The great controlable pitch prop debate.

"For me it is all very simple."

Me too!

 

you are on the wrong track if you think a cpp is going to make anything faster

 

Thats no true. Ive sailed thousands of miles with a CPP on a motorsailor. When motorsailing I could increase pitch and motor much faster than normal speed per rpm.

Perhaps even a motorboat would behave the same downwind with a CPP.

 

Think about it logically A racing boat can,t go flat out for too long or the engine goes Bang. If you put a FP prop on and pitch It for best efficiency at 90% revs you won' be going maximum speed when your most efficient. Put on a CCP and you will get the best efficiency at any rev setting you choose. If your dead level with another boat and you go 100% revs for say 5 minutes you get maximum efficiency, if the fixed pitch boat goes to 100% revs his efficiency will drop. Guess who is going to win.

 

You cant decrease power and increase pitch, speed. With sailing boats you are compensating for the added power of the sail when pitching up.

 

I' trying to say if your boats best pitch efficiency is at 90% revs you use more power at 100% revs but you get less efficiency.
90% revs =70Kts with FP
90% revs =70Kts with CPP
100% revs =72kts with FP (my guess)
100% revs = 74.5kts with CPP (my guess)

 

I dont know the science of propellor design and their selection for specific tasks.

Some are designed for top speed, some are designed for acceleration and manuvering .

 

I'm not a naval architect but I think you will find that a CPP pretty much does it all if you know how to get the best out of it. That still leaves the question of whether or not you need CPP for your boat. If you need/want it you must be prepared to pay more for it.

A tug would tend to have a finer pitched prop than a boat designed to carry gear out to an oil rig. Fine pitch gives more pull at low/no speed than high pitch.

 

exactly, cpp have lots more drag hence no top speed advantage
a purpose built fpp will beat it everytime.

 

Most cpp installs in the world are on workboats with fixed speed engines so they need the cpp which also means they can get max thrust over all the vessel speed range and also bollard pull.

 

If anyone can use it here it's myself-but unless I wreck my gear on the rocks,it's not worth spending the money to gain the mpg.

Barely idling along both 640 hp engines I get almost 3 mpg at 8 knots -that's 50'- and with the engines and props running wayyy out of their efficiency range.
If I could turn one off,feather it and run on one...who knows the mpg? 5 mpg?
When I need it I could fire the other up,and run up to 30 knots.

But at 100 hours a year-and often less,I'm not going to even use it in 2013-there is no point...

 

might be but for what type of vessel
I think for pleasure boats electric drives will prove to be more useful as they can add torque regardless of rpm to a fixed pitch prop

 

Don't know anything about what electric motors are available to power boats. I have an idea diesel electric (WW2 submarine style) might work out well for boats up to 60 tons. Any info you could let me have would be greatly appreciated. I started life as an Electrician but I never worked with a motor larger than about 2-3KW.

 

For diesel electric you should look to Northern Europe and lake boats.

Visit Steyr Diesel at the next boat show for thoughts on powerplants and modern technolgy.

http://www.steyr-motors.com/marine-...hybrid-propulsion-system-diesel-and-electric/

 

Also view the frauscher line of electic diesel boats. Ive never seen one in person. A boatshow would be good.

http://www.frauscherboats.com/en/footer-menu/specials/hybrid/