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Old 02-27-2013, 02:36 PM   #1
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25x20 or 24x22 props?

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?
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Old 02-27-2013, 04:01 PM   #2
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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.
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Old 02-27-2013, 04:18 PM   #3
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A rule of thumb I've seen says one inch of diameter translates to about 2 inches of pitch. I think I got that right. So, your comparison would be roughly a wash. But the 25 might take a little more power to swing at max rated power, so if you're concerned about overpropping, you can check that with the prop calculator link below.

This was sort of discussed a while back in the prop efficiency thread and also a bit in the later sections of the fuel mixture thread where the discussion veered into overpropping. Don't think there's an easy way to compare speeds precisely. Same for efficiency. In addition to the prop calculator, I "screwed" around with Dave Gerr's Prop-Diameter/Power/RPM chart, the prop and engine power charts for our boat, and Gerr's bollard pull formula. I was comparing a 24x20 to a 26x20 as an overprop scenario. I came up with about 7% improvement in thrust output for the bigger prop at the same engine horsepower. But it was a static bollard pull comparison. I've seen some prop efficiency charts in a googled 30 page thread on this subject that indicate the slope of the cruise prop chart gets slightly steeper with increased diameter. If I find the string I'll post a link.

www.vicprop.com/calculator.htm
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Old 02-27-2013, 04:38 PM   #4
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Probably a 3 blade 26" dia would be best but I'd probably just run what you've got if it runs up to the rated rpm at WOT. Why are you looking for max efficiency? Are you having trouble paying for fuel? Then you should find a full displacement boat or a smaller one.
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Old 02-27-2013, 05:21 PM   #5
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Probably a 3 blade 26" dia would be best but I'd probably just run what you've got if it runs up to the rated rpm at WOT. Why are you looking for max efficiency? Are you having trouble paying for fuel? Then you should find a full displacement boat or a smaller one.
Why? Because I have the props. I would always prefer the bigger and flatter. If I had the props I would go 28x14 +- in a heart beat. We only burn 2.1GPH now combined but if switching props improved that .001% I would do it on her next haul out. Look at my wake in my pics and it's hard to argue that we don't have essentially a full displacement hull. We average about 3,000+ miles a year so a little improvement adds up.
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Old 02-27-2013, 06:04 PM   #6
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Why not just call one of the many good prop shops there in SoFla tell them what you have on there now and all about your boat, and would going to a 25x20 make what kind of a difference. Then make sure you throw them a bone some time in the future. Mike Heller at Deep Blue Yacht Supply, an adjunct to Lauderdale Propellor, would be the first guy I'd call. Great place to buy very high quality zincs and any other below the waterline stuff, so easy to throw that bone to. Nice place to do biz with, no connection other than being a very loyal and happy customer.

Boat Props, Inboard Props, Cutlass Bearings, Zinc Anodes, Prop Shafts, Shaft Seals, etc.
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Old 02-27-2013, 06:05 PM   #7
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My understanding on choosing a prop is you prop for WOT engine RPM not boat speed if you want engine to have a long happy life. If you can't reach the engines specified RPM then your lugging the engine at any rpm, like a car going up a grade in to high a gear.
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Old 02-27-2013, 06:14 PM   #8
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My understanding on choosing a prop is you prop for WOT engine RPM not boat speed if you want engine to have a long happy life. If you can't reach the engines specified RPM then your lugging the engine at any rpm, like a car going up a grade in to high a gear.
That's what the experts say.
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Old 02-27-2013, 08:12 PM   #9
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Why not just call one of the many good prop shops there in SoFla tell them what you have on there now and all about your boat, and would going to a 25x20 make what kind of a difference. Then make sure you throw them a bone some time in the future. Mike Heller at Deep Blue Yacht Supply, an adjunct to Lauderdale Propellor, would be the first guy I'd call. Great place to buy very high quality zincs and any other below the waterline stuff, so easy to throw that bone to. Nice place to do biz with, no connection other than being a very loyal and happy customer.

Boat Props, Inboard Props, Cutlass Bearings, Zinc Anodes, Prop Shafts, Shaft Seals, etc.
Thanks, probably the way to go
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Old 02-28-2013, 07:13 AM   #10
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If you can't reach the engines specified RPM then your lugging the engine at any rpm, like a car going up a grade in to high a gear.

This is not so.The prop is not a transmission geared to a wheel.

All engine HP /torque graphs will show a different HP at different RPM'S.

Should one prop the boat so the HP required is less than the HP produced (10-20% less) the engine will operate far more efficiently than if propped for full tilt rpm.

Thats how every big noisemaker that needs a fixed CPS operates.

ON a new to you boat simply run to full throttle , note the RPM and pull back 10% or 300RPM , and the engine will not be overloaded.

This is because even a slight lowering of the prop speed required far less HP.

A propulsion engine may create 250 Hp at 2100,
and as a noisemaker the allowed load will only be 175HP s,,1800 standby power ,
160hp at 1500 (Brit 50 Cps) and
125HP at 1200 for cont duty prime power.

All these RPM /HP settings are fine , no lugging , no overloading , just good fuel burn.

Changing props that are "Close" is frequently done to find a sweeter spot for the engine long term operation.

Some are smoother at one RPM , some may shake at a specific RPM , a slight prop change might help with this , the efficiency difference would be hard to measure.

To avoid "lugging" simplty run at full throttle for about a minuet , note the RPM and pull back 10% or 300RPM.

500 RPM pullback on a gas powered boat.

The 10% figure or less if top RPM is under 100! Esso Maru?
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Old 02-28-2013, 10:52 AM   #11
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Prop size and the pitch/dia ratio could use more discussion.

There is a common rule of thumb that is applicable most of to a lot of the time that if carried to extremes becomes untrue. The theory is that the maximum dia always produces the greatest thrust. True most of the time but but not always.

To prove the point think of an original prop on a boat and double it's dia. Or tripple it ... or more. At some point the tip speed and surface area of the blades will have so much drag that all the power of the engine will be required just to turn the blades and w any pitch at all the engine will be over loaded. At this point there won't be a question of max thrust .... there won't be any thrust at all to consider.

Obviously if we carry the pitch to the same extreme flat blades w no pitch at all will result and no thrust will be created.

When all the variables are considered on any given boat there is an ideal pitch and diameter of a given style (design) of propeller that will give maximum performance. Change any variable and the pitch, dia or/and blade area will need to be changed to attain max performance.

So in Daddyo's situation where he's focused on max efficiency w an extremely low engine/prop loading (1 gph on a 120hp engine) there lies the possibility that to deliver 20hp to the prop there may be too much blade area for max efficiency. I'm quite sure that a switch to 3 blade props to reduce the blade area would probably work better but professional propeller people or the guys on Boat Diesel will have excellent information. A huge problem (despite what FF says) is that propellers must be (according to conventional practice of best methods) loaded for full power and maximized for full speed so when one cruises at a speed lower than low the prop isn't going to be very ideal. The only time the prop and engine are performing ideally is at full power/speed. The very low power cruise speed can be made more ideal (slightly more efficient) by over propping but no engine manufacturer recommends it. The only handy, inexpensive and recommended way to increase efficiency of a trawler to any great extent is to go slower. And of course if one is good at buying and selling boats getting a more appropriate boat for efficiency is always an option but only a 50 to 100% gain can be had going that route unless one goes to extreme hull types like an extreme AR catamaran.

So Daddyo let us know what the real experts say when you find out.
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Old 02-28-2013, 01:09 PM   #12
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I think they will be the same as the increase in diameter will off set the reduced pitch. But we will see what the experts say.
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Old 02-28-2013, 03:15 PM   #13
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What will be the same as what Phil?

I'm think'in you mean the load will be the same when the dia and blade area is matched to the pitch ?? ......
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Old 02-28-2013, 07:19 PM   #14
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"Some are smoother at one RPM , some may shake at a specific RPM , a slight prop change might help with this , the efficiency difference would be hard to measure.

To avoid "lugging" simplty run at full throttle for about a minuet , note the RPM and pull back 10% or 300RPM."

FF are you saying that you can only lug a engine at WOT & that if you back of a few rpm alls good? If there is a difference in the load applied to a engine by a prop than the wheels of a loaded truck, dozer blade or hydraulic system I'am not aware of the difference. The governor will try to hold a steady rpm on a engine as the load is increased the governor will increase the fuel delivered to maintain that rpm. When the governor at full fuel & can't maintain that rpm the engine is overloaded & if run continually it will have a shortened life. We rebuilt & then broke engines in on a dyno, you could vary the load on the engine with the dyno after running for several hours we would set the hi-idle rpm with no load then we started to increase the load till we reached the rated HP at the specified rpm. Al engines would vary from one to another but all were within the manufactures specs. We also checked the stall rpm, this was hydraulic stall & full stall where torque converter stall was included. Marine engines also have specs & the load on the prop determined by dia. & pitch will influence where the rpm falls in this spec.

As a example my engines hi-idle (no load) rpm is 3150. WOT (stall) RPM is 3000.
If I can't reach those WOT numbers it's over propped if the engines are in correct tune & the hull is clean & the boats not overloaded.

Most builders over prop their boats a bit to gain a little speed they can advertise to buyers.
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Old 02-28-2013, 08:18 PM   #15
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River Cruiser wrote;

"To avoid "lugging" simplty run at full throttle for about a minuet , note the RPM and pull back 10% or 300RPM."

FF are you saying that you can only lug a engine at WOT & that if you back of a few rpm alls good?"

There is a great deal of truth to that. In fact it's MOSTLY true (depending on how the engine's rated) if you're propped correctly. And when you're over propped it is if you back off far enough. On most engines (propped correctly)100 to 200rpm down and you can cruise 24/7 but with some one would need to back off 3 or 400rpm. Then everything below that point is loaded in the green zone. But if you're over propped one needs to back off much more to insure no dammage to the engine results but the problem is that how far you need to back off is a unknown amount. The only thing I know of to tell if your'e abusing the engine is a pyrometer in the exhaust but EXACTLY where to mount the sensor is not very exact.

River Cruiser wrote "Most builders over prop their boats a bit to gain a little speed they can advertise to buyers." Maximum speed can only be realized if propped correctly .. at rated rpm where max power is developed. At any other rpm max power will not be attained.
RC wrote .. "If there is a difference in the load applied to a engine by a prop than the wheels of a loaded truck, dozer blade or hydraulic system I'am not aware of the difference." .... Big difference. Lhe load is "soft" at lower speeds and can't achieve any significant load at all where-as a truck can handily apply full load to 1000rpm or even reduce the engine speed to a stop. Nothing like that can happen with a boat ... even an over propped boat.
The stall part of your response is'nt clear to me. Could you rearrange the words in a different way?
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Old 02-28-2013, 09:10 PM   #16
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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.
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Old 02-28-2013, 09:22 PM   #17
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Per the vicprop calculator link I provided above, the subject boat is currently significantly over propped for twin 120 Lehmans. Not that overpropping is a huge deal if you have some other data like a verified prop and engine power curve. The vicprop came out very close for our boat, which is currently propped for max rated power.
Did I miss information about daddyo's vessel like transmission ratio, LWL, displacement, beam, that allowed you to run a prop calculator?
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Old 02-28-2013, 09:45 PM   #18
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Did I miss information about skidgear's vessel like transmission ratio, LWL, displacement, beam, that allowed you to run a prop calculator?
I looked at the specs for a 48 Defever (like Daddyo's) on Yacht World. Made some guesstimates regarding LWL (42), draft 4.5. Used 50,000 and 15' beam as given in YW. Guessed it has 2:1 ratio, and 2400 max rpm....

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Old 02-28-2013, 09:50 PM   #19
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I wonder if the "draft" number is intended to be w/o the appendages such as the keel and would Daddyo's boat conform to that?
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Old 02-28-2013, 10:03 PM   #20
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I wonder if the "draft" number is intended to be w/o the appendages such as the keel and would Daddyo's boat conform to that?
The vicprop calculator says use "molded draft" (without keel, etc). Difficult to tell from the photo I used on Yacht Yorld. I knocked off 4" from the Yacht world spec and went with 4.5 feet. Difficult to tell with these semi-planing hulls.

And I just reran the numbers for a second gear ratio (2.5:1) s @ 41' lwl.....

For 2:1 ratio 3 blade 23.3 x 16.4, 4 blade 21.9x 16 (my original calculation)

For 2.5:1 ratio 3 blade is 26.6x20.5, 4 blade 25x20

Didn't realize the change in ratio would be that significant. Likely that I guessed the wrong ratio (2:1) the first time I ran the numbers...
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