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 Post subject: Prop decision
PostPosted: 28 Jun 2016, 11:28 

Joined: 24 Mar 2015, 19:18
Posts: 209
Location: Florida
1652 Weld-Craft. 550lbs dry, 73" beam. Total length with 21" transom pods is exactly 18'. '03 perfect running 50HP Johnrude mounted 3" above transom height. 10 Gal fuel tank in rear. 2 Group 31 AGM batteries up front. Trolling motor. Me 225Lbs, DW 120.

Boat tops out at 33MPH at 6200 RPM with 13" stainless 3 blade prop. That's 700 RPM over spec according to this info http://www.kirschbaum.at/ki_prop/pdf/2015/RPM%20Chart.pdf. Hole shot with pods is epic, literally up on plane and screaming over 5,500 RPM in a 3 count. Zero bow rise, it just shits and gets. Cruise is 25MPH at 4,750 RPM. Local prop pro at C&B Admiral says he can hammer out additional cup to reduce WOT 300-400 RPM for around $100 or put me on a new 15" stainless for $$$ that will drop WOT 400-500 RPM's. I would like to have a higher top speed at reduced RPM and higher cruise speed at 4,750 RPM.

We're old farts. We don't pull skiers, boat is primarily used to get us out to the party island 2.4 Mi. from my ramp at our weekend lakeplace.

What would you do?



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17' Pro-Line 115HP DI Johnrude Bay rig
1652 Weld-Craft 50HP Johnrude fish camp rig
17' Grumman canoe 3.3HP Johnrude on side mount creek runner
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 Post subject: Prop decision
PostPosted: 28 Jun 2016, 11:37 
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Joined: 12 Dec 2014, 14:32
Posts: 2039
Location: Eastern Mass
I'd try the prop calculator link in the Motors forum and see if one of the new 4-blade aluminum props from Turning Point would work for you. I put one on my 60hp, as recommended by that calculator, and the performance is unreal. The TP head Service Rep uses the same prop on his JohnnyRude 70hp on an frp boat and highly recommends these new 4-blades.

FWIW your best fuel efficiency will not be at those RPMs, typically at 3800 to 4200 RPMs, maybe up to 4,400 max on that 2-stroke.



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#1) 1st tin rebuild, 18' Lund viewtopic.php?f=21&t=36583
#2) 25' Parker refurb from EMPTY hull http://www.classicparker.com/phpBB3/vie ... p?f=15&t=6
#3) 16' V-tin rebuild viewtopic.php?f=21&t=36465
#4 Procraft SV14
#5) 16' Starcraft entirely NEW Transom Skins viewtopic.php?f=3&t=37548
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 Post subject: Prop decision
PostPosted: 11 Jul 2016, 01:23 

Joined: 23 Oct 2015, 16:29
Posts: 32
Location: Twin cities, Minnesota
If u add cup to that prop holeshkt and grip will get better you might lose top speed and you will drop wot rpm. Sounds like you want more top speed? I think you should go for the 15"


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 Post subject: Prop decision
PostPosted: 11 Jul 2016, 12:50 

Joined: 02 Jun 2013, 10:26
Posts: 166
I wouldn't go to a bigger prop, I'd go to a bigger pitch. It sounds as if you're getting too high of rpms. A bigger pitch will slow down your acceleration to plane but will increase your top end speed. A boat that size with a 50 should realistically go closer to 38-40 mph. I'm betting you're using a 10 or 12 pitch prop, I'd go to a 14 or 15.


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 Post subject: Re: Prop decision
PostPosted: 12 Jul 2016, 01:18 

Joined: 25 Apr 2015, 18:00
Posts: 6
Most prop sites show for every inch of pitch increase the rpms drop around 200. So I would go 3" to 4" of pitch increase. If you ever haul more weight, then 3". If not, and only use the boat as mostly transport, 4".

Sent from my SM-G920V using Tapatalk


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 Post subject: Prop decision
PostPosted: 03 Aug 2016, 06:55 

Joined: 24 Mar 2015, 19:18
Posts: 209
Location: Florida
Got the current prop off and found it's a 15" pitch. :oops: It's a SS 12.25-15 SST OMC #390850. Looking hard for a OMC SST 11.75-17 #390851 now to increase pitch 2" and drop RPM around 400RPM.

Used the prop wizard and it did not have my app. It asked for additional info on my MAX RPM and current prop which I gave and said their tech would get back with me. I inquired specifically about the new aluminum 4 blade prop for my app. They E-mailed me back. Very, very disappointed they did not even recommend a 4 blade I requested at all but instead recommended 2 15" 3 blade props and more ridiculously 2 13" pitch 3 blade props. I E-mailed them back and asked them what they were smoking. :lol:

Anyone have a OMC 11.75-17 #390851 laying around?



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17' Pro-Line 115HP DI Johnrude Bay rig
1652 Weld-Craft 50HP Johnrude fish camp rig
17' Grumman canoe 3.3HP Johnrude on side mount creek runner
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 Post subject: Re: Prop decision
PostPosted: 03 Aug 2016, 08:59 
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Joined: 31 May 2013, 15:51
Posts: 120
Location: Boerne, TX
You can't argue with cold hard facts. If you have real rpm data and engine red line you will make the best choice for your application.
I had the same issue with the prop suppliers I approached. Each recommended a prop that would have me on the Rev limiter all the time.
I ultimately choose to do what you are doing and go with what the boat is telling me it wants.

I'm not familiar with OMC engines so I have no feel for new prop availability. I do see OMC props on our local craigslist all the time so they are out there.
Be sure to share the results if you find the prop you are after.
Rob

Coming to you from planet Earth.


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 Post subject: Prop decision
PostPosted: 03 Aug 2016, 11:01 
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Joined: 15 Mar 2014, 16:57
Posts: 1910
Location: CT
I ran your #'s through Mercury's slip calculator and you're running at rough 16% slip, which isn't bad at all so I think you're on the right course looking to go up in pitch. If you can't find a prop you do have some slip that could be corrected by adding a cup to the prop, but I don't calculate you gaining any great amount of speed, hopefully it will reduce the role by 400 and you'll be roughly in the same position speed wise as you are in right now.



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2002 Alumacraft 1436LT w/ 1984 Mariner Tiller Converted to Remote & 55# Minn Kota Terrova 12v (removable)

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 Post subject: Prop decision
PostPosted: 04 Aug 2016, 05:43 

Joined: 24 Mar 2015, 19:18
Posts: 209
Location: Florida
Thanks!



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17' Pro-Line 115HP DI Johnrude Bay rig
1652 Weld-Craft 50HP Johnrude fish camp rig
17' Grumman canoe 3.3HP Johnrude on side mount creek runner
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 Post subject: Prop decision
PostPosted: 04 Aug 2016, 08:44 
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Joined: 15 Mar 2014, 16:57
Posts: 1910
Location: CT
http://www.mercuryracing.com/prop-slip-calculator/



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I have a marriage license and a fishing license, but I only carry one in my wallet.


For Sale - Custom Hand Tied Jigs, Bladed Jigs,Custom Rods

2002 Alumacraft 1436LT w/ 1984 Mariner Tiller Converted to Remote & 55# Minn Kota Terrova 12v (removable)

1985 Bass Tracker III - Restoration w/ 1988 Mercury 60hp
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 Post subject: Prop decision
PostPosted: 09 Aug 2016, 21:47 

Joined: 24 Dec 2011, 22:06
Posts: 966
Location: 72032
I've never seen the Mercury prop slip calculator, and just used it. Shows my slip at 5% :shock: . I don't see a need in changing anything, do you?

OP.....can you borrow a 15" to try? The old adage of 200 rpm per inch of pitch is not always accurate. Especially on these "little" motors. It might be close when you're 100+ hp but it's been my experience that it's not accurate at all on anything 50hp and smaller. Maybe just my weird experiences?


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 Post subject: Prop decision
PostPosted: 09 Aug 2016, 22:02 
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Joined: 12 Dec 2014, 14:32
Posts: 2039
Location: Eastern Mass
turbotodd wrote:
OP.....can you borrow a 15" to try?

Hmmmmmmmm I have a new Turning Point aluminum 3-blade 15" pitch prop sized to fit OMC 40-60hp gearcases ...



_________________
#1) 1st tin rebuild, 18' Lund viewtopic.php?f=21&t=36583
#2) 25' Parker refurb from EMPTY hull http://www.classicparker.com/phpBB3/vie ... p?f=15&t=6
#3) 16' V-tin rebuild viewtopic.php?f=21&t=36465
#4 Procraft SV14
#5) 16' Starcraft entirely NEW Transom Skins viewtopic.php?f=3&t=37548
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 Post subject: Prop decision
PostPosted: 15 Aug 2016, 20:50 

Joined: 24 Mar 2015, 19:18
Posts: 209
Location: Florida
" can you borrow a 15" to try? "

My first post was incorrect. I'm running a stainless OMC #390850 15" now.

Sorry for the confusion.



_________________
17' Pro-Line 115HP DI Johnrude Bay rig
1652 Weld-Craft 50HP Johnrude fish camp rig
17' Grumman canoe 3.3HP Johnrude on side mount creek runner
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 Post subject: Prop decision
PostPosted: 31 Aug 2016, 09:18 

Joined: 24 Mar 2015, 19:18
Posts: 209
Location: Florida
Update.

Found a very nice #390851 11.75/17" stainless for $129 shipped on Ebay. Top RPM now 5,700 @ 35MPH. Best part is a 3 MPH increase at cruise. Downside, a little bow rise getting on plane now. We're entertaining another couple Labor day weekend and I will see how boat performs with 4 on board. If it's OK I will sell the old 15" #390850 and recoup my $$, If not I'll keep it for running "Heavy Loads".



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17' Pro-Line 115HP DI Johnrude Bay rig
1652 Weld-Craft 50HP Johnrude fish camp rig
17' Grumman canoe 3.3HP Johnrude on side mount creek runner
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