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post #25 of Old 06-01-2011 Thread Starter
sandycohen's Avatar
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Hi All - First, thanks again for everyones input.

Two things to add to the thread.

First, I opened the boat to full throttle in idle to see what max RPM I could get to. The answer is about 3400 and the throttle control is hitting the transmission lever which in neutral is right behind the throttle control which in the real world wouldn't be a problem since it isn't necessary to go wide open in neutral. While trying to inch the transmission control down to find the end of the throttle travel, i accidentally (although I guess I should've seen it coming!) dropped the tranny into reverse at about 3500rpm - probably did more damage then the rest of this issue ever will (seriously, does anyone think that one time shift is something i need to worry about or do anything to check damage - I shifted in idle several times fwd and rev after and all seemed fine).

So, it appears in idle the engine can hit max rpm's or something every close to it.

I did get inky water in idle running high rpms the first time I opened it up wide. The 2nd time I did it againt o try and find the rpm where it starts and it didn't turn inky, but i would say the water still had a grey tint and there was greyish exhaust smoke.

To point out something else I've never said before, I normally get NO exhaust smoke, and even when i have inky water there isn't always any visible exhaust color.

The next thing I want to report is that the PO responded to an email from me asking him if he'd ever seen inky exhaust water, what his cruising rpm targets were, and if he purposefully overpitched the prop - response is below in a couple of threads:

From PO in reply to a voicemail I left him:

Anyway, black exhaust is usually caused by unburned fuel, in turn caused by engine overloading - for instance, the throttle is pushed all the way up calling for more engine speed, but the engine just can't get there because of big waves, heavy load, or the displacement hull just won't go any faster.
I think you mentioned something about max rated engine RPM. You are only going to get there if the prop is perfectly pitched for the particular boat and engine combo, which can be hard to achieve in the real world.
So, in reality, if you see black exhaust water throttle back just a little and it should go away. If you want to pull the exhaust elbow it can't hurt anything, but it might turn out to be a PITA job. Hope this helps.

My reply:

Basically since I had the boat I had been cruising around 2200rpm – I think maybe this is what you told me was your guidelines. Then I read things online that says the engine wants to be run at 80% of the rated continous rpm, which would mean a cruising rpm of something like 2800-3000. When I started doing that I got the inky black exhaust.

I’ve posted the issue on two message boards and gotten a lot of great advise. I didn’t think the issue could be pitch (what people call being overpropped I guess) since you had bought the prop special I figured you got the appropriate size one. Some people did say that some owners intentionally overprop the boat to reduce noise and fuel consumption by running lower rpm’s – did you do that on purpose?

His reply:

Yeah, the boat might be a little overpropped, but not on purpose. I installed the prop the dealer recommended, and it wasn't worth it to me to haul the boat back out, send the prop back, and install another one to get the pitch just perfect. I think 2400 RPM or so would be a good long-distance cruising speed and 2600-2700 if you were in a hurry and not too concerned about fuel consumption. It's not so much percent of rated RPM that you are looking for, as much as percent of rated power output. If the prop is perfectly pitched, the two go hand in hand, but if the pitch is a little high then you get higher power output at a lower RPM

I bought the boat largely based on my experiences with the PO and knowing that he is both more knowledgable on all systems than I am, and more careful/concientious I suspect than I am on taking care of things. So I am going to basically now use his RPM guidelines, maybe check the elbow when I get a chance, and forget about it! But, since everyone got behind me to try and figure this out, I wanted to give you more info, and if you want to keep discussing or have other thoughts, please share.

Disclosure, the PO is now probably reading this thread so be diplomatic.
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