Ok, I’m convinced to putt along at Hull speed, but

[Owen Christensen]

With two engines capable of planing, can I just run on one for a
hull speed trip? Is the spinning prop from the other engine a problem?
I want to enjoy a slow trip this summer and conserve fuel mostly because of lack of availability.
Owen C

 

[C lectric]

If you wish to run on only one engine YOU MUST FIND OUT WHAT THE GEAR NEEDS.
--Some gears can be freewheeled for a limited time only.
--Some gears can be freewheeled continuously but not over a specified shaft rpm.
--Some gears require the engine to be started after a specified time to lubricate.
--Some gears allow NO FREEWHEELING which means securing the shaft.

Failure to KNOW these requirements can cost you an expensive repair.

Get the mfgr. model and serial numbers and phone a representative.

Also if you post that info on T.F. and someone has infor for your gearbox they ay be able to help.

I will add that if your gearbox is Twin Disc that they say NO FREEWHEELING. THat may have changed which is why you need the
mfgr. name, the model name and the serial number.

 

[rslifkin]

Running on one may or may not actually save any fuel compared to running slowly on both. Although it will reduce concerns for under-loading one engine.



Whether letting a prop free-wheel at low speeds while running on 1 engine is a problem or not will depend on what transmissions you have. Some manufacturers are fine with that usage, others would require you to lock the inactive shaft so it can't turn (which will increase fuel burn, as a locked prop usually has more drag than a freewheeling one).

 

[Owen Christensen]

It’s a Hurth HSW 800V- 1.6 transmission.

 

[Nomad Willy]

First get a Suitable boat.

 

[Pete Meisinger]

Freewheeling a prop will almost certainly damage the gears and the shaft log. Locking the prop will almost certainly counter the effort of the remaining engine. In other words, too much drag.

Even in an ideal situation you will not save any fuel. It costs a fixed amount of energy to move a boat. Using only one engine will use twice the fuel.

Just run both engines

pete

 

[Ken E.]

Your Hurth owners manual will tell you if they can be freewheeled, or check their website. I have TD gears which can be freewheeled with some rules. I have experimented with running on one and there was some fuel savings. But by far the biggest savings comes from just going slower. I run both of mine now in my travels.

 

[Comodave]

First get a Suitable boat.

Why would you say that? I guess I don’t have a suitable boat either. We all have the boats we have for various reasons and there isn’t any right or wrong answers l. But there isn’t anything wrong with trying something to save fuel.

We have a planing boat and I find that running on one engine vs both doesn’t save a lot of fuel since the one engine has to be powered up more than if I run both. But individual boats do vary so if your transmissions allow it then give it a test.

 

[Rufus]

The Twin Disks in our boat require that the engine be started once per hour to relube the free spinning gears. (The lube pump is powered off the input shaft).
Regarding fuel saving at hull speed plus (8.5 knots), our 44 shows about 5% (at same hull speed)....lots of rudder to maintain heading...

 

[Owen Christensen]

Sounds like I will run both. But as far as getting a displacement hull boat, I’m happy I have this one. I was going to get a Defever, but the surveys failed. I got a perfect condition boat that can get up and go if I want.

 

[Owen Christensen]

Thanks for your story. I don’t think I’ll bother if it’s only 5%.

 

[Owen Christensen]

Thanks, I’ll run both while cruising, but if I try fishing on one motor I’ll fire it up once and a while for lube.

 

[Owen Christensen]

Freewheeling a prop will almost certainly damage the gears and the shaft log. Locking the prop will almost certainly counter the effort of the remaining engine. In other words, too much drag.

Even in an ideal situation you will not save any fuel. It costs a fixed amount of energy to move a boat. Using only one engine will use twice the fuel.

Just run both engines

pete

Yep, seems like what I needed to hear.

 

[Owen Christensen]

Why would you say that? I guess I don’t have a suitable boat either. We all have the boats we have for various reasons and there isn’t any right or wrong answers l. But there isn’t anything wrong with trying something to save fuel.

We have a planing boat and I find that running on one engine vs both doesn’t save a lot of fuel since the one engine has to be powered up more than if I run both. But individual boats do vary so if your transmissions allow it then give it a test.

Thank for that. Hull speed travel was said to double the mileage and I enjoy relaxed travel when the lake is calm anyway.

 

[mvweebles]

10+ years ago, Passagemaker Magazine ran an article about a guy with a GB42 who single-handed from California to Hawaii. To conserve fuel, he wanted to run on one engine. So he removed the prop of the other engine. This seems hard to believe even though I am pretty sure I read it - to balance engine hours, in mid-ocean, he removes the working prop and installs the prop on the empty shaft. Before he left, he backed into his slip so he would never have to go to reverse and he removed the prop-nut on the prop he used for the first half.

Small wonder he was single-handed. Sounds nuts to me.

Peter

 

[mvweebles]

Freewheeling a prop will almost certainly damage the gears and the shaft log. Locking the prop will almost certainly counter the effort of the remaining engine. In other words, too much drag.

Even in an ideal situation you will not save any fuel. It costs a fixed amount of energy to move a boat. Using only one engine will use twice the fuel.

Just run both engines

pete

Hate to burst your bubble Pete, but the sail-cats have been running on one engine to conserve fuel for a long time. While I agree with your thinking, I have read enough first-hand accounts that I suspect you're incorrect.

Peter

 

[mvweebles]

Don't even take my word for any of it.....reasearch and then research some more.

This topic comes up a bunch - does a twin and a single in the same hull use the same amount of fuel? Does a low-hp engine for displacement speeds use the same amount of fuel as a hi-hp engine throttled-down to displacement speeds use roughly same amount of fuel. Lots of hand-wringing with the twin owners mostly saying any savings is due to frictional loss of two engines vs one. Lots of guesss-work, nothing empirical.

Psneeld - rather than a 'go-fetch' call-out to do research and general 'no one is as smart as you are,' if you have something definitive, could you share some links? Calling out mis-information is fine, but do we just take your word for it that you know better? Not trying to be argumentative, but you've made a strong statement that deserves something more than 'go do research and you'll find I'm right.'

Peter

 

[Rufus]

Already someone misidentified your gearboxes AFTER you posted what it was.

Even then, some twin disc gears do NOT have to be started every hour. I owned a pair and ran many in Sea Rays that didn't.

The other thing is about running on one engine, if you try and run the same speed as on two...pretty much the same fuel consumption. But if you accept a lower speed on one, some boats have pretty significant savings, but the penalty is SPEED. This savings though is VERY dependent per boat...hard to generalize on it.

For the record, my mention of Twin Disc was in reference to a related comment in Post#7. I did not misidentify the OPs gearbox.

Regarding your second paragraph above, in my tests I purposely ran at the same speed to keep that variable constant. If you don't, a comparison is apples and oranges...garbage. Our OA clearly saves about 5% keeping the speed constant. Another forum member "TIMJET" did the same test using the same test parameters on a Carver and got similar results.

If one has a decent prop chart and an engine power chart, this is an easy test to perform...run the boat on both engines at increasing rpm from 5 knots to about 9 knots and record hull speed. Repeat on one engine. Use the prop chart and engine chart to extract fuel burn for each test run. Compare. Even easier if you have a fuel flow meter. BTW, a limiting factor is commonly rudder authority...our semi displacement OA has large surfaces so heading control with one engine inoperative is not an issue. It would be on many planing boats, in particular. Having said all this, it is indeed configuration dependent. Get the charts for your boat and go measure it...

 

[Comodave]

10+ years ago, Passagemaker Magazine ran an article about a guy with a GB42 who single-handed from California to Hawaii. To conserve fuel, he wanted to run on one engine. So he removed the prop of the other engine. This seems hard to believe even though I am pretty sure I read it - to balance engine hours, in mid-ocean, he removes the working prop and installs the prop on the empty shaft. Before he left, he backed into his slip so he would never have to go to reverse and he removed the prop-nut on the prop he used for the first half.

Small wonder he was single-handed. Sounds nuts to me.

Peter

I remember the story too. I would not try swapping props in the middle of the ocean by myself…

 

[SteveK]

I remember the story too. I would not try swapping props in the middle of the ocean by myself…

What, they do not come off easier in the water.

 

[sunchaser]

My Hurth manual says no problem shutting down one side provided temperatures are monitored. But the OP’s manual for that specific gear should be the rule. Ignore-dock talk for any fuel difference and test it yourself.

 

[C lectric]

I agree with Sunchaser. For any one here YOU need to find out from the mfgr. what the rules for freewheeling are.

Quoting Twin Disc rules for a different gearbox is not helping.

And since I have a T.D. , although old , it quite plainly says in my manual, SECURE THE SHAFT which is why I mentioned T.D. may have changed the rules , BUT FIND OUT FOR YOURS.


And I see you appear to have decided to run both engines easy.

 

[SteveK]

seems like the safe approach would be secure the shaft so it does not turn.

 

[Comodave]

seems like the safe approach would be secure the shaft so it does not turn.

Until you have an emergency and need to start and use the offline engine and the shaft is locked…

 

[markpierce]

Not needing to go anywhere fast by boat and consume 20/whatever gallons per hour, chose a single to travel at slightly below hull speed at much less than two gallons an hour.

Six knots is usually fast enough for me.

 

[socalrider]

Yeah I was going to point out that you probably want to run a bit *below* hull speed, not right on it. Maybe 80% of hull speed - some more experienced folks can describe how to tell by looking at your wake. It'll settle down considerably as you come below hull speed & you'll burn less fuel for a very small speed reduction.

 

[Don L]

you know, sailboats run around all the time with an off engine freewheeling

 

[Gdavid]

In addition to the points already made, you will want to know how your charging system is configured, if one engine alternator powers the house bank and the other powers only a starting battery, you want to be aware of which you are running from and what loads are drawing down the uncharged bank. You may just need to parallel them but you are doubling the load on that alternator and it may not keep up or run hot (the alternator, not the motor).

 

[Owen Christensen]

Yeah I was going to point out that you probably want to run a bit *below* hull speed, not right on it. Maybe 80% of hull speed - some more experienced folks can describe how to tell by looking at your wake. It'll settle down considerably as you come below hull speed & you'll burn less fuel for a very small speed reduction.

That makes sense. One of the articles at the time said my boat got 2 mpg at 8 mph and 1 at 20, with a top speed of 24mph. My goal is to enjoy nice seas and get to port fast to avoid the bad ones.

 

[jgwinks]

you know, sailboats run around all the time with an off engine freewheeling

But those are much smaller non-hydraulic gears built with that in mind. They have no oil pumps, the lower shaft and gears sit in the oil sump and splash up to the upper shaft. We also commonly put the gear in reverse to stop freewheeling, that cuts down on noise and wear. That doesn't work on a hydraulic gear.