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-   -   Re-pitching a Prop ????? (http://www.trawlerforum.com/forums/s6/re-pitching-prop-12466.html)

Tony B 11-14-2013 08:06 PM

Re-pitching a Prop ?????
 
Since we generally enjoy the slower speeds and sightseeing it almost seems crazy to have props set up for 14-18 kts in a plane when I usually only go 6-7 kts.
Has anyone here had any experience with re-pitched props? I was told that my boat's (Mainship 26 Twin 270 Cruisaders) present prop It is about 20% efficient at 6 kts. With a re-pitched prop it could be about 30% efficient at 6 kts.
I realize that my top end RPM's would be a definite No-No.

So has anyone here had any experience with re-pitched props?

Art 11-14-2013 08:29 PM

I'm registering in to keep up to date on answers. Interesting concept Tony! Pitch props for slow speed efficiency instead of full range efficiency. Hummm??

djmarchand 11-14-2013 08:50 PM

I would talk to a good prop shop and ask them to prove (with examples or theory as in Dave Gerr's prop book) why and how the efficiency would be improved.

It isn't obvious to me that for the same speed, running a prop slower with more pitch results in better prop efficiency. You do pick up a bit of engine efficiency, but not enough to matter.

David

dharriss 11-14-2013 08:51 PM

Quote:

Originally Posted by Art (Post 191776)
I'm registering in to keep up to date on answers. Interesting concept Tony! Pitch props for slow speed efficiency instead of full range efficiency. Hummm??

Complete newbie here. Lurking and loving the great info.

Why aren't there variable pitch props like planes. You could get the best out of every rpm. Just wondering. Thanks.

Art 11-14-2013 08:58 PM

I think this is going to become a great "Prop" thread that Tony B started.

AusCan 11-14-2013 09:06 PM

There are variable pitch props available , although they are expensive and can be problematic.

Re-pitching a prop can be done fairly simply and cheaply by a good prop shop, although the amount of pitch change is limited. From what I understand, the prop is just pressed or hammered to conform to the backing plate mould of the new pitch.

Still - You would want to be 100% sure this was the way you want to go. A small increase in low speed efficiency is good, but you could totally lose the ability to get up and plane. You'd likely have to change back if you ever want to sell the boat.

CPseudonym 11-14-2013 09:23 PM

I agree with talking to a good prop man Tony. If you ain't gonna be spooling it up to WOT changing the pitch makes sense intuitively. If pitching your prop to desired running speed made no sense someone will have to explain variable pitch props better to me.

Art 11-14-2013 09:23 PM

Sometime in the 60's I recall availability (don’t know for how long it lasted) to automatic/inherent variable pitch props for outboards. The prop was made of a more malleable material. In that... the material would alter pitch as the engine revved up to put boat on plane and then regain its "normal" pitch once the initial need for torque for boat to climb over bow wave had passed with the boat then on top of water and less bottom surface making contact (friction) with water. Seems that with today’s technology in materials some form of self adjusting pitch prop could be developed to coincide with rpm as well as hull-to-water friction causing need for hp or torque.

We need a real good prop engineer to chime in! :thumb:

Tony B 11-14-2013 09:35 PM

The prop shop has all sorts of charts and tables. They need to know weight of boat, Max RPM, Max HP, torque, gear ratio, and all sorts of stuff.
One can only re-pitch a prop just so much. If I decide to go that route it will be $400/prop for my 22" props.
I will never probably plane because I don't care to. The journey is more important than the destination or speed.
If I can gain fuel efficiency that also means less time spent hunting for gas during out of season travels.
Re-pitching also means I will not be under-using the twin 270HP engines at slow speeds. Right now, I open it up and gun it from time to time just to 'blow out' the engine gunk from slow speed travel.

Tony B 11-14-2013 09:38 PM

Quote:

Originally Posted by CPseudonym (Post 191794)
...........If pitching your prop to desired running speed made no sense someone will have to explain variable pitch props better to me.

Right On!!!!!!!!!!!!

AusCan 11-14-2013 09:40 PM

Here's an interesting link to info on controllable pitch props.

Controllable Pitch Propellers

Art 11-14-2013 10:01 PM

Quote:

Originally Posted by AusCan (Post 191801)
Here's an interesting link to info on controllable pitch props.

Controllable Pitch Propellers

Thanks AusCan - Good read!

CPP in relation to FPP are interesting to learn about. CPP is the winner IMHO. Cost is a factor though; especially for small craft pleasure boaters who only run 100 to 400 hours annually.

Which I think brings us back to Tony's premise - How about FPP set up for high efficiency at a certain preferred slow speed and the heck with high speed efficiency?

dharriss 11-15-2013 02:34 AM

Quote:

Originally Posted by AusCan (Post 191801)
Here's an interesting link to info on controllable pitch props.

Controllable Pitch Propellers

Thanks, great read!

FF 11-15-2013 05:17 AM

>Since we generally enjoy the slower speeds and sightseeing it almost seems crazy to have props set up for 14-18 kts in a plane when I usually only go 6-7 kts.<

Thats called a cruising prop , but usually will require more diameter , not just more pitch to eat more HP at lower shaft speeds.

This is one of the best improvements to a displacement cruiser , as the lower cruise RPM with higher loads makes a longer lived more efficient engine.

And its smoother and quieter aboard.

The TOP speed on the pin will be at a lower RPM with the cruise prop.Throttle stop is advised.

psneeld 11-15-2013 05:52 AM

Stand by for the "you are gonna blow up your engines early" guys to show up.

While cruising props and over pitching props isn't ever recommended by most people other than experienced cruisers who have been doing it forever...one poster has reported that Grand Banks was doing it straight from the factory for years.

To keep from damaging your engine, depending what it is, you may want to add extra sensors to tell you it's running characteristics. Also, possibly set the max throttle set screw on your quadrant to prevent going above whatever rpm you determine to be your new max rpm.

Tony B 11-15-2013 06:16 AM

Quote:

Originally Posted by psneeld (Post 191840)
.......... Also, possibly set the max throttle set screw on your quadrant to prevent going above whatever rpm you determine to be your new max rpm.

On my Mainship, this would be a definite 'yes'. I think the boat would tear itself to death in no time at all.




Another question.....Would I have to warm up the engine a little longer than normal before putting it into gear because of the added torque or is it going to be not than noticeable at idle speed? What I'm saying is that sometimes an engine will stall out when put in gear if still cold.

psneeld 11-15-2013 06:23 AM

Quote:

Originally Posted by Tony B (Post 191842)
On my Mainship, this would be a definite 'yes'. I think the boat would tear itself to death in no time at all.




Another question.....Would I have to warm up the engine a little longer than normal before putting it into gear because of the added torque or is it going to be not than noticeable at idle speed? What I'm saying is that sometimes an engine will stall out when put in gear if still cold.

not noticeable...

again the "your gonna blow up your engine guys" will probably crucify me...but most engine operations manuals I have read only discuss "no full throttle or no above 90% load" operations till engine has reached operating temp.

At any given rpm your engine is experiencing different loads anyway...bottom fouling, windage, more draft from weight, etc...etc...

FF 11-15-2013 07:05 AM

Warm up is best done by observing f the coolant or oil temp.

Modest throttle , say 1000 or so will speed the warm up to about 140f , where the throttle can be advanced a bit more .

At near normal temp , the throttle can be advanced to cruise. After 10 min or so of normal cruise you should be able to go hog wild with the fuel burn.

A diesel will not usually be able to idle to warm ,(too efficient) it must be under load .

Art 11-15-2013 07:48 AM

PSN, FF - Are we talking apples/oranges here regarding type engine and its use/needs with re-pitched props for continuous slow cruise efficiency??

As per OP, Tony's Mainship has Twin 270 Crusaders.

psneeld 11-15-2013 08:32 AM

ooopppps....but a lot still applies....


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