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Old 02-11-2019, 09:22 PM   #21
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Eric,

I have 28" diameter 28" pitch 4 blade prop. What is the problem with that type of set up?

Tom
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Old 02-11-2019, 10:25 PM   #22
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tpbrady,
Too much spilling of the water on the pushing side of the blades spills over the blade tips and wasting energy throwing water radially rather than aft making thrust. You probably have a very noisy boat right above the prop/s.
You’d benifit from an increase in dia and reduction in pitch.
Or an increase in blade area and a reduction in pitch.
Ask someone with more knowledge than me and they will probably agree.

Almost strange you should have that settup as most trawlers that are way off on their pitch/dia. ratio have too much dia and too little pitch.
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Old 02-12-2019, 02:47 AM   #23
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Four blades works well for the FD Coot. All the efficiency and responsiveness needed to move close to hull speed.
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Old 02-12-2019, 06:41 AM   #24
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For many boats a "square prop" (where pitch and diameter are equal) is considered ideal.
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Old 02-12-2019, 08:07 AM   #25
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tpbrady,
You’d benifit from an increase in dia and reduction in pitch.
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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Old 02-12-2019, 09:30 AM   #26
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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.
You don't fit the propeller to the boat you fit the propeller to the reduction, engine horsepower and RPM for diameter and the pitch to the calculated speed of the vessel with the fitted horsepower and reduction. Once these basic calculations produce a diameter and pitch it's time to fit the propeller to the boat. If the calculated diameter doesn't fit with a standard three blade then reducing diameter by adding blades is called for, pitch can be adjusted slightly for the vessels intended use. Blade surface also needs to be appropriate for the installation. There's nothing special about a square wheel but there's also nothing wrong with one. It is correct for lower speed vessels that the larger a diameter the more efficient the propeller will be which is also true with blades, less blades is more efficient for common production propellers.
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Old 02-12-2019, 10:28 AM   #27
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Four and five blade designs make good fans.



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Old 02-12-2019, 10:54 AM   #28
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Fish53 - I concede you are technically correct. However, in my limited experience, prop diameter has always been limiting factor. So the optimal diameter was interesting, but irrelevant. This means we have always run the calculations including a required fixed diameter and found the best outcome given that constraint.

This quote below is from the Boat Diesel prop calculator - note the comment about fixed diameter being an input. I would love to be in a situation where I had no prop diameter constraints and simply use the diameter and pitch the calculator produced, but that hasn't happened for me yet.

"Determine the correct diameter and pitch of a propeller for any given vessel and engine.
Input vessel, engine and transmission details and the program will calculate: Maximum achievable speed, estimated fuel consumption, recommended propeller diameter and pitch, a propeller pitch based on a required fixed diameter, plus it will suggest alternative propeller sizes using different gear ratios to provide better propeller efficiency."
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Old 02-12-2019, 11:01 AM   #29
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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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Old 02-12-2019, 11:12 AM   #30
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Oh, with all that said, there is a good bit of black magic involved!!
Truer words have not been spoken. This is an interesting discussion that I have learned from - I appreciate everyone's expertise.
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Old 02-12-2019, 12:50 PM   #31
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Originally Posted by Bkay View Post
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.

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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Old 02-12-2019, 02:05 PM   #32
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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!!
"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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Old 02-12-2019, 02:08 PM   #33
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For many boats a "square prop" (where pitch and diameter are equal) is considered ideal.
Very true FF.
But not typical trawlers.
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Old 02-12-2019, 02:12 PM   #34
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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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Old 02-12-2019, 02:17 PM   #35
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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.
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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Old 02-12-2019, 02:21 PM   #36
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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.
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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Old 02-12-2019, 02:30 PM   #37
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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.
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.
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Old 02-12-2019, 02:35 PM   #38
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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.
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.
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Old 02-12-2019, 02:46 PM   #39
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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
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Old 02-12-2019, 02:49 PM   #40
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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.
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?
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