Controllable Pitch Propellor

westernman

westernman

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How much fuel saving would you get in real life from a controllable pitch propellor as opposed to a correctly set up fixed fix propellor (which has to be set up so as to not overload the engine under worst case conditions - such as fully loaded ship, and heavy seas and strong headwind)?

Is it worth the complexity and cost?
 
michaelchapman said:
I was thinking of something like this:-
http://www.frydenbosabb.no/index.php?mapping=16
Click to expand...

Interesting link! I can see more products like this as we seek to maximise efficiency and new technologies, materials and manufacturing techniques evolve. I've seen a few curious ideas that have been either expensive or somewhat fragile for the marine environment, but perhaps they are starting to mature?
 
Because a controllable pitch propellor gives you an infinite number of settings between maximum ahead, and maximum backwards. It is used on (almost all) propellor driven aircraft and used on ships which have variable operating parameters/loads. In particular tugs etc. Some container ships which always go everywhere with pretty much the same load and at the same speed have a fixed propellor optimized for those conditions. The technology is simple, very efficient and robust.
A torque converter automatic gearbox is not nearly as efficient.
 
michaelchapman said:
Because a controllable pitch propellor gives you an infinite number of settings between maximum ahead, and maximum backwards. It is used on (almost all) propellor driven aircraft and used on ships which have variable operating parameters/loads. In particular tugs etc. Some container ships which always go everywhere with pretty much the same load and at the same speed have a fixed propellor optimized for those conditions. The technology is simple, very efficient and robust.
A torque converter automatic gearbox is not nearly as efficient.
Click to expand...

Sabb marine engines of Norway used to do a 30hp engine(?)/gearbox/ VP prop for small fishing boats.
 
michaelchapman said:
Because a controllable pitch propellor gives you an infinite number of settings between maximum ahead, and maximum backwards. It is used on (almost all) propellor driven aircraft and used on ships which have variable operating parameters/loads. In particular tugs etc. Some container ships which always go everywhere with pretty much the same load and at the same speed have a fixed propellor optimized for those conditions. The technology is simple, very efficient and robust.
A torque converter automatic gearbox is not nearly as efficient.
Click to expand...

Nothing new. The old Sabb engines used this system. One big advantage is that it allows the use of low powered engines as the pitch of the prop adjusts to fully load the engine at all speeds. so you can set the engine at a particular level and adjust speed by adjusting the prop. However they are complicated and the introduction of simple reduction FNR boxes and more powerful engines with wider power bands took over.

As to fuel savings, they are there because the prop is more efficient, but consumption is a function of power used so all it means is that the engine is fully loaded so produces the required power at lower revs. This can be an advantage in commercial applications such as a fishing boat, but limited value in a yacht where the usage level is low. You can achieve similar results using a self pitching prop such as a Bruntons which is at its best when motorsailing as it adjusts the pitch to take into account speed and load. There are also 2 speed feathering props which have the effect of an overdrive by changing pitch when cruising to reduce revs.
 
Tranona said:
Nothing new.
Click to expand...

I wasn't saying it was new! It is tried an trusted technology - at least 100 years old.
I think it is still relevant today as it is more efficient. I have seen 20% fuel savings claimed.

Here is a not so new example of a boat with a controllable pitch propellor - the pitch is adjusted to make the boat go backwards as well.

 
It would depend on what kind of sailing you do? If you generally leave harbour and motor off at a constant rpm until you get back to harbour then you will probably find a fixed pitch prop more efficient. If you do a lot of manoeuvring or a lot of stop/starts for example if you were fishing and moving between marks then a CPP might prove more efficient.
Unless you get one of those auto prop type things then a normal CPP will not auto adjust for load.
CPP generally prefer a fixed rpm which will be optimum for running the hydraulics and propulsion. Normally you start up, clutch in and set the rpm to full running speed. Then the thrust is adjusted with the blades and the load by the engine governor. Most CPPs fail safe to a couple of degrees of ahead pitch so as you can get home. There is also a slight power penalty to run the hydraulics, usually driven off gearbox, but this is minimal. You will likely need a slightly bigger shaft diameter than now to accommodate the pitch control. The gearbox itself will be bigger than normal as it will have the control gear attached. You might need to reconfigure the controls on the engine as you will only need two speeds (a slow start/stop/clutch in-out and a fast speed for everything else). The prop will likely be bigger due to a larger hub. You will need to service the blade seals etc and they REALLY don't like encrustations with barnacles etc or ropes.
I'm sure the salesmen will tell you different but I've only known the fitting of CPP for manoeuvring reasons not fuel efficiency. I think you would have to do a lot of manoeuvring and stop/start sailing before you recouped the higher purchase and service costs.
 
I seem to remember someone built double 'cam shells' surrounding the propellor on both sides.
A single lever would open and close the two cam shells,( shaped like a grab in a scrap yard)
The boat could go ahead, reverse and stay 'on station' without a gearbox using the single lever.
Impressive!
 
michaelchapman said:
I wasn't saying it was new! It is tried an trusted technology - at least 100 years old.
I think it is still relevant today as it is more efficient. I have seen 20% fuel savings claimed.
Click to expand...
I was trying to make the point that the so called advantages are not as valuable as you might think for the sort of usage that a yacht gets, and that developments in other areas are much more suited to modern boats. Of course the characteristics are still there, and still have a value in certain applications, principally in fishing boats where they still find a good market. A 20% fuel saving, even if it is achieved is relatively speaking neither here nor there for most leisure users of small hp engines. How much did you spend on fuel last year?
 
rustybarge said:
I seem to remember someone built double 'cam shells' surrounding the propellor on both sides.
A single lever would open and close the two cam shells,( shaped like a grab in a scrap yard)
The boat could go ahead, reverse and stay 'on station' without a gearbox using the single lever.
Impressive!
Click to expand...
It's called a Kitchen rudder, sometimes erroneously called a Kitchener rudder.

http://en.wikipedia.org/wiki/Kitchen_rudder
 
penfold said:
It's called a Kitchen rudder, sometimes erroneously called a Kitchener rudder.

http://en.wikipedia.org/wiki/Kitchen_rudder
Click to expand...


Yes that's the one.
Looks much simpler that a VP prop, and a lot cheaper.

image_zps6a49682d.jpg
 
I think it's mad that a boat only has 1 gear.

if you think about it, a modern engine ticks over at about 750 rpm and if petrol maxes out at about 5000 rpm.

Therefore, the rev band is only about 6x tickover - yet we expect it to run from 2knts to over 30 (even 40 or 50 on some boats) so we have to pitch a prop so that it slips enough at tickover, and grips enough at higher speeds. Plus "getting over the hump" requires (or more accurately would benefit from) a much lower pitch than cruising or top speed.

Ideally we'd have one prop to get us around in the marina, one for pushing over the hump, then one for cruising and one for watersports.....

I could definately see a DSG type box that could change in milliseconds and had no torque converter slip would massively improve the performance envelope - one ratio for the marina and getting over the hump, once on the plane, changing up to a watersports gearing and then up to a cruising gear.
 
If it were like a car and the 'gearing' could be done at the shaft then I think it would be commonplace, but the physics of water are such that it needs to be at the prop. It's probably more akin to changing the diameter of the wheels on your car at different speeds (amusing challange) and thus I think that the combination of cost and reliability make this more difficult where size, weight and environment are all major issues. If the prop blades could change shape and attack angle at different speeds without being more fragile, letting in water etc then it may be viable. Progress on new materials and breakthoughs on manufacture might change this? There are some on here who see the outdrive as the work of the devil now, so shoving a load of electronics, servos etc into a prop hub would likely meet with some hostility however... :rolleyes:
 
rbcoomer said:
If it were like a car and the 'gearing' could be done at the shaft then I think it would be commonplace, but the physics of water are such that it needs to be at the prop. It's probably more akin to changing the diameter of the wheels on your car at different speeds (amusing challange) and thus I think that the combination of cost and reliability make this more difficult where size, weight and environment are all major issues. If the prop blades could change shape and attack angle at different speeds without being more fragile, letting in water etc then it may be viable. Progress on new materials and breakthoughs on manufacture might change this? There are some on here who see the outdrive as the work of the devil now, so shoving a load of electronics, servos etc into a prop hub would likely meet with some hostility however... :rolleyes:
Click to expand...

But controllable pitch propellors have existed and been used for 100 years if not more!!!!

No need for breakthroughs, new materials, nor electronics - nor even electrics or hyraulics.

You will notice in the video I posted above, that the guy changes the prop pitch (all the way from forward to reverse), with a few turns on a handle.
 
ontheplane said:
I think it's mad that a boat only has 1 gear.

if you think about it, a modern engine ticks over at about 750 rpm and if petrol maxes out at about 5000 rpm.

Therefore, the rev band is only about 6x tickover - yet we expect it to run from 2knts to over 30 (even 40 or 50 on some boats) so we have to pitch a prop so that it slips enough at tickover, and grips enough at higher speeds. Plus "getting over the hump" requires (or more accurately would benefit from) a much lower pitch than cruising or top speed.

Ideally we'd have one prop to get us around in the marina, one for pushing over the hump, then one for cruising and one for watersports.....

I could definately see a DSG type box that could change in milliseconds and had no torque converter slip would massively improve the performance envelope - one ratio for the marina and getting over the hump, once on the plane, changing up to a watersports gearing and then up to a cruising gear.
Click to expand...

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.
 
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michaelchapman said:
But controllable pitch propellors have existed and been used for 100 years if not more!!!!

No need for breakthroughs, new materials, nor electronics - nor even electrics or hyraulics.

You will notice in the video I posted above, that the guy changes the prop pitch (all the way from forward to reverse), with a few turns on a handle.
Click to expand...

I think we're probably talking different scales of boat! Big shaft driven boats then I'd agree, but how much weight would that method add to the outboard on a 5M rib say? :)
 
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