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Diesel overload article vs. Nordic policy?

Diesel overload article vs. Nordic policy?

Postby Ben McCafferty » Wed Sep 20, 2017 8:31 am

Hey all,
Just read a good article I had found http://mvdirona.com/TechnicalArticles/DieselEngineOverload/AvoidingDieselEngineOverload.htm on diesel overload; it is written by James and Jennifer Hamilton and refers to advice from Tony Athens. It essentially says that many manufacturers (over)prop their boats when new for maximum speed (for marketing purposes), based on the original weight and little fuel/water/etc.

It occurs to me that Nordic Tugs is more or less obsessive about their build quality and reliability. Since they are not a "go fast" boat, I'm curious what your experiences have been regarding this over-propping question. In other words, have they propped correctly up front, at the expense of a little top speed, to give their boats maximum life and reliability? Or have they fallen to competitive pressures, needing to say that their "fast trawler" can do 17 knots or whatever?

I'm really curious to hear your experiences, especially pertaining to how your tugs have performed when loaded and when the bottom is fouled. Have you still been able to hit max rated RPM? Have you needed to prop down? Any early failures or problems due to overloading?

Thanks for the collective wisdom, talk soon.
Ben McCafferty
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Re: Diesel overload article vs. Nordic policy?

Postby Bill Stone » Wed Nov 22, 2017 2:29 pm

Sorry for the slow response. we have 42-110 with a five blade prop, Cummins QSC8.3. Max rpm rated at 2600. Since new we have consistently gotten a max of 2680 out of the engine/prop combo. I have been told that it is preferred to be slightly above the max rpm rating.
As such, in our case anyway, believe the boat is propped right where it should be.
rgs, bill
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Re: Diesel overload article vs. Nordic policy?

Postby Gary Jones » Tue Jan 02, 2018 4:49 pm

An interesting worry! My experience is as Bill Stones - our NT 32 (270 hp turbo / after cooled) runs about 60 - 80 rpm over the 2600 wot rpm, and appears correctly propped. But the real worry with "tug" diesels is more of under working the engine, and getting carbonized heads (my Cummings is non-electronic) - my surveyor says this is one of the biggest causation of diesel overhauls in recreation trawlers.
As long as you're getting at least close to the engines WOT rpm, I would tend to not be concerned. These boats are well understood by NT, plus it is not in their culture to over-rate things - also it is not like a re-powering of a sport fisherman or other more exotic engine / hull configuration, where problems definitely can happen.
Gary Jones
Twilight Time (NT 32+ #261)
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Re: Diesel overload article vs. Nordic policy?

Postby Tom Easterbrook » Sat Jan 06, 2018 5:34 pm

My Tug, when I had her surveyed when we purchased, failed to meet the WOT specs by about 100 RPM. At the time, the mechanic said it was not a problem. The bottom was clean, fuel was almost full, with about 1/2 tank of water. However, after having read advise from Tony Athens of sbmar.com, I am probably going to get my prop "tuned" this spring during haulout. According to Tony, you should be able to easily reach the rated spec and better yet achieve about 100 rpm above.
Hope this helps :D ,
Nanaimo. BC
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