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Old 10-03-2008, 01:13 PM   #1
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Prop-pitch

My props are 26/18.....the original are 26/16- i can see this* on marks on the props. We have now balancea and all its ok with the pitch 18. The next week we will proof this props. What can i do. I have the max- revolucion and the max torce of the motor but not more.
The antifouling is in a good condition and this is the moment por proof, but how?
Norbert
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Old 10-03-2008, 03:22 PM   #2
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RE: Prop-pitch

The rule of thumb is that the engine(s) should be able to reach maximum rated rpm with the props that are on the boat. Using our boat as an example, the Ford Lehman 120 develops 120 hp at 2,500 rpm. Therefore, each of our engines should reach 2,500 rpm at full throttle with the propellers that are on the boat.

Ours did not--- at full throttle one engine was less than 2,200 rpm the other one was a bit over 2,300. So the props were pitched too coarsely. The prop shop we used re-pitched both props to 16 inches as well as fixed all the other problems with them. So now we get 2500 rpm out of each engine at full throttle.

BUT..... that's not necessarily the best way to set up the props. Grand Banks generally over-propped their boats at the factory. Since the engines they used, particularly in the 1960s, 70s, and 80s, were usually run at relativley low rpms (1500-1800), using propellers with "too much" pitch allowed slightly faster cruise speeds at these lower rpms. Since the boats were never, or at least not intended, to be run at higher power settings, using the over-pitched props in theory yielded the best combination of speed, ideal rpm, and fuel efficiency.

So there are exceptions to the rule. It all depends on the power of your engines, the speed you want the boat to cruise at, the ideal rpm band of your engines for cruise speed, how easy or hard your boat goes through the water, and so on.
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Old 10-03-2008, 08:47 PM   #3
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RE: Prop-pitch

If you are overpitched, it's probably OK if you have old slow-turning diesels like the Lehmans, 6.354 Perkins, Deere, and other recycled tractor engines.* But if you have "modern" high-output engines - Yanmar, some of the turbocharged Cats, probably some of the Cummins - you'd better be able to reach rated RPM.* These engines don't seem to take to the extra torque very well and run into overheating and other problems.

Also - don't trust your tachometers.* Use a handheld tach to verify the RPM.* My boat has four very nice Stewart Warner tachs.* With the engines synchronized to within a few RPM by ear, there's about a 300 RPM descrepancy between the highest and lowest readings.
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Old 10-05-2008, 03:11 PM   #4
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RE: Prop-pitch

I disagree with Marin to some extent. I agree with Westerbeke in that they state " Westerbeke recomends a propeller that allows the engine to turn 3000 rpm underway at full throttle " and this is on an engine brochure for an engine that makes it's maximum power at 3000 rpm. I belive this to be true across the board for all full displacement hulls and slower semi- displacement hulls. If you prop by this standard you can run your engines as rated for continious power .. usually only about 200 rpm below max. If you over prop the the engine the speed that you can continiously run will be less to much less ( depending on how over proped you are ). Your maximum output and continious output will be less. If you only need 75% of the power you have you should repower with 25% less power and prop your engine for max power and load the engine to 60 to 75% of max fuel consumption. Percent of fuel consumption is directly porportional to percent of load. Over proping is in a word .. unacceptable .. if you wish to use your boat correctly.

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Old 10-05-2008, 09:56 PM   #5
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RE: Prop-pitch

True, but the Westerbeke you're referring to is a relatively "modern" engine, and is (I think) based on a medium duty Japanese engine and high RPM (3600 redline, I think?).* Definitely not an engine you'd want to overprop.* But you can't really compare it to the old "tractor" engines like Marin and I have - you can be pretty assured that thousands of those engines ran up millions of hours pulling farm machinery with the operators having no concern whether they were in a gear that would allow them to reach redline at full throttle.

But it's a "shouldn't hurt them" issue, not an "optimum performance" issue.* For example, the data sheet for my T6.354s shows best SFC of about .39 lb/bhp-hr at 2100 RPM, with the SFC increasing to .41 at 1500.* So while it would seem that "lugging" it a bit and turning the engine slower would make it more efficient, 'taint necessarily so.
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Old 10-06-2008, 04:04 AM   #6
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RE: Prop-pitch

The cruising (oversized) prop is the way to go on any displacement boat.

The preferred method is an extra inch or two of DIAMETER , as its diameter that absorbs HP , pitch simply matches speed and slip.

Extra diameter will require the engine to be operated at lower RPM max , but the engine will last longer and be far quieter.

A good Rule of Thumb for most industrial transplant marinizations is 300 or 10% less than what is observed at full throttle.

For the fly weight short life car engines Yanmar et all, DA Book would be a better guide.

With any engine with a cruise prop two precautions are helpfull.

An Exhaust gas temp gage is manditory IF the boat is used in a variety of heavy sea conditions , but not in smooth water.

A BIG SIGN near the throttle posting the top limited RPM is needed if others operate the boat.
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Old 10-06-2008, 01:12 PM   #7
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RE: Prop-pitch

Hi Chris,
The Westerbeke engine I talked about is an Isuzu that makes it's maximum power at 3000 rpm. The statement on the brochure is on all Westerbeke brochures ( that I have seen ) . About older engines the manufacturer publishes standards by which one should follow when operating specific engines. Iv'e never seen a continious hp rating carry a disclaimer that in 30 years a new rating must be adhered to. What do you and Marin think ... that crankshafts, valves, pistons ect get weak with old age? Do the little carbon guys in the metal get tired and fall into the crankcase? Do rings get Artheritus? Engines don't get old like people. One needs to replace parts that are worn beyond specs but old engines should perform as new if they are maintained properly and with a marine engine that includes loading it properly. I do wonder some about the loading issue. Ive seen farm tractors hooked to a water pump by a ditch obviously very under loaded by proper marine standards and operated for days on end. If Steve DeAntonio is wrong about loading I'd sure like to know. As for FF, don't be rediculous, diameter and pitch both have large effects on loading.

Eric Henning
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Thorne Bay AK

-- Edited by nomadwilly at 14:15, 2008-10-06
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Old 10-06-2008, 01:41 PM   #8
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RE: Prop-pitch

Chris,
I forgot something, The specific fuel consumption numbers in your post and usually where they are used refer to ratings performed at full throttle .. I think. HP ratings usually refer to hp achieved at specific rpm at WOT and if there are enough of them a curve is obtained usually called a hp curve. If you are cruising your boat at 1900 rpm your engine will not be making anywhere near the hp shown on the curve .. much less. It is usually true, however, that over proping does result in less fuel burned at lower speeds. Willard over proped the 30 footers 250 rpm .. quite a bit. Marin says GB did it too .. sinfull I say. I corrected my prop as did many on the Willard Owners Group. My new engine made 3000 rpm ( as it should ) but now it's broken in and gained 100 rpm .. so now I'm under proped! I guess the new engine makes considerably more power so now I've got to put that inch of pitch back in. That will give me the chance to experiment with my 5 blade propeller .. an old Michigan Star in nibral bronze.

Eric
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Old 10-06-2008, 09:35 PM   #9
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RE: Prop-pitch

The Perkins SFC charts are based on prop loading, so they are valid for this application.* I was actually pretty surprised when I saw them - I automatically assumed that SFC would increase with RPM since friction losses increase by (I think) RPM squared.

Not so much that the big old engines have different requirements now than they did, just that they never were as susceptable to damage from overloading as the newer engines.* The hp/displacement (and hp/weight) ratios are much higher than they were, and the new engines are extremely susceptable to damage from overloading and insufficient cooling.* Take a look over at boatdiesel.com to see the Yanmar, Volvo, Cat, and other modern engines that are extremely intolerant of abuse.*

But they are wonderful engines in terms of fuel economy, noise, and smoke.* D'Antonio is absolutely correct, though, in his warnings about overloading on these engines.* It's just that some of the old timers can handle it.

BTW - don't underestimate the loads that those water pumps put on tractor engines.* One of the great "rural myths" (as opposed to "urban myths") was that running a stationary load was bad on tractor engines (for some reason) because they weren't moving.* The reality was generally that they were overloaded, and after 10000 hours or so, even the old Ford/Deere/Perkins engines would suffer (Perkins were common in Massey Ferguson and some IH tractors).
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Old 10-07-2008, 01:19 AM   #10
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RE: Prop-pitch

Quote:
nomadwilly wrote:

Willard over proped the 30 footers 250 rpm .. quite a bit. Marin says GB did it too .. sinfull I say.*
This is a bit apples and oranges, but we "overprop" the planes we fly on every flight.* For maximum horsepower at takeoff we put the prop in fine pitch to generate the maximum rpm for maximum hp.* In cruise we back the pitch off (increase it) a lot for lower rpm, maximum cruise speed, and reduced throttle setting.* If we tried to take off (in the plane I fly) with the prop set for cruise, at full throttle we'd blow all the cylinder heads off.

Slightly overpropping a trawler-type boat can be more efficient IF the boat is going to be run at lower power settings.* Which is why most of the trawler manufacturers do this, or at least did this when the engines going into their boats were fairly slow-turning, non-turbocharged engines.

And no, I don't thing engines get weaker with age.* They wear out with age, but they are physically as strong as the metallurgy and design they were originally built with.

*
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Old 10-07-2008, 04:05 AM   #11
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RE: Prop-pitch

The real wonder is on the modern million dollar "cruising" trawler why the use of a CPP, controllable pitch prop isn't a standard.

For most its about a $10k investment , about what folks sink into the blinking toys suite , but it would have a payback during the life of the vessel.

The electric toys are 2 generations old by time the keel gets wet.

Guess its not that much of a wonder viewing the non servicible fuel tanks they get away installing , after decades of problems.

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Old 10-07-2008, 06:14 PM   #12
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RE: Prop-pitch

Chris, Thats interesting... about SFC. I don't know for sure ... I now wonder if most or all the SFC numbers in marine engine brochures / specs are projected or measured relative to typical boat loads. I can't imagine how that would be done as boats have different resistance curves. As for running old Ford engines hard .. do you know the continuous rating ?
Marin, Just like the boat the airplane can only be pitched correctly under one set of circumstances with a fixed pitch prop. Over proping will increase fuel economy but one must accept a big loss at the upper end of the range. If one is over proped 200 rpm and willing to accept NEVER being able to operate over 400 rpm below max rated rpm for a 10 to 15% reduction in fuel burn ... then it could be justified. That makes a liar of me as I said there should never be an exception to the loading rule ... but on the positive side that would make me flexible. I do think ( after reading the article on the GB 41 ) that running a planing hull at 8 knts is just plain stupid .. hull wise and engine wise ). I think it's unprofessional of all the manufacturers to recomend running a boat slow or/and fast. Most people are doing it, recomendations or not so if DeAntoiniois right most of the Trawler fleet will become hard starting smokers. I like what youv'e said politicly lately .. objective stuff, and I like your spring line holder.

Eric Henning
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-- Edited by nomadwilly at 19:17, 2008-10-07
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Old 10-07-2008, 07:40 PM   #13
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RE: Prop-pitch

Quote:
nomadwilly wrote:

Marin, Just like the boat the airplane can only be pitched correctly under one set of circumstances with a fixed pitch prop. Over proping will increase fuel economy but one must accept a big loss at the upper end of the range.
Correct.* Smaller planes like the Cessna 172, Piper Cherokee, etc. with fixed pitch props generally have their props pitched for cruise since that's what they will be doing most of the time.* As a result, they do not have the takeoff performance of airplanes with constant-speed/variable pitch*props, where the prop can be set fine for max rpm and hp and then backed off to coarse for most efficient cruise.* You can install climb props on fixed pitch airplanes. These flatter-pitched props*are popular on planes like the Super Cub and other planes that are used for bush work because they can then operate out of short fields, small lakes, off*gravel bars, etc.* But what you gain in takeoff performance you lose in cruise performance.

I do not have an answer for your query on the max continuous rating of the FL120.* However on our recent Desolation Sound trip I met a retired fellow who now lives on Vancouver Island but whose career was as an engine overhauler and rebuilder in England.* When he asked me what kind of engines we have in our boat and I told him Ford Lehman 120s he smiled and said he'd worked on thousands of them, of course in their original vehicular configuration.*

So I asked him a number of questions about the best way to run these engines, and described how we ran ours.* I told him we took it pretty easy with ours, running them at about 1600 rpm.* He said 1600 to 1800 was an ideal power setting as long as they ran to temperature.* He told me that the Dorsett engine (the base engine for the FL120) does not hold up well when run hard.* They can tolerate it for short bursts but high-rpm (this is an engine with only a 2500 rpm redline), high loading will wear them out prematurely.* This is why, he said, they proved to be poor engines in on-the-road vehicles.* They were great, however, for lower-load, lower rpm applications like tractors, cranes, generators, pumps, etc.* Which is why they are so successful as marine engines as long as they are run with the same relatively conservative settings.

He also said one of the worst things one can do to one of these engines is run it too cool.* If this is the only way it can be run, he said, then periodically running it hard to get the temperature up and "clean it out" is necessary.* But if the engines are running to proper temperature-- about 180 to 190 degrees---* in that 1600 to 1800 rpm band then there is never any need to run them hard periodically.* It will accomplish nothing except hasten the day the engine needs an overhaul.

Temperature, he said, is the aboslute key to longevity with these engines.* And religiously following the recommended maintenance intervals.* At the other end of the scale, he said the fastest way to kill an FL120 is let it overheat.* They are not tolerant of that at all.


-- Edited by Marin at 20:41, 2008-10-07
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Old 10-07-2008, 07:48 PM   #14
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RE: Prop-pitch

Here's the curves for my engines (actually for a newer model - mine are old enough that the original data sheets were printed on papyrus).* But the power and redline numbers are identical, so it seems to work.* (I don't know if they'll be readable until I try posting this - my apologies if they aren't).


*
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Old 10-07-2008, 07:52 PM   #15
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RE: Prop-pitch

Hmm... try it again a little bigger...
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Old 10-07-2008, 09:03 PM   #16
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RE: Prop-pitch

I think I've posted this before some time ago in another discussion. It's the performance sheet for the FL120. Some of it is hard to make out and I'll be honest--- I don't really know exactly how to interpret it. But FWIW....
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Old 10-07-2008, 09:51 PM   #17
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RE: Prop-pitch

Quote:
FF wrote:

The real wonder is on the modern million dollar "cruising" trawler why the use of a CPP, controllable pitch prop isn't a standard.

For most its about a $10k investment , about what folks sink into the blinking toys suite , but it would have a payback during the life of the vessel.

The electric toys are 2 generations old by time the keel gets wet.

Guess its not that much of a wonder viewing the non servicible fuel tanks they get away installing , after decades of problems.

FF
FF(and Marin), I think it is an issue of reliability and training/proficiency.* The manufacturer has to be certain that the engine is going to provide the service that they guarantee.* If you put that guarantee into the hands of the operator, then who know what could happen.* IOW, a variable pitched prop in the hands of an ignorant operator could shorten engine life drastically.* On thing that helps airplanes in this regard is gravity and self-preservation!!!!
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Old 10-08-2008, 04:02 AM   #18
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RE: Prop-pitch

" IOW, a variable pitched prop in the hands of an ignorant operator could shorten engine life drastically. "

Agreed BUT,

This is the electronic age and electric controlled truck fuel injection AND auto tranny shifting is 25+ years old in Coaches and many large (engine transplant size) OTR trucks .

Hard to believe with the electric crap on new boats to Kow Tow to the Air Police that a prop pitch adjustment setup would cost very much.

The equivelent of a FADEC (for those 200hr jet copilots) should be a $100 actuator hook up to an already electric engine.

So the delivery captain wont be able to put 20 years of wear , running full throttle , with NO cares about the engine specifications , as usual.

Bullet proof ? Maybe , but Ignorant ready ,for sure.
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Old 10-08-2008, 04:10 AM   #19
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RE: Prop-pitch

Looking at the chart you will see that at 1800rpm the prop is only capable of absorbing 90 hp.

That 90 HP is useable at under 1400rpm and will give longer life from better loading and the lower rpm.

That is the purpose of a cruising prop, quiet efficient long lived engines.

If with your hand you simply move the prop power absorbtion curve to the left , you can decide how to set up the boat.

By an EGT if you do, and use the FLANK rpm minus 300rpm rule!
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Old 10-08-2008, 02:02 PM   #20
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RE: Prop-pitch

Chris, I see your best SFC comes in at a tad over 2100 rpm. I can see why you were supprised. Mine comes in at 1800. I usually run the engine much faster than that .. at less efficent speeds but burning .87 gph .. who cares. Couldn't see anything about continious rating on your charts. My Mitsu is rated cont max hp - 35 at 3000 rpm and max intermitent hp - 37 at 3000 rpm. So from the hp curve it looks like I can run all day long at 2700 rpm IF I'm proped at 3000. .. but since my new engine has picked up 100 rpm now that it's broken in it seems I can run at 2800 continiously. Since I run at 2300 most of the time I spoze I'm probably a bit under proped myself.
Marin, I think I'd rather talk to an engineer than shop worker about operational proceedures. I think we have a good idea of the maximum load we can put on our engines. If we run an engine too cool other kinds of dammage will result. Steve D says that point is 75% of load. You guys say it's much less cause your engines are old. When my engine gets old can I under load too? Why don't you guys ask Steve on the PMM website .. I don't go there. One thing you guys could probably do to make your engines happier is to bypas or remove your oil coolers. It would be easy to install oil temp gauges and learn what the ideal operational oil temp is. If your'e far too low .. consider my idea.

Eric Henning
Willard 30
Thorne Bay AK
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