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Which Prop Pitch?

Posted: Sun Aug 04, 2013 8:03 pm
by mmorasci
I just launched my Ken Hankinson "Ultra Pierre" and have a question regarding Prop Pitch.

The boat a displacement hull with calculated hull speed about 6 1/2 Knots, displacement around 3800 lbs, and is powered by a '61 Evinrude 18HP (fit out as an "inboard") and I currently have two Props for this engine, a 9" dia. 9" Pitch, and a 9 1/4" dia. 11 Pitch. Which one do you think is best :? ?

I have spent a little time running each one and haven't "by ear" or "by feel" determined much difference. There are both 7 and 10 Inch pitch Props still available.


Re: Which Prop Pitch?

Posted: Sun Aug 04, 2013 10:17 pm
I would start with the 9x9 and a tach. and see what the RPM's are. The listed weight is 4100+ so you are a little light. I would also go for the most diameter,but I think you will be stuck with the lower unit not letting you go larger?

Some numbers from Mercury Marine.

Boat Type: Commercial Fishing
Engine Manufacturer: Johnson
Engine Type: Outboard
Engine Year: 2001
Number of Engines: Single Drive
Engine Family: 2 Cyl 2-Stroke
Engine: 2001 Johnson 15
Drive / Gear Case: Johnson 40-75 hp 2-Stroke 2.23 Ratio Std
Calculated Weight: 3800 lbs
Calculated Pitch: 5.1
Calculated Engine RPM: 5750 RPM
Calculated Boat Speed: 11.1 MPH

Re: Which Prop Pitch?

Posted: Mon Aug 05, 2013 2:42 am
by kens
for that speed & weight, the largest diameter with smallest pitch is the way to go

Re: Which Prop Pitch?

Posted: Mon Aug 05, 2013 7:08 pm
by mmorasci
Thanks for the reply's.

I may be a little light in weight as I'm not fully equipped yet and didn't take into account any "passenger/gear weight", so when loaded, I'll likely be adding an additional 800/900 lbs. And TUGMYWAY you're right, the lower unit clearance is limiting me here to not much more that 9 1/4" diameter.

I'm trying to finish up some home projects which would allow me to get out this weekend (if the Admiral agrees). I did just receive my Tiny Tach TT2A in the mail, and assuming I get it set up and registering properly, what RPM am I looking for (I haven't found any specs online, but then again, my search abilities are sometimes poor)?

With the limited choices in props available to me, will the 1/4" in diameter be a meaningful factor or is Pitch of primary importance in this case, and would it be worth it (in performance) do you think, to invest the $80ish for a 7" Pitch (remember I already have a 9 & 11)?

Just for reference purposes, I believe I was "about" 3/4 throttle when I reached my maximum hull speed last outing (no idea what my RPM was) sporting the 11" Pitch prop. Additional throttle raised the bow but didn't produce any additional speed.

I guess, what I'm after, is the most economical way to cruise at hull speed, so minimizing fuel consumption is at the top of my list. I will be doing a series of sea trials soon but want to have the best option for prop available.


Re: Which Prop Pitch?

Posted: Mon Aug 05, 2013 8:38 pm
by Mr Hot Rod
mmorasci wrote:... What RPM am I looking for ...
The service manual for your motor states that RPM should be 4000 :
When you get your tach working, post your MPH (GPS) and RPM data for both propellers at 3000 RPM and WOT.

Paul Kane
Kane Custom Boats Ltd.
Chelsea, Quebec

Building the Glen-L Hot Rod :

Glen-L Boat Video Directory : ... ctory.html

Re: Which Prop Pitch?

Posted: Sat Aug 10, 2013 6:53 pm
by mmorasci
Thanks for all the feedback so far, I'm truly appreciative of all the suggestions you folks have (this hasn't been my area of study and I feel out of me league here).

I just got off the lake and have these numbers to post.

Prop 1 - 9"dia 9" Pitch: RPM 1000 MPH (GPS) 2.4
1500 3.6
2000 5.0
2500 5.8
2970 6.5
Prop 2 - 9 1/4" dia 11" Pitch: RPM 1000 2.9
1500 4.4
2000 5.5
2500 6.5
2700 6.5
Note: the Highest RPM's listed were a result I think of limitations in the throttle linkage itself. Adjusting the butterfly by hand, I was able to "tweak" the RPM's a few hundred RPM's higher but I couldn't at the same time record MPH data. I think it still had some movement in it but the linkage was limiting it. The engine did not sound like it was topping out by any means.

Where do I go from here? Buy the 7" Pitch and try it out? Unless I go to a custom Prop I can only get 7", 9", 10", and 11" Pitch Props. It also looks like whatever I do I'll have to re-engineer my throttle linkage to get more movement out of it it.