Force 50 Prop Help

msullivan5

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Feb 16, 2015
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I am a first time boat owner with a 1989 BluFin Spectrum 1700. The boat is powered by a 1989 Force 50hp, Model # 507L9B. I would appreciate your help with a new aluminum prop. I believe the current prop is a 10 3/8" Rhand but I don't know the pitch. On the prop I have these numbers, A287265L; about 1/3 around F1287 and another 1/3rd around 291 stamped on the hub. What I have learned is it is a 14 tooth pin drive, non through hub exhaust.
The boat is primarily used for fishing but next summer I plan to pull my grandsons ages 8 & 6 on a tube.
I am looking for your advice on a prop that will give me the best gas mileage and overall performance--primarily for fishing. I am also looking for the best bang for my buck cost wise.
I look forward to insights.
Thanks!
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Welcome to Iboats.

Will need to determine firstly which size/pitch is current prop. If counting with a tach, go for a wot spin loaded as intended future use and check achieved wot rpm preferably on flat calmo no wind water cond. Compare them to engine's factory wot range to check if inside safe min-max wot rpm and then go from there maximizing prop to suit your boating needs.

Happy Boating
 

msullivan5

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Feb 16, 2015
Messages
15
Sea, appreciate the reply but part of the issue is I don't know the pitch nor how to determine it from the details on the prop. Just ordered a 2cycle tach to check rpms but without current pitch don't know how much that will help.
 

steelespike

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Apr 26, 2002
Messages
19,069
We need your lightly loaded wot rpm and gps speed to evaluate your setup. only 2 props listed here. 11 1/2 and 13 1/2"
Dry weight is around 1,000 lb and may be rated for 50 hp but maybe a little under powered.
Skidad, do you know the 50s gear ratio?
 
Last edited:

SkiDad

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Jul 18, 2010
Messages
1,518
Gear ratio is 1.62 Operating range is 4500-5500
 

msullivan5

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Joined
Feb 16, 2015
Messages
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Thanks for the info. GPS speed is 23mph at WOT with two aboard but no tach yet so RPMs is still a mystery. I'll let you know once I receive the tach. Generally, if the current pitch is 11.5 what would the advantage of going down to 10 or up to 12.5 or 13.5 pitch be?
 

Frank Acampora

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Jan 19, 2007
Messages
12,004
No advantage with either and you might hurt performance. The Force is a weak two cylinder 50 compared to other 3 and 4 cylinder 50s. On a 17 foot hull 23 MPH is a decent speed and you really can't expect too much more by spending about 100 bucks for a new prop..
 

Sea Rider

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Sep 20, 2008
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Once tach is installed will know wot rpm, probably right on, probably not. If that will be a family oriented combo could dial a pitch that revs your motor near the max 5500 rpm wot range, if solo boating will still have excellent rpm range as 2 kids don't weight that much, the only issue is that don't have a such large prop options to choose from if in pursuit of best prop maximization.

Make 2 wot tests, first with you alone and bare bone loaded boat, second same + 2 kids and check both wot rpm difference to see how far from each other both boating conditions are.

Happy Boating
 

msullivan5

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Feb 16, 2015
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Frank A--I am not looking to change the prop because I have an extra c-note. I am a new boater and made some first time boater goofs that beat up the current prop. Just want to make sure i get the best application for my use.
Anyone have an idea for the most cost effective supplier or is that not an appropriate question here?
 

SkiDad

Lieutenant Commander
Joined
Jul 18, 2010
Messages
1,518
My best deal a few years ago for an new al prop was at Deep Blue Yacht -- look for Michigan part number 072131

They are also available on this site with free shipping.
 
Last edited:

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
The tach will help a lot. Guessing slip at 14% it calculates at about 4000 rpm. @ 23 mph.
If the calculation proves to be accurate you will need to reduce pitch.
How is the hole shot now?
Have you tried different trim settings for best top speed?
 

msullivan5

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Joined
Feb 16, 2015
Messages
15
Steel-fIrst time boat owner so not much to compare to but hole shot seems fine. no hesitation and it gets up and goes. Only tilt on this motor so no trim other than manual with bar/pin on motor.
 

steelespike

Supreme Mariner
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Apr 26, 2002
Messages
19,069
Some things to check. Is the anti vent plate (just above the prop) about even with the bottom of the boat?
Try different trim positions, prop closer to the boat will plane easier but will likely allow the boat to plow. As you trim out( farther from boat),
It will gradually improve top speed to a certain point.Too much may cause porpoising and/or slow hole shot or ventilation.
Is the bottom clean and smooth
It may be cost effective to have your prop repaired.
Be sure the motor is in excellent tune.
 
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