Posts: 3472
Location: Elk River, Minnesota | anzomcik - 5/2/2018 1:00 PM
How are you getting your speed? and what brand or style tach are you using?
VMS do you think he will gain 700 rpm by droping 2" of pitch, its almost double the rule of thumb.
I almost have to question your instruments at this point, I am not saying dropping pitch wont help, i think it will.
The difference in the math i posted earlier has a concern that one of the supplied numbers is not correct.
If it were me i would find why the numbers are not closer to matching before trying a different prop.
You tach could be off and your at 6500 right now (not saying you are, useing an extreme number as a example), or your speedo could be reading 30 but your doing 40. The formula isnt close so a number is wrong.
Hiya,
I think you might have missed a couple of things I stated from earlier posts... Raising the motor will be around 300 or so, and going down 2 inches in pitch will be around another 400 give or take....thus the roughly 700 rpm increase...
The numbers are very very general as every boat is a bit different...even boats of the same brand, model, year, and even one right after the other off the line.
I have also mentioned the tach might be off, but at this point I'm not so sure... until we get some changes with the prop and height made will we see... I'd rather not send him to spend money that might not prove a thing, whereas doing a few things at home to see get him started without money.
There are a huge number of variables in this...there is even potential that the hub is spun resulting in loss of speed, or the prop just does not hook up very well at all whatsoever... Only way to know is to test and try things...
There is also a point of diminishing returns with props as well... when over pitched, the motor just cannot handle the load...it can't trim up? it cannot get even close to desired rpms.. I have tried this going from an 18 pitch to a 20 pitch on my old boat. 18 pitch would get me to 5500 rpms but the 20 pitch wouldn't even reach 4800...the motor just could not do it... Given he is not 100% sure of his numbers, all I can go off of are his numbers. The numbers I supplied above are putting the motor in a 17 pitch at 6300 rpm with a 12% slip. I agree with you that the numbers given don't add up very well.
Steve
Edited by VMS 5/2/2018 3:57 PM
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