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?

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??

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

Art said:

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??

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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.

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

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.

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.

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!

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.

CPseudonym said:

...........If pitching your prop to desired running speed made no sense someone will have to explain variable pitch props better to me.

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

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

Controllable Pitch Propellers

AusCan said:

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

Controllable Pitch Propellers

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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?

AusCan said:

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

Controllable Pitch Propellers

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Thanks, great read!

>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.

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.

psneeld said:

.......... 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.

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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.

Tony B said:

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.

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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...

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 .

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.

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

Why let something as trivial as gas vs diesel enter the conversation. Good grief guys!

sunchaser said:

Why let something as trivial as gas vs diesel enter the conversation. Good grief guys!

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Don't know if you're joshen or not! Guess you are.

Anyway, gas and diesel engine trivia makes for best understanding/comparison as to differences each engine type may provide... specially regarding altered prop configs to best accomidate altered speeds.

Controllable pitch props make sense when the machine has a wide range of speed or load. A trawler that travels 3 days to the fish grounds then pulls nets for a few days before going home. An airplane that needs full power to take off at 60mph then cruises at 180mph. A ratio of 3 to 1. A semi displacement boat that gets "over the hump" at 12kts to cruise at 17kts is a much smaller ratio.

With gas engines it is important to maintain a ratio of RPM to manifold pressure. C. Lindbergh was used by the Army Air Corp to convince pilots that they could use power settings that were "over square" (IE more inches of manifold pressure than RPM in hundreds. 23" mp at 2300 RPM is considered "square". 25" mp at 2300 RPM is "over square".) Most supercharged or turbocharged airplane gas engines can live long lives operated modestly over square but high manifold pressures and very low RPM will shorten their life dramatically. Think trying to go up a steep hill at low speed with a manual transmission in top gear in a car. This would result in very high cylinder pressures.

Naturally aspirated Diesel engines are always at 29" mp (atmosphere) minus what ever loses are incurred in the intake tract from air filter ect. Power is controlled by fuel flow. Cylinder pressure is not the limiting factor. To avoid damage you need to manage exhaust temperature. Inline 6 cyl engines have good natural harmonic vibration patterns and can usually be operated at low rpm/high power without damage IF the safe EGT is not exceeded.

Super or turbo charged diesels do have to consider balance between mp an rpm but at the low power output that most small yachts use at displacement speeds it shouldn't be a factor. Consider the Flemming 55. Two 500 hp engines. At 8 kts it will burn about 5 GPH. If we assume about 15 hp per gallon per hour that would be about 55hp to push the boat. Or, 27.5hp per engine. You could put any prop you wanted on that and as long as you never went over 8 kts through the water you couldn't hurt those engines. (I know I didn't account for head seas, wind, dirty bottom ect but for our discussion I'll ignore that. These are BIG engines.). If you propped for 8 kts at dead idle you may save (a guess here, probably optimistic) 10-20% of fuel burn. The problem would be if you tried to go faster you would be trying to make power at too low an rpm and EGT would go way up. A friend who works at Case International in the "let's see what breaks" department says that when you put a turbo diesel on the dyno and keep adding fuel until it breaks the usual result is that the head of an exhaust valve comes off and does serious, expensive damage to the engine. As an interesting aside the engines he works with do not have water cooled exhaust and the exhaust system glows red all the way to the turbo starting about 200 degrees before "she blows".

It's Settled

I spoke to 2 highly regarded prop guys in the Houston, Tx. area and they both came up with similar answers. I chose one of them.
I will be adding 1" pitch with a small cup. Everything here is a wild guess but they both said that it should increase my low speed (6-7 Kts) gas mileage by at least 15 to 20%. and put a little bit more load on my engine. Both of these were my goal.

This is not a radical change and my world won't end tomorrow if I decide to go into a plane..
The max RPS should not drop by more than 75-100.


Thanks to all for your ideas, both pro and con.

Tony B

I think that is a smart move. I would consider installing EGT gauge and check wide open temp before the change. Then I would not exceed that temp after increasing the pitch.

" it should increase my low speed (6-7 Kts) gas mileage by at least 15 to 20%"

BS. I can't believe that a 1" pitch change and some cup will cause this effect. But hey, I advised talking to a prop shop. You did and that is what they said. But ask them to prove it with examples or theory.

David

Why would I want to that? If someone came into my woodworking shop for a repair and I had to give them a hypothetical lifespan of the repair based on adhesive used, temperature, humidity, etc. I would tell them to take a flying F***.
Some things you just have to take on faith or reject it. It is only costing me another $75 to find out. This is just $75 more than the normal rebuild/repair of $650. No matter what the actual number turns out to be, It has got to be better than what I already have. How much better? I don't know and really don't care at this point. Better is still better. I It only makes sense. My total yard fees, everything included, will exceed $4,000. Another $75 gets lost in the wash.

I do however appreciate your concern

Regards

Tony B

Tony B said:

Why would I want to that? If someone came into my woodworking shop for a repair and I had to give them a hypothetical lifespan of the repair based on adhesive used, temperature, humidity, etc. I would tell them to take a flying F***.
Some things you just have to take on faith or reject it. It is only costing me another $75 to find out. This is just $75 more than the normal rebuild/repair of $650. No matter what the actual number turns out to be, It has got to be better than what I already have. How much better? I don't know and really don't care at this point. Better is still better. I It only makes sense. My total yard fees, everything included, will exceed $4,000. Another $75 gets lost in the wash.

I do however appreciate your concern

Regards

Tony B

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I am not surprised..my prop guy said should add some cup too...I explained how slow I run and we said maybe next time....real slow and it does matter less.

If someone other than you runs the boat and isn't aware they can over-rev the engine easily they will blow it up! I hate to be the "you will blow it up" guy, but them is the facts. Most boats are set up so maximum rpm's are being limited by the propeller so as to not over-rev. YOU can do it, but someone not used to your set up will assume it is not possible to over-rev the engines as the props will prevent that. "Blowing out the carbon" will involve you watching your rpm's carefully when you run it up so you don't over-rev and damage something along the way

Good call Tony and great logic too. I have the same gas engine you do and would love to hear your thoughts of how the changes have benefited you. If and when I somehow manage to grow up and stop running flat out all the time may make a similar change myself.