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Prop pitch/more speed?

RetiredC17load

RetiredC17load

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I have a 2006 2050 RL with a 115 Yamaha running a 13 pitch prop. WOT by myself, smooth lake I get right around 5100 RPM/24 mph. RPM range for the motor is 5-6K, so ideally I’d like to be around 5500. Yamaha prop pitch selector website says after inputting what I’m getting now, and with looking to increase about 400 RPM, that a 10 to 11 pitch prop is what I would need. I'm hoping that decreasing the pitch to give me more RPM is also going to give me a little more speed. Does this sound feasible? Thanks in advance.
 
I have a 2250gl 22foot long with two 25" toons with a 13.5 x 15 pitch solas alu. prop it runs 28 mph at 5600 rpms, I also have a 13.5 x13 pitch prop it runs 29 mph at 6000 rpms. Your rpms should be much higher with that prop, You need to be sure your toons are completely clean dirty toons can cause a massive lose in speed and rpms
 
I am also experiencing a drop in RPM and speed. The speed is certainly explainable by dirty toons. How does dirty toons affect RPM? I can’t see the connection.
 
I am also experiencing a drop in RPM and speed. The speed is certainly explainable by dirty toons. How does dirty toons affect RPM? I can’t see the connection.


The short answer is that dirty toons have little effect on RPM. There is something odd about that motor/prop combination. With a standard 13" prop, that motor should be exceeding 6000 RPM. Time for a tuneup/compression check or an examination of the prop. Has the prop been rebuilt to something other than 13" or otherwise nonstandard? No way is 10-11" prop a good idea.
 
PartyBarge said:
I am also experiencing a drop in RPM and speed. The speed is certainly explainable by dirty toons. How does dirty toons affect RPM? I can’t see the connection.


The short answer is that dirty toons have little effect on RPM. There is something odd about that motor/prop combination. With a standard 13" prop, that motor should be exceeding 6000 RPM. Time for a tuneup/compression check or an examination of the prop. Has the prop been rebuilt to something other than 13" or otherwise nonstandard? No way is 10-11" prop a good idea.
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If a 10-11” prop is such a bad idea why is this (11”) what Yamaha recommends on their website? If with a standard 13” prop the motor should be exceeding 6000 RPM according to you then you would be exceeding the manufacturer’s recommended operating range, which is 5-6K.
 
What hull do you have? 2 or 3 toons? Strakes? ESP? Express? Twin ellipticals? Not sure what all was available back then...
 
RetiredC17load said:
If a 10-11” prop is such a bad idea why is this (11”) what Yamaha recommends on their website? If with a standard 13” prop the motor should be exceeding 6000 RPM according to you then you would be exceeding the manufacturer’s recommended operating range, which is 5-6K.
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Provided you are not running a 2-cycle motor, you got it right, your motor should easily hit 6000+ with a standard (read 3-blade)13" prop. If you have a pontoon specific prop (or similar) or a non-standard gear ratio, then the situation could be different. With a prop designed for high load/low speed, maybe an 11 pitch is OK because they will drive slippage to a relatively low number. But in reality, your boat is too small and light to require such heroics. Also, we get about the same speed as you with our 2080 RL and 90 HP (2004 F90 4-cycle).

When we bought the current pontoon, it had a 13" 3-blade aluminum prop on it. The motor was hitting 6200-6300 RPM. Needless to say, the search for a better situation started PDQ. After checking al 3-blades in the 13-15" range, a 4-blade Nemesis is easily the best for us. Now the Nemesis 13p runs better speed than any of the others and pulls nominal 5500 RPM.


P.S I'm a retired C-141 pilot...passed through National and Pan Am on the way to retiring from United too.
 
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I went from a 17 pitch down to a 15 pitch and my RPM went up to the 5700 / 5800 range from 5300 and my top speed went from high 30s to low 40s. I was happy about that but as soon as the toons got dirty BOTH the RPM and the top speed returned to the old max, as if someone switched the prop back when I wasn't looking. Overall I'm very unimpressed with the SHO VF250 but that's the subject of another thread.

You would think that the RPM would stay and just the speed would go down but I've always said prop science is more voodoo than science. In a car they're mechanically linked so the car just burns more fuel with any additional load but I guess it's different with boats. The prop isn't mechanically linked to the water but that is even more reason to think it would just spin away, gripping the water less, and remain at the same rpm. Voodoo!
 
I would argue a prop is mechanically linked to water, it's just a fluid link. If it wasn't linked at all, you wouldn't go anywhere.

The transmission in a car is a similar link by a fluid (transmission fluid). Hydraulic pump and jacks are more examples. Even something as thin as air is "linked" to an airplane prop.

As a prop's speed increases in water, the resistance increases, and RPMs would change.

I'm not trying to be insulting, just trying to get you to think about props differently and to encourage you to keep learning about them. My reason for posting this is to help you get to the next level instead of dismissing it as voodoo. Which it is. I find the subject incredibly fascinating personally.
 
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Nautical said:
I went from a 17 pitch down to a 15 pitch and my RPM went up to the 5700 / 5800 range from 5300 and my top speed went from high 30s to low 40s. I was happy about that but as soon as the toons got dirty BOTH the RPM and the top speed returned to the old max, as if someone switched the prop back when I wasn't looking. Overall I'm very unimpressed with the SHO VF250 but that's the subject of another thread.

You would think that the RPM would stay and just the speed would go down but I've always said prop science is more voodoo than science. In a car they're mechanically linked so the car just burns more fuel with any additional load but I guess it's different with boats. The prop isn't mechanically linked to the water but that is even more reason to think it would just spin away, gripping the water less, and remain at the same rpm. Voodoo!
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Most of the reason for the appearance of black science is the number of factors in the equation. The link of a prop (air or water) is not positive or absolute, thus I do not consider it mechanical. For a typical prop (read non-racing) reasonably near it's designed operating RPM, slippage increases with load. So at the same time engine RPM may drop with load, the link to the water can be changing with the effect of not dropping RPM as much. With some boat /motor/prop combinations, higher-load RPM may not drop at all or , maybe, even cause the prop to "unload" and the RPM to jump up. Another "fly in the ointment" is that this thread was primarily about modestly-powered and smaller pontoons. Those have small case lowers (4 1/4"). Yours will have a large lower (4 3/4", I think). The combinations of prop features for those are going to be different with different characteristics.

You didn't indicate what style either you old or your new props are. That matters ...a lot. Just in the way of an example, 3-blade aluminum Yamaha props for our boat/motor have high WOT slippage, commonly more than 30%! A 4-blade aluminum Nemesis (intended for pontoons) cuts that slippage by at least half in WOT operating conditions (and almost as much in lower power situations too).

Anyway, it's not magic , just a whole bunch of moving parts with not wholly predictable results unless the operating conditions are well defined.
 
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