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Old 03-02-2013, 07:17 PM   #61
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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.

Many of us have Gerr's Prop handbook. Gerr's "The Nature of Boats" actually does a better job addressing the manner in which the other driveline components enter the mix.

Putting additional stress (load) on the engine is one of the payoffs...FF has been making this point for years. Sure, the other drivetrain components should be addressed in any analysis, although I'd bet dollars to donuts that lots of boats came out of Far East factories with no testing or analysis.

Prop efficiency discussions are all over the internet. Boat diesel members have great maintenance expertise. Engineering expertise maybe not so much.
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Old 03-02-2013, 07:27 PM   #62
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my original question was does anyone know what the efficiency and speed difference would be between the two props?

Not exactly.
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Old 03-02-2013, 09:14 PM   #63
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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.
Yes! And don't forget, if you throttle all the way back, there's almost no wake!
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Old 03-02-2013, 09:25 PM   #64
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Prop efficiency discussions are all over the internet. Yes they are, especially in stirred tank technology science and applications. It is a major field of endeavor in the real world. Boat diesel members have great maintenance expertise. Engineering expertise maybe not so much.
Unless you get into the inner circles of solid builders, strong yards and yacht/ship professionals, boatdiesel is a very good resource for us plebians. I'd not let the lack of perceived engineering talent slow you down Skidgear in tossing the over propping issue their way.
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Old 03-02-2013, 09:27 PM   #65
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The props I'm currently running are 24x22 4 blades. My spares are 25x20 4 blade. We make 6.8knts at 1400rpm with the 24s. I would prefer to run the larger flat 25s. My question is for you prop calculator types, what would my speed most likely be at the same 1400rpm with the 25s? How much more efficient would they be? Is this even reasonably calculable?
What is your reduction ratio?
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Old 03-02-2013, 09:42 PM   #66
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Gee, if one has a "true" trawler only capable of hull speed, having an engine and propeller combination just capable of hull speed is a simple and proper solution.
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Old 03-03-2013, 09:33 AM   #67
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What is your reduction ratio?

He said it's 2.5 earlier in the thread.
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Old 03-03-2013, 09:36 AM   #68
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Unless you get into the inner circles of solid builders, strong yards and yacht/ship professionals, boatdiesel is a very good resource for us plebians. I'd not let the lack of perceived engineering talent slow you down Skidgear in tossing the over propping issue their way.
Lumping everyone on TF in the "plebian" category in terms of engineering expertise might be a bit presumptuous. Thanks for invite to join Boat Diesel. Been there, done that.
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Old 03-03-2013, 10:49 AM   #69
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what is your reduction ratio?
2.5-1
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Old 03-03-2013, 12:24 PM   #70
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2.5-1
My ratio is 2.5:1 as well. My props are 28X20 4B bronze on 2" alloy shafts. Engines are Perkins Sabres 225TI. Normal cruise RPM is 1650 (4gph and 7.7 knots) to 1800 (4.5gph and 8.2 knots) RPM. 6.5 knots is 1250 to 1300 RPM. We run the boat heavy with full chain rode, fuel above 60% virtually all the time (love that ballast) and 50% + water as much as possible.

If you wanted to change from what seems a pretty good setup already, my vote would be for the larger diameter prop but your shaft angle or prop location may be different than mine. Tip space from the hull comes into play as diameter increases.
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Old 03-03-2013, 12:26 PM   #71
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My prop calculators require entry of all details, then normally give prop for full hp & rpm altho one will allow you to specify a cruise rpm.
I assume that the larger flatter wheels, turned at the same RPM, would produce less speed on maybe slightly less fuel. A 25" prop w/o pitch absorbs more hp than a 24" w/o pitch so fuel savings, if any, would be small.
Now that the thread has come fully through the usual underloading/overloading phase, I'll refer you back to the original statement that the courser prop, turned at the same speed, will propel the boat faster.
I'll stand by that.
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Old 03-03-2013, 12:37 PM   #72
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Lumping everyone on TF in the "plebian" category in terms of engineering expertise might be a bit presumptuous. Thanks for invite to join Boat Diesel. Been there, done that.
I was being as gentile as possible.

Question, do you have some verifiable data you could point me to on a recent a over propping exercise on Tier II or III engines? I can then apply some engineering analysis to it if the data is sufficient.
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Old 03-03-2013, 12:54 PM   #73
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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.
+1 I will occasionally run at wot & check rpm & sog depending on current & load all numbers should always be close. Also I'll check for any exhaust color, it's usually clear. Late last year my starboard exhaust was black when I did this & I found the 2 year old bottom paint was fouled. I couldn't do a test like this if I was over propped. I also couldn't run at wot to make a lock before a tow, had to do this last year for about 3 miles when the lock master said he would get me thru if I hurried up. I'am no engineer only a mechanic who has rebuilt many different diesels, & after watching operators & truck drivers on jobs you can pick out pretty easily the ones that'll have the most breakdowns. A lot of times it's the ones that think they are doing what's right.
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Old 03-03-2013, 02:20 PM   #74
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I was being as gentile as possible.

Question, do you have some verifiable data you could point me to on a recent a over propping exercise on Tier II or III engines? I can then apply some engineering analysis to it if the data is sufficient.
Why the ugly insults? I've assessed my own pre-tier boat and there's no question the drive system is easily up to the task. By the way, this uninformed "plebian" is an engineer by education, and an engineering test pilot by training (USNTPS) and occupation. I've analyzed and flight tested drive systems on rotorcraft and airplanes for 35 years....twins to a single main transmission, twins to a combining gearbox into a main transmission, twins to a single prop, reengining with oversized engine(s), repropping, re-rotoring and transmission swaps. That includes hands on experience with instrumentation, ground test stands, water brakes, electric brakes, transmission run stands, loads testing and analysis, endurance testing, performance testing, and associated data analysis. Conversions live with red lines and limits all the time. Boat drive systems are not all that different, and comparatively, the analysis is pretty much a no brainer. Assessing the suitability of the drive system isn't difficult. Predictng the performance/efficiency improvement is not so precise. The problem I'm encountering with boats is that published propeller data is scarce (unlike airplanes, and even rotorcraft rotors where vast amounts of information are available to the public from NASA/NACA testing).

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Old 03-03-2013, 04:18 PM   #75
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[QUOTE=skidgear;138652]Why the ugly insults? ?? The problem I'm encountering with boats is that published propeller data is scarce.

Not really but it resides within each boat builders domain. With the Tier II and III engines of today all sorts of confidential onboard data is available for prop selection and tuning. Since this data is boat specific there is little use for me, you or most of the others on TF to know what happens on a Fleming 65, GB with IPS or a Nordhavn 68 as various prop combinations are tested and evaluated - other than mere interest. If not done already, you may want to go to one of the quality builders and talk with them about how they size and select props in conjunction with getting the engine supplier on board for warranty purposes. It is a fun thing to do.
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Old 03-03-2013, 08:00 PM   #76
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Tom if one had the same engine, gear ratio and prop driving whatever boat at just ablut the same speed I would think a comparison would be worth making.
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Old 03-03-2013, 08:11 PM   #77
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The props I'm currently running are 24x22 4 blades. My spares are 25x20 4 blade. We make 6.8knts at 1400rpm with the 24s. I would prefer to run the larger flat 25s. My question is for you prop calculator types, what would my speed most likely be at the same 1400rpm with the 25s? How much more efficient would they be? Is this even reasonably calculable?
People can guess and argue who is the most qualified to guess, but if you really want to know how these props will perform on your boat, the most reliable way to find out is to do some testing with the current props, switch to the new ones and repat the testing under the same conditions.
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Old 03-03-2013, 08:16 PM   #78
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My ratio is 2.5:1 as well. My props are 28X20 4B bronze on 2" alloy shafts. Engines are Perkins Sabres 225TI. Normal cruise RPM is 1650 (4gph and 7.7 knots) to 1800 (4.5gph and 8.2 knots) RPM. 6.5 knots is 1250 to 1300 RPM. We run the boat heavy with full chain rode, fuel above 60% virtually all the time (love that ballast) and 50% + water as much as possible.

If you wanted to change from what seems a pretty good setup already, my vote would be for the larger diameter prop but your shaft angle or prop location may be different than mine. Tip space from the hull comes into play as diameter increases.
I don't know if ours are the same but my spacing is huge. I could and would turn a much bigger wheel if I had one.
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Old 03-03-2013, 11:37 PM   #79
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Daddyo,
I see you do have room. I basically assumed you didn't have much. Most boats don't.

I think greater efficiency for you would lie in greater diameter and less blade area. More specifically a 3 blade prop. The only good reason for a 4 blade prop is if a 3 blade would be not enough blade area to absorb the power. If you're over propped to the degree that you want you could predict the dia and pitch of the new prop based on your present propeller's performance. You could predict the new prop that way but it won't be more efficient if you increase the dia too much. The tip speed may be too high and your power may be lost just dragging the blades through the water.

I see your boat is designed to be a single engine boat as well.

You have "X" amount of power. Part of it drives the pitch and part drives the area of the blades. Too much of either much efficiency will be lost. Look at the optimum dia/pitch ratio's and see if the larger 3 blade produces a pitch/dia ratio that is in the most desirable range. Then consider you're present props and see how optimum they are re the pitch/dia ratio. If they both in the most desirable range the new 3 blades will be more efficient. And other obvious conclusions can be made. You may want to sacrifice max dia for a better pitch/dia ratio.

Also it looks like your boat is closer to a FD hull than I had remembered. Your aft section is swept up considerably but I also see that your boat is bow down in the slings.
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Old 03-04-2013, 07:41 AM   #80
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Actually she's sitting a little bow up in the slings. The slope in the hull gives that illusion. The keel matches the waterline. That FD hull is the observation I had posted on several other threads. I agree that if there is any thing such as an almost FD then I think she would be it. I think she is much more full then semi which explains a lot about her economy. Yes she could be a single although I'm pretty sure none were ever built. If I ever have to do a rebuild or re-power then I will convert her to a single with thruster.

I still can't get my head around the prop thing. Adsorb power, etc. ????
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