6BT370 – propping advice

[Woo]

Last year I relaunched a boat after a refit and fitting it with a pair of reman program 370B engines. The boat is planing design, quite narrow, very deep V, and quite heavy, and doesn’t carry heavy fishing loads particularly easily do to the nature of the design (narrow).

Prop guys were advised “355hp @ 3,100rpm” and upon relaunch, and we were close, achieving 3,050 on one, 3,080 on the other (corrected to accurate).

Of course, they seldom get lighter or a cleaner bottom, so it fairly quickly got back to juuuuust on 3,000rpm. Maybe a touch lower on one, touch higher on the other. This is in a typical “as used” heavy state. EGT is a lot higher than I’d like @ “fast cruise” rpm of 2,500. I know I’m overloaded, and want to correct it.

Now, with a filthy bottom (very bad, and an extreme hopefully never to be repeated level of filth) its not getting anywhere near even 3,000, so we are slipping it ASAP.

What I’d like to know is what does the brains trust think? I am thinking ask the prop guys for at least 150rpms. You guys might think more rpm are warranted? I’m not “afraid” of rpm, just wanting to get the most from the engines for the longest period of time.

Topic

[Woo]

No not at all Bill.
Our rudders, shafts, and props are all covered in PropSpeed.
However, it’s been an extreme season for growth in our marina, and a shortfall of usage hasn’t assisted with keeping it clean.

1 user thanked author for this post.

Bill Desmarais

[Bill Desmarais]

Curious,

Is anti fouling applications for marine growth prohibited in Australia?
There is some decent products for metal on the market.

[Woo]

Thanks gents. To clarify, she’s not struggling at all to “get on top”, in fact quite the opposite, it is a bullet up onto plane and feels exceedingly sporty. But, the boat loses speed readily. The running gear on this is over-built and when it gets worm growth, you feel it….. Shafts are 2″, and the rudders are deep and large, that’s just the way the builder specced them.

That said, we undoubtedly have more boat than we expected to have, and indeed, more than I was aiming for. Those last minute decision to have granite bench tops and a huge house battery bank etc got it heavier that planned.
Props and weight expectations discussed earlier in the rebuild process. My memory may be a little sketchy and I could well have asked them for 3,150rpm, but the weight got me.

I’m not struggling for 2,900 at working weight, it is getting just over 3,000 – but I am way low on rpm (maybe down to 2,850 – I don’t risk holding it flat to see) with it currently as it is now with a filthy bottom. It’s a worst case scenario and is not working or going anywhere until cleaned.

With reply to Rob, it is impossible to know how a particular vessel will be affected by marine growth over time. Some suffer effects far more readily than others and this one is the latter. I feel I need to get another 150rpm and use it more often.

[Bill Desmarais]

This topic brings up a good question about the the “governors” on the mechanical B’s.

When I repowered the 25 T Jason lobster boat I ( Tony engineered the power train for me) we chose the ReCon 6BTA 370 with a ZF63A gear, 1.5:1 ratio spinning a 20X20 DQX prop with a 1.75″ shaft.

370 hp for this particular 25 ft. downeast lobster boat style hull was overkill but the 370 is the “best bang for the buck versus the 270 or 330 versions and I wanted the extra rpms (3,000) “just in case” to run @ 30 knots. The ZF63A gear has a max diesel hp limit of 315 hp and I could not fit larger than a 5″ exhaust in the hull with a muffler so we limited the max hp by grossly under propping the engine.

The plan was to “lower the max hp” of the 370 thru purposely under propping the engine to 3200 rpm.

The sea trial WOTs rpms photo tached out at 3090 +- (not 3200 as planned) and EGTs never got above 700F.
Yes the 370 was de-tuned according to the EGTs.

The boat scooted right along with this setup. (20X20 prop). To be absolutely safe and sure the engine was fully “de-tuned” I had the prop shop take an extra inch of pitch out and now the prop is 20X19. Boat runs perfectly and plenty fast for my needs and is the fastest 25 ft lobster boat around.

With the 20X19 prop photo tach rpms did not go up…still around 3100 rpms. EGT is never higher than 700F 2500-3100.

I talked with Tony about this and he told me although the governor will allow the engine to spin up over 3300 + rpms with “no load”, If the governor senses a “load” the engine wont spin much past 3100 no matter how much it’s under propped.

This is my understanding of the “governor” on the mechanical 6BT/BTAs.

Bill D

[Tony Athens]

Here is another way to think about a boat that seems to harder & harder to get up on top as time goes on…………It’ basically impossible to under prop these engines as long as once you are up on plane you are happy with the vessel speed at 2600-2700 RPM or less.. But as you have seen 1st hand, they are very easy to over prop them……….If you now barely squeak 2900 RPM (or less) under your actual working weight, you are still grossly over propped.

Until you can make over an accurate 3100+ RPM under these conditions & make that EASY, you need to realize that more prop work needs to be done, or you need to start fresh with new props.. Also, it would not be the 1st time that someone may have too much boat and not enough engine vs. their need for speed.. Reaching that over 3100+ RPM’s is critical to good life & performance with these engines, and then running at 2600-27 RPM or less and meeting you need for speed is vital ..

[Rob Schepis]

It sounds like you gave them tight specs – 355hp at 3,150 on a hard to plane boat would have been safer – and the prop guys fell short of your tight spec, so a “double whammy” .. You want to make 3,125-3,150 and make it easy under all compounded dirty/loaded conditions. You state you are no where near 3,000 now so won’t another 150 still fall you short?

I’ve seen pics of your vessel, I know she is “tip top” in many ways…

[Author]

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