Hydraulic Gearboxes

[bedouin]

Going round the boatshow last week - discussing an engine replacement - one of the items that kept coming up was a hydraulic gearbox.

These seem to be larger and more expensive than mechanical boxes, but have the advantage of being able to deliver full power in both directions.

Do hydraulic gearboxes have any other benefits or disadvantages? Are they quieter, smoother or help with shaft alignment?

 

[Billjratt]

They are mechanical (at least mine is). just hydraulically actuated, so a small lever requiring little movement or pressure contols big cone clutches rather than clunky meshing gears and a manual selector.

 

[Gordonmc]

Hydraulic boxes run hotter so an external oil cooler is needed, or at least it was on the Borg Warner Velvet Drive I had on a Thorneycroft/BMC T90 35 hp engine.
No oil cooler on my present 46 hp engine with a mechanical box.

 

[sailorman]

More resilient, used on hire boats a lot due to abuse. can go from ahead to astern quickly & repeatedly.
there is a loss of power when measured against a mechanical box but generally much quieter

I have a PRM 150 on my Nanni 4150 & a TMP 1200 prior to that on my Perkins 4108

 

[bedouin]

So how do they actually work?

Is there still a direct mechanical connection (bits of rotating metal) between the crankshaft and the prop shaft or do they use hydraulics to transfer the power (e.g. like a torque converter in the old style automatic boxes).

And if it uses hydraulics - can you go "the whole hog" and decouple the engine from the propshaft (using flexible pipes) and so save all the hassle of engine alignment? I guess that is a different sort of drive so more expensive

 

[sailorman]

http://www.powerandmotoryacht.com/engines/everything-you-need-know-about-your-boats-transmission

http://the-norfolk-broads.co.uk/printthread.cfm?Forum=38&Topic=29105

 

[npf1]

Note that they often need a shaft lock to prevent the prop rotating freely when sailing, as putting them in reverse will not lock the prop.

 

[jsl]

Nicholson 35s of the earlier 1970's have/had whole-hog hydraulics. The arrangement allows the engine to be under the cockpit and aft of the prop. I don't think the system is/was very popular: C&N changed to a V-drive as soon as they could get one from Hurth. Nowadays, the solution to the design challenge would be a saildrive leg - and some Nic35's have been fitted with these. I am pretty sure no-one has retro-fitted a hydraulic box.

 

[Tranona]

So how do they actually work?

Is there still a direct mechanical connection (bits of rotating metal) between the crankshaft and the prop shaft or do they use hydraulics to transfer the power (e.g. like a torque converter in the old style automatic boxes).

And if it uses hydraulics - can you go "the whole hog" and decouple the engine from the propshaft (using flexible pipes) and so save all the hassle of engine alignment? I guess that is a different sort of drive so more expensive

Think you may be confusing a hydraulic operated gearbox, which as said already uses a pump to engage the gears rather than mechanical means and hydraulic drive. The latter uses the engine to pressurise a hydraulic system and a pump drives the shaft. In the latter system you can place the engine anywhere and is often used on canal hire boats so the engine can be placed sideways under an aft deck. However big power loss and expense so rarely used on yachts which are usually configured to use an in line engine gearbox directly linked to the shaft (or a saildrive if the boat is designed for that).

If re-engining then makes sense to use the same basic setup and mechanically operated gearboxes are fine in the sort of horsepower ranges used. Alignment should not be a problem with a well installed engine in a modern (ie GRP) boat. The bogey reputation is a hangover from the old days of flexible wooden boats that changed shape in the water and rigidly mounted engines and shafts. Add the rough running of many old engines and you end up with every little movement transferring to the structure of the boat. Modern installations have overcome most of that.

 

[Danbury]

As I understand it... the big problem with a full hydraulic solution (engine produces high pressure fluid, that is then routed to a hydraulic motor attached to the prop shaft) is that they are inefficient with large amounts of heat produced.

I am currently trying to work out how I could replace my aging diesel engine with an electric solution... With the advent of electric vehicles and forklift trucks, it's all reasonably standard parts these days... but if you want more than about 20hp, you're looking at 2 motors. Connecting 2 motors that may rotate at slightly different speeds is a conundrum though !

What we really need is an azipod solution that is small enough for a 40ft boat...

 

[bedouin]

The particular issue that started the discussion is that when re-engining I would normally need to switch from an RH to LH prop (or is it the other way round).

However I am told that hydraulic gearboxes don't have the same problem and can run either way round - so as the additional cost of a hydraulic box is less than the cost of a decent prop it is an option to fit a hydraulic box and retain the current prop.

If the only reason were to save the cost of the prop it would not be a sensible approach - but I was wondering if the hydraulic box would give additional benefits.

 

[Tranona]

It would be unusual to use exactly the same prop size on a new engine unless the power rating and shaft speed are virtually the same. Depends of course on the prop, but some feathering props can be assembled to run either way and pitch can be adjusted.

Just as an aside, if you are selling the old engine it may be more valuable with all the stern gear.

 

[bedouin]

It would be unusual to use exactly the same prop size on a new engine unless the power rating and shaft speed are virtually the same. Depends of course on the prop, but some feathering props can be assembled to run either way and pitch can be adjusted.

Just as an aside, if you are selling the old engine it may be more valuable with all the stern gear.

Yes that is part of the question.

The new engines would suit the old prop if they could drive it the right way round, and with a slightly higher than normal reduction. The current prop is a 3 blade Gori folding that would cost between £1500 and £2000 to replace. However one of the reasons for having such an expensive prop is that it was hard to match the engine to the boat and with more revs at the prop shaft I could get away with a cheaper replacement.

I don't expect to sell the engine as a going concern - if I thought there was more life in it I would be keeping it myself

 

[fisherman]

The particular issue that started the discussion is that when re-engining I would normally need to switch from an RH to LH prop (or is it the other way round).

However I am told that hydraulic gearboxes don't have the same problem and can run either way round - so as the additional cost of a hydraulic box is less than the cost of a decent prop it is an option to fit a hydraulic box and retain the current prop.

If the only reason were to save the cost of the prop it would not be a sensible approach - but I was wondering if the hydraulic box would give additional benefits.

Certainly the PRM will run either way, but you would surely buy one with the right rotation? If run in astern you will find the emergency get you home option is also astern.

 

[bedouin]

Certainly the PRM will run either way, but you would surely buy one with the right rotation? If run in astern you will find the emergency get you home option is also astern.

In the section of the market I am looking at (25-30HP) no one makes I have found has the option of a mechanical box that rotates the way I need - so the options are hydraulic or replace the prop.

 

[PCUK]

The light operating pressure required means that you can use a lightweight single lever control to operate a hydraulic box but not generally mechanical boxes.

 

[charles_reed]

Nicholson 35s of the earlier 1970's have/had whole-hog hydraulics. The arrangement allows the engine to be under the cockpit and aft of the prop. I don't think the system is/was very popular: C&N changed to a V-drive as soon as they could get one from Hurth. Nowadays, the solution to the design challenge would be a saildrive leg - and some Nic35's have been fitted with these. I am pretty sure no-one has retro-fitted a hydraulic box.

Hydrostatic drive - used in flameproof FLTs, Travelifts amongst other things. Unfortunately at a price of about 18-20% power loss.
There are a few genuine hydraulic boxes - basically using the front of an automatic car gearbox, most however are (as Tranona says) merely hydraulic-actuated using cone clutches.

 

[charles_reed]

The light operating pressure required means that you can use a lightweight single lever control to operate a hydraulic box but not generally mechanical boxes.

a sweeping assumption, but OK it's your opinion.

 

[billcowan]

I have a PRM hydraulic box.
being British made, it leaks, my bilge is always oily and if I forget to fill it up once a year I suddenly lose drive at an inconvenient moment.
The boat was built in 1980, but the box is a 1998 replacement. 16 years and only 1500 engine hours, not good engineering really.

 

[fisherman]

I have a PRM hydraulic box.
being British made, it leaks, my bilge is always oily and if I forget to fill it up once a year I suddenly lose drive at an inconvenient moment.
The boat was built in 1980, but the box is a 1998 replacement. 16 years and only 1500 engine hours, not good engineering really.

Output shaft oil seal, easily changed, less than £20. Once with me, about 5000 hours, might be to do with rust on the coupling which runs in the seal, maybe renew that?