Going slow in a fast trawler

My “trawler” has a theoretical hull speed of just under 10 knots. I’ve got two 1350hp V12 diesels.

At just under 9 knots I’m burning 40lph (10gph) giving me 0.85nmpg. If I bump up to ten knots my total consumption nearly doubles, dropping my efficiency to 0.5nmpg.

Or course if I’m in a hurry I cruise at 23 knots, you really don’t want to know the rate then...

High Wire said:

Miami to Bahamas is like 60 miles with 200 plus gal. How could you not make it?

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This is my thought too!

Woodland Hills said:

This is my thought too!

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Also agreed. Even with my boat I'd have no concerns about making the Bahamas run up on plane. For the closer islands, I could run there and back in a day and have enough fuel for the round trip. And my range at fast cruise is shorter than most on here (I figure about 160nm with 25% reserve plus a few unusable gallons).

I am flabbergasted by the 7 Kts fuel chart.

Before doing anything do some test runs and verify that figure. My semi planning 38,000# Loaded 42' boat got about 2- 3 NMpg at 8 Kts.

Also try 6 kts Something not right.

I'm all for going slow and sipping fuel. My first inclination was to say it's futile in this boat.

Then I went snooping and found some data that might be relevant

https://www.backcoveyachts.com/pdf/BC30_Yanmar_6LPA_STP_315mHp_E001.pdf

Interesting that the Back Cove 34 with a single has performance roughly similar to the Sabreline 34 with two of the same motors. If you take that chart and halve the mpg and gph numbers I think it'll apply to the OP's boat pretty closely.

It looks to me like 3+ mpg is available at 1200 rpm or thereabouts.

Try it out.

That said, I would do all my trip planning at 1 mpg and plan to get back and forth across the gulf stream at speed.

Edit: just checked the link and its a Back Cove 30. Close enough.

boating rich said:

When we do our next local trip 50-70 nm, I will give six or seven knots a try with full tanks. Do you think I should try six or should I see enough extra fuel mileage by just going the hull speed. I guess this is one example where analog engines can’t give you instant answers like their digital new ones. Not that I am complaining

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I just reading through the thread now late.....
but a thought on how to more quickly and perhaps more precisely test different power settings would be to temporarily feed from a small tank, where your quantity before and after can be accurately measured...or even weighed to get really precise....like say a gallon or three?

Wifey B: Going slow in a fast trawler.

Sounds too much like this:

The distance may be 60nm from FL to the Bahamas , but that is a straight line distance , it might be near 80 or 90 miles as the Gulf Stream is being crossed at 6K.

Going slow in a fast trawler is ....

Kinda like going to the bank to check your balance in a dump truck.

You’ll get there but prolly wish’in you had driven the Corolla.

Ignore the hull speed BS.

The fastest economical speed will be the speed below the point where the bow begins to lift and waves form at bow.

Some folks use "hull speed" generically, when they actually mean "maximum theoretical hull speed" -- aka 1.34 x SQRT(LWL).

Beebe makes a point that best economy is often down around 1.0x to 1.1x versus 1.34x.

FWIW, we found decent economy -- with our sorta-faster definitely-not-a-trawler by going "slow." Generally our slow cruise was more RPM-driven, not so much speedometer-driven. That usually meant anywhere between 7.5 (1.0x) and 8.5 kts, anywhere between 900-1200 RPM, depending on wind and current. We could approach 2 NMPG in best of circumstances, but it was more often in the neighborhood of 1.25-1.5 NMPG... while we enjoyed the coffee.

-Chris