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Old 28-08-2012, 23:51   #1
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How Do Variable Props Work

i mean how do the controls work to turn the blades. Is there some kind of switch or lever or something on the console next to the power/gear levers? How does it reach the prop? My ignorance is complete...
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Old 29-08-2012, 05:23   #2
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Re: How Do Variable Props Work

The blades on folding props pivot out to tht "drive" position by centrifugal force.
Feathering props have gears between the shaft and the blades--which crank them into the drive position.-----Basically.
Ckeck out sites for Maxprop and Martec.
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Old 29-08-2012, 05:53   #3
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Re: How Do Variable Props Work

Are you asking about variable pitch props that allow you to change the pitch underway or props that fold or feather when you are under sail.

If the later, then the process is pretty much automatic. I have heard of at least one brand that may require shifting reverse to neutral or some similar combination to make the prop fold but no buttons or levers involved.

If you are referring to a variable pitch prop that allows you to change the pitch of the prop underway to match power and speed requirements then there is a lever at the helm to do this. This lever would replace the shift lever since no longer need a reverse gear but achieve reverse by adjusting the prop pitch until the angle is in the opposite direction.
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Old 29-08-2012, 05:58   #4
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Re: How Do Variable Props Work

Right. There are feathering props which use gears -- like MaxProp. Basically, then the load is off, springs feather the blades. The load overcomes the spring resistance and opens the blade to the driving position. Then there are feathering props which us a mechanical linkage -- a Scandinavian job the name of which escapes me. These props also have manually variable pitch - very handy.

But then there is also the Autoprop, which has neither gears nor linkage. It uses a balance between hydrodynamic force and centifugal force to not only to feather and unfeather the prop, but to change the pitch according to load.
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Old 29-08-2012, 23:16   #5
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Re: How Do Variable Props Work

so a manually adjustable prop requires a matching gearbox to go with it ?
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Old 29-08-2012, 23:31   #6
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Re: How Do Variable Props Work

Maxprop doesn't have springs - at least mine doesn't. Its the centrifugal force that turns the blades from zero pitch (feathered) to maximum pitch (preset), when the forward or reverse gear is engaged.
When motoring forward and killing the engine its sometimes necessary to put the gear in reverse to get the prop to feather while sailing.
This is a feathering propellor. The pitch is only adjustable by taking the prop apart and resetting the internal gears.

A folding, or yacht propellor has its blades thrown out by centrifugal force when the shaft turns and the blades just fold aft when the shaft stops turning. The pitch like a fixed propellor is a function of the shape of the blades and you can't change it yourself

There are other props called Autoprop, Gori et al which are fully pitch-self adjusting props according to direction, load, engine speed etc

Variable pitch prop systems, like Hundestat, are complicated and expensive, but allow the control, as said by a lever at the helm so the pitch can be fine tuned for the conditions you are encountering. These are not common in small yachts as the mechanism is quite big and heavy. Steve Dashew used one on his 78ft Beowulf

You always need a gearbox or clutch of some sort...even with a variable pitch prop you need some method of disconnecting the engine from the shaft.
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Old 29-08-2012, 23:45   #7
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Re: How Do Variable Props Work

Quote:
Originally Posted by charliehows View Post
so a manually adjustable prop requires a matching gearbox to go with it ?
I fitted a 'TRIDENT' variable prop to my 35ft sloop some years ago, i used a dog clutch only so i could run the refrigeration without the shaft turning.

You cannot change pitch under load as the load on the blades will bend the actuators inside the hub. The pitch was changed by a sliding tube on the outside of the shaft, when you moved the pitch lever the tube slid up or down the shaft, the end of the shaft actuated links to each of the 3 blades.

I loved it the no drag was great for sailing. Trident were KIWI built....
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Old 30-08-2012, 05:27   #8
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Re: How Do Variable Props Work

Manually (or hydraulically) adjusted props come in some interesting configurations.
I worked on a 90ft, north sea type trawler yacht, that had a 5"diameter, hollow steel pipe prop shaft, that turned within a sealed fore and aft pressurized oil bath.
Shaft was powered by 2 big Volvos which sat raised on either side, and drove in a "V" configuration with 8" wide toothed belts.
Front end of shaft was carried on a pilot bearing, whose mountig bracket carried the hydraulic ram to which the prop pitch mechanism operated down thru the prop shaft, to a "finger" assembly which controlled the blades.
Really neat engineering package.
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Old 30-08-2012, 05:31   #9
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Re: How Do Variable Props Work

There's a few different types of setups. You can have an automatic control of the pitch like Brunton's Autoprop, (Trident is some kind of "hybrid" dunno) or manually controlled propellers. The latter is called CPP, controllable pitch propeller. Here's a link to longish thread about the subject.. The Controllable Pitch Propeller, a summary. - Boat Design Forums
and you don't need a matching gear box in some of them..
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