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Old 15-06-2016, 10:44   #1
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Idle Speed Cruising?

Strange question of the day:

I like going fast on the water and I spent much of my working life doing just that as a yacht captain and running crew boats in Louisiana, but someone else was always paying for the dead dinosaurs to power the boats.

I also don't mind going slow, and it has some advantages over going fast, especially if you're running in places like the ICW or rivers. You get to see things the go-fast boats miss. I once ran an old classic yacht that wouldn't go faster than 10 mph (not knots, mph)if it went over Niagara Falls and I did three trips from Cape Cod to Ft. Lauderdale in her. I also owned a 26-foot sailboat and if I could make good 5 to 6 knots I was feeling that I was making good time and now, since I'm retired I don't have to be anyplace at any given time..

I need to get back on the water again but with COPD and three stents in my coronary arteries I don't think I can physically handle a sailboat again. But in looking around for boats recently I've seen some real good deals on small powerboats with single Mercruisers and one with a big outboard.

Now I know all the bad mouthing that's done about trying to run a planing hull at displacement speeds, and that there would be no savings on fuel doing so, but here's my question:

Since I'm going to be running almost exclusively on the ICW and rivers like the St. John's in Florida and the Tenn Tombigbee I really don't need to be a speed demon. If a small powerboat like a Wellcraft or a Tiara, for example, could make five knots at idle speed wouldn't there be an appreciable savings in fuel costs? I mean when you run the ICW there are probably a few hundred miles where you're running at idle speed because of wake restrictions, anyway, so why not do it as a way of life? And you'd STILL have power to git up and go if you needed to to get into shelter from an approaching storm.

Any thoughts, folks? Is it practical?
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Old 15-06-2016, 10:59   #2
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Re: Idle Speed Cruising?

Yes a power cruiser at 5-6 knots can be pretty economical.
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Old 15-06-2016, 19:15   #3
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Re: Idle Speed Cruising?

A diesel engine would not like that at all, but a gasoline engine is a different animal and not mind it so much. Still, gas or diesel, all engines have a sweet spot where rpm's and torque cross which is usually the best fuel efficiency. I don't think any engine at idle is running anywhere near its fuel efficiency best-operating speed, so I would tend to doubt your premise. As an example, take a car at idle and let her roll. Her GPH will be much higher than if you are running the engine at it's most efficient operating speed. The same principle would apply to boats I would think.
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Old 15-06-2016, 19:22   #4
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Re: Idle Speed Cruising?

Have you considered displacement hulls?

First example I came across -
Displacement Power Boats to 30'~ Small Boat Designs by Tad Roberts
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Old 15-06-2016, 20:35   #5
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Re: Idle Speed Cruising?

Running at idle will shorten the life of your powerplant.

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Old 19-06-2016, 21:03   #6
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Re: Idle Speed Cruising?

Gas engines load up at idle from incomplete fuel burning. They generally are getting more fuel at idle and low rpm so they wind up quickly when asked to do so. Also more of the gasoline will get past the rings and into the oil. New, electronically controller gas engines may be better, but there goes reliability.
Diesels aren't bothered as much, especially mechanically injected engines. When I was a fisherman, I ran long hours at 700 rpm without problems.
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Old 19-06-2016, 21:14   #7
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Re: Idle Speed Cruising?

The answer of course is... run a smaller engine. Then you can cruise along at that small engine's most efficient RPM.

Like: For trolling a large bass boat might hang a small gas outboard on the transom.

You'll have the larger engine when you want/need the speed.
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Old 19-06-2016, 23:13   #8
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Re: Idle Speed Cruising?

I wouldn't pay too much attention to the people who say idling diesels is bad for them--with COPD and three stents your diesel is going to outlive you.

We did the Great Loop in a GB 42, which idled at 4 knots, but we typically went 7 in open waters and 6 in the restricted speed zones. If we went 8 knots, the mileage dropped about 40%.
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Old 20-06-2016, 04:41   #9
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Re: Idle Speed Cruising?

Quote:
Originally Posted by Yachts66 View Post
A diesel engine would not like that at all, but a gasoline engine is a different animal and not mind it so much. Still, gas or diesel, all engines have a sweet spot where rpm's and torque cross which is usually the best fuel efficiency. I don't think any engine at idle is running anywhere near its fuel efficiency best-operating speed, so I would tend to doubt your premise. As an example, take a car at idle and let her roll. Her GPH will be much higher than if you are running the engine at it's most efficient operating speed. The same principle would apply to boats I would think.
Neither engine type likes running at idle for too long. Of course, as long as you open them up every couple of days, it takes care of most of the issues.

Then again, you can always get a sail boat, leave the sails in bags and use the motor.
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