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Old 02-28-2013, 10:38 PM   #21
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Per the vicprop calculator link provided above, the subject boat is probably over propped for twin 120 Lehmans (wasn't sure about max rpm and gear ratio). I'm an over propping afishianado.
2.5-1 ratio 2500 max rpm. Yes it is over propped but I prefer it that way. Does the calc say what it should be if I used the 24" wheel or the 25" wheel?
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Old 02-28-2013, 11:12 PM   #22
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2.5-1 ratio 2500 max rpm. Yes it is over propped but I prefer it that way. Does the calc say what it should be if I used the 24" wheel or the 25" wheel?
I was revising the last post when you added this new info. So I went back again and ran the calculator for 2500 rpm and 2.5 ratio...41 lwl, 50,000, 15' beam, 4.5 draft... Calculator spit out:

3 blade 26x 19.6 4 blade 24.4x19.2

It is overpropped, but not nearly as much as when I first ran it with 2:1 ratio..that's what happens when one assumes...yikes!

A note at the bottom of the calculator says the rule of thumb of 1" diameter equals 1.5 to 2" of pitch is ballpark. So .6 inches of additional diameter x 1.5" pitch is .9'' change in pitch. 19.2 minus .9 = "18.3 pitch....

25 x 18.3 is a wag if propping a four blader for max power.

You might be able to force the calculator by feeding in some artificial numbers for power, rpm etc. Slick tool, easy to use. Check it out.
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Old 03-01-2013, 12:28 AM   #23
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Looks good to me.

It's probably 2.57-1 and 2500 rpm.

Re the over propping I think people do it for noise reduction.

And most trawler people think there's something virtuous about lower rpm.

I like to hear a diesel engine work ... especially a DD.
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Old 03-01-2013, 06:26 AM   #24
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The very low power cruise speed can be made more ideal (slightly more efficient) by over propping but no engine manufacturer recommends it.

Engine Mfg want the smallest load on the engine to get it to live past the warentee period.

GB seems to overprop, , along with many other builders to increase the comfort on board and engine life.

Simply take a look at ANY "prop/ hp graph" vs shaft rpm and see how rapidly the prop load falls compared to HP aviliable from the mfg.

That power required drop is why every engine can operate without lugging with a small (10% rpm) pullback.

Of course all is within reason, prop a 2100rpm engine so it cant pull 1400, and it will probably still be overloaded 10% down.
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Old 03-01-2013, 09:31 AM   #25
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Looks good to me.

It's probably 2.57-1 and 2500 rpm.

Re the over propping I think people do it for noise reduction.

And most trawler people think there's something virtuous about lower rpm.

I like to hear a diesel engine work ... especially a DD.

I think that more and more owners will begin to over prop for efficiency. Engine manufactureres be damned. And more and more data will begin to pool on sites like this, which will help folks set a safe red line on the tach (or position on a throttle stop). I'm not a fan of the 10% rule of thumb....there is data out there for most engines that can provide a fairly accurate limit setting.

I'd say most owners who operate their trawler style cruisers at low speed think there is something virtuous about operating their boats efficiently....especially those who have oversized engines blubbering down in the hole. A bigger prop and perhaps lower engine rpm happens to be a small step in that direction

As you say, if you like to hear a diesel truly work, overpropping is your thing. Not sure that necessarily equates to noise reduction. A different kind of noise, perhaps.
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Old 03-01-2013, 11:44 AM   #26
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If these overproppers were really looking for efficiency they'd be buying smaller boats and more efficient boats .. or even sailboats run under power. Look at the other thread discussing cruising speed. The OP has a 34' boat w 260hp. The boat needs about 40 to cruise. The people who buy these boats new have enough money to buy whatever amount of fuel necessary. Like an old Cadillac ... worth less than the same age economy car.

"I'd say most owners who operate their trawler style cruisers at low speed think there is something virtuous about operating their boats efficiently...."

True .... a very small amount ... and efficiency is very vogue these days. Good point but not objectively sound. Yea I know "objectivity" be damned. But if one ALWAYS runs at a very light load overpropping actually increases the load and permits running at a lower rpm for the same speed and hence the small increase in fuel efficiency. Like FF says ... it works ... but to what degree and w what restrictions and limitations?

Objectively the "small amount" isn't worth the downside. If you run an overpropped engine hard big trouble could result but if propped correctly you almost can't hurt them.

But in the overall scheme of things perhaps it really dosn't make much difference. You OP guys are wearing me out.

When I said "work" I meant singing along at 400rpm down from rated speed, propped correctly doing real work ... NOT lugging. That's stupid and I doubt if anyone here is actually lugging.
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Old 03-01-2013, 12:04 PM   #27
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I like to hear a diesel engine work ... especially a DD.
Me too! A lot of folks (rag baggers) like the sound of water slapping against the hull but to me the sound of a well tuned diesel is pure music and should not be drowned out by music!
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Old 03-01-2013, 08:46 PM   #28
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[QUOTE=manyboats;138080]If these overproppers were really looking for efficiency they'd be buying smaller boats and more efficient boats .. or even sailboats run under power.

Objectively the "small amount" isn't worth the downside. If you run an overpropped engine hard big trouble could result...

But in the overall scheme of things perhaps it really dosn't make much difference. You OP guys are wearing me out.

When I said "work" I meant singing along at 400rpm down from rated speed, propped correctly doing real work ... NOT lugging. That's stupid and I doubt if anyone here is actually lugging.[/QUOTE

Don't want a smaller boat. Want smaller engines, different gear ratio, and bigger props on the one I have. That's definitely too expensive to be worthwhile. I'd estimate 5% is possible with a reprop alone.

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

The objective is to wear you out and make you an ardent supporter.
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Old 03-01-2013, 09:11 PM   #29
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Old 03-01-2013, 10:25 PM   #30
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I've been making this argument for over 5 years. I guess that's why I'm getting worn down. That and the fact that the over proper's are presenting this practice as a responsible way to drive a boat and many are reading this stuff and buying into it. If it wern't for me they'd only have one opinion and most here don't know much about this. Listening to you guys would/does sound like you know what you're talking about and they would take this over propping as the golden thing to do. At least w me continuing to post against it they have a 2nd opinion to consider and may objectively research the issue and make their own decisions.

Well I'll stand down and not continue the crusade but I can't support it. You said it yourself .... 5%. Thinking about the downside elements 5% isn't enough to justify over propping in my opinion. Just not worth it.

I will say though that over propping w/o going to extremes can work fairly well if great care is taken not to over load the engine and if the correct prop is installed before an attempt is made to sell the boat.
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Old 03-02-2013, 06:42 AM   #31
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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
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Old 03-02-2013, 08:23 AM   #32
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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.
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Old 03-02-2013, 10:09 AM   #33
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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?
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Old 03-02-2013, 10:50 AM   #34
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Why? Look at my wake in my pics and it's hard to argue that we don't have essentially a full displacement hull. .
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.
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Old 03-02-2013, 11:07 AM   #35
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As you say, if you like to hear a diesel truly work, overpropping is your thing. .
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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Old 03-02-2013, 12:06 PM   #36
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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.

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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Old 03-02-2013, 12:15 PM   #37
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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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Old 03-02-2013, 12:17 PM   #38
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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.
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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Old 03-02-2013, 12:27 PM   #39
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And if you're worried about the next buyer...reprop it when it goes up for sale.

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.
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Old 03-02-2013, 12:35 PM   #40
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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.

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.
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