prop test on 30 ft boat: http://www.catamaransite.com/files/propeller.pdf
The Campbell has far less drag than the Michigan.
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Old 07-02-2018, 10:12   #4
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Re: 12 HP Yanmar Diesel question

Depending on waterline length, 5 kts May be hull speed.
However I’d bet lunch that this boat that was just bought’s prop is covered in growth, and needs nothing more than a good scraping.
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Old 07-02-2018, 11:29   #5
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Re: 12 HP Yanmar Diesel question

I have a Yanmar SB12 in good condition (finally, after much work!) on my Bristol 29.9. It pushes the boat at 4-5kts depending on conditions. Right now we are motorsailing at 5.5kts in 5kts of wind on the beam. Any headwind kills our speed motoring....we struggled to make 2.5kts a few days ago with 20kts of wind on the nose.

In my opinion, the SB12 is a bit small for this boat, but I like it for two reasons:

1) We sail, anytime sailing is possible. Motoring is not the best way to get my boat from A to B. We are an auxiliary sailboat, with just enough power to motor where we have to, to get in and out of inlets with a 2-3kt tidal stream, etc. If you just want to get from place to place on a schedule, consider a larger engine?

2) You can't ask for a simpler, sturdier workhorse of an engine. Easy to work on, easy to learn, and parts are, for the time being, still pretty available. I've learns a lot overhauling mine.
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Old 07-02-2018, 13:19   #6
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Re: 12 HP Yanmar Diesel question

I concur with the above posts. Prop must be clean and correctly sized otherwise no reasonable amount of hp will move the boat optimally; check these first!
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Old 07-02-2018, 13:56   #7
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Re: 12 HP Yanmar Diesel question

Thank you all for your comments and suggestions. I am as concerned with power for getting out of situations as much as for getting out to sail in a timely manner. The suggestions and comments are similar to what I was thinking, but not having power expertise with no inboard experience, I wanted to see what the experts say. The weight of the boat (10,700 lbs.) seems a bit heavy for the small engine that was put in this craft. I do not want to replace it, just get the most out of it. I would love to get 6 Kts out of it. I know that the prop is the original small two blade so I am sure that adds to the situation. The hull has great anti-fouling paint, however at the end of summer at time of pull there is some of "extra" on it and that all adds up. She is a beautiful craft and sails like a dream.
Thanks again for all of your help. I have more questions for later. Meis
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Old 07-02-2018, 15:02   #8
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Re: 12 HP Yanmar Diesel question

I calculate it will go 5.38 kts with 12 HP. 5 kts with 9 HP, 4.5 kts with 7 HP
Thats using 11000lbs and 25' waterline length in the calculations.
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Old 07-02-2018, 15:02   #9
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Re: 12 HP Yanmar Diesel question

Bristol 32 (22 feet waterline) 10,000lbs Yanmar 2gmf 2 cylinder 18 hp good for 6 knots flat out, 5 knots cruise. Most 32's go for the 3 cylinder, 24 hp, but I find this suits our needs.

Propellor is vital, vital.
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Old 07-02-2018, 15:03   #10
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Re: 12 HP Yanmar Diesel question

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Originally Posted by jmmeis View Post
Thank you all for your comments and suggestions. I am as concerned with power for getting out of situations as much as for getting out to sail in a timely manner. The suggestions and comments are similar to what I was thinking, but not having power expertise with no inboard experience, I wanted to see what the experts say. The weight of the boat (10,700 lbs.) seems a bit heavy for the small engine that was put in this craft. I do not want to replace it, just get the most out of it. I would love to get 6 Kts out of it. I know that the prop is the original small two blade so I am sure that adds to the situation. The hull has great anti-fouling paint, however at the end of summer at time of pull there is some of "extra" on it and that all adds up. She is a beautiful craft and sails like a dream.
Thanks again for all of your help. I have more questions for later. Meis
Yes, the two bladed prop will limit what hp you will extract from the engine; a three bladed correctly sized will give better motor performance at the expense of light air sailing performance.

As to needing power to get out of a sticky situation - don't overthink this. It is rare that a life treating situation will occur due to a small engine - yes, it is possible but it is rare. More often a larger engine simply makes it easier or makes you lazy. Attention to potential dangers, developing weather conditions and sailing knowledge will save your butt more often than raw hp.

I had a 10 hp single cylinder (Buhk) in a very heavy 28' Giles Wanderer for many years and it served me very well, even when it was down on power (due to age etc). Sure you couldn't punch into 20+ kts on the nose but you could still work to windward, tacking (motorsailing) with a baggy "cruising" main. The point being that when sailing, you never expect to be able to go straight into the wind so why have extra hp just to do that unless you want a motorsailor. If so, then you need a bigger engine .
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Old 07-02-2018, 15:31   #11
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Re: 12 HP Yanmar Diesel question

Start with the low-hanging fruit and check the prop for growth. A very small amount of growth on it will slow the boat down to a surprising degree. And the growth can happen very quickly.
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Old 07-02-2018, 17:13   #12
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Re: 12 HP Yanmar Diesel question

I’m going to argue.
A three blade prop will not give more speed than a two blade, assuming both are correct.
It will or may give more “bite” or traction if you will and may stop and accelerate the boat better, it may have less prop walk and less vibration too, but should not be faster.
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Old 07-02-2018, 17:15   #13
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Re: 12 HP Yanmar Diesel question

After the prop is cleaned, we then need to know what his full throttle RPM is I believe before anything else, he may have a clogged exhaust elbow, or even a half collapsed fuel line or all kinds of things if he isn’t making RPM, and of course the wrong prop too.
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Old 07-02-2018, 17:22   #14
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Re: 12 HP Yanmar Diesel question

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I’m going to argue.
A three blade prop will not give more speed than a two blade, assuming both are correct.
It will or may give more “bite” or traction if you will and may stop and accelerate the boat better, it may have less prop walk and less vibration too, but should not be faster.
In calm weather and water I agree but with headwinds the 3 blade, with greater surface area (bite), might push it better. I'd stick with a two blade for less drag under sail though.
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Old 07-02-2018, 17:29   #15
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Re: 12 HP Yanmar Diesel question

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I’m going to argue.
A three blade prop will not give more speed than a two blade, assuming both are correct.
It will or may give more “bite” or traction if you will and may stop and accelerate the boat better, it may have less prop walk and less vibration too, but should not be faster.
Luv a good argument .

I agree that a perfectly matched two bladed prop can give the same speed but IME, most two bladed props are selected for minimal drag rather than best efficiently under power.
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