Prop Differences

Bkay said:

I have great respect for this commenter's experience, but I'm not sure I agree with this statement. I believe you need to start with the largest diameter you can fit with a minimum of 10% standoff between propeller tip and hull - 15% is even better. If you don't have that standoff, then increasing diameter is not helpful.

Assuming you have the largest diameter propeller you can fit - then you need to look at other variables if there is some change you need to make. Frankly, I agree with the comment above that your square wheel may be an ideal compromise.

If you can run your engine up to WOT with a full load, then I'd have to ask why you think you need any changes to your setup. Is there some problem you are trying to solve or just curious?

I stand by to be corrected by those wiser than me.

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Thanks,
It appears to me that the dia/pitch ratio can be more important than diameter alone. On a faster boat square props (dia and pitch the same) works well as there’s not enough time for the water to flip around the blade tip and thus change sides. And changing sides looses lots of thrust. The only prop that can do square well at trawler speeds is one w narrow skinny prop tips and more than usual # of blades. Just like on airplanes, especially gliders. Long wingspands and small wing tips wa very short chord. Higher aspect ratio wings and props.
But yes one is limited on a trawler for max dia. And w a given blade design or profile shape and number of blades one can optimize. More blades can deliver a higher aspect ratio for a given dia but the dia insribes a circle that acts much like blade area ... disc area. Just like blade area considering one blade the whole prop is limited or blessed by the disc area loading.

So for max thrust not considering speed one looks for the max dia, then an ideal blade area. That will dictate #of blades with typical prop designs .... for reasonable economy. To buy props w unusual blade shapes or aspect ratios gets expensive so usually a typical prop is used. And one chooses the #of blades to achieve the optimum blade area. And that is usually dictated by the power delivered to the shaft. Then of course there’s rpm, boat speed and some other variables. Then you look for the ideal pitch/dia ratio for the speed of the boat. Assuming a typical 8 knot trawler one would look for about 2/3 to 3/4 as much pitch as diameter. Adjusting the dia, pitch and blade area will bring one close to the bulls eye for good efficiency. Ideally no trawler should have only half as much pitch as dia. That will be good for a max bollard pull but lacking for pushing a boat w some speed on.

Here is an example of high aspect ratio 4 and 5 blade props that should be very efficient. But props like this can only be used if there’s sufficent blade swinging clearence. The 4 blade below was on a big motor sailer and the one below on my boat. But it didn’t work well at all. Too much blade area and was only able to achive 2000rpm .. needed 3000. But the reason I posted these pics is to a non-typical high aspect ratio prop. Most trawler props are fairly low aspect ratio

I have no credentials and am not a marine engineer so the above is only my opinion as is usually the case.

Ski in NC said:

There is a sweet spot with prop diameter that depends on all the relevant boat parameters: Length, weight, hull shape, engine hp, engine rpm, gear ratio, desired speed, etc, etc.

Too small a diameter, and to some extent too little blade area and the "slip" will be too high. Slip is defined as theoretical speed (pitch and prop rpm) compared to actual attained speed.

All props have some slip since they are churning in a fluid.

Too much diameter and the skin friction begins to eat shaft hp. This friction can result in a loss of speed greater than the gain from reduced slip. At some point a big dia prop just won't fit, or blades too close to hull (noise).

So somewhere between small diameter and large diameter is the right diameter.

Once diameter is selected, you pick pitch. Pitch is much like a gear ratio, you pick that to get engine rpm to the desired spot. Most boats that is to get a little above rated rpm at full power.

Oh, with all that said, there is a good bit of black magic involved!!

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"Skin friction" is a function of blade area and not diameter and is determined by the hull section on FD hulls, SD is a different and more complicated calculation.

FF said:

For many boats a "square prop" (where pitch and diameter are equal) is considered ideal.

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Very true FF.
But not typical trawlers.

Fish53 wrote;
“Skin friction" is a function of blade area and not diameter and is determined by the hull section on FD hulls, SD is a different and more complicated calculation.”

OK I’ll bite
What has the hull section got to do w props?
That ought to be interesting.

Fish53 said:

"Skin friction" is a function of blade area and not diameter and is determined by the hull section on FD hulls, SD is a different and more complicated calculation.

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True, but increase diameter and you increase blade area, shape kept similar. Also as you increase diameter, the velocity increases with radius from center. Skin friction is a function of area and velocity.

Nomad Willy said:

Thanks,
It appears to me that the dia/pitch ratio can be more important than diameter alone. On a faster boat square props (dia and pitch the same) works well as there’s not enough time for the water to flip around the blade tip and thus change sides. And changing sides looses lots of thrust. The only prop that can do square well at trawler speeds is one w narrow skinny prop tips and more than usual # of blades. Just like on airplanes, especially gliders. Long wingspands and small wing tips wa very short chord. Higher aspect ratio wings and props.
But yes one is limited on a trawler for max dia. And w a given blade design or profile shape and number of blades one can optimize. More blades can deliver a higher aspect ratio for a given dia but the dia insribes a circle that acts much like blade area ... disc area. Just like blade area considering one blade the whole prop is limited or blessed by the disc area loading.

So for max thrust not considering speed one looks for the max dia, then an ideal blade area. That will dictate #of blades with typical prop designs .... for reasonable economy. To buy props w unusual blade shapes or aspect ratios gets expensive so usually a typical prop is used. And one chooses the #of blades to achieve the optimum blade area. And that is usually dictated by the power delivered to the shaft. Then of course there’s rpm, boat speed and some other variables. Then you look for the ideal pitch/dia ratio for the speed of the boat. Assuming a typical 8 knot trawler one would look for about 2/3 to 3/4 as much pitch as diameter. Adjusting the dia, pitch and blade area will bring one close to the bulls eye for good efficiency. Ideally no trawler should have only half as much pitch as dia. That will be good for a max bollard pull but lacking for pushing a boat w some speed on.

Here is an example of high aspect ratio 4 and 5 blade props that should be very efficient. But props like this can only be used if there’s sufficent blade swinging clearence. The 4 blade below was on a big motor sailer and the one below on my boat. But it didn’t work well at all. Too much blade area and was only able to achive 2000rpm .. needed 3000. But the reason I posted these pics is to a non-typical high aspect ratio prop. Most trawler props are fairly low aspect ratio

I have no credentials and am not a marine engineer so the above is only my opinion as is usually the case.

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What is the story with the 5B? I have seen those on very old power boats but have zero experience with them. Have always been curious how they performed.

Ski in NC said:

What is the story with the 5B? I have seen those on very old power boats but have zero experience with them. Have always been curious how they performed.

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I had an OEM 5 blade on my Mainship Pilot II. The earlier pilots had issues with noise as the prop diameter was too big. So Mainship put a 5 blade on as well as did a complete hull design which included a prop pocket. It worked. A close friend had an older Pilot 30 and it was definitely an improvement in noise and performance with basically the same engine.

Ski in NC said:

True, but increase diameter and you increase blade area, shape kept similar. Also as you increase diameter, the velocity increases with radius from center. Skin friction is a function of area and velocity.

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You're thinking disc area not blade area, two very different things. Diameter does not necessarily increase blade area where it does increase disc area as diameter increases. Blade characteristics should be chosen to provide the necessary blade area for a vessels hull section to maximize efficiency and provide adequate thrust. This is for FD hulls as stated.

can`t explain the difference, i think it`s already been done here, but i know we went from a 4 blade michigan, to a 4 blade thompson prop, made in vancover, bc, for ocean trollers. it has a kind of a straight sided blade, instead of the figure 8 design, also the thompson is thicker, probly 1/4" with abt a 45* sharp edge. made so when it is hit by driftwood, it just chops it up, with no damage to the prop! it`s a 26x22, been on our 41 full keel skookum, built in port townsend. we`ve had the boat/prop for 28 yrs...clyde

Nomad Willy said:

Fish53 wrote;
“Skin friction" is a function of blade area and not diameter and is determined by the hull section on FD hulls, SD is a different and more complicated calculation.”

OK I’ll bite
What has the hull section got to do w props?
That ought to be interesting.

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Hull section is the square area that needs to be pushed through the water by the propeller. I'll respectfully suggest you reference pages 122-123 of the Propeller Handbook for a more detailed explanation. Interesting enough?

As it was explained to me by the better prop shops (General, Domineer Propeller, Mich Wheel) that the main goal is to get the full rpm at wide open throttle.... basically.

The pitch is a function of efficiency and we set it up for that WOT position. If we could change the pitch we could get better efficiency at lower speeds and a faster get away from idle. While that's quite common in airplanes where they need the extra power to get off the runway, we don't have that luxury (for the most part), nor do we need it. Now, if you have a go fast boat, there needs to be some consideration to get the boat on plane.

The number of blades are varied for smoothness and the ability to reduct the diameter. The disc area ratio (DAR) also effects this and can give more push at slower speeds (aka tugs).

For my single Mainship, most came with a 26" 5 blade 19 pitch .9 dar and operated very smooth and efficient. I tried a 26" 4 blade 22 pitch .8 dar which spec'd out with full throttle and produced very little difference. However the 5 blade seem to perform a tad better in speed and certainly smoother.

There's also the cup, which apparently not a big thing in trawler props so can't comment on that one.

Nomad Willy said:

Very true FF.
But not typical trawlers.

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I should also state that a square wheel simply means the diameter and pitch happen to be the same number nothing more or less. There is no magical benefit to a square wheel either the diameter, pitch and blade characteristics are correct for a given vessel or they are not, that simple.

Anyone mention David Gerr's prop book?


https://www.amazon.com/Propeller-Ha...195434728041cb09edb35c58521e5a&language=en_US

ben2go said:

Anyone mention David Gerr's prop book?


https://www.amazon.com/Propeller-Ha...195434728041cb09edb35c58521e5a&language=en_US

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Yes I referenced it in #12 and #40 and have been using it to respond frequently, I have several other references but his is the easiest to read and use. I essentially quoted it in #42.

Fish53 said:

Yes I referenced it in #12 and #40 and have been using it to respond frequently, I have several other references but his is the easiest to read and use. I essentially quoted it in #42.

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Ski wrote;
‘What is the story with the 5B? I have seen those on very old power boats but have zero experience with them. Have always been curious how they performed.“

Ski,
The story is actually a bit funny.
Found the prop at a 2nd hand boat stuff store. In the Wallingford district in Seattle. Was called the Second Wave .. I think. Went out of business about 10 yrs ago. I actually know what it was .... a Michigan Star. Michigan sold them in the 50’s I heard but for what application I don’t know. Probably to boaters that thought more blades was what the big boys do and must be good.
I was attracted to the high aspect ratio blades. My normal 3 blade is 18” dia and the Star was 19”. Only one inch more. I had plenty of clearence for it and it was cheap. WOT usually gets me 3000rpm and the star only pulled 2000.
I made one adjustment in pitch but I was fooled by my optimism. At that point it was obvious even to me that it would take many changes to get it loaded right and soon after I bought that prop we moved to Alaska. There I had tidal grids for fairly easy and cheap prop changes but i would need to make the mods myself or take the prop over to Ketchikan. I could have sawed off the blade ends 1/4” at a time but that would probably ruin the aspect ratio advantage. Lost interest. It’s around here somwhere .....

And then even if I was totally successful I may cut my fuel burn from 1gph to .9gph. And Willy has no vibration issues at all so .....?

Fish53 said:

I should also state that a square wheel simply means the diameter and pitch happen to be the same number nothing more or less. There is no magical benefit to a square wheel either the diameter, pitch and blade characteristics are correct for a given vessel or they are not, that simple.

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Yes Fish53,
Like hull speed. It has a number attached to it so it must be magic. Why else would they attach a number?
If we all got real familiar w speed/length ratios we could comunicate better but?

Nomad Willy said:

Yes Fish53,
Like hull speed. It has a number attached to it so it must be magic. Why else would they attach a number?
If we all got real familiar w speed/length ratios we could comunicate better but?

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I referenced SL ratios in the displacement thread and it got nowhere. If I could type faster or had more time I'd have been tempted to delve deeper but like other social platforms one or two sentences seems to be the norm. I like to review technical data before I open my mouth in fact I reread the entire Propeller Handbook before even posting here, even though I was already familiar with the subject.