Which Prop Pitch?

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mmorasci
Posts: 46
Joined: Wed Mar 03, 2010 3:45 pm
Location: Coeur d Alene ID

Which Prop Pitch?

Post 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.

Mike

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TUGMYWAY
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Location: Glendale,AZ
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Re: Which Prop Pitch?

Post by TUGMYWAY »

Hello,Mike.
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
Details
Calculated Weight: 3800 lbs
Calculated Pitch: 5.1
Calculated Engine RPM: 5750 RPM
Calculated Boat Speed: 11.1 MPH

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kens
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Location: Coastal Georgia

Re: Which Prop Pitch?

Post by kens »

for that speed & weight, the largest diameter with smallest pitch is the way to go
Oak..........the juice ain't worth the squeeze :D :shock: :o :)

mmorasci
Posts: 46
Joined: Wed Mar 03, 2010 3:45 pm
Location: Coeur d Alene ID

Re: Which Prop Pitch?

Post 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.

Mike

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Mr Hot Rod
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Location: Chelsea, Quebec, Canada
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Re: Which Prop Pitch?

Post 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 : http://www.kanecustomboats.com

Glen-L Boat Video Directory : http://www.kanecustomboats.com/pages/vi ... ctory.html

mmorasci
Posts: 46
Joined: Wed Mar 03, 2010 3:45 pm
Location: Coeur d Alene ID

Re: Which Prop Pitch?

Post 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.

mike

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