Technique for "trailing" an engine

[celt]

I am in the process of purchasing a '66 41' Hatteras convertible with two Chrysler 440CID 300HP engines (two years on rebuild). Given the price of fuel and the fact that I am not in a hurry to go places during my 3-4 month summer cruises in northern New York and the Great Lakes I would like to have the option of running the boat at hull speed on one engine (trailing the engine). Does anyone know how that can be managed with these engines (unfortunately I do not know the transmission manufacturer) which are apparently original to the boat?

 

[MikeP]

Normally if the engine is not running, the tranny is not being supplied with oil pressure since tranny oil pressure is provided by an oil pump driven by the input shaft from the engine. If an engine is not running but the boat is moving, the flow of water through the prop will cause that shaft/tranny gears to rotate. Since there is no oil flow in the tranny, it will cause failure of the tranny.

There are two ways to deal with this. Either devise a way whereby the non-running engine's prop shaft is locked so it cannot rotate or run that engine at idle so as to provide lubricant in the tranny. Either way will ensure no damage to the tranny. Running the "other" engine at idle won't use much fuel at all and doing this is easier than locking a shaft AND it allows you to alternate the engines easily every few hours or whatever.

 

[Maynard Rupp]

Those transmissions are most probably Borg Warner Velvet Drives. I was told that free spinning those particular transmissions is OK. The book even says so. I don't like doing that. If you take a 1"x6" hardwood board,(Oak), hole saw a hole the dia. of your shaft in the middle, drill two 1/4" dia. holes vertically through the board, one on each side of the big hole. Now saw the board in half through the middle of the hole. Install 2 long 1/4" bolts, (or carriage bolts), with washers and wingnuts through the smaller holes you drilled. Now try the board on your shafts and adjust the length so it will fit in the boat. Install the side with the bolts sticking up under your shaft. Place the other half over the bolts and install the washers and nuts. When you clamp down, the amount of wood you removed when you sawed the board in half will allow you a good clamping preassure on the shaft. If you forget and start the engine, when you put it in gear you will smell wood burning, but the only damage will be to your ego. I always keep one of those devices aboard.

 

[NAN-PO 41]

Celt,
I would question how much fuel you would save by running 1 engine harder to get hull speed than by running both engines easier to get the same speed. Especially if you idle the second engine. My 1966 41'tc is very fuel friendly at hull speed with both engines running.
Fred

 

[DCMY #92]

You won't be happy cruising on one engine.

We have '65 41DCMY that I suspect has the same hull as the boat you are considering. She is diesel, so may have larger wheels than the gas boat. Over the years I have run on one engine due to prop damage or transmission problems. I have done so with the deal wheel locked, with it windmilling and with it removed.

With the prop locked the boat won't make hull speed no matter how much power you put on the good engine. In addition it is very difficult to steer and takes a great deal of opposite rudder deflection. There is no way you will save fuel this way.

With the prop windmilling you don't need as much power on the good engine to move the boat, but she still handles like crap. No way will you enjoy cruising like this and I doubt you will save any fuel. Plus, I'm not so sure the Velvet Drive that you likely have will like it. My manual says it should not be allowed to windmill. This may differ on some models depending on the style of reduction gear.

With the prop removed the economy is good and handling at hull speed is normal. Slow speed handling and docking is another matter. She does not handle anything like a single screw boat. You can't turn the boat into the good engine without significant speed, so you end up doing allot of 270 degree turns away from the good engine. There is no way you want to remove a wheel just to save a little fuel. If you want to go slow with this kind of economy you might consider a single screw boat rather than this one.

 

[celt]

Hi guys, thanks for your input!

1. I like the idea of keeping one engine at idle since it will provide less drag than a fully stopped engine and is ready if you need it for speed and maeuverability. The one problem is I would be adding time on the hour meter (resale).

2. The idea of building that wood brake has it's merits as well. It is inexpensive and would not damage the transmission if I made a dumb mistake. It also does not add engine hours to the hour meter.
The downside is that the engine is not available should you need it quickly.

3. The ideal solution seems to let the shaft free wheel. I don't understand why that would be a problem if the transmission manufacturer recomends it? Can you explain your reasoning? The point may well be moot if the transmission is one of those that are not recommended for free wheeling.

Thanks, Celt

 

[celt]

Sorry Fred and DCM#92, I had my reply framed before I saw your responses.

Fred: Do you have gas engines and what kind of mpg do you get at hull speed with both engines? My definition of hull speed for the 41 is 6 Kts to 8 kts - the square root of the waterline length X 1 (most fuel efficient) or X 1.34 (max hull speed for a displacement boat).

DCM#92: I take your point as well. The hulls for the 41' DC and 41 convertable are the same. All of my previous boats were single engine diesels but I got tired of being run down by thunderstorms and want the capability of speed when I need it. I also want the ability to maneuver better around the dock since I am now 70 and the reflexes are slowing down. Your advice is the same as Fred's - run her on both engines and reduce speed if I want to conserve fuel and forget the addition of hours on the meters. My concern there is about resale. Since she is gas powered, any hours over 600 tends to scare off buyers, no matter what the actual condition of the engines.

Thanks for the input, Celt

 

[REBrueckner]

I agree that BW are ok to free wheel and I also agree I would never do it...it's just to risky....There have been threads here previously about running on one vs two engines..some by me...it's not worth it....a few with detailed fuel consumption data...

Also, freewheeling vs a fixed/locked prop may not yield any efficiency whatsoever..you can read different results on prior posts here...I believe the jury is still out on that one...

When I tried runinng on one engine for several weeks, I ended up sticking a piece of hardwood length wise between the shaft coupling transmission bolts and the engine stringer to prevent rotation...Allisons should not be rotated when off....the advantage was that when I wanted to crank up the second engine, a moment in reverse let the wood drop away into the bilge while I was still at the helm, and I was all set for normal operation.

 

[NAN-PO 41]

Celt,
My boat has diesels so I am sure the fuel mileage will differ from your gas powered boat. That being said my fuel economy is in the 2mpg range at hull speed ( about 8-9 kts. ) The engines are running not far above an idle at that speed. ( enough to maintain temp ) I do find that if the weather kicks up a bit the boat will handle much better if the speed is brought up ( 14 kts or so ). She tends to wallow around at the slower speed.
As far as the resale value is concerned I personally worry less about that and more about enjoying the boat while I can. I am 60 years old and every day I can spend on the boat ( especially with my family ) is a gift.
JMHO Fred

 

[Nonchalant1]

However you do it, when you do this, calculate your fuel use in gal per NM on one engine at a given speed. Then run at the same speed using 2 engines. I am guessing that your fuel use will be so close to identical that you won't be able to calculate the difference.

What it takes to move a boat at a given speed is X units of horsepower. It doesn't make much difference if that all comes from one engine or half from each of two engines. Trawlers with singles achieve better fuel economy by going really slow, frequently way below hull speed. You can get much better fuel economy by going really slow too, unless you have diesels that need to reach operating temps and can't at those slow speeds.

Doug

 

[REBrueckner]

Nonchalant makes good points.

Add to that the fact that single screw trawlers usually have relatively larger diameter props relative to the boat size and you have the essential insights into their relative efficiency...