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Old 09-24-2016, 11:31 AM   #1
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Propellor sizing???????

One of the things on the boat that I have a hard time with is propellor size. When we bought our boat 20+ years ago it came with a 19x21x3 blade prop which you could over-rev by 500rpm over the recommended 2800rpm. We had 2" of pitch added which dropped the max rpm's to 3100.

We would like to gain some speed and reduce our running rpm(2300rpm), our prop now stands at 19"x23" when I do a prop calculation with Victoria Propellor the recommend 24.8"x17.1"x3 blade. As you can see the diameter is quite a bit larger and the pitch is smaller, what I don't understand how this would increase the boat speed to what they say.

If any one else has a PT38 sedan with a 135 Perkins 3.6544M, I would love to hear what prop size you have and what rpm and speed you cruise at.

I have included copies of the prop calculation and a photo of our running gear. Thanks for helping me wrap my head around this, the internet has not been to helpful.
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Old 09-24-2016, 04:43 PM   #2
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I'm not a prop expert, nor have I played one on TV. From what I understand, increasing the diameter reduces slip / cavitation through the water. Simply, if you reduce the diameter of a prop enough, the boat doesn't go anywhere as the prop is slipping. So a larger diameter means less slip.

The bigger issue or road block to going faster has to do with hull form and bow wave. From your attachment, you want 9 knots now in a 35' (WLL) boat. That's a hull speed ratio of 1.5. Betting it's going to take a lot more HP to plow the boat through the water at 9 knots. For your boat, as you go above 6 knots a bow wave forms. The faster you go the larger the bow wave (until the boat planes). It takes energy to form a wave. The bigger the wave, the more energy (HP and fuel) required.

That's how I understand it anyway.

Ted
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Old 09-24-2016, 05:44 PM   #3
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Rochepoint,
Have you got room for a 24" prop? If so just go w the calculation. I'd favor the 24x18 3 blade. The four blade is only needed if a three blade lacks the blade area to load the engine right. Bigger is better to a point but you risk having too little pitch and too much blade area eating too much power just turning the prop from water friction over the blade surfaces. Too much pitch causes too much tip loss. The water spills over the tips of the blades doing little work .. pushing water aft.

High speed boats can benefit from a square prop (same dia and pitch (numerically) because the water hasn't got time to spill over the blade tips. For trawlers a pitch of about .75 of the dia. (Very approx) seems best. Mine is 18X13. Guys often fall into the "dia is everything" school of thinking and get something like a 28 X 15 prop. This prop will be good at a static pull w no way on but the large surface area of the blades will eat up the power and lower efficiency w any speed.

So it's a ballance game of speed/blade area/pitch/dia.

And then loading the engine w the capability of getting 50 - 100 rpm more than the engine's rated rpm. There's some limitation here as props are pitched on forms and you can't get 18 1/2" pitch where your ideal rpm may fall .. say 25 - 50 rpm over rated. With your size prop I don't know how much rpm change occurs w 1" pitch change. You may be stuck w 50 rpm below rated or 125rpm over. In that case I'd go w 50 rpm under. The above is just my opinion.

A professional opinion can be had calling Michigan Wheel. They are good on the phone w advice and knowledgeable information about the differences between prop styles and types. I recomend using their service.
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Old 09-24-2016, 06:51 PM   #4
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Nice presentation, very succinct. I would ponder how much speed is going to be available with finding the correct wheel when the power is rated at 135 hp on a boat this size with these dimensions.
Looking at the computer hull speed of some 7 knots, this little engine is going to huffing and puffing to provide what is desired. In other words, one has to wonder if as Eric indicated, everything will start to go down hill at a point.
In our case, anything after 7 knots has us getting the skis out and on!!

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Quote:
Originally Posted by Nomad Willy View Post
Rochepoint,
Have you got room for a 24" prop? If so just go w the calculation. I'd favor the 24x18 3 blade. The four blade is only needed if a three blade lacks the blade area to load the engine right. Bigger is better to a point but you risk having too little pitch and too much blade area eating too much power just turning the prop from water friction over the blade surfaces. Too much pitch causes too much tip loss. The water spills over the tips of the blades doing little work .. pushing water aft.

High speed boats can benefit from a square prop (same dia and pitch (numerically) because the water hasn't got time to spill over the blade tips. For trawlers a pitch of about .75 of the dia. (Very approx) seems best. Mine is 18X13. Guys often fall into the "dia is everything" school of thinking and get something like a 28 X 15 prop. This prop will be good at a static pull w no way on but the large surface area of the blades will eat up the power and lower efficiency w any speed.

So it's a ballance game of speed/blade area/pitch/dia.

And then loading the engine w the capability of getting 50 - 100 rpm more than the engine's rated rpm. There's some limitation here as props are pitched on forms and you can't get 18 1/2" pitch where your ideal rpm may fall .. say 25 - 50 rpm over rated. With your size prop I don't know how much rpm change occurs w 1" pitch change. You may be stuck w 50 rpm below rated or 125rpm over. In that case I'd go w 50 rpm under. The above is just my opinion.

A professional opinion can be had calling Michigan Wheel. They are good on the phone w advice and knowledgeable information about the differences between prop styles and types. I recomend using their service.
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Old 09-27-2016, 03:27 PM   #5
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Thanks Ted, Eric & Al
The 9 knots I put in the calculator was just dreaming. Basically I was just trying to understand why a bigger diameter and what the benefit would be, looks like it would increase the efficiency of the propellor. Wondering what effect a larger diameter/less pitch would have on prop walk in reverse as the boat is terrible to back up, it has a huge prop walk to starboard.
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Old 09-27-2016, 04:15 PM   #6
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Quote:
Originally Posted by rochepoint View Post
Thanks Ted, Eric & Al
The 9 knots I put in the calculator was just dreaming. Basically I was just trying to understand why a bigger diameter and what the benefit would be, looks like it would increase the efficiency of the propellor. Wondering what effect a larger diameter/less pitch would have on prop walk in reverse as the boat is terrible to back up, it has a huge prop walk to starboard.
Guessing a larger diameter prop bites better in reverse also. I guessing the same or worse prop walk.

Ted
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Old 09-27-2016, 05:55 PM   #7
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All good advice so far. Going to a larger diameter prop is generally more efficient, especially if pitch is greater than diameter.

Make sure you have the space to do this. You need a minimum clearance of 15% of prop diameter between prop and hull. (See pic below)

But will it be worth the cost??
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