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Old 02-10-2013, 11:22 PM   #101
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Bottom fish are the best. Yellow Eye Rockfish, Ling Cod and of course Halibut.
In that order.
OMG! Again, I agree with Eric! (I must get a check up PDQ.
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Old 02-11-2013, 10:22 AM   #102
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I spent oodles of time creating comparative spreadsheets, picking my engine, calculating pitch (thanks Dave Gerr), and installing it. Almost as much fun as using the boat.
I've been toying with the idea of cruise propping our hugely overpowered, semi-planing boat. I'm confident that the engine overload issue is easily addressed with some simple operating limits. The idea is to redefine the operating envelope toward the lower speed end. Anyway, I looked at Gerr's books and didn't see comparative data showing thrust/rpm/power curves for various prop sizes and configurations. Anyone know of a source for generic propeller thrust/efficiency data in chart or table format? I have a prop curve for the boat, but of course it's based on the rated power for the engine.

Thanks
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Old 02-11-2013, 10:32 AM   #103
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I've been toying with the idea of cruise propping our hugely overpowered, semi-planing boat.
Thanks
  • What engines?
  • Post question with boat, engine and current prop details on boatdiesel.com - they also have a prop calculator
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Old 02-11-2013, 01:26 PM   #104
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  • Post question with boat, engine and current prop details on boatdiesel.com - they also have a prop calculator
Thanks. I'm familiar with the boatdiesel calculator. It's designed for selecting the "correct" prop. Don't believe it will be of much use for an over-propping analysis across a range of boat/engine speeds. I'm really looking for comparative prop efficiency data. Thought I'd ask the pros here first.

By the way, there's a half decent prop calculator at the address below. I ran it for our boat and it was on the money for diameter, but about an inch too shallow for pitch. (They have a note about using 90% engine power for the calculation, so that probably accounts for the difference). Another great tool is Dave Gerr's Propeller - Diameter/Power/RPM chart....allows you to play around with gear ratios, engine power, and prop diameter. Combine that with a decent engine curve and a prop curve, and the possibilities regarding an over prop begin to emerge. Problem is it doesn't clearly define potential fuel savings. Anyway...

www.vicprop.com/calculator.htm
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Old 02-11-2013, 02:55 PM   #105
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skidgear,
I't's been stated here on the forum by many that ther'e is savings from over propping but it's very small.
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Old 02-11-2013, 05:22 PM   #106
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skidgear,
I't's been stated here on the forum by many that ther'e is savings from over propping but it's very small.
Yes, I know what owners of low powered, primarily hull speed boats have said. But there's virtually no data. And like you, I prefer documentation, or at least a solid analysis. And since my boat configuration is in another power/prop category, I'm taking a look for myself.

The extent of "very small" must vary with the extent of the over prop. What, exactly, is that relationship. That's a question which is very similar to those you posed in your prop efficiency thread...I'm just relating it directly to the perennially popular overprop proposition.

So, have you run across any useful data in your search?
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Old 02-12-2013, 07:16 AM   #107
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So, have you run across any useful data in your search?


Sorta hard to get hard data on the extent of engine life extension by properly loading an engine.

There are many tales of woe from the fish guys (bigger engines and noisemakers) about the disaster of underloading.

A 25% decrease in fuel burn is of no consiquences on a 200 hour a year 3GPH boat.

How long between engine rebuilds IS a concern.
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Old 02-12-2013, 09:21 AM   #108
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So, have you run across any useful data in your search?


Sorta hard to get hard data on the extent of engine life extension by properly loading an engine.

There are many tales of woe from the fish guys (bigger engines and noisemakers) about the disaster of underloading.

A 25% decrease in fuel burn is of no consiquences on a 200 hour a year 3GPH boat.

How long between engine rebuilds IS a concern.

More conjecture. I'll see if the Skenes tables you mentioned in another thread have any useful information.
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Old 02-12-2013, 09:36 AM   #109
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"More conjecture"

National Fisherman has done a number of articles on the price of under loading .

And given a number of techniques to help service life.

Perhaps a search of their archives , although most pro fish guys do have larger engines 400-700hp being common.
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Old 02-12-2013, 11:16 AM   #110
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"More conjecture"

National Fisherman has done a number of articles on the price of under loading .

And given a number of techniques to help service life.

Perhaps a search of their archives , although most pro fish guys do have larger engines 400-700hp being common.

I was referring to your 25% fuel savings statement, not engine service life, which for a recreational boat I'd consider more of a byproduct from a cruise prop. Quantifying relative fuel savings with a sound analysis is my objective. While I do have a spare set of larger props, I want to understand what's possible before going through the trouble of swapping them out....it's this winter's mental gymnastics exercise.

Again, my question is in regard to finding data in chart or tabular format which I can use to quantify through simple analysis, the relative benefits of one propeller over another. Stating that the savings is very small, or stating that it's 25% (fairly large) is conjecture.

I just googled for tables from Skeene's that might be on the net and found nothing specific, so my next stop is the library. But from references that I did read, I don't expect to find much more than what I have from Gerr's books. I might be able to back into some comparative data by messing around with horsepower and gear ratios in the prop calculator that I mentioned earlier. I'd prefer to avoid making my own charts via tedious calculations, but it appears that's probably the next stop.

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Old 02-12-2013, 11:34 AM   #111
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Again, my question is in regard to finding data in chart or tabular format which I can use to quantify through simple analysis, the relative benefits of one propeller over another. Regards
Skidgear

The way the pros (commercial Post type charter fishermen, offshore racers etc) do this is to first understand that each boat is different. Then they do as you are, dial in on paper their best prop setup and have a prop on either side of the diameter and pitch equation to test as well to see if any benefits arise by going bigger or smaller. I had this exact test done years ago on a high speed boat. Many choose to go for quiet and smoot props which are not necessarily the most fuel efficient. I've a friend who tinkered with this for years after his boat was initially - "perfectly" propped. Your quest can be frustrating and expensive.

Accurate fuel flow and speed measurement is a must. But, and a very big but, at what boat load conditions do you plan ot run the majoriity of the time because the prop selection goes to hell as you add or subtract tons of water, gear and fuel or hit a small stick that bends a prop.

What is your boat, desired perfect prop speed range and engine(s) combination?
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Old 02-12-2013, 12:37 PM   #112
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Skidgear

The way the pros (commercial Post type charter fishermen, offshore racers etc) do this is to first understand that each boat is different. Then they do as you are, dial in on paper their best prop setup and have a prop on either side of the diameter and pitch equation to test as well to see if any benefits arise by going bigger or smaller. I had this exact test done years ago on a high speed boat. Many choose to go for quiet and smoot props which are not necessarily the most fuel efficient. I've a friend who tinkered with this for years after his boat was initially - "perfectly" propped. Your quest can be frustrating and expensive.

Accurate fuel flow and speed measurement is a must. But, and a very big but, at what boat load conditions do you plan ot run the majoriity of the time because the prop selection goes to hell as you add or subtract tons of water, gear and fuel or hit a small stick that bends a prop.

What is your boat, desired perfect prop speed range and engine(s) combination?
For discussion purposes say the boat is 30,000 pounds, semi-planing, twin turbo 250's, with 2:1 transmissions. The target "efficiency" speed for discussion purposes is roughly 8.5 knots. Prop speed is currently 750 rpm at that hull speed. It should change incrementally with larger, more efficient props.

But keep in mind that I'm not thinking in terms of optimizing the "perfect/correct" prop as seems to be the case in your charter boat example. The cruise prop scenario at its most basic, installs a bigger "wrong" prop while keeping the rest of the drive train unchanged...same engines...same gear ratio, etc. So, start with a perfect prop and increase the diameter an inch at a time. Then plot the fuel specifics across the speed range for each new prop size. I understand that efficiency gains will be small (and boat specific)...but that's not the point of the exercise. Documenting an examplle of what we're really talking about when it comes to cruise props is what I'm trying to quantify.....curiousity if nothing else.

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Old 02-12-2013, 04:44 PM   #113
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GB slightly overpropped all their boats back when the typical powerplants were one or two FL120s or FL135s. As they began putting larger and larger engines in their boats starting in the mid-80s i don't know if they continued the overpropping practice or not.
They still do. My 2009 47' EU was over-propped by about 2".
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Old 02-12-2013, 04:54 PM   #114
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...just for grins I ran some numbers through Gerr's "bollard pull" thrust formula using one half of the total horsepower (58 SHP) required to move the hull at its calculated hullspeed as a constant. That horsepower number was "derived" by jacking around with the vicprop calculator.

Per the Gerr formula, thrust for one 24" generic prop (absorbing 29 SHP) came out to about 938 pounds. Thrust for one 26" generic prop @ 29 SHP is about 1005 pounds. And thrust for one 28" generic prop @ 29 SHP is about 1056 pounds. That's about 7% thrust improvement going up to a 26", and about 12.5% jumping from a 24" to a 28". Of course, thrust is what moves the boat. My boat has room for 26" props....14% total improvement in thrust for a given horsepower....

A point of interest...perhaps...
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Old 02-12-2013, 04:55 PM   #115
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Sorry I'm coming in a little late here...

Yes, with diesels you really need to ditch the whole concept of mixture. All that matters is that there is enough air to fully burn the injected fuel. It's fine to have too much air, but if there isn't enough, you start to get black smoke and all you are doing by injecting more fuel is making more black smoke. Throughout a diesel's whole power range, there is always more air than needed right up until it reaches max power. That's the upper curve in the charts that David posted. If you try to push above that line there is no longer enough air in the cylinders to completely burn the fuel.

The whole concept behind a turbo on a diesel is to jamb more air down it's throat so you can successfully more fuel. If you can jamb in more fuel, then all of the sudden that power line moves up 'cause you can burn more fuel.
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Old 02-12-2013, 05:36 PM   #116
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They still do. My 2009 47' EU was over-propped by about 2".

No way!!!!

Go back to post #30 where it states "Overpropping is not adult behavior."....surely you are mistaken....

I also think I read from some zealous TF poster...that no way would a manufacturer ever overprop a boat...
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Old 02-12-2013, 05:44 PM   #117
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Sorry I'm coming in a little late here...

Yes, with diesels you really need to ditch the whole concept of mixture. All that matters is that there is enough air to fully burn the injected fuel. It's fine to have too much air, but if there isn't enough, you start to get black smoke and all you are doing by injecting more fuel is making more black smoke. Throughout a diesel's whole power range, there is always more air than needed right up until it reaches max power. That's the upper curve in the charts that David posted. If you try to push above that line there is no longer enough air in the cylinders to completely burn the fuel.

The whole concept behind a turbo on a diesel is to jamb more air down it's throat so you can successfully more fuel. If you can jamb in more fuel, then all of the sudden that power line moves up 'cause you can burn more fuel.

Aren't there other factors involved here like cylinder temperature that determine combustion eficiency? Dosen't the compression, temp, ratio of reactants also determine the type and amount of pollutants and of course if to rich raw fuel emitted into the air and or water.
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Old 02-12-2013, 05:46 PM   #118
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I resemble that person psneeld.

GB may do what ever they want to do and because they do X,Y or Z dosn't mean it's good practice. I'm very surprised to hear they do over prop though.

Engine manufacturers don't recommend it.

By the way I'd rather you called me by name than "zealous". It's rude.
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Old 02-12-2013, 06:43 PM   #119
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GB may do what ever they want to do and because they do X,Y or Z dosn't mean it's good practice. I'm very surprised to hear they do over prop though.

Engine manufacturers don't recommend it.
I wish Ocean Alexander would take my boat back and over prop it. I'll even tell them where to place the "throttle" stop...or the red line on the tach if they insist on being cheesy about it.
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Old 02-12-2013, 06:54 PM   #120
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I wish Ocean Alexander would take my boat back and over prop it. I'll even tell them where to place the throttle stop...or the red line if they want to be cheesy about it.
are you guys sure about this? I think its common for the yard to prop boats more for the shipping weight than the loaded outfitted weight. Would make sense since they have no control over how the customer will outfit or load the vessel unless he stipulates during the build. If this were true then when loaded the boat would appear to be over propped wouldnt it.
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