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View Full Version : What effect does propeller diameter have on perfor



tiupc
01-28-2012, 07:21 AM
I have found lots of references to the effect pitch has on performance but nothing on the relationship to prop diameter. What difference could be expected from props 14" x 25P vs 16" x 25P. More or less speed or RPM ?

Thanks

mattyboy
01-28-2012, 07:48 AM
rule of thumb for every inch of pitch or diameter = 200 rpm

so going from a 14x25 to a 16x25 will be approx a 400 rpm loss

there are other factors like slip and features like cup and rake that will also effect performance and handling

gcarter
01-28-2012, 08:07 AM
What matty said, but w/this caveat;
The reduction in RPM w/the larger diameter probably also means a reduction in slip, so you may actually pick up performance.

Morgan's Cloud
01-28-2012, 09:06 AM
If we're talking specifically about a single engined smaller boat, like ours around here , would I be correct in assuming that a larger diameter prop would also exacerbate the prop torque/lean to port/stbd issue ?

mattyboy
01-28-2012, 10:11 AM
morgan's cloud yes in general if the same type blade design is used yes

but as george said a larger diameter may have less slip the solas titan is a good example on the volvo aq boats . they have no ill handling traits and have less slip so i did see an overall increase in controlable wot speed

mattyboy
01-28-2012, 11:38 AM
let me clarify my statement

i was running a 14 ish 22 to 26 pitch props on the volvo 16 each had handling quirks the best prop was an ultra 24 but it still got squirely at high speed the ultra was a 14 or 14 1/4 i moved to a 15 5/8 21 pitch solas and picked up a higher controlable top end speed and lost all bad handling traits no ventilation no torque no cavitation or blow out in any water conditions. the solas was not as fast as the ultra but i could not stay at that speed for any length longer than a few seconds.
i had a large 15 something round elephant ear prop it was a pos and torqued the boat badly.
props are like a black science blade design number of blades cup rake labbing all things that have a say in performance

BUIZILLA
01-28-2012, 01:36 PM
standard rule of thumb is diameter is for accel and torq bite, pitch is for speed and trim

f_inscreenname
01-28-2012, 03:01 PM
Bad torque issue, try a Left if running a Right hand prop. My boat is a bear with a right. Even broke a steering cable trying to turn at WOT one time. Put a left on it and it drives like power steering. Let go of the wheel at WOT and it goes straight or what ever way I point it.

oledawg
01-28-2012, 05:04 PM
Interesting thread...cleaning up in my garage and found a very nice four blade 14 x 25 Solas prop that I haven't tried yet on my 22C. I have tried a labbed 23P, a Mirage 26p+, and a Mirage 27P, all three blade, with the latter being the best handling...not the fastest, but rock steady at WOT. The 23 is fastest but get really squirrelly and cavitates a lot when accelerating. So, what are thoughts on the 4 blade 14 x 25? I have a new engine going in, a 482 stroker 454 that should be cranking a minimum of 500 hp, and wonder if the four blade might have any advantages. The 27P also holds the RPM under 5,000 rpm which is meaningful to me since I have blown two engines in the past two years :bonk:

gcarter
01-28-2012, 05:44 PM
The best all-around prop I ever ran on the Minx was a 15" diameter X 22" pitch Hydro Q-4.
Incredible stability, and NEVER blew out.
I highly reccomend them.
I believe the four blade design and the large diameter were instrumental in its success.

MR MAGOO
01-28-2012, 05:56 PM
The larger diameter prop can carry more weight. Small light boats generally perform better with smaller diameter propellers. Heavier vessels require more disk area (diameter and/or # of blades) to carry the weight and keep prop slip numbers in check.

oledawg
01-28-2012, 06:27 PM
Soooo, what about the 14" x 25P 4 blade? Thoughts? :pimp:

tiupc
01-28-2012, 08:57 PM
Thanks for all the helpful information. I had concerns about RPM effect changing from a 14 x 25P Alum to a Solas Titan 21P CR for my volvo drive. Speed was not the most important; control and not over reving was. It was at the suggestion of good people like yourselves that I bought the Solas Prop. Up in Canada it seems very hard to get the solas props as Marinas will order(special) but take several months to get and they can't guarantee you will get it. I ended up ordering from the US. Now all I need to do is wait for the ice to melt in April to try it.

Trevor

mattyboy
01-29-2012, 12:52 PM
the solas is a prop worth having in your arsenal if you have a volvo aq classic. not sure what your power plant is but i was just able with about 310hp to turn a 21 to about 4900-4950 on a 1.6 drive. the size and grip on a solas gives them a point or two above what's stamped on them . a 21 acts like a 22 or 23 .you are going to have to deal with the torque tab if you have a 250 or 270 not a big deal a yammaha ob tab will work the 280 or 290 it's not so much of an issue

oledawg
01-29-2012, 02:36 PM
OK, so it sounds like my Solas prop could actually be sort of like my Mirage 27P except that it has 4 blades instead of three, correct? Guess I will bolt the puppy on and check it out :wink: