Extend range, lower fuel burn, folding props?

Hi, does anyone have any experience of folding props on, for example twin Cat 3208TAs?
My thinking is that running one engine instead of two would increase range dramatically at displacement speed and the folding props would eliminate any drag or risk of damage to the idle engine and gearbox.
Has anyone done this?
Is there a good reason not to look into this?
Thank you all.

Folding props are mostly designed to give lowest drag under sail.

Their ability under power is less a concern.

The Luke feathering prop is the only exemption to this problem , as it was designed way back for large motor sailors.

The proper , but quite expensive solution is a CPP .About $10K each and a heavy yard bill for a complex installation.

IT would never pay .

Automatic Feathering Propellers - Paul E. Luke, Inc.

www.peluke.com/marine-hardware/boat-props/
Automatic Feather Propellers, P. E. Luke. Each propeller is custom crafted from a heavy duty aluminum bronze alloy. Full service boatyard, East Boothbay, ...

Re

The same principles apply surely?
We have a sailing catamaran and have folding props.
When one engine is running the other prop folds to reduce drag and we cut consumption.
The only difference is the hull shape.

A boat under power requires a certain amount of thrust to reach any given speed. You can get the required thrust from one or two props, but a power boat will use almost the same amount of fuel in most cases. If using just one prop, then the engine turning that prop will will have to run at higher rpm and be more loaded, and it will use more diesel, if you want to travel at the same speed.

I discovered this after sea trials for my OA50, but it is not at all surprising when you think about it in terms of getting the thrust required. For my heavy SD hull plowing through water the little bit of extra drag from the unused prop/strut was not measurable to one decimal place in gph consumption for speeds in 6-8 knot range.

Now, for very slippery and long skinny hull, which could include quite a few catamarans, the prop drag could well be a significant proportion of total hydrodynamic drag, which of course increases rapidly with speed. You may well see a measurable difference. But a 'dramatic' difference? No.

Going slower always saves fuel, and that can be a dramatic difference. Now, as you slow down you will reach a point with twins when the engines aren't turning fast enough or sufficiently loaded to reach a minimum operating temperature. At that point just running one does make sense. For my sea trials that was under 6 kn, and in practice that is something I normally would not do.

Many of the wing engines on single engine trawlers have either folding or self pitching props to reduce drag. Bay Pelican has a self pitching Max prop on its wing engine.

.but a power boat will use almost the same amount of fuel in most cases.,

Engines selected for twin engine boats are frequently bigger for higher speeds , the fast trawler concept.

Even for just a displacement style hull, at modest speeds the fuel burn with a single will be better as the loading on the engine is higher and therefore more efficient.

The problem is the twin prop either locked or worse allowed to rotate is an amount of drag equal to a speed brake dragged of the same diameter.

30 inch prop? . drop a 30 inch diameter pail over the side , tie it on first.

The reason most boats show zero improvement is the better fuel burn in offset by the higher drag.

One experiment could be to simply remove one prop, and do a graph of speed vs fuel burn with a Flowscan,

Bay Pelican said:

Many of the wing engines on single engine trawlers have either folding or self pitching props to reduce drag. Bay Pelican has a self pitching Max prop on its wing engine.

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Just a point of clarification- Max Prop is not "self pitching." All Max Prop models must have their pitch set manually. This does not change unless intentionally altered. Auto Prop can be considered self-pitching however.

amelis,
Check the archives. There's much much more about this.

Perhaps you should consider a different boat that has a better range?

But removing a prop for a specific trip works well .. so I've read. And of course installing two smaller engines w half the power would work even better but $$.

Actually I know a guy that delivered an old Hatteras that had one bad engine. After a few hundred miles, they were in a place where they could get a diver to pull the prop off the bad engine, both to cut the drag and so they didn't have to put something on the shaft to keep it from turning (Allison trans requirement). They went several hundred miles further, including a Gulf crossing. At any given speed, below hull speed (could get the boat to plane on one) no noticeable difference from when the engine was later functional, and only a nominal but measurable difference with the prop off. This was a big heavy boat, to be sure.

I once ran a Mainship 430 with 3116 Cats , equipped with FloScans about 150 miles on one engine (due to a neutral switch failure I couldn't figure out), a boat we chartered many times and put many hundreds of miles on. Again, at a given speed, no noticeable fuel consumption difference. What it would be with a feathering prop, I don't know for sure but I can't ever seeing getting a payback on the retrofit. There is a reason you don't see recreational twins fitted with them. A 30" bucket will have much more resistance than a 30" prop, BTW, but it would still be interesting to drop a series of five gallon jobs over the back and see what if any effect there was.

The question of folding or feathering prop is moot for those engines. Nobody makes such a prop that can absorb the 375 hp that they put out.

And FWIW in many cases the fuel savings are minimal. My favorite example is Bob Lowe, the former PNW boat yard owner and owner of a restored twin engine Grand Banks. He published detailed data on rpm, speed, fuel burn with one and two engines, with the dead prop freewheeling and fixed. There wasn't much difference, but in all cases the boat used more fuel AT THE SAME SPEED with one engine. Here is the link Dreamer

There is an apocryphal story about a guy who cruised his twin Grand
Banks from Florida to California. To save fuel he removed one prop, cruised half way, shut down, dove, switched props and finished.

Sailboats usually pick up 1/2 kt or so while sailing with a feathering prop so I suspect a feathering prop would show some improvement on a twin engine trawler, maybe 10% over a dead prop, but not for your engine.

David

Nobody makes such a prop that can absorb the 375 hp that they put out.

Click to expand...

Actually, not true, unless you are talking about the type that folds up perfectly smoothly, and that might still not be true, I just can't think of one off hand. . They use controllable pitch props on some ships, for that matter. Do a search, most are from Europe.

Edit: whoops, that didn't take long..

http://www.varifold.info/

I think everyone can accept that the only way to save fuel is to run the boat slower, do not expect one engine to burn less at the same speed. One way to run slower in a twin engined boat is to shut off one engine. Now to reduce drag of the stopped propeller fit a folding Max prop. A pair of 28" diameter three-blade max props will be $17,000.00 plus installation. You could just buy one and switch them occasionally to equalize engine hours. But even $8500 will buy a fair amount of fuel.....

djmarchand said:

There is an apocryphal story about a guy who cruised his twin Grand
Banks from Florida to California. To save fuel he removed one prop, cruised half way, shut down, dove, switched props and finished.


David

Click to expand...

I heard the same story, but it was California to Hawaii in the version I saw.

ronlord said:

I heard the same story, but it was California to Hawaii in the version I saw.

Click to expand...

David Butler made two singlehanded crossings from Hawaii to Seattle. The first was in Iolani, a GB42 woody, in 1992 he brought her to Seattle to sell. Passage time was 15 days 9 hours, running at 7 knots with one Lehman at 1500 RPM and the other shut down with prop off. He carried an extra 400 gallons of fuel in 5 gallon cans (!) and had two days fuel left when he got to Seattle. He stopped mid-passage and changed the prop from one shaft to the other, to equalize engine hours.

In 1996 he made the same trip in a 60,000 lb 47' Cheoy Lee called Holokai. She carried 2000 gallons of fuel for twin Perkins power and he didn't need to shut one down, at 7.5 knots she burned 75 gallons a day.

The article covering this is in PassageMaker Vol 2, No 3 (Fall 1997).

Bruntons Autoprop, they talk a good show but very little hard information is available (one Passagemaker article years ago.) Also very expensive. You would think if they were trying to market them to powerboaters, they would do some real testing...

fstbttms said:

Just a point of clarification- Max Prop is not "self pitching." All Max Prop models must have their pitch set manually. This does not change unless intentionally altered. Auto Prop can be considered self-pitching however.

Click to expand...

Guess I must have meant self feathering?

djmarchand said:

The question of folding or feathering prop is moot for those engines. Nobody makes such a prop that can absorb the 375 hp that they put out.

And FWIW in many cases the fuel savings are minimal. My favorite example is Bob Lowe, the former PNW boat yard owner and owner of a restored twin engine Grand Banks. He published detailed data on rpm, speed, fuel burn with one and two engines, with the dead prop freewheeling and fixed. There wasn't much difference, but in all cases the boat used more fuel AT THE SAME SPEED with one engine. Here is the link Dreamer

There is an apocryphal story about a guy who cruised his twin Grand
Banks from Florida to California. To save fuel he removed one prop, cruised half way, shut down, dove, switched props and finished.

Sailboats usually pick up 1/2 kt or so while sailing with a feathering prop so I suspect a feathering prop would show some improvement on a twin engine trawler, maybe 10% over a dead prop, but not for your engine.

David

Click to expand...

David
Good info and link. I decided to plot Dreamer's info as a simple chart of speed versus NMPG. And then I added data for Insequent. Same story - running both engines gives improved fuel economy. Note that I did need a 9 deg rudder angle to go in a straight line with just one engine, no doubt part of the explanation for the poorer performance on one engine.

Insequent does better fuel economy due to longer waterline length and modern Tier II engines that have an electronic governor.

The only reason to run just one engine is to go really slow for extreme range.....