variable pitch

Does anyone know of a variable pitch prop being used under fifty feet. With the price of fuel going up it seems to be a better answer. the drag of the bigger hub would be a big consideration on a prop relatively speeking small. In bigger vessels the prop and shaftsize, the variable pitchhub n shaft are less of drag consideration.
YOu would loose some performance ina boat under fifty, because you need the bigger shaft and hub. But it seems to me that for what you would loose in drag, you would gain in control of ecomics on fuel bill and engine life.
Anyone know of examples? any thoughts?
Captword

 

Most electric trolling motors are variable pitch. The high skew/blade span in the blade allows the composite/plastic prop tips to twist off under high loading. Common concept for small vairable pitch units that has been around in the literature for a long time. I'm not sure if anyone is making them in the size you need. Larger ones tend to have fatigue/impact problems.

 

See http://www.hundestedpropeller.dk/index.htm . They do variable (controlable) pitch propellers as small as 20".

But I fear your boat will be a wreck before fuel gains pays the price of the CP prop, hollow shaft and the pitch control unit.

 

BTW, just imagine a boat with the same technology, but 7% smaller, 3 % thinnier. it will weight 10 % less. and will go 3% slower. (46.7 ft instead of 50 ft, 13.6 ft instead of 14 ft beam)

This boat will save 10 % fuel compared to the original. And will even be of lower cost since cost tend to be in relation with weight.

 

Oh and another thing. Because of the the way pitch is distributed on the blade of a CP prop, you never get to the maximum efficency when operating off the design point.

For example, a CP and FP prop both may have an efficency of 75% on the design point J. At half J the CP might be 50% and the FP 45%. The difference is that the CP has more thrust/less torque. For a small power boat the cost difference in fuel would rarely offset the initial penality in cost and weight.

 

What kind of applications are you talking about -- fast powerboats, trawlers, sailboats, etc? As mentioned here before, Sabb of Norway is still making diesels paired with variable props, suitable for sailboats or trawlers under 50'.

 

Just wonderin with some of these 50 sportfishernen that are being produced are getting loaded with monster motors that swing monster props and just blow by you, like you got the dock still tied to the stern. I got a customer that i am drawing a set of plans for a fifty footer. He asked about the variable pitch props. He heard from a friend that some one was putiing a set on a monster sportfisherman. I think he said one of the ninety footers, maybe built in florida. not sure. I told him that i didnt think that they are applicable yet in below 60 really. But i told him i would ask around and see what you guys would say. If variable pitch proppelrs would work?
HOward

 

Variable vs controllable pitch

I think there may be a bit of confusion regarding the terms "variable" and "controllable". In the context of propeller design, they are not the same. Much of this thread has responded about controllable pitch (CP), where the blades can be mechanically rotated into a different pitch setting to better match speed, rpm and pitch. I doubt that was the characteristic to which your client referred, as they are rarely found on go-fast boats. The thrust loading vs speed of a free-running boat really does not justify CP capability. It is typically only when you need high thrust at low speed does a CP prop make sense.

Variable pitch (VP), on the other hand, is something you see in fast craft. VP props have a fixed change in pitch from the blade's root to the tip. For example, traditional stock propellers of, say, 24" pitch have 24" pitch at every radius from root to tip. VP props might be designed with 80% of the pitch at the hub, increasing to full pitch in the mid-radii, and finally a reduction of pitch to 90% at the tip. The design objectives here are to better match local pitch to the water velocity the radius actually sees (called a "wake-adapted" design), as well as to "off-load" the tip to reduce pressure pulses near the hull (a cause of noise and vibration).

So, if I understand your client's original intent correctly, there is a good reason to look at "semi-custom" designs from the major prop builders which do include variable pitch (and possibly camber and new blade section shapes). Controllable pitch is something left to the high-thrust/low-speed guys (tugs, trawlers) and for commercial vessels where a 3% savings in fuel means real money.

Regards,

Don MacPherson
HydroComp

 

Thank you for the reply that was the answer that i had basically figured to get. It a shame that they havent come up with an aplication yett for a go fast boat. If this guy decides to charter it though.... If a cp prop could be made to apply the cost savings would add up. Not quite as fast as commercial tug though. If he where to run an average of a hundred trips a year at roughly 200 gallons a day, in a years time that would be 600 gallons a year saved. With fuel prices rising as fast as they are, the fuel savings would pay for the cp prop. Probably loose a litle on the cruise and or top end.
thanks again Capt howard
ps any engineers or prop designers that read these threads, the new opportunity could be in cp props if you can come up with a way of making them apply to medium to high speed boats. i know that it would help with slow trolling baits.

 

Please remember that Sabb has stopped production of their older 2 cylinder putt-putt engines and are now coupling their CPP gear to Lister engines.

 

there are many advantages to v.p and c.p , however as don mcpherson pointed out some confusion does exist with the terminology. hunderstedt propellors of denmark, market what is a controllable pitch prop , and market it as variable pitch.the blades are rotated within the hub by means of a control rod, which is worm driven in a thrust box either manually or hydraulic.many other companies market almost identical units.this may be common only in europe, i am not sure. either way, this type of prop when used in conjuntion with an exhaust pyrometer can save fuel, quite significantly. for single screw instalations they are great, the stern of the vessel can , using ahead and astern, coupled with forward and reverse pitch, be swung round in either direction. in most comercial applications,they are used in place of a gearbox, low rev engines only of course.in many sailboats the pitch is set from neutral tofull feather to reduce drag and prop spin under sail. this can be used as a paddle wheel and will spin the boat in its own length.having said that, they are expensive,heavy as four normal instalations,require way too much maintenance too.after 50000 miles using them in sail and comercial craft i would only consider them for a tug. the latest designs of fixed and folding props are so efficient, the fuel efficiency actually exceeds c.p. props.go towww.autoprop. com, or bruntonspropellors.com to see some amazing specs. these props are not available in the states so please dont construe this as advertising.

 

BTW, if you want to chase efficiency, why not contra rotative props instead of controllable pitch props.

The advandage is that is a known solution for fast boats, as both vovlo (duoprop & IPS) and mercruiser (bravo 3) do this.