(Newbie) why don't Boats use Multi-Gear Transmissions?

Hi,

I have always wondered this: Why don't boats use a multi Gear Transmission like a Cars do?

Works for cars, saves fuel and lowers the engine RPM. Seems like it wold be a good idea for Boats.

Or is that just not the case?

Thanks
Kalagan

 

Have a look through this thread. It might answer your question.
http://www.boatdesign.net/forums/diy-marinizing/why-dont-boats-have-gears-24747.html

Rick W

 

They do. It's a perfect variable ratio.

 

Good idea Kalagan!
Please draw us a feasible setup so we can discuss it.
I've heard that houseboat owners have used automatic gearboxes to help getting their sluggish boats into plane.

 

For one- car transmission don't have a thrust bearing or can't handle full torque load under constant use like a boat. Perhaps in a very small boat.
On a boat a variable pitch or controllable pitch propeller would do same function.

 

Hi CDK,

I guess that is basically what I was suggesting, using an Automatic Transmission like cars do. If it helps big old House Boats get on a plane, why not other Boats?

I guess my thought was to just replace the normal direct drive boat transmission with an Automatic Tranny like a car. obviously you would have FWD, Neutral and Reverse, but the FWD would have multiple lower gears to help get you started and then higher gears for when you are crusing.

I an not an engineer, so the basic concept is as far as I can take.

Thanks
Kalagan

 

Kalagan, like mydauphin remarked, there is no thrust bearing in an automotive transmission and maybe the torque load is a bit higher than in a car.

But the real reason is something else.
Because the prop has only a limited grip on the water, there is always 10-50% slip, so there is no absolute need for an automotive gearbox. As a result, the transmissions intended for marine application are much simpler.
There is a whole range of gearboxes, adapter rings, engine couplers, bell housings etc. from which you can construct a drive train without the need to make your own parts.
If you install an automotive engine and gearbox in a boat, you are faced with numerous problems for which there are no of the shelve solutions. Your gearbox may not have the proper gear ratio and will end with a splined shaft or flange for which you cannot buy a suitable coupling.

Should there exist a supplier for adapters between automotive and marine transmissions, the number of boats with a cheap car transmission would surely increase.

 

eg. ZF supplies 2-speed marine transmissions.
http://www.zf-marine.com/Transmissions/SelectionGuide.cfm?Speed=2-Speed&Page=16

 

This is a chart of performance hot rod tranny's that I found. Looks like the average ratio in high gear is 1:2. I can't see how this will be applicable in a boat though.

 

Larger boats sometimes use an electric transmission, virtually identical to that in a diesel locomotive.

The engine powers a generator. The generator output goes through a variable frequency 3-phase power inverter, and that powers an electric motor which powers the prop.

This does absolutely everything that you could want a boat transmission to do. The loss in efficiency from running through a motor and a generator AND a vfd is more than made up for because the engine produces more torque at any RPM than the prop requires, in every case except flat out full throttle. In most cases, the engine produces a LOT more than necessary torque.

So the engine in the case of the electric transmission generates power, and the transmission makes best possible use of it.

 

There's NO loss in efficiency (meaning % engine output available at the prop shaft) when going to electric transmission, in fact they are MORE efficient, leading some to believe that they are universally applicable. The very reason they are used in rail locomotives since the 1930's is that they are MORE efficient (less lossy) than big gears churning around in thick oil.

I's just WAY more expensive and often much heavier to do an electric transmission than a simple mechanical one. The rail locos are getting in/out efficiency % in the high 90's, which would be impossible with a mechanical transmission with say, 12 speeds. The efficiency gains would be smaller compared with a mechanical tranny with fewer speeds, but there nonetheless. It just makes even less sense economically and for weight.

Jimbo

 

Jimbo,

My comments on efficiency were with regards to a first-pass examination. Most electric motors or generators are in the 80%-90% efficient range. Then figure another 80% for the electronics, you get 0.8*0.8*0.8=51.2% efficient in just the transmission itself for a worst-case scenario. A gear box by itself is more efficient than the electronics.

However, you are right. The total system efficiency is higher because the engine doesn't have to turn so fast most of the time, and thus doesn't have to burn fuel that isn't being used.

It's the difference between the engine CAN DELIVER so much torque at this rpm, or the prop IS USING so much torque at this rpm.

 

Three phase induction motors (>100 HP) are quoted around 93% efficient, with high efficiency types exceeding 95%. Large induction alternators are around the same. As you get bigger with motors and alternators efficiency increases

With larger gearboxes, the opposite is true; the bigger they are, the more lossy. Keep in mind that even a 'modest' 100HP induction motor is going to weigh ~1000 lbs; ditto for the alternator. And you still need an engine to drive it all, so light it is NOT. Cheap it is NOT. But it is very efficient mechanically.

Jimbo

 

Jimbo,

I'm not arguing with you. I love the idea of electric transmissions. My original point was, even if the electric transmission seems to be less efficient than the mechanical, the opposite is usually true. You seem to be arguing with me but we're both on the same side.

 

The tried and true way is a CPP controllable pitch propeller, so the HP absorbtion can be varied a bit.

Gori has a 2 pitch prop, selectable under way that is a retrofit , and does not require a hollow shaft.

I think for most folks having the shaft made to fit an outboard prop might be a simple solution.

Changing props would be a wet job , but only require minor time & tools , no underwater prop puller.

And many prop shops will loan a prop to test with , so "dialing in" a new boat would be a snap.

FF