Controllable Pitch Propellor

[cryan]

Two distinct discussions going on here.

#1 Controllable pitch propellers.

#2 Muti speed transmissions.

Post by Cryan sums up #1 perfectly. In addition ducted propellers are not to be confused with Kitchen rudders. A 'Kort' nozzle can be used with either a fixed or controllable pitch propeller, why? Commercial vessels which employ ducted nozzles generally have a very full run aft even to the extent of a bulbous stern which robs propeller and rudder of clean water. Use of a 'Kort' nozzle dramatically improves propeller and rudder efficiency. Big bonus of ducting is improvement in bollard pull.

#2 Propellers move boats NOT engines and are only 100% efficient at ONE point on the prop curve. ZF tried two speed transmissions and they were a complete flop, ie low ratio for getting over the hump and higher ratio once on the plane, but ratios configured so that high ratio did not overload engine at WOT.

You will find in application data for leisure rated diesel engines, only fixed ratio propellers are permitted in order to prevent potential danger of overloading.

Kort nozzles only increase efficiency up to a certain speed(which if memory serves me right is about 17 knots) after that you loose any advantage. If we are talking planing vessels here then you will go a long way to beat the efficiency of water jets. Having driven a few jet powered rescue craft over the years I personally don't feel they suffer at low speed manoeuvring either. Given the choice I would certainly choose jet over an outdrive leg .
I would also worry as to whether a CPP unit could be built robust enough at the small scale required by most leisure craft.

 

[rustybarge]

Kort nozzles only increase efficiency up to a certain speed(which if memory serves me right is about 17 knots) after that you loose any advantage. If we are talking planing vessels here then you will go a long way to beat the efficiency of water jets. Having driven a few jet powered rescue craft over the years I personally don't feel they suffer at low speed manoeuvring either. Given the choice I would certainly choose jet over an outdrive leg .
I would also worry as to whether a CPP unit could be built robust enough at the small scale required by most leisure craft.

What put me off jets is the high revs needed to pootle along. Nothing happens at 1000 revs, 2000 still nothing, 3000 ah starting to move.....3500 4 kts yeah!

 

[Latestarter1]

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Kort nozzles only increase efficiency up to a certain speed(which if memory serves me right is about 17 knots) after that you loose any advantage. If we are talking planing vessels here then you will go a long way to beat the efficiency of water jets. Having driven a few jet powered rescue craft over the years I personally don't feel they suffer at low speed manoeuvring either. Given the choice I would certainly choose jet over an outdrive leg .
I would also worry as to whether a CPP unit could be built robust enough at the small scale required by most leisure craft.

You are perfectly correct as drag of all these appendages offsets venturi effect of nozzle as speed increases. Bulbous stern design does not take kindly to being driven much more than 15 knots. Likewise the larger hub of CP screws up efficiency of performance planing vessel.

Water jets just fine as long as somebody else picking up the fuel bill........WOT they work great, part load plain nasty.

As I said CP is no, no, anyway for recreational motors.