diesel longevity at low speeds

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diesel longevity at low speeds

Postby Bajabuzz » Tue May 17, 2011 3:33 pm

Hi All, I attended a seminar at Trawler Fest Anacortes last week and Bob Smith was the speaker.
We got on the topic of larger higher HP motors being used at lower RPMs.
Bobs opinion was that this is not a good idea and the life of the engine will be shorter.
You cant seem to buy a trawler built in the last 15 years that does not have a higher HP motor in it.
My personal vessel is not a trawler (1988 Ventura 37 Convertible) and has twin 250hp Detriot 8.2t engines.
It does seem to get very good economy at 8 knots (2.5 - 3 nmpg) for this kind of boat. It burns 15 gph at 15 knots with a top speed of 19 knots. When I run the vessel at 8 knots for extended periods (3+ hours) and then get the boat on plane I do not see any smoke or visual clues the engines are loading up. 8 knots = 1,500 rpm on my vessel.
Anyone with personal experience with higher HP motors having increased failures due to running lower RPMs?

Thanks in advance for any help.
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Re: diesel longevity at low speeds

Postby NewMoon » Thu May 19, 2011 8:28 am

Our engine is a Volvo KAD44P-DP-EDC, an electronically controlled 24-valve straight six that puts out 260hp from only 3.6 liters, WOT 3900 RPM. We cruise at 1350-1400 RPM most of the time, pushing our heavy 26-footer at 6.5 knots, just fast enough to make the engine work a bit, and thus stay up in a normal temperature range. We run it at 3200-3400 and 16-18 knots from time to time. So far no problems running at relatively low power, in 13 years and 4300 hours.

My understanding from the VP pros is that long idling or otherwise running at very low power would be hard on it, as it would run too cold.
Richard Cook
New Moon (Bounty 257)
"Cruising in a Big Way"
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Re: diesel longevity at low speeds

Postby Bajabuzz » Thu May 19, 2011 10:23 am

Thank you for your reply.
Sounds like I just need to keep it in the normal operating range and run it up every few hours.
I posted this on boatdiesel as well and the consensus was the same as yours.
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