Prop size for 4LHSte
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Prop size for 4LHSte
In the "Gear" topic, Mike Ohlstein mentioned he was using 20x22 4 blades with the 2:1 gears. Any cup? Any difference between port & stbd? Nibral? What kind of performance? I switched out my gears and I have a borrowed set of wheels and I'm looking for some input before I order new wheels. I have a FBC - fairly light, composite decks, no generator, no radar, etc. Thanks BC
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mike ohlstein
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Light cup. Right now I can cruise at 22.5 and top out at 25 knots. I run the same props on both sides. I know that the gear ratios are slightly different, but I just don't think that it matters enough to tinker with it. I used to have the same size props in Nibral and was able to make another knot at cruise and at the top.
I believe that too much cup makes the boat less responsive when backing down. I believe that too much pitch makes the boat unmanagable at the dock. And I believe that too much diameter makes the bow ride too proud.
The largest diameter prop that you can reasonably use on this boat is 21". If you go with a three blade prop, 21X23.5 is about as far as you can go.
If you're a fisherman, you might be interested to know that when trolling..... I have caught more fish eight feet behind the boat, with the 20X22 four blades at 7.5 knots, than with any other set-up. Others may disagree, and not every boat is the same, but the wake patern that this combination produces is (to me) a superior set-up.
Other than that, if I was retired and living in a warm weather port....with access to a lot of props, gears, a lift, and a mechanic..... I would surely be tempted to try the four bangers with 1.85:1 (or so) gears and a 20X20 (or so) set of props.
I believe that too much cup makes the boat less responsive when backing down. I believe that too much pitch makes the boat unmanagable at the dock. And I believe that too much diameter makes the bow ride too proud.
The largest diameter prop that you can reasonably use on this boat is 21". If you go with a three blade prop, 21X23.5 is about as far as you can go.
If you're a fisherman, you might be interested to know that when trolling..... I have caught more fish eight feet behind the boat, with the 20X22 four blades at 7.5 knots, than with any other set-up. Others may disagree, and not every boat is the same, but the wake patern that this combination produces is (to me) a superior set-up.
Other than that, if I was retired and living in a warm weather port....with access to a lot of props, gears, a lift, and a mechanic..... I would surely be tempted to try the four bangers with 1.85:1 (or so) gears and a 20X20 (or so) set of props.
Billy,
Mike and I run the same set up in roughly the same boat. I agree with much of what he says, and have this to add.
You need Nibral. Our boats take a lot to push, as the deep v is not a particularly efficient hull. In exchange for that you get a good or better than average ride. Those props work hard. You need the Nibral. It is the equivalent to using stainless vs. aluminum props on an outboard. When you are pushing a lot of hp, you will need the stainless. I have blown a blade off of a straight up bronze prop. Just cracked and flew off. That is not the way to go.
for the same reason as above, you need some cup. I think I have between a 3-5 cup, but it has been a couple of years snince I messed with it. While I cannot thus will not argue or go against Mike on the handling questions, I think my boat handles great. You will need to sacrifice some pitch for cup or you will get cavitation behind the blade, ie: slippage, and that is all fine and good except it will pitt the blade and that is not good. You may have to "play" with it a bit, and that is where the "art" of prop picking/specification really crosses from math to art.
Lastly, I am a huge fan of 4 blades. You will benefit from the increased responsiveness, and efficiency at cruise. It will also be smoother, and will yield more control. Look, except for Bob Lico and Harv, you are not concerned about your top end run speed. This is not a fountain. You want efficient cruise. Period. 4 blades are better than 3 for that.
I run 22-23 knots speed over ground all day long logged by the GPS. That is a pretty heavy load of fuel, w/ riggers and a hard top with open array radar, blah, blah, blah...
Dug
Mike and I run the same set up in roughly the same boat. I agree with much of what he says, and have this to add.
You need Nibral. Our boats take a lot to push, as the deep v is not a particularly efficient hull. In exchange for that you get a good or better than average ride. Those props work hard. You need the Nibral. It is the equivalent to using stainless vs. aluminum props on an outboard. When you are pushing a lot of hp, you will need the stainless. I have blown a blade off of a straight up bronze prop. Just cracked and flew off. That is not the way to go.
for the same reason as above, you need some cup. I think I have between a 3-5 cup, but it has been a couple of years snince I messed with it. While I cannot thus will not argue or go against Mike on the handling questions, I think my boat handles great. You will need to sacrifice some pitch for cup or you will get cavitation behind the blade, ie: slippage, and that is all fine and good except it will pitt the blade and that is not good. You may have to "play" with it a bit, and that is where the "art" of prop picking/specification really crosses from math to art.
Lastly, I am a huge fan of 4 blades. You will benefit from the increased responsiveness, and efficiency at cruise. It will also be smoother, and will yield more control. Look, except for Bob Lico and Harv, you are not concerned about your top end run speed. This is not a fountain. You want efficient cruise. Period. 4 blades are better than 3 for that.
I run 22-23 knots speed over ground all day long logged by the GPS. That is a pretty heavy load of fuel, w/ riggers and a hard top with open array radar, blah, blah, blah...
Dug
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mike ohlstein
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Regarding Nibral---I was informed that originally Nibral Props were made with thinner blades because they were stronger and the thinner blades allowed themy to turn faster.
But over the years this changed and bronze and nibrals are now made at the same thickness.
I was also understood that unless you are turning over 4000 rpm nibrals may not be necessary, above 4000 you risk throwing a blade with bronze.
This information was passed to me over the years, not sure its validity.
But over the years this changed and bronze and nibrals are now made at the same thickness.
I was also understood that unless you are turning over 4000 rpm nibrals may not be necessary, above 4000 you risk throwing a blade with bronze.
This information was passed to me over the years, not sure its validity.
dug keep me out of this.i have posted before my boating situation is much differant than anybody else.i can pull my boat out of the water 5 times a day if i wanted too experiment with props.i have 50years experiance on the water and use that to make my 31bert as efficient as possible.i believe nibral is the only way to go when you looking to dial in your 31bert at peak.two thing i want to say when you increase maximum speed you also increase cruise.i cruise all day at 29.5 to 30 knots.the next important observation is when you get the 31bert out of the water with the first 3 feet never touching and the boat riding on the inner strakes (about 28kns.) the speed goes up dramaticly.the 5blade prop -no good,the 4 blade with cup was good but a little more drag than the 3 blade(with the same blade area as the 4 blade).yes my props are radical but i put time and effort into getting peak efficienty with 5 dollar diesel why not!
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