I am replacing my dead diesel engine in my sailboat.

I wish to replace it with a new diesel and an electric motor in parallel hybrid.

This would allow me to:

  1. Run the propeller using the diesel engine only.
  2. Run the propeller using the electric motor.
  3. Generate power by running the diesel and motor as a generator with a disengaged propeller.

In order to to do this I need a clutch so that I can disengage the propeller, please see the attached drawing.

enter image description here It would look kinda like this. enter image description here

Clutch requirements:

  • Bi-directional - must be able to go forward and reverse.
  • Ability to engage/disengage remotely, electrically or mechanically.
  • Stay engaged with springs. A power would need to be applied to disengage the clutch.
  • Torque >= 600Nm.
  • Does not need to be able to engage/disengage with differential shaft speeds, although it wouldn't hurt.

9.5488 x Power (W) / Speed (RPM) = Torque (N.m) =>

9.5488 x 50 000 (W) / 800 (RPM) = 596 Nm

I know that I won't get full 50 kW from the Diesel engine at 800 rpm shaft speed but I am using it as a worst case scenario, especially since the electric motor can generate a lot of torque during acceleration and develop much more than 20 kW for short periods of time.

My question is, do you have any suggestions regarding what clutch I could use?

I have spent days now searching the internet and I am simple out of my league. Please help with links to products, ebay or anything else that would help me find a clutch that would work.

I would love to be able to use something that is readily available such as the clutch that is between the engine and the gearbox.

I appreciate your help, thank you.

  • $\begingroup$ If you want to run off the electric motor and not the diesel, wouldn’t you need a second clutch between the diesel and the pulley? $\endgroup$
    – Eric S
    Dec 25, 2017 at 15:12
  • $\begingroup$ A cautionary comment - your idea is "feasible" given sufficient R&D. However, please consider the reliability factor - anything not built to the highest standard is subject to failure from many causes in a marine environment. Perhaps more pragmatically, your marine insurer will be very unlikely provide any coverage for your vessel unless all critical equipment meets normal industry standards. Standard marine gearboxes for the power range you mention are available in mechanical and hydraulic versions. Hydraulics may be the most compact, versatile and appropriate option to investigate. $\endgroup$ Dec 25, 2017 at 22:52
  • $\begingroup$ When I run off the electric motor and not the diesel, the diesel would be in 'Neutral' and would just freewheel. Regarding the reliability, the only component between the Diesel engine and the propeller that could fail and leave the boat without propulsion is the added clutch. Therefore I believe a spring engaged clutch is the best bet. If there is no power I just wouldn't be able to disconnect the propeller to run the diesel and motor as a generator. That would be the only functionality lost. Of course if the clutch fails completely then it needs replacing. So I will carry spares. $\endgroup$
    – Daniel L
    Dec 26, 2017 at 2:38
  • $\begingroup$ I added a 'Gearbox' label to my drawing above since that was not made clear enough. $\endgroup$
    – Daniel L
    Dec 26, 2017 at 2:50

4 Answers 4


You'd need two clutches by the looks of it. One to engage and disengage the generator and one for the propeller.

Otherwise you'd be always generating power with the generator which will add a load to your motor and when you try to spin the electric motor it will spin the diesel engine.

Another question is how you'd want the clutch to be activated, by hand or should it be powered.

Either way in the range of the torque you want, large motorcycle clutches are your best bet I think.

Or if you have access to tools you could attempt to make one, but that would require things like a lathe and you'd need to buy springs.

Also another issue will be storing the energy the generator will produce, but I won't get into that for now.


Consider a gearbox such that either the engine can be connected or the electric motor and possibly both at the same time, but the main advantage is that while one is connected the other is free.

  • $\begingroup$ Hi, Thank you for reading my question. That wouldn't solve how to make the engine run the motor and generate power. The propeller needs to be disconnected so that it would work without moving. $\endgroup$
    – Daniel L
    Dec 25, 2017 at 13:42
  • $\begingroup$ You can get a ‘box to do that... Maybe epicyclic with the required ratios between sources and prop as well. $\endgroup$
    – Solar Mike
    Dec 25, 2017 at 13:44
  • $\begingroup$ I would really need it to be a clutch as seen in the photo. $\endgroup$
    – Daniel L
    Dec 25, 2017 at 14:18
  • $\begingroup$ Sorry, but it doesn't really exist in small boat sizes. But there are recently some smallish pto/pti marine transmission options specifically targeted at hybrid setups. But for now, these things start to be available around 500 hp installations. Wait ten years and that will probably change. One very serious issue for small craft is engine and drive packaging. 2 feet doesn't sound like a lot, but few boat can accommodate the extra space needed to do this. I've got about 5/8 inch of shaft showing from coupler to shaft seal. Also check here $\endgroup$
    – Phil Sweet
    Dec 26, 2017 at 4:36
  • 1
    $\begingroup$ Torque (friction) of unconnected (unpowered, open circuit) electric motor is low enough that there's no need to disengage it when diesel is active. So I'd go with one-way clutch bearing after the diesel (where you have the gearbox), so that diesel can propel the electric motor and the propeller, but electric motor won't propel the diesel. Then a gearbox between the axis where the clutch is in your schematics, to allow reversing. $\endgroup$
    – SF.
    Jan 26, 2018 at 14:10

electric clutches, as used for example to engage and disengage air conditioner compressors in cars, are abundantly available in a variety of sizes and power ratings.


It sounds like you actually need to disengage the drive at two points.

  1. To disengage the engine from the drive altogether so you can run with just the motor without the engine acting as a brake
  2. To disconnect the propeller so you can use the engine/motor as a genset.

As you have shown a gearbox connected to the engine in your diagram a combination of a clutch and neutral gear as found in car engines would meet case 1. this really depends on what clutch/flywheel/gearbox sets are available for your engine. If you're using a road vehicle engine this should be pretty easy to find. Marine engines don't necessarily have clutches and gearboxes as the propeller is only loosely coupled to the load and inherently acts a bit like a torque converter in an automatic gearbox.

For case (2) you may find that a dog clutch is the simplest and most reliable solution and will probably also be easier to find as a stand-alone component.


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