[Nismo350Z33]

Could anyone please tell me why multi-forward gear transmissions aren't used on most yachts? The only time I've seen them are usually on race boats. The advantages I think are pretty good with quicker acceleration and better fuel economy if the higher gears have a less than 1:1 gear ratio.

[Mark in SBA]

Because they use controllable pitch propellers should there be a need to match prop torque requirements to engine loading. CPPs eliminate the reverse gearset, allow an infinitely adjustable load matching, and can "feather" should an engine shut down. A lot of CPPs are now controlled in unison with the engine with a microprocessor-based engine management system.

I know Limitless ditched the fixed pitch props when they had problems maintaining proper engine loading using either the diesel or diesel/electric modes. The fixed props were pitched for 25 knots using both the Caterpillar 3616 and 3516/electric drives together in "boost" mode. Whenever the electric boost mode was dropped offline, the 3616 was improperly loaded, i.e., lugging, at 17 knots.

[PMLI]

Limitless originally had CPP in her specifications from the start. The yard had miscalculated her weight and she became 200 tons more than her design so the speed and range contract became an issue.

The yard had asked the owner to reduce his speed requirements which they declined. Thealternative was for the Captain to give up the CPP for the more streamlined FPP's and propellers that were pitched for maximum contract speed. Unfortunately, the whole project was designed around CPP to work with the Diesel and the Diesel-Electric systems.

On acceptance sea-trial the yacht exceeded her 25 knot contract speed by 1.4 knots (26.4 kts) and all requirements of the propulsion system were exceeded including a maximum light speed at one time of 27.5 knots. However, she ended up with a smoke problem during the switch-over from Electric to Diesel mode due to the FPP loading.

The owner agreed to retrofit CPP in the future which has recently been done here in Germany. Unfortunately, the Chief Engineer decided to take the working Siemens Electric propulsion equipment out and replace it with a smaller unit of his choice. The Diesels were also derated at the same time, so with all of the above, the vessel does not perform as original.

CPP should work well in an applicaton like this as the CPP pitch can be reduced in order to take the load off the Diesels while their RPM slowly increases along with their turbos and ship's speed.

Most large yachts do not have the capapility to feather their CPP.

[Mark in SBA]

Thanks for filling in the blanks. The entire FPP/CPP saga makes sense now, and understandable given the D/DE design. All I know about the original and revised setups came from the Wood Report articles.

If I understand the architecture correctly, substituting a CPP changes a whole bunch of limitations with these D/DE setups. I can't begin to fathom the complexity of the control software managing all these mode transitions.

I was suprised to read the 3616 main engine drags the electric motor along in diesel only mode. I'm sure the frictional losses and residual back-current protection are easily managed, but I figured they'd throw a clutch in there prior to the reduction gearbox to take the electric motor mechanically offline.

[Nismo350Z33]

Many thanks for the info guys! This sounds very similar to CVT (continously variable transmissions) in cars. There are no set gear ratios and the transmission finds the optimal gear ratio for whatever situation. The torque loads are never too little or too much.

[PMLI]

Continuously Variable Transmissions would work well in yachts where the propellers pitched for max speed and performance. When the engines are running at idle or maneuvering speed the props would in essence, down shift and produce less load on the engines. This would give the same effect as CPP but without the instant forward/reverse/forward/reverse that some throttle jockeys require.

Unfortunately, the automotive units use belt and cone configuration that is usually good up to about 1.4L engines as larger horsepower would break the belts. Not until recently, has anyone designed a fully gear driven CVT, or IVT (Infinitely Variable Transmission) that can be built for any horsepower.

I have recently patented this CVT/IVT which should be released any day now.