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

[Kalagan]

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

[Rick Willoughby]

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

[mark775]

They do. It's a perfect variable ratio.

[CDK]

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.

[mydauphin]

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.

[Kalagan]

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

[CDK]

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.

[liki]

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

[mudman]

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.

[kroberts]

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.

[Jimbo1490]

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.


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

[kroberts]

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.

[Jimbo1490]

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

[kroberts]

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.

[FAST FRED]

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

[fasterboatman]

To all on this thread: For years, may folks used vdrives with various automotive automatic transmissions, most also used custom version that removed the converter, but still utilized the shift points. I grew up near St Louis and spent many days in old V-drive flatbottoms running turbo400s, and powerglides. All of them had manual valve bodies. The goal was to have a big pitch prop for high end, and the low gear for getting onto plane and idle quality(carbs and blowers!). See http://www.rexmar.com/page133.html for a current version of the t400 for vdrives.

I think there are still some advantages to this approach, but the weight issue needs to be overcome.


Jeff D Rauscher

[jakeeeef]

Thanks for all those answers!
I've had to change my login name due to a new email address, but I'm the originator of the post. The thought was bugging me again, so as I do every once in a while, I put 'why don't boats have gears ' into google and suddely realised i had found my own post from months before!

At the time I thought Id got no responses as Im used to forums that email you once someone has answered your thread. I'm glad it has generated so much interest and that it's not just me that keeps returning to this simple question.

The reason I'm thinking about it again is this week I'm optimising my prop on a small RIB.

I have to use one prop for pulling wakeboards/ quick acceleration and another for top speed. That strikes me as a bit like only being able to use one gear in a manual car each day! One day you might pick second if you were just going to the shops, the next you might pick third if you were going on a long motorway trip. Picking 4th or 5th would never really be an option as you wouldn't be able to pull away from standstill effectively enough. And imagine how bad on fuel it would be! I know this is an exaggeration due to the different mediums cars and boats operate in, but come on! This is stone age technology we're still using in our boat engines isn't it? The prop I use for skiing is my second gear, the one I use for top end is 3rd. But I want access to gears 1, 4, 5 and 6, and I don't want to buy four more props and I dont want to have to take the engine out of the water and get a spanner out to change those gears!

I can't help thinking that with gears in my RIB I could select a lower gear to tow things and a higher one to cruise distances under low load and get away with running a bigger pitch in a general purpose application.. and only one prop to limit buggeration factor. I could use a relatively high pitch prop but get away with it whilst retaining decent acceleration by gunning it through the gears

The engine would not need clutch as a sequential motorcycle gearbox can be shifted in and out of gear without one (though might very briefly need a bit of load taking off the throttle while accelerating up through the box).

The more I think about it the more I want to put a motorcycle engine and trans in a boat! I accept that I was optimistic on my guess at motorcycle economy at WOT, but if the cruising rpm can be reduced, economy will surely be better?

Another of the benefits is in the price of the technology. I know it needs adapting, but look at the price of an insurance write off 1000cc 4 stroke sports bike, compared with a 140hp for stroke outboard motor. And the sportsbike's got brakes and wheels and tyres and plastics and a fuel tank etc. you could sell to fund the marine drive parts.

I maintain that boat engines only typically have one forward gear because of the complexities that existed as they were being developed in the first decades of the 20th century. A 1910 boat engine with gears would have probably been weak and unreliable and therefore unacceptable in the marine context, though I bet there were a few in those days due to the low engine power available.... single gear boats have persisted because of the early reliability issues and the fact that the slight compressibility of water has let us get away with it... to a certain extent. This is just the direction in which they have evolved.

I maintain that if you were inventing a planing boat from scratch now with a clean sheet of paper and it needed to work effectively and controllably and economically as possible at all speed ranges from 0- 50 knots, it would probably have gears.

Complexities like gearboxes are cheap now compared with the price of fuel. The era of geared boats for all is long overdue in my book!

[Kalagan]

I am the creator of this post and I have never changed my email address or changed my login User name.

While this last post is well written and thought out, it was not posted by myself, User ID = Kalagan.

I just wanted to clarify that, but thanks for the great input on the topic.

Kalagan

[apex1]

What was so well thought out? this:
>>>The era of geared boats for all is long overdue in my book<<<

It remains a stupid and unnecessary idea!
A CPP is cheaper, more reliabe, proven and off the shelve available.

But this question pops up every other month.

[Frosty]

2 gear automatic type transmissions have been tried and have been used on offshore power boats for 30 years.

[FAST FRED]

I can't help thinking that with gears in my RIB I could select a lower gear to tow things and a higher one to cruise distances under low load and get away with running a bigger pitch in a general purpose application.. and only one prop to limit buggeration factor.


Any you think a custom multi speed tranny will be cheaper than just a spare prop?

With conventional shaft & mechanical gear the power losses are only 2 or 3% ,efficiency untouchable with any other setup.

FF

[jakeeeef]

I am the creator of this post and I have never changed my email address or changed my login User name.

While this last post is well written and thought out, it was not posted by myself, User ID = Kalagan.

I just wanted to clarify that, but thanks for the great input on the topic.

Kalagan

My apologies Kalgan, I mistook this one for a similar thread which I originated some time ago asking the same question. It is at http://www.boatdesign.net/forums/diy-marinizing/why-dont-boats-have-gears-24747.html