Hydraulic Drive for max efficiency

I agree. Efficiency and hydraulic are mutually exclusive. Hydraulics have some advantages, like using the same pump to run a variety of motors. Also, in application when machinery gets overloade, a relief valve regulates the maximum force that can be applied. Otherwise, they are heavy and less efficient than gears..

 

Maintenance on hydraulic systems can not be done on site except for the smallest systems - say 50hp or so. Above that and you need a shop just to take things apart. I can take the shaft couplers off a 400hp propshaft with a couple small wrenches. Ever try to take a 2" hydraulic fitting off a machine? You need serious tools and room to swing a sledgehammer to do that. Got one of those on your boat? The maintenance tools would weigh as much as the diesel. And then there's the fluid capacity. Probably need about a gallon per hp. And you need a tank to drain the system that is not the system reservoir. So about 800 gallons of fluid tankage for a 400 hp diesel - compared to a 2" propshaft? At $25/gallon, an annual fluid and filter change will set you back about $10,000.

Note that shaft traction trucks and trains, and diesel-electric traction trucks and trains are both viable with long commercial histories*. But not diesel hydraulic traction drives. The reasons are largely the same. Hydrostatic drive equipment is invariably used where propulsion is a proportionately small component of the total power budget, and power multiplexing and priority control is important. It is also an effective, if expensive, way to approach tier IV compliance in diesel equipment using computer management of the hydraulic systems. But this is the opposite of the op's simplicate and remove unnecessary stuff. Electric hybrid systems are making broad inroads on traditional hydraulics applications now.

*The first diesel ship was a diesel electric drive - 1903. The first diesel electric railcar was put in service in 1914. Mercedes Benz introduced a successful diesel electric truck in 1923.

 

In the evoltion of the combat tank transmission and stearing systems, there are vehicles where output of the engine was split into two outputs, with one driving a geared transmission, and the other driving a hydraulic transmission, typically a close-coupled pair of swashplate units. Their respective outputs was recombined in a epicyclic gear set. The hydraulics provided much of the drive at low gears, but proportionally less as the geared half of the system geared up from 1st to 8th, for example. The very poor efficiency of the hydraulic side was balanced by the high efficiency of the geared side. The entire assembly had mediocre efficiency in 1st gear, and acceptable efficiency in top gear.

 

This is what I was referring to as being known as "Hydrostatic Drives " in North America.
They are used frequently in farm machinery and sno-cats here including Piston Bullies made in Germany.

 

There are hardly any hydraulic drive legs that use underwater motors. Most of them are buildt like an outboard leg with a mounting plate, keeping the hydraulic motor dry on the inside of the hull. The ones that do use underwater motors are a hydrodynamic nightmare, not really usable as a sailboats propulsion.

I expect that if the legs are "worn out" the engines are too, so your baseline would be the cost to either repair or replace both. You don't say what engine-saildrive combination you have, so I can't comment on repairability. Most Catalacs had Yanmars, engine and transmission spare parts shouldn't be a problem.

I am confident to say that unless the leg casting itself is compromised (broken, corroded, etc.) anything else can be brought back to spec if necessary.
Of course sometimes it's just cheaper to buy another preowned saildrive that's in good order and service that instead of replacing all the internals of the existing one.

I don't know what you expect from going hydraulic, but unless you just happen to sit on a bunch of free parts and expertise it won't be cheaper then repairing what you have or replacing with the current in production version of the same.

 

Create heat to make a piston move .Turn it into rotary movement.Then rotate swash plate to make linear fluid move . This linear fluid move (like piston ) is switched to another rotary movement. After that we use that rotary movement to rotate another hydraulic pump : e.g. there is a screw or jet drive .

 

And finally we need thrust. Maybe the best idea is make shortcut .
Special hull shape that accommodate combustion chamber between bottom and water.Every explosion is a propulsion force. No gearboxes at all.

 

Original poster definitely has an experience with hydraulics that's not based in any reality I've seen.

With that said the thread ressurector has a motivation I've seen from a lot of guys with sail drives... a heavy desire to become free from the maintenance of a sail drive. Have no direct saildrive experience, but back in the mid 2000s we lived adjacent to a boatyard that could haul wide boats. It was also on the circuit used by guys doing the northern circumnavigation route. Even though our local harbor had 0 sail drives the local diesel shop stocked a pair of spares. Over 5 years I'm not sure a big cat with saildrives was hauled without dropping at least one for repair or replacement.

Wouldn't be the first person to wish there was a solution to the sail drive conundrum. Just not positive the solution is hydraulic gear legs. Tried to do the math on total bowthruster run time on my sleipner hydraulic thruster. Betcha a week motoring is more total run time than a decade of thruster use. Might not be a longer served unit even compared to the sail drive systems.

While I'm hesitant to suggest a hydraulic propulsion drive system, some of the prices referenced above seem a bit steep. My current boat has about a 75hp deck system (oversized for my boat but a mirror system of my now former big boat). Also has a 12hp system for refrigeration and a roughly 20 hp system for the bowthruster. And have over the years done systems that go up to 250 hp. It does take some bigger tools, and there is certainly a weight penalty, but we service them routinely on a diy shoe string budget. Pulling power off a small sail boat isn't going to be big hydraulics by any stretch of the imagination. Granted it does seem awful complicated compared to a shaft drive.

 

I'll try to exemplify the costs on the basis of the Catalac 10M wich is the model that had factory fitted saildrives. Most had 2GM20 16hp engines, some the 3 cylinder 25hp. If I look at something new that's similar prices range from ~10 000 for 2 cylinder ~16hp to ~13 000 for the 3 cylinder 20+ hp. This price includes tax, the saildrive and the molded fiberglass mount pan for the engine+saildrive combo. Total cost would be 20-26k plus shipping, whatever you pay to have it installed and the associated bits and pieces like for example exhaust hose.
To drive both hydraulic legs one needs a new 4 cylinder 40+hp engine, that's 8-10k for a bobtail. Two drivelegs of the appropriate lenght to fit the props plus the hydraulic motors are another ~8k, plus pump, cooler, pipes, etc. End result for is in the same 20-26k range with increased installation efforts.

A new saildrive unit in this power range costs 3-4k and its doubtful you can have the hydraulic drive leg cost less. What you save from buying two individual smaller engines vs. a single bigger one you have to invest in the hydraulic driving side of the equation.

On the maintenance side I see this benefits:
1. a hydraulic leg is fixed solidly to the hull, so no diafragm seals to replace every 5-7 years.
2. You can get the leg in bronze or stainless if you so wish, no more special Al antifouling paint.
3. No integrated water intake to unblock.

Both still need anodes, leg oil changes and shaft seal checking.

Just for kicks I did find a manufacturer that offers a hydraulic drive leg with underwater motor, it also kicks up. Drive Leg - Hercules Hydraulics https://herculeshydraulics.co.uk/drive-leg
Here's a more typical leg with the motor inside the hull. Drive Legs - Ocean Marine Systems https://oms.ltd/product/drive-legs/