Utilizing just one screw for increased range

[Kquaine]

Hey everyone - in an effort to get better fuel economy and therefore greater range, would operating on just one screw help? My thought is instead of operating 2 screws at a low rpm, maybe 1 screw at medium rpm might be more economical and provide for longer range. Also running a single screw would run that engine at a higher temp, minimizing carbon build up. Obviously the net fuel usage with 1 engine at the desired over the ground speed needs to be less than 2 engines at the same over the ground speed to save fuel. I am putting on fuel flow sensors to provide the fuel usage data. I have a 1988, 67’ CPMY with 12v71 with turboMy main questions are as follows:1) Has someone already done this experiment and what were the results?2) I am assuming that the unused screw will be back driven. Is there any issues with that? Transmission cooler? Gear wear? Etc.3) Other concerns?Thanks for reading and any useful input you may provide.

 

[Pascal]

I only ran one long trip on one engine, with an 84 footer a few years ago after a shaft snapped in the Exumas. I looked at my fuel log and we got 0.52 nmpg on one engine on the way back vs 0.50 on both. And that was with the prop on deck so no drag… since the boat had electronic C32s the burn numbers are very accurate

If it takes X hp to move the boat at Y speed, it will take pretty much the same amount of fuel to make that HP whether with 1 or 2 engines. Yes the second engine has losses due to friction etc but the drag of the dead prop plus the angled rudders waste a lot of fuel.

The only way you will save fuel is if you slow down way below hull speed and in that case running on one engine will keep it at higher temp.

Most transmissions incl Alisons require the shaft to be locked.

 

[Glory]

If Pascal's prop was turing i would bet the burn would increase a bit. The bigger concern is the lube for the trans on the engine that is not running. Engine not running trans pump not running, and the prop is still turning. There would be some oil splash, some rotating equipment is particularly submerged in oil, might be ample, might not.

 

[Captain Ralph]

Read all the material from the clutch mfg. Our old TD514s recommend filling the case full of oil since the oil pump will not operate without the engine running.

I believe the extra drag from the rudders trying to keep the boat straight, forcing extra drag, pretty much kills this idea above the drag of a locked or even a free spinning wheel.

I remember reading about trawlers using a wing engine to get home. First concern when back in port; Any damage to the mains clutch before the question of what was wrong with the main....

 

[Sadey]

You can buy a lot of fuel with the money it’s going to cost installing FF gauges as well.

 

[whoover]

You can also buy alot of fuel with the money it takes to rebuild burned up gears. If you require more range maybe extra tankage would work for you.

Walt Hoover

 

[Bob Quinn]

There is a lot of talk on here about fuel usage right at hull speed. Not a lot of talk about when you back down from that speed, but I have a feeling the fuel usage decreases quite a bit if you back off 2 or 3 knots. That might be worth researching.

on our 53 most people quote 1 NMPG at hull speed. We actually got 1.25 on our maiden 400 mile voyage home. We ran around 1000 or 1200 rpm which I think kept us under hull speed just a small amount.

Trawlers you might notice run more in the 6 to 8 Knots per hour range. There has to be something to that (hulls are different of course too).

I know there are calculators out there that predict HP usage for different speeds.. That would give a good idea on fuel use. I know engine temp is a concern though too.

I'm probably stating the obvious but it would be interesting to know the effects of backing down.

 

[Pascal]

Slowing down from HS makes a huge difference, the problem is that extended running a very low RPM isn’t ideal for the engines due to low operating temperature.

I don’t have exact data for our Hatts since they rarely have flow meters but I have run a number of boats with electronic engines and accurate built in flow meters. No trawlers, all planning boats. in every case slowing down down from hull speed by 20% cut fuel burn by half. Huge difference.

But again the problem is keeping the engines hot enough. If you really want to extend your range on a long trip by slowing down below HS, running on one engine will work although the fuel savings will not be from not running the second engine but from the lower speed

 

[jim rosenthal]

I think the basic problem with this idea (running on one engine) is that these boats were really not designed to operate that way. If you have no other way to get home, you have no choice, but I think trying to run the boat on one engine creates more problems than it is worth.

 

[Pascal]

I think the basic problem with this idea (running on one engine) is that these boats were really not designed to operate that way. If you have no other way to get home, you have no choice, but I think trying to run the boat on one engine creates more problems than it is worth.

These boats handle very well on one engine thanks to the keel. You can get going in a straight line from a dead stop on one engine with opposite lock and releasing the rudders as you accelerate. The only hassle is really to lock the shaft

That said I can’t see many scenario where it s worth the hassles. For instance my 53 can do 450-500nm with gen running. Why / where would I run to run longer than that non stop?