25x20 or 24x22 props?

Why would anyone who is aware enough to purposely overprop their boat...purposely lug the engine?

The mistake is thinking that matching the boat , engine prop and cruise speed is lugging.

The boat is ONLY "overproped " if the operator attempts to operate outside the range the boat was set up for.

To assist in not harming the engine a slight bit of operator understanding is required , the RED line on the tachometer has meaning , as does the RED line on the EGT gauge.

"Thinking about the downside elements 5% isn't enough to justify over propping in my opinion. Just not worth it. "

Would moving the cruise RPM from 1900 , down to 1500 be worth it?

There is lots more to Efficiency than just fuel burn

And if you put a physical stop so no knucklehead could push your boat to the "old pin"...overpropping responsibly is no big deal.

If you have done the math and taken some verification measurements...many would agree it's not only OK but more commonly done than what some would have us believe.

If you are still worried abut engine health...you can install an array of monitoring gauges that will tell you what your engine thinks of the whole deal.

And for those that say 5% isn't worth the effort...if it's part of an overall improvement...then it most certainly is...at least to them.

With the way manufacturer's play with "duty" ratings...and essentially the same engine...I'm not so sure that Cruisepropping (overpropping is a dity word obviously) doesn't fit right into the scheme of things anyway.

psneeld wrote;

"And for those that say 5% isn't worth the effort..." Effort wasn't my word. Has almost nothing to do w effort. Hardly any involved. It's risk.

"And if you put a physical stop so no knucklehead could push your boat to the "old pin"...overpropping responsibly is no big deal." That's true but I'll bet only an extremely small percentage do. But done that way I think it's OK .. as long as the position of the throttle stop is carefully chosen.

FF Very true. Over propping isn't overloading unless one runs the engine hard enough to actually over load it and a EGT will probably tell what engine speed that that occurs. "To assist in not harming the engine a slight bit of operator understanding is required , the RED line on the tachometer has meaning , as does the RED line on the EGT gauge." Indeed and if such understanding is applied over propping should be OK. FF says "There is lots more to Efficiency than just fuel burn" Like what?

Daddyo said:

Why? Look at my wake in my pics and it's hard to argue that we don't have essentially a full displacement hull. .

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I think your non wake is more a reflection of the very low speed you choose to run. Throttle it up to 1700 - 1800 RPM and you'll kick up a rooster tail like the rest of us DeFevers and get there a lot quicker. My throttle setting goal is to never let a Nordhavn less than 55' pass me.

skidgear said:

As you say, if you like to hear a diesel truly work, overpropping is your thing. .

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Not mine. The best diesel/drive train health test is to now and then run at max rated RPM to see if it overheats, shakes, shudders or otherwise objects. For sure during a "new boat" seatrial not being able to achieve rated RPM without problems is a red flag.

Over prop all you like, but don't always count on a savvy buyer accepting your over propping (non) rationale. Especially on a boat made since the turn of the last century.

sunchaser said:

Not mine. The best diesel/drive train health test is to now and then run at max rated RPM to see if it overheats, shakes, shudders or otherwise objects. For sure during a "new boat" seatrial not being able to achieve rated RPM without problems is a red flag.

Over prop all you like, but don't always count on a savvy buyer accepting your over propping (non) rationale. Especially on a boat made since the turn of the last century.

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Yes, I completely understand the current prevailing wisdom. But if I found an overpropped boat that had all the supporting engine and propeller data for setting a new red line, or throttle stop, or even better an EGT system with supporting data....I'd say to myself, theres an owner operator who has his stuff together. Now if it's overpropped with no rationale or operating guidelines, then I'd be concerned. That's why I don't care for the back off 10% rule of thumb.

Your drive train health test could just as easily be run at the new red line or throttle stop or EGT limit. The prevailing wisdom and the definition of "savvy" is going to change if fuel prices keep rising.

And if you're worried about the next buyer...reprop it when it goes up for sale.

skid says;

"The prevailing wisdom and the definition of "savvy" is going to change if fuel prices keep rising."

Yes and there will be a lot more full disp trawlers w no more power than they need. But in our culture that will be a long way off.

skidgear said:

Yes, I completely understand the current prevailing wisdom. But if I found an overpropped boat that had all the supporting engine and propeller data for setting a new red line, or throttle stop, or even better an EGT system with supporting data....I'd say to myself, theres an owner operator who has his stuff together. Now if it's overpropped with no rationale or operating guidelines, then I'd be concerned. That's why I don't care for the back off 10% rule of thumb.

Your drive train health test could just as easily be run at the new red line or throttle stop or EGT limit. The prevailing wisdom and the definition of "savvy" is going to change if fuel prices keep rising.

And if you're worried about the next buyer...reprop it when it goes up for sale.

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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"...

skidgear said:

And if you're worried about the next buyer...reprop it when it goes up for sale.

Click to expand...

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.

manyboats said:

skid says;

"The prevailing wisdom and the definition of "savvy" is going to change if fuel prices keep rising."

Yes and there will be a lot more full disp trawlers w no more power than they need. But in our culture that will be a long way off.

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I'm not so sure about the "more full displacement trawler" (style coastal cruisers). For the 35-45' segment I'd guess the efficiency push will result in designs with a single engine large enough to drive a planing or semi-planing hull to 14-15 knots...no more. Light weight materials and construction methods will be the other part of the efficiency equation. For example, solid FRP bottoms will fade away. The deckhouse structure will still have the "salty" look that strokes folks egos. Painfully slow, full displacement hulls will continue as a niche segment. But the ones that do get built will have properly sized engines as you say. Interesting subject for a new thread.

psneeld said:

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"...

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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.

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

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?

sunchaser said:

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.

Click to expand...

Read carefully...it says overpropping OR ignoring ABYC suggestions ....

sunchaser said:

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.

Click to expand...

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.

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...

psneeld said:

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

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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.

sunchaser said:

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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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.

skidgear said:

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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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.

sunchaser said:

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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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....

psneeld said:

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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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.

sunchaser said:

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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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...

sunchaser said:

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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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.

Greetings,
Any answer to post #43?

sunchaser said:

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.

Click to expand...

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?

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.

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.

Greetings,
Use your cursor... Eel slap!