Prop diameters and pitches

[jake]

Well thanks for everyones help with my warning buzzer sound, it turned out to be a over reving problem. I have one question though my current prop is 14.5 x19p and the lad reconmend to use a 12.75 x21p. I wonder if I should be using a larger diameter with a 21p or does the diameter decrease with the pitch increase.

Thanks

[jake]

Well tried it out last night and lots of over reving and maybe hit 10mph before buzzer went off. I am starting to think th White lk guys don't know much about props.

[Mick]

Jake

a Merc optimax should be able to rev roughly to about 5700 rpm before you are in to an over revving issue. In an 18 foot boat you should max out at about 50 mph.

I re-read your earlier post and if that info was correct (not a typo) and you are hearing that alarm at 4200 rpm and 30 mph, that is defintely not an over revving issue.

Getting the correct pitch in your prop is a trial and error issue. Your 19 may in fact be fine, a 21 may in fact be fine also. Either way, 4200 rpm is nowhere near the engine's capacity.

[CNs]

Jake,

For your boat (165 lowe FM) with a 135 opti, the merc prop finder suggests a 3 blade 29 pitch or 4 blade 28 pitch.

Go to this site and check for yourself

http://sites.mercurymarine.com/portal/p ... ema=PORTAL

select the prop selector option fill in the info and find the correct mercury prop.


the other thing is that you hub kit could be spun. The prop fits onto a hub. this hub is design to break before the prop, shaft, or lower unit. So if you or the previous owner (if there was one) hit something the hub might need to be replaced. This might cause over reving. you definitely have the wrong prop though.

[Paul Shibata]

Be very cautious before you commit to a 28 or 29 pitch prop. There is no way your setup would be able to turn a wheel that big despite what the prop selector may have said. 19-23p with a similar diameter prop is most likely within your working range.

[Mr.J.]

Isn't this fun?

My new to me boat came with a 15 pitch prop. Great hole shot but not much top end speed. 37 MPH @ 5300 RPM WOT.

I too am trying to find best prop for my boat. I used the Mercury site that Norm had posted. Got a 19 pitch prop. Now 38 MPH @ 5000 RPM WOT

Punched the new numbers into the Mercury prop selector and it now recommends a 17 pitch prop.: roll:

Hit and miss is fine but pricey.

[Wallyboss]

The only way to find the right prop is to go for a ride, WOT trim out for the best feel possible. Check your RPM's and see what the max RPM for your engine is. For every +1" of pitch(comparing the same diamater) you will drop about 200 rpms. I did all that to mine a couple of weeks ago and I had to go from a 17" to a 19". Now all trimmed out properly I ma hitting my max RPM right on the button.

MrJ did you try trimming your motor up. Cause your numbers seem off ???
I have Norm's old boat with a 75hp(I think that you have the 115??) and I now hit 45 mph at 5500rpm's WOT. I did also get the motor raised by 2 holes on the bracket.

[mikemicropterus]

jake, your buzzer issue is not the prop or overrevving if it is going off at 10 mph.

Ask wallyboss and mick about their set-up or not being set-up properly. Some marine dealers don't set-up the boat properly and if the customer doesn't know how the boat is supposed to perform, they can own it for many years before it is set-up properly, as is the case with championnorm and when it is it becomes and new boat. This was the issue in both mick and wallyboss' case.

This is a bone of contention with me as I see this far to often. Once you get over 70 mph like my boat does and have that experience you come to understand what settin-up a boat is all about. But sadly a lot of marine dealers in this area a devoid of this knowledge and thus the boats they sell are not performing up to their potential. This year I hope to get another 3 to 4 mph just from adusting the engine height some more and using another prop with more cupping. Boat set-up and prop selection is a science and some marine mechanics out there are not scientists of failed science in school. If the mechanic does not own a boat or has never owned a boat how do they get this experience.

My 9.9 on the back of my fisherman gets close to 40 mph on gps, why because I rebuilt the engine and it makes close to 25 hp and spent a season selecting props to get max rpm. With a 54lb thrust trolling motor , flippin deck, 2 29 series deep cycle batteries, my tackle and my wife it still does 22 mph. Why because it is propped for max rpm.

The set-up needs to be addressed first before a prop is selected.

I suggest you get it to a good shop and let a good mechanic determine what the issues are first.

You can ask fellow hawkers about me and my capabilities. I need to see the boat and set-up before I can make an educated response to your issues. Also I will figure out what the buzzer issue is.