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Old 10-11-2020, 04:22 PM   #21
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I have come to that same conclusion as this discuss has gone on

I have gone through the Bahamas chart guides and they are way closer than I thought.

I carry 240 gals so I can go 200 miles on plane.

I agree with the real world testing suggestions and I am going to do 70 miles at Thanksgiving at 90% of hull speed and see how it does. It will be relaxing...
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Old 10-11-2020, 05:19 PM   #22
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I have come to that same conclusion as this discuss has gone on

I have gone through the Bahamas chart guides and they are way closer than I thought.

I carry 240 gals so I can go 200 miles on plane.

I agree with the real world testing suggestions and I am going to do 70 miles at Thanksgiving at 90% of hull speed and see how it does. It will be relaxing...
Can you achieve that speed in gear?
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Old 10-11-2020, 10:03 PM   #23
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Traveling at <90% of hull speed, especially by shutting one engine down and running on the other, should amaze you regarding fuel savings... compared to 100% hull speed running either one or both engines.
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Old 10-12-2020, 12:29 AM   #24
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One other follow up question on the idling comments..you are right that six knots is idle with both engines. Given I have the no drip, would you suggest I tie one shaft so it doesn’t freewheel. Is is rubber bungee enough? Seems that would break if I accidentally fired the engine up.
If securing a prop shaft for underway operation, a flat strap wound around the shaft multiple times. That is how the Coast Guard does it.

As for your question about your shaft seal: Refer to the seal manufacturer directions.

No, a rubber bungee is not enough.

As other mentioned, do tests to know the results.
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Old 10-12-2020, 12:41 AM   #25
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Traveling at <90% of hull speed, especially by shutting one engine down and running on the other, should amaze you regarding fuel savings... compared to 100% hull speed running either one or both engines.
We're talking about the boat in question running at less than 6 knots. I don't think he can do it for the hours required in ocean waters, including crossing the Gulf Stream. I think if he's in gear he'll exceed that. I also don't think his engines will come close to any theoretical efficiency at his slowest speed.

His Thanksgiving test will be interesting but even it will leave some questions as it's likely to be in protected waters, not in rougher seas or Gulf Stream, and for 14 hours and not for days. Even the no wake rules allow that you may need more speed, more power, to maintain headway and going from the US to the Bahamas, you definitely may.
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Old 10-12-2020, 07:43 AM   #26
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Traveling at <90% of hull speed, especially by shutting one engine down and running on the other, should amaze you regarding fuel savings... compared to 100% hull speed running either one or both engines.
This works on gassers, but the difference is much smaller with diesels, as they lose far less efficiency at light load.

Regardless, too far below hull speed on a boat that's meant to go fast can be a problem handling-wise in rough water. Even worse with 1 engine shut down and 1 rudder not having prop wash over it.
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Old 10-12-2020, 07:52 AM   #27
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We're talking about the boat in question running at less than 6 knots. I don't think he can do it for the hours required in ocean waters, including crossing the Gulf Stream. I think if he's in gear he'll exceed that. I also don't think his engines will come close to any theoretical efficiency at his slowest speed.

His Thanksgiving test will be interesting but even it will leave some questions as it's likely to be in protected waters, not in rougher seas or Gulf Stream, and for 14 hours and not for days. Even the no wake rules allow that you may need more speed, more power, to maintain headway and going from the US to the Bahamas, you definitely may.
True
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Old 10-12-2020, 07:54 AM   #28
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This works on gassers, but the difference is much smaller with diesels, as they lose far less efficiency at light load.

Regardless, too far below hull speed on a boat that's meant to go fast can be a problem handling-wise in rough water. Even worse with 1 engine shut down and 1 rudder not having prop wash over it.
True - Another plus for gassers!! Truly a trade off for everything...

"This works on gassers, but the difference is much smaller with diesels, as they lose far less efficiency at light load."
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Old 10-12-2020, 09:19 AM   #29
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True - Another plus for gassers!! Truly a trade off for everything...

"This works on gassers, but the difference is much smaller with diesels, as they lose far less efficiency at light load."
It's not a plus, honestly. It's a massive negative for gassers. Running slow with 2 diesels running will typically still burn a bit less fuel than the same boat with gassers and 1 shut down.
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Old 10-12-2020, 12:12 PM   #30
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Yes
But there are a lot of variables.

How close the props are to the keel/ctr.line.
How big the rudders are.
How big/long the keel is. (directional stability)
Which way the props turn.
Windage from cabin/bridge ect.

Re one engine a list the beneficial way may help.
But the biggest variable will usually be prop spacing.

Depending on how much you wish to run single perhaps you should consider a single ... or a single engined FD boat.
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Old 02-07-2021, 08:34 AM   #31
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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...
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Old 02-07-2021, 11:02 AM   #32
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Miami to Bahamas is like 60 miles with 200 plus gal. How could you not make it?
This is my thought too!
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Old 02-07-2021, 11:20 AM   #33
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This is my thought too!

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).
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Old 02-07-2021, 11:45 AM   #34
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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.
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Old 02-07-2021, 10:06 PM   #35
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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/B...15mHp_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.
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Old 02-07-2021, 10:18 PM   #36
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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
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?
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Old 02-08-2021, 12:30 AM   #37
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Wifey B: Going slow in a fast trawler.

Sounds too much like this:

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Old 02-08-2021, 07:31 AM   #38
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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.
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Old 02-08-2021, 10:51 AM   #39
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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.
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Old 02-08-2021, 12:35 PM   #40
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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.
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