Going forward in reverse

[VicS]

Still trying to figure out what I video'd and how exactly it works... What does part #46 in that diagram do? I'll wait until morning before wanting to understand the answer.

The little one labelled #46 is the gear that transmits the drive from the vertical shaft to the output shaft.. also labelled #46

The sliding sleeve drives the vertical shaft in one direction when its engaged with the gear above it and in the other direction when engaged with the one below it

 

[Martin_J]

Thank you.. Couldn't sleep until I'd figured that one out.

That's why my MD2040 with 120S saildrive doesn't have that part. The final drive is straight downwards through the vertical shaft and into the leg..

In the case of the gearbox in your picture in this post.. the drive is from vertical shaft through #46... to #46 on the horizontal final drive shaft.

And in both cases... either upper gear #8 or lower gear #8 drives the shaft in forwards or reverse depending on whether the cone has been pushed up or down.

 

[maxi77]

I must admit I fail to see why one would have different ratios for ahead and astern, after all most props have the same pitch and diameter which ever way they are turned.

 

[Tranona]

I must admit I fail to see why one would have different ratios for ahead and astern, after all most props have the same pitch and diameter which ever way they are turned.

On the contrary, many boxes have different forward and reverse ratios - usually very close so it does not matter. But on my TMC box, for example you can get a choice of two forward ratios (2:1 and 2.6:1) but the reverse is the same for both. Usually not a problem for reverse as you do not need maximum speed in reverse. Many feathering props (like my JF) can be set with different pitches forward and reverse. On the other hand a Kiwi prop has adjustable pitch forward so that they can use the same basic prop for a variety of applications, but fixed pitch in reverse which causes problems in some applications because it is very coarse.

Life's never simple with engines/boxes/props!

 

[pvb]

What is all this about a ratio problem.

If its a Volvo 2002 I think you will find that it has a MS2 gear box !

I think you will find that the gear ratio is the same forwards and reverse.

Look at the design of the box and how it works .. it cannot be different!

and looking at the diagram just now (thanks VicS).. it's actually the same part number for the gear above the cone as it is for the one below #8

Well, in fact, the majority of MS2B gearboxes are the "R" version, which has a 7 degree downward angle on the output shaft. To achieve this angle, I believe that the gear at the top has a different diameter from the gear at the bottom, and therefore the ratio in one direction will differ from the ratio in the other direction. There is a less common "L" version which has a straight output shaft and has identical ratios.

The part identification code 8 in the diagram relates to the whole gear set, not to individual gears.

 

[Norman_E]

and looking at the diagram just now (thanks VicS).. it's actually the same part number for the gear above the cone as it is for the one below #8
Cool..

Talking of reversing.. Many years ago I met a French boat that had their wheel gear reversed.. Skipper said he felt happier that it felt more like a tiller when steering!

That reminds me of a man who used to turn up at fetes when I was a boy. He had a bicycle adapted with geared steering so that the handlebars turned the wrong way. He challenged anyone to ride it for sixpence a go, with a prize for anyone who could cover a short course without falling off.
Of course every boy who could ride a bike tried it, and fell off, mostly within about 5 yards. One boy however tried it a lot of times and eventually mastered it and won the prize.

After that he got on his own bike to ride home......

....and promptly fell off.

 

[VicS]

Well, in fact, the majority of MS2B gearboxes are the "R" version, which has a 7 degree downward angle on the output shaft. To achieve this angle, I believe that the gear at the top has a different diameter from the gear at the bottom, and therefore the ratio in one direction will differ from the ratio in the other direction. There is a less common "L" version which has a straight output shaft and has identical ratios.

The part identification code 8 in the diagram relates to the whole gear set, not to individual gears.

But surely its only the output shaft which is angled down. That will affect the gear set #46.

The input shaft remains horizontal so the forward and reverse gears that form part of gear set #8 will be the same.

 

[pvb]

But surely its only the output shaft which is angled down. That will affect the gear set #46.

The input shaft remains horizontal so the forward and reverse gears that form part of gear set #8 will be the same.

Looking at the diagram more closely, you may well be right. Incidentally, I see from the VP online shop that it's still possible to buy the gear set - err, £5092.

 

[Martin_J]

This thread should be entitled.. How it works - The gearbox.

 

[Burnham Bob]

i connected my yanmar the wrong way round frst time with the morse control working in reverse and seemed to have as much power as needed going forward with the lever in what should have been reverse until i swapped it round. however, if gearboxes do have a right and wrong way why do engine manufacturers specify right or left hand propellors?

 

[maxcampbell]

Many years ago I met a French boat that had their wheel gear reversed.. Skipper said he felt happier that it felt more like a tiller when steering!

As a 16 year old I borrowed a sports boat that had the wheel steering opposite to the required turn - set off from the beach, went for a slow turn, which quickly became a spiral until I wrenched off one of the blocks the steering line went through

 

[Tranona]

i connected my yanmar the wrong way round frst time with the morse control working in reverse and seemed to have as much power as needed going forward with the lever in what should have been reverse until i swapped it round. however, if gearboxes do have a right and wrong way why do engine manufacturers specify right or left hand propellors?

They do. The hand of the propeller is determined by the direction of rotation of the shaft when in forward gear. So if you re-engine (for example from old Volvos to new Betas) the shaft rotates in the opposite direction in forward so you have to change the prop.

 

[scottie]

This box is an outdrive completely shortened in vertical shaft length
The gear set no 8 is in constant mesh the drive being taken through the cone clutch which engages in the top or bottom gear as selected via a spiral gear in the vertical shaft which drives the bottom gear and in turn the larger gear on the out put shaft .

 

[PaulJS]

What about thrust?

Just one caution here folks, although maybe only relevant to the bigger stuff I work with.
If the prop thrust is transferred from the prop shaft to the hull in the gearbox rather than in a separate thrust block, will the thrust bearings not be direction dependent? Running ahead in astern gear might damage these in a relatively short time or adversely affect gear mesh clearances.

Paul

 

[VicS]

Just one caution here folks, although maybe only relevant to the bigger stuff I work with.
If the prop thrust is transferred from the prop shaft to the hull in the gearbox rather than in a separate thrust block, will the thrust bearings not be direction dependent? Running ahead in astern gear might damage these in a relatively short time or adversely affect gear mesh clearances.

Paul

Look at the design of the MS2 box and I think you will find that does not apply. May be not with others either
What hast been suggested is running the box with reverse selected but with an "incorrectly" handed prop.

When travelleing ahead the thrust on the box will still be the same .. ie propshaft pushing forwards.

The area , with the MS2 box, that concerns me is the power following the "wrong" route through the gear set#8 with any resultant thrust on the vertical shaft being in the "wrong" direction.

 

[Wunja]

That reminds me of a man who used to turn up at fetes when I was a boy. He had a bicycle adapted with geared steering so that the handlebars turned the wrong way. He challenged anyone to ride it for sixpence a go, with a prize for anyone who could cover a short course without falling off.

There is a trick to doing that: cross your hands over - left hand on right handlebar and vice versa.

 

[ianj99]

A friend has just fitted a new propellor, only to discover she (gentlemen please be civil) ordered it wrongly handed. Sorting this out properly will entail more cost and fiddling about (ie. hauling the boat out again), so we're trying to decide if there's any good reason why she shouldn't spend this year using reverse gear to go forward, and vica-versa? The engine is a Volvo 20hp (probably a 2002 by the age of the boat) driving a conventional propshaft. We think that it could even be good for the gearbox, as the reverse cone is likely to be much less worn than the forward one, so it might even extend its life a bit. Any thoughts?

My ZF10 / HBW 100 gearbox can be used in either direction to go ahead and was in fact running astern to ahead.
The downside was increased gear whine which was annoying.

Fitting a RH Kiwi prop and running the gear box ahead (gearbox lever forward) to go ahead eliminated this whine.
The ratios are different but not by much and the lower astern ratio is better for the Kiwi prop.

 

[PaulJS]

Look at the design of the MS2 box and I think you will find that does not apply. May be not with others either
What hast been suggested is running the box with reverse selected but with an "incorrectly" handed prop.

When travelleing ahead the thrust on the box will still be the same .. ie propshaft pushing forwards.

The area , with the MS2 box, that concerns me is the power following the "wrong" route through the gear set#8 with any resultant thrust on the vertical shaft being in the "wrong" direction.

Regarding my earlier post about thrust, having enlarged the drawing I reckon you're absolutely right there VicS.

Did give me an idea for getting rid of the difference between "telegraph ahead" and "gearbox ahead" though, if it's possible to invert the actuating lever and slightly reroute the gear-change cable pushrod this should mean that the actuating cam would be turned the opposite direction thus giving a "gearbox astern/vessel ahead" result when putting the telegraph ahead.

 

[Gypsyjoss]

5+ Years in Reverse!

For 5 years I ran my first boat, an ex flotilla Cobra 850, having a Yanmar 2GM20 engine/gbox and a very large 2 bladed reversed pitch prop with the gearbox direction reversed (unbeknown to me).
It was a pig to motor in a straight line astern so I never did after the first couple of tries. Forward speed was fine because the prop was oversized to compensate for the reduced ratio. This anomaly was only pointed out to me in the 'For Sail' year. No damage was apparently done and I sold the boat on without correcting this 'bodge' repair from a previous life.

If your gearbox has got a lower ratio in reverse, but you have the same size reversed pitch prop as the original, there are two things two things to consider apart from possible damage to the box.
1. You will have to run the engine at higher revs to get the same forward speed.

2. You will have far less control of the boat when going astern

Pete

 

[jubro]

Thank you to everyone for responding to this thread. There's much more useful information here than I ever thought we'd get. I suspect we'll probably give it a try (after swapping the gear cable to the opposite end of the morse lever), but aware of the side effects: may need more revs to compensate for small change in ratio; possible increased gear whine; and check gearbox oil, temperature, noise and vibrations regularly for signs of excessive wear.

In the meantime, if you see an X-342 called Xarifa coming out of the Hamble stern first, you'll know why!