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Old 03-06-2011, 21:31   #1
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White Smoke

I need some advice. I have a Niagra 31 with a GM20FC Yanmar engine and SD20 saildrive. There is only 350 hours on both. I recently replaced the 2 blade folding Volvo prop with a 3 blade Flex-o-fold. On the first trip I noticed that there is a continuous but small amount of white smoke at cruising rpm's. I spoke with the mechanic at our marina who told me that most boats smoke. This answer did not inspire much confidence so I am reaching out to the boating community for help. Should I be worried?? The book I have on diesel engines speaks about lack of compression, air in fuel lines and unburnt fuel. I cannot see it as an engine problem as it still runs as smoothly as before switching props. I am thinking it has to do with the pitch of the blade...which I have to confess that my understanding of is a bit weak. I had given the manufacturer all the specifications and they had assured me that they supplied other owners of Niagra 31's (same engine and saildrive) with the same prop. Should I be worried? Is their something wrong? Other than this I love this prop...help! Thanks ahead of time.
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Old 04-06-2011, 00:30   #2
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Re: White Smoke

The first eliminator for belching White smoke - water giving off steam - water is mixed with the exhausted gasses following combustion.
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Old 04-06-2011, 00:31   #3
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Re: White Smoke

Besides steam, white smoke is also an indication of a lean mixture
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Old 04-06-2011, 00:48   #4
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Re: White Smoke

David, The GM20 FCs have a Swirl type combustion chamber - at 350 hours - Lean mixture on this type of engine ? how does that occur ?
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Old 04-06-2011, 00:56   #5
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Re: White Smoke

I'm sorry, I don't know the specifics of your engine. I just know what white smoke might indicate.
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Old 04-06-2011, 01:42   #6
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Re: White Smoke

A diesel with white smoke indicates water in the combustion chamber. How much white smoke is the question. A little water in the fuel will do it. So will a bad head gasket. From there it goes on and on.

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Old 04-06-2011, 01:49   #7
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Re: White Smoke

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Originally Posted by Ironhorse74 View Post
A diesel with white smoke indicates water in the combustion chamber. How much white smoke is the question. A little water in the fuel will do it. So will a bad head gasket. From there it goes on and on.

Brad
Thanks Brad,
In the tropics during the rainy season - very high humidity will mean the diesel's air intake will contain water vapour - and the exhaust will emit White Smoke = steam.
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Old 04-06-2011, 02:31   #8
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Re: White Smoke

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Thanks Brad,
In the tropics during the rainy season - very high humidity will mean the diesel's air intake will contain water vapour - and the exhaust will emit White Smoke = steam.
My weather at the moment :-
Max Temperature 31 - 33 C
Min Temperature 24 - 26 C
wind speed 15 - 35 km/hr
Relative Humidity 80-90 %
Amount of rainfall 260-320 mm
rain 18-22 days this month.
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Old 04-06-2011, 03:10   #9
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Re: White Smoke

Check the raw water impeller. If it is damaged it may not be pumping enough water through the system. The raw water can reach boiling temperature and show as white smoke. Good luck.
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Old 04-06-2011, 05:20   #10
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Re: White Smoke

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Check the raw water impeller. If it is damaged it may not be pumping enough water through the system. The raw water can reach boiling temperature and show as white smoke. Good luck.
If the raw water supply is insufficient , then the engine's heat exchanger will not cool the engines fresh water supply. Then the engine's temperature gauge will show a very high reading ??? Not so?
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Old 04-06-2011, 06:03   #11
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Re: White Smoke

A certain amount of steam escaping with the exhaust is perfectly normal. My first guess would be that the new prop has more cut to it and the engine is working harder, thereby putting out more hot exhaust and more accompanying steam.

Does the boat go the same speed at the same RPMs as before? That would be the first indicator that you have a somewhat different pitch to the prop.

Is this the first time the engine has been operated in a while, or was it running shortly before changing props?

When was the impeller last changed?

I wouldn't go tearing anything apart before sorting more things out. Most importantly, watch the engine temp. That will be the first indicator of a cooling system problem and/or engine overloading due to over proping.

Do you have the specs for the old and new prop?
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Old 04-06-2011, 07:58   #12
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Re: White Smoke

Hi Twistedtree,

The engine has been run regularly and there was no white smoke prior to the prop change. The boat goes faster with lower rpm's with the new prop. I use to do 5 knts with 2800 rpm and now I do 6+ with 2500 rpm. The impeller has around 25-30 hours. This engine does not have a temp guage just a temp alarm. I do not have the specs on the old prop.

I have taken the boat out on two trips since the prop change and there is no change in the amount of white smoke. At first I thought is was just exhaust steam as that is about how much white smoke there is but I can smell the diesel in it.

For maintenance I change the oil at the beginning and end of the season. I run the engine regularly over the winter and keep the tank full as well. BTW the tank was also changed when the engine was put in.

To me it seem to be related to the prop pitch...maybe the engine does not have to work as hard?? I just worry I may be damaging the engine.
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Old 04-06-2011, 08:05   #13
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Re: White Smoke

have you discussed this problem at length with your yanmar dealer/distributor?? i have yet to meet a diesel that is left for any time at all on water without having white-grey smoke come from exhaust. mine does that, my perkins does that, also.make sure all is in good shape and run the engine under load for a while. smooth sailing.
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Old 04-06-2011, 08:13   #14
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Re: White Smoke

The white smoke does have the appearance of exhaust steam as it dissipates a couple of feet from the transom yet it does have a diesel smell to it. I will try to contact a Yanmar dealer. Thanks.
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Old 04-06-2011, 11:02   #15
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Re: White Smoke

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To me it seem to be related to the prop pitch...maybe the engine does not have to work as hard?? I just worry I may be damaging the engine.
By far, the change in prop pitch is the most likely culprit since that's what changed. Your change in RPM vs speed confirms that the pitch is different, but from what you describe (boat moves faster at same or lower RPM), the engine is working harder, not less. It has to produce more HP at lower RPMs to make the boat go faster than it did before.

Whether the engine is overworking, I don't know. On power boats the test is to confirm that you can attain full rated engine RPM, preferably a few hundred RPM more. That tells you that the load from the prop matches the output capacity of the engine. If you can't hit full RPM then you are over-propped. I can't see why that test shouldn't apply equally well in your case, and it's real easy to do.

Regarding the smell, your exhaust should smell like diesel - that's pretty obvious. I'm not sure how you would tell if it's the visible gas (steam presumably) or the invisible gasses in the exhaust that you are smelling. So the smell may not tell you much. Is it noticeably smellier than before? More smell would actually be consistent with the engine being more heavily loaded. If you are getting black smoke then you are definitely over-propped, but it doesn't sound like that's the case.

I'd do the full-RPM test and take it from there. Everything points to this simply being steam in your exhaust. As long as you are not over-propped with the new prop, I think you are good to go.
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