Use of hydraulics to better locate engine

[Velella99]

Hi there. I have a 45 foot steel Roberts boat with a typical engine /gearbox/flange coupling/stern tube-cutless bearing setup. The engine is set low relative to the prop location of course. It's got not-too-bad access for maintenance but...could be a lot better.
I'm going to be re-powering in the medium term and it struck me that I may be able to locate a hydraulic motor on the prop shaft with the engine located in a better place for access and other services**...which may not necessarily be elevated. Engine would be hooked up to a shaft driven hydraulic pump (servicing the prop shaft motor) and likely a **watermaker pump and a **shaft driven alternator as well. Any thoughts out by here on this concept?

Sent from my SM-G920I using Cruisers Sailing Forum mobile app

[nauticalnomad]

I changed my engine and shaft to hydraulic propulsion as it previously ran through a vee drive and back out under engine. My boat was originally built without an engine. I replaced an 45yrs old Perkins 40hp shaft driven direct drive with a new Nanni 43Hp set up. Im not satisfied with the power etc as i notice the loss. Not much loss but i notice it. I shouldve used the 50 or 60hp engine but then more fuel etc. As the boat sails faster under sail than motor its only used when in the marina. The hydraulic hoses also put out alot of heat which makes it uncomfortable inside when motoring with no wind for a crossing of 3 or 4 days.. It is however quiter than the previous clunky shaft and drive setup.

[transmitterdan]

The hydraulic pump/motor/reversing valve will have much higher losses than the old transmission. So you will need probably a 25% greater horsepower diesel with more cooling to get rid of the extra heat. Also will need more fuel tankage if you want to maintain the same motoring range. Depending on the type of pump and motor the relationship between engine RPM and propeller RPM may not stay fixed. This could lead to some interesting problems.

The cost would be probably two to three times as much as a regular repower using conventional transmission gear. Plus it will most likely reduce the resale value making the financial equation even worse.

[model 10]

A generator and electric motor like a train?

[bvander]

There are hydraulic coupled drives out there, and they do have advantages, but the major down sides are hose maint and power loss.
Better system is a hybrid, diesel drives a generator which then, through a controller, drives a DC Motor. Some catamarans have gone this route (one diesel, two drive motors).
Check out Beta Marine, I believe they have done some work in this area.

[tkeithlu]

I considered an engine-hyraulic motor setup for my Bruce Roberts trawler, in part with the thought of including power steering (no wheel!) and in part in order to improve the engine placement and drive shaft angle. I turned away from it because of several factors. Primary was the 25% or so efficiency loss, and secondary was the added weight and cost of the hydraulic pump and motor, plus the controls. I ended up mounting the engine as far forward and as low as possible (with a 21' drive shaft) and moving the house batteries to under the floor of the forward cabin. The balance was then right, but there was a negative consequence. With an open main salon and wheel house design, plus that arrangement, the boat ended up so light that it needed 6,000 pounds of ballast to meet off-shore standards. I think the conclusion is that this and other drive train ideas, such as using a constant velocity universal joint drive shaft and diesel-electric drive would have been adopted widely by now if there were not prohibitive obstacles. The ideas are just too easy to arrive at for them not to have been tried and found lacking. Diesel-electric works on mine sweepers, because reversing the field with a very large switch can stop them from flank speed in 1 1/2 lengths, but the Navy uses our money when it pays at the fuel dock.

[Stumble]

Originally Posted by Guy

A generator and electric motor like a train?

It has been beaten to death, but basically you accept an efficency loss in the drive train for a much larger and heavier generator than the engine it is designed to replace. Alternatively you have to accept much less power out of the engine and less efficency.

The real issue is that a direct drive Diesel engine is so fantastically efficient it's hard to beat unless you have very odd usage requirements.


Trains by the way have 18 drive wheels. A mechanical transmission for them would be incredibly complicated. Which is why they used steam power then electric, power can be routed without mechanical transmissions.

[bvander]

Ref electric hybrid here is one example of a cruiser who did just that.
http://www.annapolishybridmarine.com...Propulsion.pdf

With modern DC Stator electrical motors you do gain some efficiencies.
The diesel gen set is typically smaller than the diesel propulsion unit. Also an electric motor has a better torque profile than a diesel engine, which is usually optimized at cruising RPM. Big advantage for Cats, as one diesel motor vice two, and DC motors in the hulls much less space requirement.
No real weight advantages as need extra batteries.
There is typically a fuel saving.

[Terra Nova]

Originally Posted by transmitterdan

The hydraulic pump/motor/reversing valve will have much higher losses than the old transmission. So you will need probably a 25% greater horsepower diesel with more cooling to get rid of the extra heat. Also will need more fuel tankage if you want to maintain the same motoring range. Depending on the type of pump and motor the relationship between engine RPM and propeller RPM may not stay fixed. This could lead to some interesting problems.

The cost would be probably two to three times as much as a regular repower using conventional transmission gear. Plus it will most likely reduce the resale value making the financial equation even worse.

This...while decreasing reliability.

[valhalla360]

Originally Posted by bvander

Ref electric hybrid here is one example of a cruiser who did just that.
http://www.annapolishybridmarine.com...Propulsion.pdf

With modern DC Stator electrical motors you do gain some efficiencies.
The diesel gen set is typically smaller than the diesel propulsion unit. Also an electric motor has a better torque profile than a diesel engine, which is usually optimized at cruising RPM. Big advantage for Cats, as one diesel motor vice two, and DC motors in the hulls much less space requirement.
No real weight advantages as need extra batteries.
There is typically a fuel saving.

For the same available power, the diesel gen set is NOT SMALLER. It must be larger both in HP and size to produce the same HP at the prop.

The main criteria driving the choice of HP is being able to maintain a steady speed while fighting a strong headwind or bashing into waves. In that situation, the conventional diesel is putting out it's peak continuous HP with relatively small losses in the transmission. A diesel electric would have greater losses converting mechanical energy to electrical energy and back to mechanical energy, so the generator would need to be rated for more HP. (shaft and prop losses would be roughly the same)

When people put in electric motors and use lower HP, they are accepting less capability but that lower capability could just as easily be achieved with a smaller HP conventional drivetrain.

The instant torque is nice around the dock (assuming you add expense by providing a battery to create a true hybrid, without the battery to provide the surge, the electric motor can only put out what the generator is producing) but since it is exceedingly rare to use max HP docking, it doesn't typically drive the HP requirements.

Now if you were designing a ferry that crosses a 100m channel dozens of times a day, top speed may not be critical but acceleration and deceleration could benefit from that instant torque with a modest battery bank that fills back up while the cars are loading.

This is also why electric cars often have lower HP electric motors. It may only take 50-60hp for a small car to maintain freeway speeds but a 50-60hp conventional drivetrain would be horribly sluggish in stop & go traffic as peak torque typically requires the engine to wind up to a higher RPM. With electric motors, peak torque is available from a standstill, so even with lower HP, it can still feel peppy off the line.

Hydraulic has been tried but it's more complicated and expensive and as others have suggested, likely to reduce resale value as it will be an oddball system.

[Velella99]

Thank you very much for all your replies. I must admit to being a newbie on the cruising scene so apologies if this thread and subject has been done to death. I can see from your comments that the shift from mechanical to fluid to mechanical or mechanical to electrical to mechanical has an penalty of between 10 and 30% efficiency compared to your standard engine gearbox drive systems. Based on your comments that means that you've got to have a compensation larger engine and accordingly higher fuel consumption and fuel storage. I guess that puts paid to the idea so I thank you for your comments.

Sent from my SM-G920I using Cruisers Sailing Forum mobile app

[tkeithlu]

Sorry to rain on your parade. Don't let it stop you from thinking outside the box; there are novel solutions to problems out there. But when the idea is big/expensive, Cruisers Net is a great place to discuss it before you commit to something that may cost you a lot of money to correct. About half the odd-ball ideas I had for my boat turned out well; the others had to be ripped out and replaced.

[Cottontop]

And when hydraulics go bad, a messy nightmare to troubeshoot and repair.

[Stumble]

Originally Posted by Velella99

Thank you very much for all your replies. I must admit to being a newbie on the cruising scene so apologies if this thread and subject has been done to death. I can see from your comments that the shift from mechanical to fluid to mechanical or mechanical to electrical to mechanical has an penalty of between 10 and 30% efficiency compared to your standard engine gearbox drive systems. Based on your comments that means that you've got to have a compensation larger engine and accordingly higher fuel consumption and fuel storage. I guess that puts paid to the idea so I thank you for your comments.

Sent from my SM-G920I using Cruisers Sailing Forum mobile app

It's actually worse than that. A generator needs to operate at a specific rpm to generate 50/60hz power. So the motor must be sized to generate X amount of power at that rpm, where a direct drive has a throttle and can match the rpm of the engine to the power demands.

So if you want a generator capable of putting out 35kw of power at 1800rpm, you need a monster power plant compared to a motor that can put out 5kw at 1000rpm, and 35kw at 4000rpm.

There is a lot of variables, but figure a generator with the same output needs to be at least double the physical size and weight as a direct drive.

[Velella99]

Very good people. Thanks very much for your considered and detailed responses.

Sent from my SM-G920I using Cruisers Sailing Forum mobile app