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Old 07-02-2018, 18:36   #16
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Re: 12 HP Yanmar Diesel question

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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 isnít making RPM, and of course the wrong prop too.
All true however the OP hasn't (yet?) mentioned any issues with reaching max RPM at WOT.

My limited experience with older small single diesels is that the dynamic RPM range is not as great as when using larger engines. For instance, an 8 hp Arona I had in 30' steel boat (heavy) was really only operated at 2000 RPM when going slow and 3000 when powered up while my current 2GM20 in a 31' in easily driven hull can be operated anywhere between 1200 and 3400 as conditions dictate.
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Old 07-02-2018, 18:39   #17
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Re: 12 HP Yanmar Diesel question

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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.
Nothing wrong with that-------after all, it's a sailboat.
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Old 07-02-2018, 19:36   #18
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Re: 12 HP Yanmar Diesel question

I used to have a CS27 with a 12 Hp Yanmar 1GM. Great engine would start first turn over but not much power. It came with a two bladed fixed prop from the factory so I assume it was sized correctly.

In flat water I could get about 5 knots, any head wind or wave action and it would struggle to make 3.

Your boat is a bit heavier but this will give you some guidance, do the things already mentioned by others and I am sure you will see some noticeable improvement.
Good Luck!
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Old 08-02-2018, 09:55   #19
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Re: 12 HP Yanmar Diesel question

I agree...a 12hp diesel should be able to push you 5 knots.

I'm wondering if your boat was repowered? I had a repowered boat that was switched from atomic 4 to a volvo diesel. To save money, they kept the original shaft and prop, which was obviously undersized for a diesel. Diesels spin slower than gas engines, so usually have larger diameter props. This can also be accomodated via the transmission or going to a three blade, but at some point someone has to put some thought into it so that the prop/prop speed are appropriate.

At what RPM are you running? Is that WOT?
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Old 08-02-2018, 10:05   #20
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Re: 12 HP Yanmar Diesel question

Further to my previous reply about undersize props...the opposite can also be a problem. A prop too large or too aggressive may require too much power to increase rpm. This is quickly noticed since at WOT the rpm remains low. I actually had this problem too, where a previous owner had replaced the factory 2 blad prop with an aggressive 3 blade, thinking it would be better. The A4 would not rev past 1500 rpm,

And finally, it could be something as simple as a lack of fuel flow. Be sure to check your filters!

Good luck, and please let us know what happens.
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Old 08-02-2018, 10:09   #21
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Re: 12 HP Yanmar Diesel question

A 12 hp diesel is plenty for that boat. Have a qualified diesel mechanic check your engine. Sounds like a faulty "rebuild."
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Old 08-02-2018, 10:31   #22
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Re: 12 HP Yanmar Diesel question

agree on checking prop and bottom for growth, when was the last time bottom was cleaned.
also what rpm can you get from the engine.
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Old 08-02-2018, 16:31   #23
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Re: 12 HP Yanmar Diesel question

Try this to see if you have the right prop:https://www.vicprop.com/calculator.htm. Sometimes the diameter & pitch is stamped on the hub. Engine should be ok IF it was recond properly in 2010 & IF it was maintained properly. Will it rev to 3000 rpm under no load? & what does it rev to under load? For instance i put a too big prop on ours ( YSM8) & it would only get to 1800 rpm whereas it supposed to be able to reach 3200 rpm.At 1800rpm it only puts out 4hp so you arent utilising the engine to its potential.
Let us know
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Old 08-02-2018, 17:46   #24
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Re: 12 HP Yanmar Diesel question

What is your current prop size?

For reference, when I had the Yanmar 2GMF (13 hp) on my 31 footer, it would do 6.2 knots in calm and down to 4.7 knots in moderate seas. After I re-engined to the 2GM20f (16 hp), top speed went up to 6.6 knots. Hull speed is 6.9 knots.

Troubleshooting goes like this, check rpms at WOT (preferably with an external meter, the tech is probably not reliable at this age) in neutral. Then in reverse and forward. Observe the engine and the rpms. Have a diver clean up your prop. Test run. Measure the prop. Measure the compression. Fix what needs fixing.
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Old 08-02-2018, 18:16   #25
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Re: 12 HP Yanmar Diesel question

I haven't read all the posts, but I DO have an Adams 31 running a Yanmar YSE12. No purist here, and would love a much larger engine, but finding the same thing as you when I bought her many years ago (ie 3.5 knots top speed, spotless under), I picked up a second hand prop of similar pitch but about an inch larger diameter and gave it a try. Long story short, the old prop was never refitted and is now just the spare. Can now motor at or very near 5 knots all day (or about 5.5 by wringing its poor neck for short bursts).
Having said that, the boat and particularly the prop itself MUST be absolutely spotless now, or the engine overloads (black smoke). Wasn't the case with the original prop of course. Headwinds likewise call for careful throttle use to avoid overload (I will usually sail to windward if there is room, or motor while tacking hard-sheeted if it is tight, like in the long narrow channel between my marina and the ocean)
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Old 08-02-2018, 18:27   #26
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Re: 12 HP Yanmar Diesel question

Kind of unrelated but maybe interesting to some; ya know what happens if you keep increasing the size (diameter) of your prop, assuming unlimited horsepower availability?
I wouldn't have guessed it but....apparently the boat rotates instead of the prop!! This was demonstrated in Sydney a very long time ago (maybe a hundred years - I dunno) in a popular tugboat competition aimed at identifying the most powerful tug. Props kept getting bigger until one day, sporting a new monster prop, the skipper gassed his tug and it promptly rotated upside down and sank (this story retold from an old "Sydney Afloat" magazine).
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Old 09-02-2018, 13:33   #27
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12 HP Yanmar Diesel question

I think that belongs in the joke thread

On the old Corsairs, the joke was when they hit the starter button it was a toss up to see if the fuselage or prop would spin.
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Old 09-02-2018, 13:43   #28
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Re: 12 HP Yanmar Diesel question

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Originally Posted by mowerandy View Post
Kind of unrelated but maybe interesting to some; ya know what happens if you keep increasing the size (diameter) of your prop, assuming unlimited horsepower availability?
I wouldn't have guessed it but....apparently the boat rotates instead of the prop!! This was demonstrated in Sydney a very long time ago (maybe a hundred years - I dunno) in a popular tugboat competition aimed at identifying the most powerful tug. Props kept getting bigger until one day, sporting a new monster prop, the skipper gassed his tug and it promptly rotated upside down and sank (this story retold from an old "Sydney Afloat" magazine).
Yes I think I saw a video of a Tug almost rotating, big power, big prop. The Oyster boats of east coast US were noted for their huge props turning slow rpm and great economy. Like 28-30 ft boats with a 3 ft diameter prop. I sailed Mexico with a HP Physicist who wanted one. After returning from Mexico he bought an old oyster boat.
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