| Author |
Message |
    
Keith Quarrier
Member
Username: kquarrier
Post Number: 9
Registered: 09-2013
| | Posted on Saturday, October 20, 2018 - 12:02 pm: | 
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I have a homemade 15 1/2' flat bottomed launch with a 5hp one cylinder farymann k30 diesel. 2400rpm. Presently I'm running it with a 3 blade 12"x12p. I'm getting some black soot on the transom. I've had the injector checked out- seems fine. I'm wondering if I should be using a smaller propeller- maybe a 3 blade 11" x 8? I called Farymann and they said to talk to the boat manufacturer- that's me. Any help on what size prop I should be using would be great- and if anyone has one.....
Thanks, Keith |
Jim Parrott
Senior Member
Username: jim_parrott
Post Number: 195
Registered: 06-2009
| | Posted on Saturday, October 20, 2018 - 05:18 pm: |
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Keith,
I have used prop calculators in the past with good results. I just looked up one on the internet that looks like a good one.
Click on the link below. Hope this helps.
Jim
https://www.vicprop.com/planing_size.php?action=calculate |
Keith Quarrier
Member
Username: kquarrier
Post Number: 10
Registered: 09-2013
| | Posted on Saturday, October 20, 2018 - 06:23 pm: |
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Jim,
Thanks for replying. I've used vicprop - just not sure if it works for my old motor and boat.
Keith |
Jim Parrott
Senior Member
Username: jim_parrott
Post Number: 196
Registered: 06-2009
| | Posted on Saturday, October 20, 2018 - 09:10 pm: |
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Keith,
I found a manual online that says your engine should turn up 2500 rpm. Make sure to use max rpm when using the calculator. I put in my boat specs and it was pretty close called for a 13 x 14 for a 4hp 1000 rpm engine. 12 X 12 seems big for a 2500 rpm 5 hp engine to me. That would definitely make it smoke. Try borrowing a smaller prop from a friend or a prop shop if possible.
Good Luck
Jim |
Keith Quarrier
Member
Username: kquarrier
Post Number: 11
Registered: 09-2013
| | Posted on Sunday, October 21, 2018 - 06:45 pm: |
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Jim,
When using vicprop I've been putting in that the tranny is 2-1 reduction. Looking at the specs again, I think it might be 1-1 which changes the prop to 8". I'll look for a smaller prop- and I thank you for encouraging me.
Keith |
miro forest
Senior Member
Username: miro
Post Number: 1008
Registered: 11-2001
| | Posted on Monday, October 22, 2018 - 03:32 pm: |
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On my fantail I use a 13 x 13 prop three blade .
The engine runs very nicely at about 700 RPM. It is a 2 cylinder St Lawrence and is rated about 6 HP at around 1000 RPM.
On some hull testing experiments that I did 2 years ago ( had another boat pull me at 2, 4 and 6 mph and measured the pull on the line in lbs) my copper jacket engine rated at 3 HP at 975 RPM, actually produced about 1/2 HP at the propeller.
The prop was a 9 x 9 in., two blade
Most of these marine engines have a fairly flat torque curve so HP = Torque x Speed.
Since torque is ( more or less constant) then running your engine at 1250 RPM will produce 2.5 HP.
So, I'm sure that you will find much better performance and the engine will run better with a smaller prop - it will get the RPMs up and let the engine run in a better load range.
And if you are really keen, there is a book "Propeller Handbook" by Richard Gerr 1989 International Marine / Ragged Mountain Press, Camden , Maine. which is by far the best that I have ever read - even better that Weston Farmer's stuff.
Miro |
Keith Quarrier
Member
Username: kquarrier
Post Number: 12
Registered: 09-2013
| | Posted on Monday, October 22, 2018 - 05:35 pm: |
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I checked my tranny today by turning it by hand. Seems that it is 1:1 or maybe 1.2:1. Plugged my numbers into vic prop again. and came out with 9x7 three blade and 9.6x7 two blade. Anyone have one this size that I could purchase? RH 1" shaft.
Keith |
Jim Parrott
Senior Member
Username: jim_parrott
Post Number: 197
Registered: 06-2009
| | Posted on Tuesday, October 23, 2018 - 10:47 am: |
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Keith,
Sometimes you can pick up props on eBay for a good deal. Many prop shops have used props and can adjust the pitch plus or minus 2� as needed. |
Keith Quarrier
Member
Username: kquarrier
Post Number: 13
Registered: 09-2013
| | Posted on Monday, November 05, 2018 - 07:26 am: |
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Propeller update. A few weeks ago I bought a 2 blade 9"X10 prop. The engine sounded wonderful with it- full range of rpm,and no soot on the transom, but it was like a dead fish in the water. It would pick up speed after awhile, but I doubt it could have propelled the boat in a head wind. Yesterday I tried a 3 blade 11"X9 prop. Much better pickup and full rpm's and no soot. Not sure if it's the 'perfect' prop, but I'm going to stick with it.
Keith |
miro forest
Senior Member
Username: miro
Post Number: 1012
Registered: 11-2001
| | Posted on Monday, November 05, 2018 - 02:23 pm: |
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Your initial guess was a 3 blade 11x8 and you managed to find a 3 blade 11x9
I think you're better at figuring out props than most . You've also discovered that props and their performance especially for slower turning engines, include a "magic" factor, known only to the "magicians" , and they generally don't tell their secrets.
miro |
Keith Quarrier
Member
Username: kquarrier
Post Number: 14
Registered: 09-2013
| | Posted on Monday, November 05, 2018 - 05:09 pm: |
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Couldn't upload pics of my boat. I have a Mac. |
