Feathering prop?

[Ski in NC]

Second the above. A twin running on one will have asymmetric thrust and will need rudder to hold heading. Boat will be going a bit sideways in the water, along with a cocked rudder, neither of which is good for efficiency.

And the folding/feathering/etc props will not have the best blade shape for efficiency.

If you want maximum range, get a single engine. Carry tools and spares and it will be plenty reliable. Want more reliability? Get a single with a wing engine. Put a folding prop on the wing.

 

[DDW]

The drag of spinning vs. braked prop has been extensively tested by the sailboat crowd. The answer is "it depends". Number of blades, solidity ratio, drag on shaft, all have an effect. On a 4 blade high solidity powerboat prop, spinning might be better. A braked prop on a power twin has another effect, very disturbed flow over the rudder behind it, which might make half of the steering ineffective.

 

[garycasey]

We used to cruise on one engine when we need extra range and it made a significant difference. We went from 7 knots on two engines at 1200 RPM to 6 knots on one engine at 1200 RPM. Fuel flow from 12 LPH to 6.5 LPH.



The WOT for our engines was 3000 RPM so a load was not an issue at all and our transmissions did not require the engine to be running while free wheeling.


We used to alternate the engines about every 4 hours to keep the operating hours and wear the same. Did hundreds of hours like that.

I agree with Jetstream in that his numbers are about right and the logic behind it works. He goes from 1.7LPM at 7 kts to 1.1LPM at 6 kts. Sure, some of the gain is just due to the speed reduction, but certainly not all. It would be interesting if he had done the same thing, but adjust the speed of the working engine to maintain the same boatspeed. I don't have any data, but I'd be willing to bet that at those low speeds half the fuel consumed just keeps the engine spinning, with only half being used to propel the boat. That's the big reason to run on one engine, and a lot of people do it. However, there is no free lunch. The propulsion efficiency will drop when running on one engine. That is because the total effective prop area drops in half. The efficiency (reciprocal of slip) will be about 65% on two engines, but maybe more like 55% on one. So you gain a lot on engine efficiency, but lose propulsion efficiency. Sure, a feathering prop has less drag than a freewheeling prop, but its driving efficiency would be less. A complex tradeoff.

 

[clyde]

...a feathering prop on a power boat? reason? you gonna add a mast too?...clyde

 

[ProMaritime]

When I checked into it 8 years ago the MAX prop for a 47' sailing cat was $2500 each whereas the fixed prop was about $500 each. Now I understand the MAX for the same boat is about four grand.

I'd like to put them on my 47 power cat for long-range cruising but I understand that to get the most efficiency your need to extend the rudders due to the distance between shafts on the cat.

I have a friend who shingle-engined a couple of trawlers from Hawaii to Seattle by taking one prop off and then putting on scuba gear and switching to other shaft enroute. He got unreally range on internal fuel.