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Old 01-04-2008, 12:58   #31
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A question to go along with the same motor. My port motor gets harder to start as it get colder. Anything above 80* ambient, and it starts easily. The cooler the harder, and when it does start it emits white smoke, and then the smoke clears. Also same engines sounds like a deisel. While the strbrd engines is quiet like a gas motor?
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Old 01-04-2008, 17:31   #32
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Originally Posted by imagine2frolic View Post
A question to go along with the same motor. My port motor gets harder to start as it get colder. Anything above 80* ambient, and it starts easily. The cooler the harder, and when it does start it emits white smoke, and then the smoke clears. Also same engines sounds like a deisel. While the strbrd engines is quiet like a gas motor?
You might want to pull out the injectors and get them serviced.
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Old 01-04-2008, 18:00   #33
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No, you have to understand the different terms being used and sadly, not ALL the story is told in the specs with some. But youy specs shoudl tell you several different key points.
Max RPM is a short term unloaded rating. It is the Maximum RPM that the engine will rev to before the Govoner cuts in a limits the engine from going any further. This speed is not something you want the engine to be able to achieve normaly. You then have a "short term" maximum rating. This is where the Power peaks on tha Graph and is what most manufacturers will use as the HP rating. But this RPM is still not a continuous rating. The continuos rating is the RPM where the engine will happily run all day every day with out over heating the engine. Which spec is being used, depends on the engine manufacturer.
So the easiest way of determining all this is a simply industry rule of thumb. Reve the engine unloaded and see what the rev counter tells you. Place in gear and do the same. The max differnce should be around 10% by rule of thumb. If you sit within 10%, the bet is you are pretty close to spot on. It's not me that came up with it. It is a general inudstry recognised rule of thumb that brains bigger than mine have used for years.
I have heard and understand that theory. Here is the flaw.

In order to utilize max HP, one must be able to achieve the rated RPM for that HP. That takes torque.

If you look at the torque ratings on that engine, you will find that it drops off dramatically after 2500RPM (which is the ideal cruising RPM):
http://www.yanmar.com.au/marine/gm_s...0_TechData.pdf

If the salt water hits the fan and you need the HP (for short periods of time) you must be able to reach that HP by the use of torque. The dirtier the bottom of the boat or the harder the wind is blowing or choppier the seas, the less ability a vessel will have to reach the RPM that will deliver needed HP when it is needed most.

I delivered a Catalina 36 (from Florida to San Diego via Panama) with that exact same engine. Before I left, I insisted that the owner re-pitch the prop so that I could attain max RPM. The vessel had just had a bottom job done and I could only get 3200RPM out of the engine in calm conditions and it smoked like a pig.

The prop was re-pitched and I was able to get (almost) 3700RPM out of it. The governor kicked out at 3750 (out of gear). There was no more smoke.

As it turned out, I had zero wind from the Bahamas to Panama and I was forced to motor for 900 miles. That motor was quiet and smooth at 2500RPM and it sipped fuel. The owner told me that he got 3/4G per hr @ 6.5kts out of that engine. After re-pitching the prop, it burned just under 1/2G per hour @ 7kts.

I was extremely impressed with that engine.

The real problem is, the dirtier the bottom of the vessel is, the less apt it is to be able to reach the potential that the engine has. If it is slightly over-propped, this problem is worse.
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Old 01-04-2008, 18:59   #34
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I think we all know how hard it is to pedal your bicycle in a very high gear. Well that's what it's like for your boat engine when the prop is too big or has too much pitch. You strain to pedal and your engine strains to turn the prop.
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Old 01-04-2008, 22:31   #35
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Kanani, I am not sure what your argument is? I think we are on the same page. I am saying no less in RPM than 10% of maximum intermittent RPM. The closer to Max continuous RPM you can get the better.(Please note the two differences) There is a difference between Continuous and Maximum RPM that the engine is govorned to. And there is often a difference between the Govorned RPM and what the throttle stop maybe set to.
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Old 02-04-2008, 06:59   #36
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So - it seems that is is better to be a bit underpropped, so you can still make decent rpms in conditions such as stiff headwind / chop - rather than having the engine struggle, if overpropped. Is this correct?

If so, what are the other implications of being a bit (not severely) underpropped, ie reduced "normal condition" cruisimg speed, tendancy to over rev engine to go faster, etc?

just trying to learn here/ make adjustments, before the boat goes back in the water!!
Note. I think my engine is overpropped 15X10 prop on a Yanmar 2Gm 15hp peak, 13 continuous) Km2a 2.62 - 1 tranny. boat is sailboat 30'LOA, 25' LWL. As I considered repowering with a 20hp, and the prop calculator shows I should have a 14x10 prop, even with 20 hp, i assume Iam overpropped at 15 hp and a 15x10 prop! engine did smoke at higher rpms, but had no tach so don't know my rpm levels! the calculator shows I can use a 15X7.6 pitch or 14x8.6 with the current engine!!
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Old 02-04-2008, 09:12   #37
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Kanani,

Thanks, I bought some new injectors, and will install them. Then have the old ones serviced. I also have a thread under propullsion if you would take a look about some 3 blade feathering Boyen props. Advice from all is welcomed......
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Old 07-04-2008, 20:59   #38
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prop info

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Originally Posted by Inlet Marine Services View Post
As a Yanmar dealer,over proping is a common problem. The first thing we look for is can it hit 3600 rpm. Get me your gear ratio, water line length, gross weight, prop size, how many blades

Ok as follows
gear ratio. 2.61-1
LWL 27.9
Gross weight 11k
prop 15" 3 blade Max Prop

HP 27
shaft speed 1303 rpm

Hull painted blue

Thanks I am ready to float this boat. Al
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