Re-pitching a Prop ?????

[Darylat8750]

AkDoug,
He is adding pitch, so full throttle will be less rpm, not more. Experienced prop guys I've talked to say that with a 2 to1 gear ratio 1 inch should make about 200 rpm difference. So what Tony is doing with the added cup sounds reasonable to me. I'm looking forward to hearing results.

 

[Tony B]

CP

The actual data may be a while coming. I don't have a flow scan meter for actual accurate data accumulation. We are already 9 months late in our 'running away from home" plans. Right now we are the final stage before our disappearing act - the dreaded yard routine. Hopefully we will be out of here by Wednesday and heading back to our marina. Then I want 2 weeks for shakedown to make sure all of the work done actually works.
Then as weather permits, we will be heading from Galveston Bay, Tx to Biloxi, Ms. for the rest of the winter. I suspect that my overall averages will be enough to tell whether it made a noticeable difference in fuel consumption or not. As far as performance is concerned, I might actually notice the difference in power when pulling in and out of my slip in idle.

Would it be acceptable to create a Tony B Foundation with a collection plate here on the forum? In no time at all, I will have enough money to get a flow scan meter and I can give immediate data. Just sayin'.

 

[koliver]

When I re-powered from a pair of 145 hp engines to a pair of 200 hp engines, my mechanic suggested I re-pitch my props to lower the operating rpm of my engines and improve efficiency.
I had 23x13 props. I was getting 5 gph at 7 knots on the old engines. I re-powered and before re-pitching my fuel efficiency improved by a noticeable amount. I guess 10%. A couple of years later I re-pitched to 23 x 17, lowered the rpm from 2700 to 2000 rpm at 8 knots, now I get 4 gpm at 8 knots. I estimate the re-pitching saved another 5%. My mechanic told me the engine life will improve due to running slower, so its all on the win side.

 

[CPseudonym]

Tony, I've no flow scans either and no interest in acquiring them. Fuel mileage averages and "how it feels" in the seat of the pants is good enough for me. I'm pretty simple and low maintenance that way, kinda the California way.

You know what's funny? Before we bought a boat the wife didn't like the idea of a slow boat and I could go either way on the issue. Now that we have a planing boat she likes slow cruising over fast because the exhaust noise disturbs her reading

 

[Tony B]

CP

I generally find that most women like a slow motion. I think the speed and noise is a testosterone thing. Gets the blood pumping.
Also, women seem to be more noise sensitive than men. My wife could hear a mouse fart. It's very annoying to me when she complains about noise because I don't hear all that well.

 

[CPseudonym]

Thanks for the wisdom, I'll give it a try in a couple hours

 

[FF]

There big if efficient,, but water lift mufflers might quiet the beast.

On most gas cars the exhaust can not be heard , so it cant be that hard on a boat.

 

[Tony B]

I don't know jack about motors which is obvious by now.
Anyway, when I bought the boat I joined a Mainship forum of sorts on Yahoo Groups. One of the older moderators had done the Great Loop and had kept meticulous records of his trip of almost 6,000 miles. He had flow scan meters which I was told were very accurate. Between the Flow Scan gal/hr data and his GPS he came up with these charts which there is no reason for me to believe that they are anything but accurate. On my little day trips and fuel sounding stick data I pretty much track with his charts.
Just by sheer coinkydink, he had the same make, model and year as my boat. A true twin.
Anyway, attached is some of what he sent me.

 

[RickB]

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.

Diesel engine RPM is limited by the governor. The limiting rpm is called "high idle" and is what you check when you put the gear in neutral and advance the "throttle" to maximum. It is a normal condition and won't hurt anything.

If the prop were to limit rpm at some point below the high idle rpm then you might have problems. The condition you describe is a prop capable of absorbing more power than the engine can produce. In that case the governor fuel stop is supposed to limit the amount of fuel it is possible to deliver.


Running the engine at its designed "high idle" speed is not going to create an instant disaster or even one in the near future. It just sounds scary to some people because they have never done it before.

The rpm at which normal "full throttle" operation occurs is set by the high speed screw, it limits speed lever movement and nothing else, it only sets the point at which the governor is happy with the engine rpm and delivers just the right amount of fuel to keep it there.

In any event, when a prop limits engine rpm, you have an overloaded engine and a lot of smoke. The prop is NOT supposed to be a governor.

 

[timjet]

I'm of the opinion that an overloaded engine is overloaded at all rpm's. If you are unable to get rated WOT rpm due to prop pitch then the engine is overloaded period, at all rpm's.

If Tony believes this then he must decide if the slight increase in pitch is worth the potential damage to the engine vs the increase in fuel efficiency. It could very well be, but I don't think there is any chart or formula anywhere that can tell you this. Entirely a judgement call.

 

[Nomad Willy]

timjet,
I think you would be right if the load curve was linear but it's far from linear. So at lower engine speeds on a moderately to slightly overpropped engine at WOT the engine obviously will be overloaded. As rpm is reduced the load decreases faster than amount of fuel injected. At some point rpm, load and the amount of fuel injected becomes ideal like it would be if one had a variable pitch prop and knew how to use it. Below that ideal point the load drops off faster than the fuel injected and from there on down the engine is underloaded with less than optimum fuel injected.

Unless you have an overpropped engine your engine will be underloaded 9999.999% of the time as the only time fuel mixture, load and rpm will be ideal is at WOT .... w a correctly propped engine.

So most all of us most all the time are running underloaded. Increasing the load at lower than rated rpm increases efficiency a small amount so FF and all the other overproppers are right .... it increases efficiency below a certain rpm that nobody knows for sure where is. No engine manufacturer recommends it as it's a potentially dangerous practice and has little return.

 

[GFC]

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.

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

I'm certainly not an expert on props and I've read through this thread with interest. I'm curious about something.....how did you determine how efficient your current prop is at 6kts? Was this from some chart somewhere or by measuring fuel economy?

What led you to determine it was 20% efficient at 6kts?

 

[Tony B]

The 20% figure I was given by two different prop people at 2 different occasions. I have no scientific backing for this.
But consider this: Think of a prop as a multi-tool. It will do most OK and one or two things real well. Again, I have no knowledge of the following statements, but this is what logic tell me: The boat was designed to cruise around 16 knots. It also can idle and can go WOT about 4000 to 4400 RPM's. The designer will use a prop that will serve him best at cruising speed - 16 kts. Probably 3000 to 3400 RPM. So, at a hair above idle speed - around 6Kts, where I want to be, I am at the lowest end of efficiency this engine/prop arrangement can possibly be. So not only is this the most inefficient speed, it is also the most under-loaded speed I could go. Remember, the prop is not a 'fits all speed' kinda thing.
Given this, and not knowing any better, I will take both their word for it.
Adding 1" pitch and a small cupping, I am not to concerned about popping wheelies idling out of my slip.
The cupping and added pitch will add a little more load to the engine and bring it closer to it designed load. It will still be far far away, but closer.
The logic suits me just fine and I am still very comfortable with it. Especially when I run on one engine only.

 

[FF]

I am at the lowest end of efficiency this engine/prop arrangement can possibly be. So not only is this the most inefficient speed, it is also the most under-loaded speed I could go.

Perhaps with a diesel,underloading is a really bad deal, but with your gas engine once the RPM is up enough not to need extra enrichment (as it does at idle) the engine efficiency is not really worse than at other speeds.

The efficiency can be aided by making sure that if you have a 4BBL carb the secondaries are vaccume operated , not with a mechanical linkage .

You will run on the tiny primaries , for good metering and enough air velocity for the carb to function well.

With a distributor engine a cable can advance the spark, with electronic a small brain box is required.

Maybe 1000 rpm would do the trick? Should be delightfully Quiet!

 

[psneeld]

The 20% figure I was given by two different prop people at 2 different occasions. I have no scientific backing for this.
But consider this: Think of a prop as a multi-tool. It will do most OK and one or two things real well. Again, I have no knowledge of the following statements, but this is what logic tell me: The boat was designed to cruise around 16 knots. It also can idle and can go WOT about 4000 to 4400 RPM's. The designer will use a prop that will serve him best at cruising speed - 16 kts. Probably 3000 to 3400 RPM. So, at a hair above idle speed - around 6Kts, where I want to be, I am at the lowest end of efficiency this engine/prop arrangement can possibly be. So not only is this the most inefficient speed, it is also the most under-loaded speed I could go. Remember, the prop is not a 'fits all speed' kinda thing.
Given this, and not knowing any better, I will take both their word for it.
Adding 1" pitch and a small cupping, I am not to concerned about popping wheelies idling out of my slip.
The cupping and added pitch will add a little more load to the engine and bring it closer to it designed load. It will still be far far away, but closer.
The logic suits me just fine and I am still very comfortable with it. Especially when I run on one engine only.

The chances of you doing your engines great harm I think is pretty small...

I've posted it before...but what the heck.

Finishing up my 11th year running a 26ft Shamrock with a carbed 454, and (don't remember exactly" but way bigger, overpitched, cupped, 4 bladed prop. We do this for pulling power...not speed or efficiency....

As an assistance towing boat it gets probably less care than most rec boats...but does get 200 hr oil changes with "whatever" oil is cheap.

The motor is 12 years old and get around 400 hrs on it every year. It sits for hours idling and hour upon hour at 2000-2500 rpm towing vessels 8-50 feet.

I has at least 20 hours of running with the overheat alarm going off and op temps of between 225-250...blowing coolant all the while I'm towing.

The worst 2 hrs of it's life were when I had to survive a 3 knot current, pulling a 15x40 barge in a Nor'easter. The normal top end RPM is about 4200 and that night I had it on the pin for over 2 hrs and the rpm never got above 3000...overloaded a bit?

When pulling boats off sand bars...often the motor again goes from 2000-3500 rpm in seconds until the prop is cavatating so bad and the boat shaking so much I can't read the instruments. Often the boat will slingshot rearwards into another sand bar and stop the motor dead.

I have run the boat for weeks with nothing but duct tape holding the water in the exhaust tubes and once ran it for a week or better with a pencil sized hole in the side of the water pump from sand erosion when we used to dredge slips with the boats.

The motor burns no oil, doesn't leak coolant, it has had several alternators and starters...but with exhaust hoses and pumps throwing salt water around the engine compartment for days...what the heck. Some electrical parts and a set of exhaust elbows...but the main motor and parts are 12 years old this winter.

So all you "you're gonna blow her up" guys...have at it...I know better what the average motor can take...cruising around in a pleasure boat unless the captain it a total deadhead is nothing at all like commercial service.

Somehow if Tony overprops by a couple hundred RPM and gets 5000 hrs out of his gas engines...I think he'll be happy.

And my engine is no fluke...the other Shammies in the fleet don't get quite as hard as service as my boat because they have more open water coverage...but they aren't far behind.

 

[Baker]

My advice is that you will likely not notice a thing! 1 inch and a little cup is not gonna do much(not something she says).

Secondly, damage is being done when the engine detonates/pings. Detonation is what will happen IF the engine is overloaded. So if you hear pinging, then back off. I seriously doubt you will at displacement speeds. But you may during the transition to plane if you choose to. Obviously, a couple of pings aren't gonna kill your engine. But over time, they do take a toll and at some point will burn a hole in the piston. A manifold pressure gauge would be optimum. Not sure if they sell such a thing for car engines and likely would require some custom rigging.

The above statements are made for gasoline engines only. As far as diesels go, I would have to somewhat agree with Timjet in theory. You have to realize we are both pilots so if there is no procedure or no literature to back it up, we have I hard time doing it. You are "outside the lines" of how the operation of the engine was intended. You are a test pilot. A lab rat. A guinnea pig. Do I think operating an engine such as a Ford Lehman with an over propped pitch at slow speeds will harm it??? I really don't think so. But in the end, I don't know. And nobody else does either. And I am conditioned to operate within the guidelines so I don't feel comfortable outside of them. At the very very least, I would like to have an EGT gauge. Just my opinion.

Rickb, you have given us the facts of how a diesel governor works. I'd like to know your opinion on operating an engine like a Ford Lehman at lower power settings with an over pitched prop? I realize it is a wide open question with variables. Just looking for your gut feeling.

 

[Tony B]

............. damage is being done when the engine detonates/pings. Detonation is what will happen IF the engine is overloaded. So if you hear pinging, then back off. ....you may during the transition to plane if you choose to. Obviously, a couple of pings aren't gonna kill your engine. But over time, they do take a toll ..........

Ignorant as I am about engines, I guess you brought up a good point about the pinging. I will have my boat buddy/auto mechanic on board when I go into a plane and hold it there for a while.
I'm not anticipating any problems though. The prop guy said this was not a radical change, but I would rather be safe than sorry.

Thanks again

Tony b

 

[Al]

Greetings

Yes, this forum thread subject is one favorite when gathered to discuss efficiencies of fuel burn. In a couple of other forums where the subject was in play the following site was given as a reference as to correct wheels by formula.

Vicprop - Propeller Calculator

Given the formula it was found that our boat was right on the mark for the correct wheel size. Now the WOT is 3000 RPM with a 3:-1 gear.
so at 2400 RPM (which meets the 3/4 throttle model as optum, or so I have been told) the boat is about at hull speed of 6.7 knots.
Information that has been offered says one inch in diameter equals about 2 inches of pitch. If the information read on this forum is actuate each inches of pitch increased equals 200 RPM reduction from WOT. So one inch of diameter increase will reduce the WOT by 400 RPM. The "Cup" factor is designed to reduce slippage right? If so then is that a factor in adding or subtracting RPM or is it just a house cleaning addition. I have had cupped wheels but don't or can not confirm that cupping counted on that score.
As our engine is running at 175 degrees and should be around 190, adding an inch of diameter and being talked into another inch of pitch o would seemingly bring the WOT to 2400 RPM and have us reduce the throttle to around 2000 to achieve the same hull speed of 6.7 Knts.
Plus the increased work load would increase the operating temperature closer to the 190 degree mark. If not, a hotter thermostat. Needless to say, doing nothing with a wheel change will see the thermostate replaced.
Good thread, lots of meat on this table-Thanks for sharing.
Al (Bridge to Nowhere) Alaska

The midget fortune-teller who escaped from prison was a small medium at large.

 

[RickB]

Rickb, you have given us the facts of how a diesel governor works. I'd like to know your opinion on operating an engine like a Ford Lehman at lower power settings with an over pitched prop?

The whole point in selecting a powerplant is to match its output to the boat's operating parameters. If you already have a boat with an engine that provides the highest speed at maximum continuous power then it sure looks to me like a well matched hull, prop, and engine.

Trying to achieve a higher speed at some intermediate power level at the cost of no longer having full rated power available in any condition seems kind of pointless to me.

Horsepower is what moves boat hulls, horsepower is converted to thrust, without changing the gear ratio, pitch, and prop diameter to achieve the highest level of thrust at an rpm that matches desired performance then what have you got?

There is a parameter in marine propulsion called a "margin." We rarely see that mentioned in recreational boat discussions but, I believe, that concept is at the root of many of them. If you don't have a power margin, that is a reserve of power that can be delivered to the prop to produce thrust and do useful work, then the slightest wind, wave, prop or hull fouling will deteriorate boat performance so far as to make it all but a pointless exercise of fuel conversion.

With regard to playing with the gas engine output, this version of the cruise prop fantasy is like using a constant speed prop on a recip aircraft engine. If you don't have the instrumentation, and knowledge, to manage the power output correctly you will probably get pretty good at doing engine exchanges. There is a reason P&W produced those clever circular slide rules to calculate power settings.

 

[Tony B]

The whole point in selecting a powerplant is to match its output to the boat's operating parameters. ..........

Rick, I certainly do appreciate your input and this is where I am caught in somewhat of a loop using circular logic with myself. My situation is totally different than you guys with 'real' trawler. I have a 36' Mainship aft cabin model. The manufacturer calls it a Motoryacht. I have twin 270 HP engines for a total of 540 HP and we like to do trawler speeds and not planning speeds. It just seems to me that a boats cruising speed should be matched with its props. Since we normally cruise well under the designed cruising speed I thought I could get a closer match by re-pitching my props. I realize that adding 1" of pitch with a small cupping is not a drastic change but it might do something for my low end power, steering, control and fuel consumption since the factory prop is made to operate in a totally different range.
This whole experiment is only costing me about $75 more than just repairing my props.

Please tell me where my logic is flawed. I still have time to change my mind.

Thanks in advance

 

[Tony B]

Greetings

......... In a couple of other forums where the subject was in play the following site was given as a reference as to correct wheels by formula.

Vicprop - Propeller Calculator .

Al

Thanks for the link, unfortunately, it didn't work for me. Some of the important infor that came back to me such as speed and blade pitch was way off for "Displacement and Semi-Displacement Hulls" Main ship provided a 22-20 pich prop and Vicprop came up with 22-28. Then I tried using the "Planing Hull" and then selecting "Cruiser". Vicprop came up with 22-16 and again, Mainship provided a 22-20. I attribute that to the idea that A), There is no such thing ad a semi-displacement hull. You are either full-displacement or not. and B) There are so many variations of planning hulls, it's difficult for a computer with one set of questions and parameters to fit all.
I tried it first using the manufactures specs and equipment provided just to see how close vicprop would come. Since they are so far apart, there would be no validity in trying again to see what would happen if I changed it to fit my cruising habits.

thanks again
Tony B

 

[Nomad Willy]

Tony I hate to say this but it looks to me like your boat is not at all suited to you're boating style.

If you did some research and good planning selling your present boat for what it is worth and going shopping would seem the best way of solving your problem.

Then your options would be more or less unlimited.

We just bought another house and am selling the other w too many stairs. If you buy and sell reasonably well the cost of making the switch should be limited to the broker fees, taxes and such. Make an estimate of those costs and evaluate if it would be worth it to you to get a much more suitable boat.

Now that the market is up and fuel prices are much lower this should be a good time to make the switch.

What boat would you like?

 

[RickB]

Please tell me where my logic is flawed.

The logic escapes me.

If you want your current boat to go slower and burn less fuel but don't want to change engines, gearboxes, and props ... pull the throttles back.

 

[Al]

Eric and now Rick B.

Both of you are hitting the same button that drew the conclusion for myself as the thread was reviewed.

Eric-the concern with the 'Market being up' is with twin gas pots, that is the reason the market is up. folks wanting to either sell and trade into diesel or just sell due to operating cost.
Somewhere I read a thread by yourself suggesting that if hull speed was the goal, then engines choice would include only those that would drive the hull at hull speed. IE: in the case of Tony, around two 55 hp or even less diesels. I am paraphrasing the comment.

That has been a solution that I have kicked around regarding the value of purchasing a larger twin screwed gas pot such as those being discussed on the Chris Craft forum, and doing the above.
Ebay often has marine diesel sets for sale as an example, or re powers are often available given the opportunity to spread the word amongst diesel engine outlets. Just saying.

Rick B is more subtle maybe blunt, yet addresses the ultimate option other than sell/buying.

Al

 

[Tony B]

I think we are all getting a little too literal, including myself. When we go out day tripping we usually travel at 6 kts. Sometimes I like to goose it a little and get up and go even for a very short period uf nothing else just to blow out every clean. My intend was with this question was not to try to get diesel efficiency but to find out if anyone here has had any experience with a minor change in pitch and cupping for low end performance. I appreciate all of the responses but I think we all are drifting a little.
So now that I have learned a lot, and I mean a whole lot from you guys, let me rephrase the question slightly.
How much effect will adding 1" pitch and a small cup to a 22-20 prop on a 21,000 lb. boat with twin 270 crusaders with a max RPM of 4400 and a max speed of around 22-24 knots. What will the overall effect be?
I still have till tomorrow morning to change my mind.
ANNNND, thanks to all.

 

[sunchaser]

Tony

You have received much information, some of it on point for diesel engines which you do not have.

Why not keep your life simple and cheap and ignore the prop shop guys who want your money and just pull back the throttles as suggested by Rick?

 

[Art]

I think we are all getting a little too literal, including myself. When we go out day tripping we usually travel at 6 kts. Sometimes I like to goose it a little and get up and go even for a very short period uf nothing else just to blow out every clean. My intend was with this question was not to try to get diesel efficiency but to find out if anyone here has had any experience with a minor change in pitch and cupping for low end performance. I appreciate all of the responses but I think we all are drifting a little.
So now that I have learned a lot, and I mean a whole lot from you guys, let me rephrase the question slightly.
How much effect will adding 1" pitch and a small cup to a 22-20 prop on a 21,000 lb. boat with twin 270 crusaders with a max RPM of 4400 and a max speed of around 22-24 knots. What will the overall effect be?
I still have till tomorrow morning to change my mind.
ANNNND, thanks to all.

Tony - Do as you see fit, IMHO. At $75 for a test - why not? Good Luck!

 

[Tony B]

My plans just got more complicated. I have a 22-20 on the boat which I assumed was standard original equipment. I just got a post on a different forum (Mainship Groups - Yahoo) where the poster stated that he has a 22-23 prop from the factory with the same make, model, engine and gear ratio I have. That puts my 1" additional pitch to a 22-21 which is still 2" short of his. I tried calling Marlow Mainship last week and had one heck of a time getting past the computerized options. Finally got a salesman that was totally uninterested in ancient history. I cannot find the actual size from the factory published anywhere. So, I then can say that my props are not factory issue and why would the previous owner change from that?

 

[Al]

Tony
Perhaps a review of the Christ Craft forum regarding "Connie 500" (I may have miss named this thread. The subject is the running on a single of twins. The point is both fuel economy and the effectiveness. You may learn a diffrent intent that will be supportive of the goal you seek. If you do review that thread, please respond to this thread as to what you will or should absorb. Interesting data and information offered.
Cheers,
Al
* Politics is the gentle art of getting votes from the poor and campaign funds from the rich, by promising to protect each from the other.
~Oscar Ameringer

 

[Baker]

Tony, 1 inch and a little cup will likely not be noticeable to you at all.