Two speed gearbox

[Harmony]

Before I ask my question let me say I do NOT want to go faster.

Recently I had a problem with the tickover it was too high and being a newbie, mooring was a problem (ask the bloke next to me). After a trip to the dealers (engine is under warranty) this was rectified and no the tickover is about 700rpm in gear, so far so good.

It made me wonder why gearbox are not 2 speed ie. slow to fast and dead slow.

It made me think of an old DAF that my farther had that worked with a centrifugal gear system the worked by the more revs the more the circumference therefore the faster the momentum.

Please don't think I am one of those people that want to travel at ballistic speed If I wanted that I would have bought a boat for sea use not 20 ton of steel.

On a last note PLEASE try to keep your comments simple and not too condescending this is a genuine question.

 

Thanks

[Albion]

Before I ask my question let me say I do NOT want to go faster.

Recently I had a problem with the tickover it was too high and being a newbie, mooring was a problem (ask the bloke next to me). After a trip to the dealers (engine is under warranty) this was rectified and no the tickover is about 700rpm in gear, so far so good.

It made me wonder why gearbox are not 2 speed ie. slow to fast and dead slow.

It made me think of an old DAF that my farther had that worked with a centrifugal gear system the worked by the more revs the more the circumference therefore the faster the momentum.

Please don't think I am one of those people that want to travel at ballistic speed If I wanted that I would have bought a boat for sea use not 20 ton of steel.

On a last note PLEASE try to keep your comments simple and not too condescending this is a genuine question.

 

Thanks

 

 

The main reason that I can think of is cost and complexity. To introduce another layshaft and change mechanism into a mechanical marine box would cetainly add to the cost and complexity and, probably, the size of the box also.

The DAF that you quote used a Continuously Variable Transmission done by large rubber belts and was very complex and huge in size for the amount of power transmitted. There are much better, more modern, systems on cars now using a sort of toothed chain in an enclosed and lubricated gearbox (look up any car that quotes a CVT auto box) but these are sophisticated, expensive and use modern electronics to control them correctly in modern driving/emissions conditions.

Roger

[Alec Gatherer]

My experience of marine gearboxes is limited to Lister & TMP; in both these cases the input shaft speed is exactly the same as the output shaft speed, so the word gearbox, though true in its strictest sense, would make its function easier to understand if it were referred to as a forward/neutral/reverse box.

The reduction on these boxes, if any, was achieved by using a simple gear reduction drive downstream of the gearbox output shaft.

My impression is that given that the speed range required on canals (dead slow to very slow) is easily handled by the engine rev. variation alone; any difficulty in slow travel or gaining top (!) speed might indicate a propellor compatibility problem?

[alan_fincher]

any difficulty in slow travel or gaining top (!) speed might indicate a propellor compatibility problem?

Or clearly in some cases just selection of an engine that was really far too powerful for the boat in question.

 

I'd say most boats, (well narrow boats at least), where slow speed control presents a problem, may well be over-engined, (assuming idle speed is correctly set up).

[nb Innisfree]

Or clearly in some cases just selection of an engine that was really far too powerful for the boat in question.

 

I'd say most boats, (well narrow boats at least), where slow speed control presents a problem, may well be over-engined, (assuming idle speed is correctly set up).

 

Trouble is propellor size is a compromise, should be calculated to absorb max engine revs while pitch is calculated to give slow speed at tickover, which leaves in between a bit of a lucky dip. I can see where a two speed would come in handy to reduce revs at normal cruising speed. Innisfree tho slightly overpropped cruises at 3mph @ 1400rpm. When engine is up to temp and alternator output has reduced it would be nice to up the ratio and reduce to 1200rpm, or less.

 

Never satisfied, always looking for that little bit extra!

Edited June 11, 2009 by nb Innisfree

[Yoda]

I don't think it's a question of more gears or too big an engine but of simplicity. The more complex the boat, the more to breakdown. So don't add liabilities to your boat where basic setting should do the trick.

[nb Innisfree]

I don't think it's a question of more gears or too big an engine but of simplicity. The more complex the boat, the more to breakdown. So don't add liabilities to your boat where basic setting should do the trick.

 

What a horse?

[RLWP]

What a horse?

 

I don't have a lot of experience, but if you think a horse is "trouble-free" you could be in for a surprise.

 

Richard

[Biggles]

If varying the prop speed to engine speed was an issue, surely it could be overcome with a hydraulic drive and furling valve. Can't see how this can be much good in a NB though.

 

I get 3mph from my Lister on tick over but I am over propped as Listers like this. To go slower I just knock it into neutral as required. Simple.

[David Schweizer]

A two speed gearbox is not beyond the realms of possiblity, many tractors have just two gearbox speeds, operated hydraulicly, basicly slow and fast (or realistically slow and not quite so slow) The Ford I used to occassionaly drive had a two way switch to change gear, one side had a picture of a hare on it, and the other side had a picture of a tortoise.

 

Fords clearly thought that the literacy levels of agricultural workers were somewhat stunted!!

Edited June 11, 2009 by David Schweizer

[Timleech]

A two speed gearbox is not beyond the realms of possiblity, many tractors have just two gearbox speeds, operated hydraulicly, basicly slow and fast (or realistically slow and not quite so slow) The Ford I used to occassionaly drive had a two way switch to change gear, one side had a picture of a hare on it, and the other side had a picture of a tortoise.

 

Fords clearly thought that the literacy levels of agricultural workers were somewahat stunted!!

 

There was an outboard engine produced years ago with a 2-speed box, I don't know how useful it was.

The other answer is a variable pitch propellor, but 'canal size' versions tend to be 2-bladed for simplicity so might be too small to transfer enough power, multi-bladed probably get rather pricey and they would perhaps be a bit too vulnerable to damage.

 

Tim

[colin stone]

Some gearboxes will take a "trolling valve". Essentially, reduces/bleeds off the hydraulic pressure so allowing more slippage in the gearbox. Only used at slow speeds. Fishing vessels often have them to maintain a slight headway, 1 or 2 knots, while hauling. Considered one for a while (£1000ish), but gambled that I would not need one. So far, so good and have good steerage way down to a knot, which is when it would have been used.

[Tony Brooks]

If varying the prop speed to engine speed was an issue, surely it could be overcome with a hydraulic drive and furling valve. Can't see how this can be much good in a NB though.

 

I get 3mph from my Lister on tick over but I am over propped as Listers like this. To go slower I just knock it into neutral as required. Simple.

 

 

Or even a swash plate pump with a variable swash plate. That would give a variable reduction ratio but I prefer a good sold shaft rather than pipes of oil just waiting to burst.

[Biggles]

"trolling valve" that's what I meant, not furling valve

[WotEver]

"trolling valve" that's what I meant, not furling valve

 

Or just tie a couple of trolling bags to the stern

 

Or drop the anchor...

 

T

[Guest]

Many years ago, when I was a kid there used to be an engine advertised in "Practical Boat Owner" which had two forward gears. IIRC it was a petrol inboard. I am unsure as to why it was fitted with two speeds.

[Sir Nibble]

Before I ask my question let me say I do NOT want to go faster.

Recently I had a problem with the tickover it was too high and being a newbie, mooring was a problem (ask the bloke next to me). After a trip to the dealers (engine is under warranty) this was rectified and no the tickover is about 700rpm in gear, so far so good.

It made me wonder why gearbox are not 2 speed ie. slow to fast and dead slow.

It made me think of an old DAF that my farther had that worked with a centrifugal gear system the worked by the more revs the more the circumference therefore the faster the momentum.

Please don't think I am one of those people that want to travel at ballistic speed If I wanted that I would have bought a boat for sea use not 20 ton of steel.

On a last note PLEASE try to keep your comments simple and not too condescending this is a genuine question.

 

Thanks

Try reducing speed a lot earlier to leave more time to get the way off the boat.