Dual engine's -- one prop?

[TinyMan]

Hello, I am looking to see if anyone has any ideas on how to accomplish my latest wacky dream.

I want to have the redundancy of two motors, with the reliability and economy of a single motor.

I am going for efficiency first and foremost, as this boat will be a long range cruiser and I want to get the most nm out of every gallon possible.

Most of the time when cruising, a single engine running at 60% could provide enough umphf to get along at 9knts+. But the hull could go 15knts+ with two motors running at 100%.

I'd like to keep the ability to run at that top speed to get out of the way of weather, etc. But I'd like to spend most of the time running just one of the motors at cruise for economy sake.

So, I am thinking that some sort of coupling (hydraulic maybe?) between the two motors that connect them to the final drive shaft. This box would also do the reduction to final prop speed.

It'd be nice if this magic box was a two speed, so as help get the prop to spin at the right speed when both engines are running full out, and also when a single engine is loafing. A variable pitch prop could help a lot with that though, but they cost $$$ and are a major reliability issue.

The box would have to disconnect the un-powered motor when running in single engine configuration.

Anyone know of such a device?

[marshmat]

Truck-grade planetary automatic transmissions, perhaps, with hydraulic torque converters? These usually have an electric- or mechanically-controlled mechanism to lock the converter when cruising, and decouple it when accelerating/coasting. One truck converter/gearbox on each motor, and then a simple (but still custom) gearbox to couple them? Such a system gets you a lot of heavy, complicated mechanicals though, that will reduce efficiency.

I've toyed around a few times with the idea of putting a large, efficient DC brushless electric on the propshaft, with 2 engines of different sizes spinning phase-coupled generators. It would be about a 5-10% efficiency loss over direct drive but you can then use a smaller engine to cruise on, and keep it in its most efficient speed range. This seems to me to be a pretty good configuration for what you're talking about. (Add a few high-capacity batteries and you can cruise engines-off in locks, no-wake zones, etc.)

[cyclops]

Use a automobile rear wheel drive axle. Going down a hill the wheels feed power into the transmission. It will be a mechanical bag of right angle motors feeding the auto differential. It will and does work. All you need is a locking pin setup in each side of the wheel shafts to stop rotation of the unused engine. All off the shelf and a lot of dead weight. Even if the 2 engines run normally for desired top speed. 2 engines in a boat and its heavy and does not dock as easy as a real twin engine driven boat.----engines feed right angle drive boxes, which feed each axle shaft, meet at the differantial, which feed a transmission, which drives the prop. Enjoy.

[Willallison]

There was an article on just such a system in Passagemaker magazine a few months back. If you go to their website www.passagemakermagazine.com and go to the back issues page you should be able to find the issue in question.
But the question has to be why? If you're starting from scratch and insist on the redundancy of a second engine, why not enjoy the same safety margin provided by a second prop?

[TinyMan]

Willallison,
I am interested in gaining the efficiency of a large prop, plus if I had two props and was only planning on cruising using a single engine, I would be dragging that other prop through the water.
I'll go digging for that info from PassageMakeer though, thanks!

[Willallison]

Hmm - fair point, but don't forget that in calculating the prop size / pitch etc you'll have to take into account the power of both engines (assumining that you plan on using both at least some of the time), in which case it will be massively oversized when running on one....

[TinyMan]

That is why I mentioned something about having a two-speed unit, or having to use a variable pitch prop. I don't see a way around that requirement without running into this over-propping issue you talked about.

[cyclops]

Persistant aren't you? Good luck on a water cooled 2 speed marine with a 2:1 ratio.

[FAST FRED]

Check with the scrap yards that do Gov work. The tranny you desire is common on LST and usually has 2 GM 6-71's coupled to a single shaft. Last I saw the complete packjages were $6000 for bots engines & trannys rebuilt to gov specs.

A 6-71 could be swapped for a 3-71 to give you the 60hp at cruise with some efficency.

FAST FRED

[cyclops]

What is the approximate weight of the complete power package?

[gasdok007]

simpler.Cal Connell of rybo runner fame had a set up with twin engines and twin shafts where you could run both shafts off one or two engines. Detroit guy, somehow used OIds Toronado transmissions.

[FAST FRED]

"What is the approximate weight of the complete power package?"

With each 6-71 coming up at 3000lbs , the tranny would be (guess) another 1000 minimum,
So the total would be in the range of 3 tons.

Any good industrial belt will handle 10 hp for extended times . 6 belts would run 60 hp which might be enough for a non plaining boat.
Chain is far better for transmitting HP and hundreds of HP can be run with ease.

Chains are noisy , expensive and really prefer an oil bath to get the longest service life.
The ability to change gear ratios with a simple sprocket change is interesting to contemplate.

It usually is "3rd time lucky" when selecting the "best" prop, perhaps it would be easier to change chain sprockets?

FAST FRED

[650xs]

do it all with hydraulics one engine two props or two engines one prop shaft with variable prop, nice installation put any where,if use very good efficent pumps and bilge motors power loss not alot worh a look ,ive seen it done and works well..................

[cyclops]

In the real world and the real boat . Size the propellor correctly so the type of acceptable efficiency by present day standards is attained . That should keep some of you very busy.