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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a forum dedicated to diesel engines and their applicable accessories.


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  #11  
Old 07-04-2009
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Quote:
Originally Posted by Gary M View Post
I found a web site with the maintenance manual, mine is on the boat. It lists monitoring the exhaust colour and if it is black is says you are overloading the engine and this should be avoided as it will shorten engine life.

Diesels like to be worked but within limits. It also says if you are running for long periods of time at slower RPMs that every 2 hours you should rev the engine up several times to clear it out.

Web site can be found here
Yanmar Manual

Gary
Yeah, I have the same manual and have read that. When it happens, it's pretty obvious it's not good for the engine. I simply didn't understand WHY it was happening. However, as I think about it more, I realize that the opposing forces probably ARE lugging the motor, as if I were going up a hill in too high a gear. Obviously, I can't downshift on a boat, so the only option is to decrease rpm. Although, that analogy doesn't really work, because reducing RPM in the same gear wouldn't work in a truck. I'm so confused ! Just when I thought I knew what I was talking about, I proved myself wrong. Hate when that happens!
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  #12  
Old 07-06-2009
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I'm suprised you didn't notice issues with backing the boat. I got a similar encrustation at the end of last season, just on my prop. The first symptom noticed was the boat wouldn't back up worth a hoot. That alone didn't cause the penny to drop, but when the performance going forward was not as expected, I immediately suspected a prop load of barnacles, ony because my slip neighbor had told me the same thing happened to him the year before when I had no problem at all on the other side of the marina.
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  #13  
Old 07-07-2009
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Quote:
Originally Posted by L124C View Post
Yikes, thats not a drive train, it's a reef Poor little Yanmar! I've sometimes noticed that my motor (same one) will smoke at 3000 RPM if I'm fighting a strong tide and/or headwinds. If I back off the RPM, it seems fine. The motor purrs at 3000 otherwise, so I assume it's not a prop issue. This has never made sense to me, as the motor shouldn't "know" what the opposing forces are. Right? They should simply make the boat go slower. Or, are the forces somehow "lugging" the motor? Hypothetically, if the boat was tied to the dock, shouldn't the motor turn effectively at 3000 RPM? Dock, line, boat and dock cleat failure aside of course Someone please explain
Depends if you mean having the motor turning the prop when at the dock or running in neutral. If in neutral then it should theoretically run to it's max governed speed.
However in the context of turning the prop, forces come into play.
If you are stationary and turning the prop, then the blades angle of attack to the water are at maximum so it has maximum load and it's static (the boat not moving rpm will be lower).As the boat begins to move forward the props angle of attack relative to the water is decreased so the load on the prop is less and the motor can spin it easier, faster the boat through the water the lesser the AoA on the blades.
So if your are motoring into a headwind your boat speed will be less so the AoA will be greater.
For instance if I am motoring my boat and try to go to full power too early then I will get smoke but if I allow the boat speed to increase and I incrementally open the throttle I can achieve full power and no smoke from the engine.
Same with a propeller driven aircraft, on the ground your max static rpm will be lower then what you will achieve when you are flying. In an aircraft this effect is more easily seen as if you place the aircraft in a dive to increase air speed you can actually over speed your motor if you do not reduce power.

Mychael
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Old 07-07-2009
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  #15  
Old 07-09-2009
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Quote:
Originally Posted by Mipcar View Post
For instance if I am motoring my boat and try to go to full power too early then I will get smoke but if I allow the boat speed to increase and I incrementally open the throttle I can achieve full power and no smoke from the engine.
I think this is also because you are dumping more fuel in the cylinders than can be burnt efficiently until the pistons get up to speed.

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Originally Posted by Mipcar View Post
Same with a propeller driven aircraft, on the ground your max static rpm will be lower then what you will achieve when you are flying. In an aircraft this effect is more easily seen as if you place the aircraft in a dive to increase air speed you can actually over speed your motor if you do not reduce power.Mychael
Interesting. Good analogy, and it IS easier to visualize with an aircraft. So, with momentum, the force of the air/water pushing the front of the prop aids in increasing the RPM and reduces the load on the motor. Correct? Is this what you mean by angle of attack (as the pitch of the prop isn't changing, (with the boat anyway))? Makes sense, but I never thought about it (obviously, physics isn't my strong suit!).

Last edited by L124C; 07-09-2009 at 12:46 AM.
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  #16  
Old 07-09-2009
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Quote:
Originally Posted by L124C View Post


Interesting. Good analogy, and it IS easier to visualize with an aircraft. So, with momentum, the force of the air/water pushing the front of the prop aids in increasing the RPM and reduces the load on the motor. Correct? Is this what you mean by angle of attack (as the pitch of the prop isn't changing, (with the boat anyway))? Makes sense, but I never thought about it (obviously, physics isn't my strong suit!).
Think of it more in the way of sailing you have actual wind, what you get if you feel the breeze standing on the shore and the apparent wind you get when you are sailing, which is a vector of the wind from it's true direction and from what you create by your own forward movement.
So, if you prop is turning (under engine) but your boat is not moving through the water then it's leading edge of the blades is getting maximum resistance from the water, trying to "cut" the water if you like.
As your boat makes forward motion the prop is "cutting" the water at a different angle (remember the wind vector analogy) and this reduces the load on the engine.
I've never had it explained to me in the terms of the air/water "pushing" the front of the prop, however that could essentially be correct, as when we stopped an aircraft engine in flight and then reduced our air speed, at some point the prop would stop "windmilling" and be stopped by the compression of the motor.. If we then dived to high speed we could usually (not always) forced the prop to begin rotating again, so I guess you could say it was pushing the prop.

Mike

Last edited by Mipcar; 07-09-2009 at 04:27 AM.
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  #17  
Old 07-10-2009
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JSL3,
I have run into the same problem here in NJ. With all the rain we had in the spring the boat hung from the mooring under-used . The sun came out and I looked back to see that black exhaust cloud! I just figured it was time to don the wet suit and go for a swim. 5 minutes with the paint scraper and the problem was solved. A side note. My friend and I both being frugal, decided to test out a couple of the "urban legend" Anti foul remedies. He painted his prop with a Sharpie, I used Lanocoat. Somewhere on this site you can find someone extolling their virtues. Fugeddabout it! We will be doing manual prop cleaning every 5-6 weeks for the rest of the season. (not a big deal, really).
Bruce
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  #18  
Old 07-17-2009
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Quote:
Originally Posted by bruceyp View Post
JSL3,
I have run into the same problem here in NJ. With all the rain we had in the spring the boat hung from the mooring under-used . The sun came out and I looked back to see that black exhaust cloud! I just figured it was time to don the wet suit and go for a swim. 5 minutes with the paint scraper and the problem was solved. A side note. My friend and I both being frugal, decided to test out a couple of the "urban legend" Anti foul remedies. He painted his prop with a Sharpie, I used Lanocoat. Somewhere on this site you can find someone extolling their virtues. Fugeddabout it! We will be doing manual prop cleaning every 5-6 weeks for the rest of the season. (not a big deal, really).
Bruce
I suppose my biggest problem was under use. Since the lastest cleaning I've been out 2 times (once a week basically) and all is well obviously. While the boat was out of the water two weeks ago I did try the 'urban legend' anti foul remedy of vaseline. No results yet due to the short time period.

Bruce,
When you scrape, do you just hold your breath or use other means of breathing under water?
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  #19  
Old 07-20-2009
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Yes, I just hold my breath. Sometimes if I want to be extra diligent I will use a snorkel and mask. Otherwise I just feel my way. Carefully! I guess gloves might be a good idea, but I do without.
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