Prop decision

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nowgrn4
Posts: 209
Joined: 24 Mar 2015, 19:18
Location: Florida

Prop decision

Post by nowgrn4 » 28 Jun 2016, 11:28

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/20 ... 0Chart.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?
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

Xtremeboats
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DaleH
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Prop decision

Post by DaleH » 28 Jun 2016, 11:37

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.
#1) 1st tin rebuild, 18' Lund http://forum.tinboats.net/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 http://forum.tinboats.net/viewtopic.php?f=21&t=36465
#4 Procraft SV14
#5) 16' Starcraft entirely NEW Transom Skins http://forum.tinboats.net/viewtopic.php?f=3&t=37548

dootech
Posts: 32
Joined: 23 Oct 2015, 16:29
Location: Twin cities, Minnesota

Prop decision

Post by dootech » 11 Jul 2016, 01:23

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"

ericman
Posts: 185
Joined: 02 Jun 2013, 10:26

Prop decision

Post by ericman » 11 Jul 2016, 12:50

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.

bplayer405
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Joined: 25 Apr 2015, 18:00

Re: Prop decision

Post by bplayer405 » 12 Jul 2016, 01:18

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".

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nowgrn4
Posts: 209
Joined: 24 Mar 2015, 19:18
Location: Florida

Prop decision

Post by nowgrn4 » 03 Aug 2016, 06:55

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?
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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Ebug
Posts: 121
Joined: 31 May 2013, 15:51
Location: Boerne, TX

Re: Prop decision

Post by Ebug » 03 Aug 2016, 08:59

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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onthewater102
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Prop decision

Post by onthewater102 » 03 Aug 2016, 11:01

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.
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

nowgrn4
Posts: 209
Joined: 24 Mar 2015, 19:18
Location: Florida

Prop decision

Post by nowgrn4 » 04 Aug 2016, 05:43

Thanks!
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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onthewater102
Posts: 2124
Joined: 15 Mar 2014, 16:57
Location: CT

Prop decision

Post by onthewater102 » 04 Aug 2016, 08:44

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

turbotodd
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Prop decision

Post by turbotodd » 09 Aug 2016, 21:47

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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DaleH
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Prop decision

Post by DaleH » 09 Aug 2016, 22:02

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 http://forum.tinboats.net/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 http://forum.tinboats.net/viewtopic.php?f=21&t=36465
#4 Procraft SV14
#5) 16' Starcraft entirely NEW Transom Skins http://forum.tinboats.net/viewtopic.php?f=3&t=37548

nowgrn4
Posts: 209
Joined: 24 Mar 2015, 19:18
Location: Florida

Prop decision

Post by nowgrn4 » 15 Aug 2016, 20:50

" 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

nowgrn4
Posts: 209
Joined: 24 Mar 2015, 19:18
Location: Florida

Prop decision

Post by nowgrn4 » 31 Aug 2016, 09:18

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".
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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