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Old 03-02-2013, 12:36 PM   #41
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Originally Posted by psneeld View Post
Exactly...every time a forum member says you're really screwing the pooch for coloring outside the lines whether overpropping or ignoring ABYC suggestions gives some of us NO CREDIT for being smart enough to handle the situation from either the buying or selling end of things.

To them I say ..."enjoy staying in the box"...

ABYC - I wasn't aware they had propping guidelines. If they did, what would they say about intentionally wrong, oops, overpropping?

Fear not PS, I give you the credit you are due.
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Old 03-02-2013, 12:44 PM   #42
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I thought just a week back someone said they had a new Grand Banks that the calculators and actual performance suggested it was overpropped...

Hope he pipes up soon!!!!

As far as real world experience....I've got a little...

I have put about 5000 hours on a 454 big block in a 26 Shamrock for an assistance towing company. Towing also overloads the crap out of most engines when they are already overpropped. Many experts suggest that marine gassers ralely last much more than 2-3000 hours...well our fleet of 4 boats has proven them wrong a long time ago.

The boat I run...about a 1000 hours ago (now over 5000 total hours)...in a Noreaster towing a runaway barge back to a marina...I had the engine on the pins for over 2 hours till I made it to safety...the engine should turn 4400-4800 if my memory serves correctly...We prop it so it never turns more than about 4200 and that night with the barge...I was at the pins and she was maxed at 3000 rpm....never overtemped...1000 hours later and still towing.

The overproping we do makes quite the stump puller out of the 26 shamrock...but we give up probably 5-6 knots at the top end. It also give us better fuel economy while towing...again...with no extra instrumentation or throttle stops (which I often hit while pulling someone off a sandbar but usually back off quickly because the prop cavatates and the boat shkes so bad I cant read the tach anymore...

So argue away chaps...but people do it...more than people would know because lot's of commercial guys do it...for a good reason...it helps in situations it's called for and hopefully done responsibly...etc...etc
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Old 03-02-2013, 12:45 PM   #43
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Greetings,
Here's a question, which might have been answered already but I missed it: We're talking about a non variable pitch prop here. Is there an optimum rotational speed (RPM) where the prop is most efficient or is it efficient through a broad range of RPM?
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Old 03-02-2013, 12:49 PM   #44
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ABYC - I wasn't aware they had propping guidelines. If they did, what would they say about intentionally wrong, oops, overpropping?

Fear not PS, I give you the credit you are due.
Read carefully...it says overpropping OR ignoring ABYC suggestions ....
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Old 03-02-2013, 12:53 PM   #45
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Over propping is a very expensive worn out gimmick to follow if you have to buy new and bigger props and then hope all is well a few years down the road when selling time comes. Fuel savings are spurious at best and no reputable boat or engine builder today supports intentionally wrong propping as opposed to installing a right sized engine prop combination to begin with.

If you want to run slow, just pull back on the throttles.

I would never purchase new props...used all the way. Almost a wash if you sell the ones currently on the boat. Yes, manufacturers are more cognizant of efficiency these days. But they were idiots 15-20 years ago when they built thousands of boats with oversized twins. That's mostly what this discussion is about. Most of those overpowered twin hulls are very very nice boats....they're just pigs on fuel when operated where the manufactuere designed them to run. Some of us don't feel compelled to live with their mistakes and would reset the operating envelope toward slow speed efficiency at the expense of speeed. Simple engineering exercise that can be done very safely with a little planning and some data. Keep in mind that airplanes have had redlines forever.

I have pulled back on the throttles...not good enough. Probably the best option for some of these older boats is to convert the turbo engines into NA's if parts can be found cheaply. Then do the prop swap.
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Old 03-02-2013, 01:04 PM   #46
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If you run with the throttles pulled back all the time...overpropping and adjusting the throttle limit makes perfect sense.

I still think that is all manufacturers do with the different "duty ratings"...drop the rpm limit whick I would think would adjust a few other parameters along the way and a natural overprop situation develops per their specs...not sure but maybe...
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Old 03-02-2013, 02:20 PM   #47
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As far as real world experience....I've got a little... on a 454 big block --We prop it so it never turns more than about 4200 and that night with the bargesituations it's called for and hopefully done responsibly...etc...etc
I found the same identical thing on a 454 gasser. 4200 was the sweet spot and gave me a smoother top end of around 46 mph in a 24' Searay. I'm not sure marine diesels have the same tolerance for lugging that our beloved 454 gassers do.
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Old 03-02-2013, 03:00 PM   #48
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I found the same identical thing on a 454 gasser. 4200 was the sweet spot and gave me a smoother top end of around 46 mph in a 24' Searay. I'm not sure marine diesels have the same tolerance for lugging that our beloved 454 gassers do.
No one is really suggesting the torture tests like I am used to...but even the little commercial tugs I operate have all sorts of overproppping, hull reconfigurations, etc...etc...that some forum members would argue till they are blue in the face "wouldn't/don't work"...yet they do.

Sure...there's a lot of backyard boat-adjusting that doesn't work too....but broad sweeping statements that manufacturer's know best is shutting your eye's to a whole world of reality out there.

As a private trawler owner, I feel responsible number crunching and living within those tolerences we set for ourselves gives us the ability to improve out individual needs over some generic formula the manufacturer sold to the first buyer.
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Old 03-02-2013, 03:20 PM   #49
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I would never purchase new props...used all the way. Almost a wash if you sell the ones currently on the boat. Yes, manufacturers are more cognizant of efficiency these days. But they were idiots 15-20 years ago when they built thousands of boats with oversized twins. That's mostly what this discussion is about. Most of those overpowered twin hulls are very very nice boats....they're just pigs on fuel when operated where the manufactuere designed them to run. Some of us don't feel compelled to live with their mistakes and would reset the operating envelope toward slow speed efficiency at the expense of speeed. Simple engineering exercise that can be done very safely with a little planning and some data. Keep in mind that airplanes have had redlines forever.

I have pulled back on the throttles...not good enough. Probably the best option for some of these older boats is to convert the turbo engines into NA's if parts can be found cheaply. Then do the prop swap.
Regarding your "they were idiots" comment (does this then mean the current owners?):

There is a currently active thread on boatdiesel called "Boost Pressure Significantly Different ---" that may be relevant. Tony's response #21 regarding different sized props says "it takes the same HP to move the boat at 8 knots regardless of RPM."

You may want to take up your point with him. I'll watch for your post there.
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Old 03-02-2013, 03:32 PM   #50
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Regarding your "they were idiots" comment (does this then mean the current owners?):

There is a currently active thread on boatdiesel called "Boost Pressure Significantly Different ---" that may be relevant. Tony's response #21 regarding different sized props says "it takes the same HP to move the boat at 8 knots regardless of RPM."

You may want to take up your point with him. I'll watch for your post there.
Reread what you posted very carefully...is there no efficiency and other good attributes to be gained from running at lower RPM if you don't exceed any undesireable limits?

Remember, the original poster like me is overpropping diesels rated at 120 hp that we are only extracting 20-40 hp from usually...not 80-90 hp from a 100 hp engine....
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Old 03-02-2013, 04:02 PM   #51
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Remember, the original poster like me is overpropping diesels rated at 120 hp that we are only extracting 20-40 hp from usually...not 80-90 hp from a 100 hp engine....
The boatdiesel thread references a Cummins 6BT rated at 250 HP where the owner is using 50 to 100 hp. Pretty similar power ranges I'd say to what you and Twisted are bringing up. It still seems accurate to say that at an 8 knot boat speed, the same HP is required whether say an engine RPM of 1400 or 1700.

But go ahead, post your over propping theories on the boatdiesel thread.
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Old 03-02-2013, 04:41 PM   #52
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The boatdiesel thread references a Cummins 6BT rated at 250 HP where the owner is using 50 to 100 hp. Pretty similar power ranges I'd say to what you and Twisted are bringing up. It still seems accurate to say that at an 8 knot boat speed, the same HP is required whether say an engine RPM of 1400 or 1700.

But go ahead, post your over propping theories on the boatdiesel thread.
I don't need to, I live them there theories.....

And I can accet that a certain HP is required to move a boat at a certain speed...what I'm not sure about and I haven't had anyone including experts explain carefully enough is exactly how and when a certain hp is extracted at what efficiency (and at what rpm/prop size/hull weight/shape). Boatdiesel or otherwise.

All I know is that a bunch of hardworking commercial guys I work with swear by it in certain situations. I wouldn't call them amateurs...
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Old 03-02-2013, 04:48 PM   #53
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Regarding your "they were idiots" comment (does this then mean the current owners?):

There is a currently active thread on boatdiesel called "Boost Pressure Significantly Different ---" that may be relevant. Tony's response #21 regarding different sized props says "it takes the same HP to move the boat at 8 knots regardless of RPM."

You may want to take up your point with him. I'll watch for your post there.
No need to get nasty and personal. I'm not a member of Boat diesel and have no interest in rejoining. Thanks for the recommendation, though.

Tony should have said that it takes the same thrust to move the boat....there is a difference and that is precisely what we've been discussing.
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Old 03-02-2013, 04:54 PM   #54
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Greetings,
Any answer to post #43?
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Old 03-02-2013, 04:54 PM   #55
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Over propping is a very expensive worn out gimmick to follow if you have to buy new and bigger props and then hope all is well a few years down the road when selling time comes. Fuel savings are spurious at best and no reputable boat or engine builder today supports intentionally wrong propping as opposed to installing a right sized engine prop combination to begin with.

If you want to run slow, just pull back on the throttles.
I agree. Just like a fuel management system would never pay off for me, spending a couple thousand dollars on the wrong sized props with the hope of saving fuel and then putting the originals back on when it comes time to sell makes no sense whatever.

As for those who claim to have gained considerable efficiency by overpropping, these are subjective, not objective opinions. It would be very difficult and take some expensive test equipment to make an objective test.

Remember the magnets you put on your car's fuel lines to magnetize the fuel and get better mileage? Lots of people swore that they worked. Could it be they were just a little more gentle with the gas pedal?
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Old 03-02-2013, 05:25 PM   #56
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Simple calculation to compare the static thrust of two comparable props at the same horsepower. I've done Gerr''s bollard pull calculation for 24x20 and a 26x20. The static thrust differential is about 7%. How well that translates across the speed range goes to the RT Firefly question. I posted the formula a while back...check it out.
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Old 03-02-2013, 05:34 PM   #57
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Thinking that merely "overpropping" by some amount is a simple solution is ill advised. Of course, if you go over to a much more authoritative forum such as Boat Diesel, you can begin to understand (and be forced to argue about in an informed fashion) that the prop is only part of the equation, to get the desired result outside of the box defined by the boat's original drive train set up. Working back up the system, shafting, reduction gears and engine tuning all come into play.

Mere over propping is a recipe for shortened engine life. It puts stress on the engine all the way through the power curve. Guys try it a lot to get better "hole shot" acceleration out of their outboard boat, or more speed out of their big sportfisherman as well as yes, better "bite" at lower speeds for fish-on-the-line maneuvers.

So if you want to really learn and converse about this issue, go over to Boat Diesel, and/or get yourself a copy of Dave Gerr's "The Propellor Handbook".

Same applies to the "Underloading"thread too.
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Old 03-02-2013, 06:30 PM   #58
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my original question was does anyone know what the efficiency and speed difference would be between the two props?
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Old 03-02-2013, 06:37 PM   #59
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Greetings,
Use your cursor... Eel slap!
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Old 03-02-2013, 06:42 PM   #60
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