Propeller Diameter vs Pitch

[BillD]

Question for the prop/DE hull experts on the forum.

Maybe no perfect answer to my question.

Assume two identical DE boats. Let's assume the hull is the builder's 33 footer.

The prop aperture of the hull can accommodate up to a 28" prop with standard accepted clearance.

Each is the same weight, 14,000 lbs.

Each has the same engine, 480 hp, rated 3300

2.5:1 gear with a 2" prop shaft

Cruising rpms for recreational use 2600 rpm.

Prop calculator suggests a 4 blade 26X31 prop or the equivalent

4 blade 28X28

rule of thumb........assume 1" of diameter equals 2.5" of pitch.

rule of thumb the 26X31 is the equivalent of 28X28

solid cruise speed is of most importance, not wot speed.

I've heard from builders and long time DE owners that a 4 blade square prop is "best" for a keel boat.

I've heard from builders and long time DE owners that the square prop being "best" for a keel boat is a myth.

Which prop would you choose?

FWIW,

Bill D

 

[F/V First Team]

I've done this exact test, only my control subject was 36' and weighed 15,000 lbs with 410 hp at 2100. The propellers mentioned were the ones used. At idle the speed was within 0.1 mph, at top end there was 4 mph difference with the 28x28 being the slower of the two propellers. Acceleration through the entire curve was similar as was "smoothness".

26x31 3 blade 1.75" bore left hand bronze propeller

vs.

28x28 4 blade 2" bore (with bushing installed) right hand bronze propeller

Long story short, the 28 is on the shelf back to gathering dust being kept in reserve as the spare, SPARE work prop.

 

[BillD]

I've done this exact test, only my control subject was 36' and weighed 15,000 lbs with 410 hp at 2100. The propellers mentioned were the ones used. At idle the speed was within 0.1 mph, at top end there was 4 mph difference with the 28x28 being the slower of the two propellers. Acceleration through the entire curve was similar as was "smoothness".

26x31 3 blade 1.75" bore left hand bronze propeller

vs.

28x28 4 blade 2" bore (with bushing installed) right hand bronze propeller

Long story short, the 28 is on the shelf back to gathering dust being kept in reserve as the spare, SPARE work prop.

Interesting,

"Rule of thumb" would dictate the 28" would be a better choice ?

 

[F/V First Team]

Well rules are meant to be broken, also not quite sure where the extra half inch came from in your theorem, always was 1 inch of diameter was worth two pitch when I was growing up.

At least we aren't doing it like in the old days with the long stroke big bore engines, where the outside diameter of the flywheel was the diameter of the wheel and the inside diameter of the flywheel, with the spokes, was the pitch.

Chugga chugga chugga

At the end of the day speed is based on pitch, sure blade area has some things to do with it, but if you can sling more distance on less area you're going to be able to do a much faster job.

Like the little skinny guy with a small spade at the construction site all hopped up on coffee and a package of that five hour energy crap vs the big burly dude in the hard hat two sizes too small with the square tipped shovel. Five spade fulls in the same time as two big squares is more material moved.

 

[BillD]

"At the end of the day speed is based on pitch, sure blade area has some things to do with it, but if you can sling more distance on less area you're going to be able to do a much faster job"

Understood,
If you want to spin it faster...maybe a 2:1 gear??
As long as the torques is available.

 

[Powderpro]

I would choose the larger diameter prop, so I would go with the 28x28. Pitch does equal speed, but a more efficient prop also equals speed, and the 28" diameter prop is going to be more efficient (less prop slip) than the 26" prop under the conditions you describe. Now if the boat were super light weight and designed as a race boat, the larger diameter would probably not be an advantage. My 34 Calvin at sea trial was so light, I think the 26" diameter was able to perform as well as a 28" would have, but add weight to it, and I think the 28" would have outperformed it.

I have built boats with 24" props, 26" props, and with a 28" prop. The 28" prop outperformed the smaller props in fuel efficiency, cruise and top speeds. If I was building a light weight 33' race boat, I would consider the 26" prop, but for real world uses, in my opinion and from my personal experience, the 28" prop would deliver the best fuel economy and cruise speed.

I cannot understand why adding 3" of pitch, but reducing prop efficiency would net you 4 more mph in top speed on a 15,000 lb boat. And a 1.75" shaft is quite small for that much torque and size of prop.

 

[F/V First Team]

*shrug*

Numbers do not lie

And the shaft is plenty big for what the boat does and is exposed to

With 5,000 lbs of traps stacked on deck she is a little slower to get up to speed but has all of it at top end, probably adds about four seconds to the acceleration time.

 

[petrel]

What is the rule of thumb for shaft size? I've got 2" shafts in my DMR turning 28" wheels slowly w/ 2.5 to 1 reduction. The wheels are over-square, but a couple of things have kept me from trying a pair of 30X30's instead of 28X34's. One is that I wonder if 30" diameter would be pushing my luck on 2" shafts and the other is what Travis was talking about- speed. I don't want to risk losing a knot or two of cruise speed as we are far from racing as it is. I've always thought, however, that the two inch shafts much be somewhat easier to turn than 2.5" shafts would be w/ marginal power, and they contribute to the efficiency of the boat, which is remarkable w/ two strokes.

Any thoughts here on shaft size/ prop diameter/ horsepower/ reduction and so on- what's safe? The 1.75 inch shaft apparently works for First Team, although he hasn't mentioned whether or not he has ever cracked one. There's a really fast (non DE) boat in Hatteras that used to run 650 hp 892's w/ 2 inch shafts and 28x34's, I think w/ 1.5 to 1 reduction. But he did crack a few shafts over the years....

 

[captainlarry84]

Diameter vs Pitch: this topic will get a lot of attention and our theories and knowledge will vary a lot so....No fight boys.....LOL. To start off where I come from 1" of pitch carries the same load a 1" of diameter, meaning a 24 X 24, 23 X 25, or 25 x 23 all carry the same amount of load, but not performance. In addition when going from a 3 bladed wheel to a 4 bladed wheel we drop 1" of pitch to make up the load difference, therefore a 24 X 24 3 bladed wheel carries the same load as a 24 X 23 $ bladed wheel.

Lets take an in-depth look at Bills original post and break it down. To better understand propellers lets look at the components and terminology used in propellers:

Diameter: This is the overall width size of the propeller or as stated the diameter. Which is the same as the diameter of a circle. The diameter is measure from one outer tip to the other outer tip and directly through the center or hub of the propeller. On all propellers the diameter and pitch is always stamped either between the blades on the hub or on the face of the hub. The first number is always the diameter. Therefore a 24 X 22 stamped propeller means that the diameter is 24 inches.
Even we as layman can measure the diameter. Diameter of the propeller plays a big roll in overall performance. On most vessels when wheeled are propped correctly they have the maximum diameter allowable to get the largest blade surface. Large blade surface is important in order for the propeller to catch enough water and allow the pitch (twist) of the propeller to do it job and push the vessel forward. In theory the larger the blade area the less slippage a propeller has.

Pitch: Pitch is what pushes you forward. In a perfect world with zero slippage for each inch of pitch and every revolution your vessels shaft turns the boat should move forward 1". Therefore a One shaft revolution with 22" pitch and 100% efficiency in theory will move a vessel forward 22. But as we all know there are many variables which get in the way and reduce this number.
Things such as keels, struts, propeller pockets are just a few of the items that lower our 100% efficiency number.
Pitch is the second number on the stamp of a propeller; so on the same 24 X 22 propeller, the second number of 22" indicates 22" of pitch or twist in the blade. Unlikely diameter pitch cannot be measured by a simple ruler, as pitch is the gradual twist of the propeller blade. On a flat plan an inch of pitch would be about 1/8" of rise at the outer edge. The rise of pitch however changes through the blade because the blade must end at the root of the hub of the propeller, which is the center.
Pitch must be measured by a propeller shop. The propeller is place on a jig and a dial indicator is dropped down the blade to measure the pitch. Pitch is usually what we need to adjust when re-coning or resizing a propeller to gain or lose load on the surface of the propeller blades. The more pitch the more load.
On many of the newer boats due to high horsepower and diameter clearance problems pitch may exceed diameter by as mush as 12" or more. So a 24" X 36" propeller is not uncommon on some of our high performance Sportfishing Boats to use. Propellers such as these are not commonly seen on keeled bottom or displacement bottoms, as these types of hull bottoms cannot provide enough water flow to propellers blades with such massive pitch.
Although an inch of pitch may not sound like a lot it is. On most diesel engines with a revolution range of 2800 RPMs. @ wide open throttle (WOT) the following change can occur:
2:00 transmission 1@ of pitch = 100 RPM change
1:75 transmission 1" pitch = 150 RPM change
1:50 transmission 1@ pitch = 200 RPM change

Blades & Blade Area: This is where the work gets done. On our powerboats propeller blades usually are either three (3) or four (4) blades depending on there use. On some vessels even five (5) blades are used. In theory the more blades the smoother the propeller, as the load is disperse over a larger propeller blade areas, thus making the load more manageable. As a rule the fewer the amount of blades the faster the propeller is at top end. Most boats leave the factory with three bladed propellers because of this factor.
With the amount of propeller blades pitch always changes. For example if you can turn a 24" X 24" three bladed propeller to full RPMs and wanted to go to a four bladed propeller, your four bladed propeller would most likely be a 24" X 23". The pitch being reduced to make up the load difference with the surface area of the extra propeller blade. This also explains why the three bladed propellers are faster, as it has one additional inch of pitch, which as stated earlier is the force that move your vessel forward. As you come down in RPMs to the Cruise RPMs on most vessels you pick up efficiency due to less slippage with the extra blade.



With all of this knowledge how do we pick the correct combination as originally requested in the first post by Bill D:


Each has the same engine, 480 hp, rated 3300

2.5:1 gear

Cruising rpms for recreational use 2600 rpm.

Prop calculator suggests a 4 blade 26X31 prop or the equivalent

4 blade 28X28

My math @ 2600 RPMs Based on a maximum diameter of 28" based on hull aperture I would est the efficiency of the 28" propeller at 80% & the 26" propeller @ 70 %. These are my rough numbers just based on prior knowledge... The reason for reduced efficiency with the 26" propeller is less blade area Y& with less blade area and the keel the odds of 26 of diameter feeding 31 of pitch at best is 70% . However the 26 X 31 is larger so I believe the equivalent wheel would be a 26 X 30 The speeds are as following:

28 X 28 propeller @ 2600 RPMs @ 80 % efficiency would be 19.2 Knots est

26 X 31 propeller @ 2600 RPMs @ 70 % efficiency would be 18.5 knots est

26 X 30 propeller @ 2600 RPMs @ 70 % efficiency would be 17.9 knots est

I would therefore request the 28 X 28 propeller. Lastly in heavy seas and heavier loads the larger propeller diameter will maintain less slippage so performance would additional increase.

The people rest their case

 

[F/V First Team]

Original shaft, no issues at all with the running gear.

I've torn the transmission clutches loose a few times, but I've since cured that particular issue.

I'm stepping out of this thread

I supplied my numbers and apparently they aren't what the people want to see. It works for me with my set up, but then again I've got torque to spare.

If you have the torque, wind the pitch to it.

All I have to say

 

[Powderpro]

Speaking of numbers not lying, for the 2012 salmon season I burned $2,394 in diesel. For the 2011 season, I burned $4,491 in diesel. That's a difference of $2,100 in my pocket and I actually caught more fish in 2012. Granted, the 2012 price per gallon was about $.20 less per gallon than 2011, but still, that's a lot less fuel burned in 2012 vs. 2011. The difference you ask? Length of boat, hp, and prop diameter.

2011: 32' Wegley, JD 375hp, 24" diameter dynaquad prop.

2012: 37' Wegley, Iveco 420hp, 28" diameter dynaquad prop.

Boat hulls are identical except the 5' in length, and the 37' only weighed about 1,000 lbs more than the 32'. Yet my fuel burn with the 37' hull and 28" prop was almost half of what the 32' hull with 24" prop delivered. Plus, the 37' boat was about 6 knots faster top speed and about 5 knots faster cruise. No comparison in performance between the 2 props. Granted it's a 4" difference in diameter, but what's true of 4", is also true of 2". In the real world, in 95% of circumstances, the larger prop is going to be the better choice.

There is a reason you can add pitch when you reduce the diameter of the prop. It's because the smaller prop is less efficient and is slipping more going through the water.

 

[captainlarry84]

wheel sizes

What is the rule of thumb for shaft size? I've got 2" shafts in my DMR turning 28" wheels slowly w/ 2.5 to 1 reduction. The wheels are over-square, but a couple of things have kept me from trying a pair of 30X30's instead of 28X34's. One is that I wonder if 30" diameter would be pushing my luck on 2" shafts and the other is what Travis was talking about- speed. I don't want to risk losing a knot or two of cruise speed as we are far from racing as it is. I've always thought, however, that the two inch shafts much be somewhat easier to turn than 2.5" shafts would be w/ marginal power, and they contribute to the efficiency of the boat, which is remarkable w/ two strokes.

Any thoughts here on shaft size/ prop diameter/ horsepower/ reduction and so on- what's safe? The 1.75 inch shaft apparently works for First Team, although he hasn't mentioned whether or not he has ever cracked one. There's a really fast (non DE) boat in Hatteras that used to run 650 hp 892's w/ 2 inch shafts and 28x34's, I think w/ 1.5 to 1 reduction. But he did crack a few shafts over the years....

Petrel, I would love to give you my thoughts on your vessel. I need the following:

Vessel size make & model DM
28 X 34S 3 or 4 blades
Current speed & RPMs at Cruise
Current speed & RPMs a WOT

2 shafts are the minimum recommended size for wheels from 28 to 32

Therefore even though you may increase diameter you would decrease the pitch so the load would remain the same. But efficiency may increase.

 

[petrel]

I believe you Travis and I didn't think you had cracked a shaft, but I had to ask... Send me a PM and let me know if you think I'm on the right track. Of course I don't have torque to spare, so maybe not.

 

[petrel]

Capt. Larry,

28x34 4 blades. 70% blade area per Prop Scan
56' DMR- hard chined semi-displacement, slow planing sort of boat, about 40,000 pounds w/ fuel but no pax
14 knots at 1900
About 19 knots wide open 2300 or 2400

Thanks,
Brian

 

[captainlarry84]

My math

Capt. Larry,

28x34 4 blades. 70% blade area per Prop Scan
56' DMR- hard chined semi-displacement, slow planing sort of boat, about 40,000 pounds w/ fuel but no pax
14 knots at 1900
About 19 knots wide open 2300 or 2400

Thanks,
Brian

Based on your performance you are getting 68% efficiency with your current 28 wheels @ 70% blade area. If you when to a 30 wheel your would get 32s of pitch not 30s and your you would get most likely 78 & efficiency giving you a speed of 15.60 knots. Therefore you would pick up 1.6 knots.
If you go with the 30 x 30s which I think you will over turn it would be 14.64 knots @ 1900 RPMs.

Therefore if you can turn the 30 x 32s which carry the same load at 28 x34s you would have a marked improvement. With the 30 x 30s you would do little at best.

 

[captainlarry84]

Wot

About 19 knots wide open 2300 or 2400

At 2300 RPMs you are getting 75% efficiency at 19 knots with your 28 X 34 wheels

At 2300 RPMs I would est efficiency @ 85% your would get 20.61 knots. With 30 X 32 wheels.

 

[petrel]

Thanks, Capt. Larry; I was thinking I would have to give up four inches of pitch for two inches of diameter.

 

[captainlarry84]

No the rule is 1 of pitch = 1 of diameter in load. On DQX propellers because the blade area is greater then it may be more like 1.25 to 1.50s of pitch to = 1 of diameter.

 

[Capt Rich]

Hey Larry,
Glad to see you have a bite on KRISTEN!
Wish I could have her !
Anyhow , I found a 22.5x21 dynaquad that Im
Thinking of using on BARBS. Just wondering what your thoughts are?
She turns 2500 on the clock no load. Runs 2400 with the 21x10 and a # 1 cup.
I don't know if the prop will give me more cruse speed?
What do you think?

 

[captainlarry84]

Hey Larry,
Glad to see you have a bite on KRISTEN!
Wish I could have her !
Anyhow , I found a 22.5x21 dynaquad that Im
Thinking of using on BARBS. Just wondering what your thoughts are?
She turns 2500 on the clock no load. Runs 2400 with the 21x10 and a # 1 cup.
I don't know if the prop will give me more cruse speed?
What do you think?

You mean it runs at 2400 with a 21 x 19 not a 21 x 10. You will noit turn that big of a propeller. Case closed. As I stated the max wheel with a 3208 350 Cat & a 1:15 gear is yout 21 X 19.