Installing a Hydrostatic Drive in my Sailboat

[dickindvn]

Looking for help in determining prop pitch to drive my 23' sailboat using a 3600 rpm 10 hp air cooled diesel to drive a reversable hydrostatic pump to a hydrostatic drive motor @ 1200 rpm. Want to mount an outboard lower end unit to hull to hold and drive the propeller. Any help on propeller info will be appreciated. Thank you.

[DCockey]

You need:
1) Efficiency of the hydrostatic system or torque/power of motor at desired hydraulic motor speed.
2) Lower end gear ratio. Typically they are in the 1:2 to 1:2.5 range.
3) Maximum diameter propeller accomodated by the lower end unti.
4) Estimate of the resistance characteristics of the sailboat.
With that information an initial propeller can be specified using standard propeller analysis such as outlined in Gerr's book, or possibly an on-line calculator.

With 1200 rpm for the hydraulic motor and an outboard lower unit your propeller speed will be in the 480 rpm to 600 rpm range. This is around one-quarter to one-fifth of the speed of an outboard engine propeller at wide open thottle. Maximum prop diameter with a lower unit from a 20 HP outboard for example will probably be around nine to ten inches.

You might want to consider getting some assistance from someone knowledgable about propeller selection to determine if a there is a suitable propeller available given the combination of propeller speed, maximum propeller diameter, and boat characteristics before proceeding with the installation. This is probably why you posted your question here. You many need to sort through the responses to decide on which advice to follow.

[CDK]

Not a good idea Dick.
Deafening noise from the single cylinder diesel with a hydraulic pump and lots of heat to get rid of. Very poor fuel economy too!
The outboard lower end must be from a large engine, otherwise there is no room for a slow spinning prop.

[philSweet]

like CDK said- you've got a huge RPM mismatch to contend with- 6000 RPM powerhead replaced with a 1200 RPM powerhead. So you would need, at first guess, a 50 HP leg , and a pretty high pitch. Figure 16-20 fps absolute slip at WOT and see where that takes you. If the swash plate is adjustable, you may be able to get the slip a bit lower than that and still be happy with the overall versatiliy of the system. Overall, you'll have a hard time beating a nice little 2 stroke four HP on a transom mount. If you are thinking that you can recapture the efficiency you loose in the hydraulics with better prop efficiency, you can't, because the oversized leg will eat up all your gains in increased friction losses. Low slip is not really desireable either. It costs you versatility, and that's what small craft are most in need of.

[michael pierzga]

Ive got 25kw hydraulic thrusters. Very inefficient use of power. Very complex installation. Very expensive gear. Very high maintenance schedule.

On a small craft I would prefer to have an electric motor powering an outboard motor type leg with a small diesel generator recharging the electric motors battery bank and giving life support to the ship. . Simple and it works.

[Submarine Tom]

Originally Posted by dickindvn

Looking for help in determining prop pitch to drive my 23' sailboat using a 3600 rpm 10 hp air cooled diesel to drive a reversable hydrostatic pump to a hydrostatic drive motor @ 1200 rpm. Want to mount an outboard lower end unit to hull to hold and drive the propeller. Any help on propeller info will be appreciated. Thank you.

18 X 15 would do it but your going to need a reeeeeeally long leg.

-Tom

[PAR]

Why would you be willing to accept all the power losses associated with a drive assembly like this? Then there's the weight, complexity and convolution issues to consider.

[marshmat]

Hi Dick,

If you try to do hydraulic drive on the cheap, you invariably end up with an inefficient, high-maintenance mess of a system.

It's possible to do a pretty good job of it, too. Even the best hydraulic drive will never be as efficient as a simple shaft, but it does let you put the engine in a weird out-of-the-way corner. It can make sense if the engine's main role is to drive a hydraulic windlass and hydraulic deck gear, with auxiliary propulsion as an occasional duty.

The trouble is, a good hydraulic system is really, really expensive. So expensive that they're very hard to justify unless you absolutely must have the engine in a weird location, AND if a small outboard is not a viable option. A 10 hp pump, by itself (no engine, hoses, motor or shaft), costs about the same as a complete 10 hp outboard motor.

That said, if you must go with hydraulics: An outboard lower unit is probably not a good candidate. They don't much care for salt water, they have oil that needs regular changing (for which you have to haul the boat), and they take a limited selection of small-diameter, high-blade-area propellers that are generally optimized for light, fast boats. You would likely be better off with the hydraulic motor driving a conventional shaft, in which case the starting point for prop size, given a 3:1 pump:motor reduction, would be something like 15"x11".

[Submarine Tom]

You can't do hydrostatic drive on the cheap, in any way. It'll cost you in power, money, time, weight, etc.

Stupidest idea I ever heard of, but what do I know.

All the power to 'ya.

Good luck.

-Tom

[dickindvn]

Hi All,

Thank you all for your candid and insightful facts. A lot to think about. Although the facts appear to be slanted against what I was hoping I could make work. Again, Thank You.

[Submarine Tom]

dickindvn,

I have no doubt you can make it work but is that really what's in question here?

-Tom