Stock 8 1/4 or 6 inch pitch on Force 15 hp?

[Old Gamefisher]

I have a 14' aluminum Gamefisher boat with a 1990 (Sears) Force 15 hp motor. Given to me by someone who bought it used and never used it, the motor hadn't run in 10 years. After some new parts and work, the motor runs like a top (I hope I haven't jinxed myself), and gets the boat up on plane easily with two of us in it. I have no idea what RPM it's turning, and what speed the boat's going at WOT, but I'm happy with it.

The prop that's on it now is an 8x6, but the owner's manual says that 8x8 1/4 is standard. The current prop is a little beat-up, so I'd like to get a new one, and probably carry a spare (lots of logs where I go). Both props are a bit hard to find. I plan to carry a little more gear in the future, maybe along with a hunting buddy who weighs more than the current passenger, my girlfriend.

Should I go back to the standard pitch, or stick to the 6"? It's a 10-spline shaft, if anyone has one for me to buy.

 

[steelespike]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

Without actual rpm info its pretty near impossible to make an accurate judgement.You may be able to borrow a shop tach or perhaps buy a tinytach.
about $40, easy to install and will fit any 2 cylinder 2 stroke.Speed by gps is also an excellent reference as well.
Without the info I would be inclined to recommend the 6" prop with the caution to ease off a little when lightly loaded.Without the info your better off
under propped as it is easier on the motor as long as you observe the caution
I suggested.I would strongly reccomend a max rpm and speed test for prop selection.Both for economy and motor health.

 

[Old Gamefisher]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

Thank you, wise one! That was very helpful. I just ordered a TinyTach.

 

[Old Gamefisher]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

Another question. The motor came with two wings that attach to the anti-ventilation plate. They don't appear to be stock, as they're not shown on the parts diagram (and there are no holes on the AV plate as it appears on the diagram). I have left them off for now. Any benefit to putting them on the motor?

 

[steelespike]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

Hold off on the wings(foils).They can cause as many problems as they cure.
One thing you can work on is motor height.Add 1/4 sticks to the top of the transom.one at a time till just before venting(cavitation)becomes a problem.Be sure it pumps water and has a good grip.You may not get much height with a beat up prop but you may be able to get it repaired so you can get good test readings.Probably won't gain much but under ideal conditions
maybe a mph.

 

[hwsiii]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

Gamefisher, I find that motor to turn 5,500 to 6,500 with a 1.57 to 1 Gear Ratio. If it was doing 6,500 RPM with a 8" x 6" if you change to a 8" x 8 1/4" prop in the same model it would now be turning 4,800 RPM, wayy below recommended RPM. Check my findings on RPM and gear Ratio, to be sure I am right. But the RPM difference is there if I am right on RPM and Gear Ratio.

 

[Old Gamefisher]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

You may be right, but I can't find an 8 x6 prop, only 8 x 8 1/4. Any ideas where to look? I think I've exhausted the internet.

Then again, as suggested above, it might have been spinning too fast with an 8 x 6 and a light load.

 

[hwsiii]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

THis is my recommendation of the prop for your boat with a total weight of 1,200 pounds, boat, motor, fuel passengers and everything.

H

 

[Old Gamefisher]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

Thanks for crunching the numbers. Where is that program? The boats only rated to 500 or 600 pounds, though (hard to read the placard), and it can't weigh more than 200, so a 1200 estimate is high.

Where to get that 8 x 6 prop?

 

[hwsiii]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

Here is a prop for it. But wait for the tiny tach to see what RPM you are turning first. The numbers that I have shown you come from a custom program that I wrote for finding the correct prop for all planing boats.

http://www.marineexpressusa.com/proddetail.asp?prod=710-FP715

H

 

[steelespike]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

Thanks for crunching the numbers. Where is that program? The boats only rated to 500 or 600 pounds, though (hard to read the placard), and it can't weigh more than 200, so a 1200 estimate is high.

Where to get that 8 x 6 prop?

If there is a prop shop in your area you may be able to have your 6 repaired.
See how it works first. If we can get numbers that make sense then we can use them to determine a prop.No need to repair the 6.
Do you have access to a gps? We'll need rpm and speed to tell if a prop is
working right.

 

[Old Gamefisher]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

The TinyTach is on its way. I planned to get a new GPS (gave my old one away to a girl who built her own sailboat!) just before duck season, but I guess I'll get one now. May not give an accurate water speed, because the tide or current is always moving.

Thanks for the lead on the 6 pitch!

 

[fishrdan]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

How heavy is your rig, loaded? From the pics in your other post it looks pretty light.

I have a similar boat (1436 Tracker jon boat) that I'm running a 7.5HP Gamefisher on and it planes just fine with the stock 8x8 plastic prop. My loaded rig is about 400# (hull, batteries, troller, outboard, gear,etc) add me 230# and my kid 80# and it does fine. Throw in a 200# adult instead of my kid and the hull is at max capacity, still planes fine but just goes slower. The only time I've had problems planing is up at altitude, 9000'.

I have a Stingray Jr. on my outboard like is shown in your pic, so maybe that's why my rig planes so easy. It came with the outboard and I never took it off since it seemed to work fine with my boat.

If you think a 8x8 prop is going to work on your outboard (don't know how much weight you are going to have in the boat) check this out.

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=280338929577&sspagename=STRK%3AMEWAX%3AIT&viewitem=

I was going to bid on it but decided to look for a 8x6 prop for high altitude.

 

[hwsiii]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

Gamefisher, to get correct GPS speed all you do is make two runs 180 degrees apart from each other, add the two together then divide by two. I have some Boat information forms and speed test forms already made up and they have the RPM already filled out all you do is insert the speeds at those RPM, if you would like them, just send me your email address.


H

 

[Old Gamefisher]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

OK, folks, the Tiny Tach arrived and I installed it on the motor; it seemes to work well. At WOT with just me, a fuel tank, and some safety gear aboard, the motor's showing 5930-6010 RPM. That would tell me that I should not go to any steeper pitch than the 8 x 6 prop that's on it now. Agree? I don't have a GPS yet, so I don't have a speed for you.

Or does it mean that my engine's not making enough power?

 

[hwsiii]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

Gamefisher it is perfect.
Stay with what you have.

H

 

[steelespike]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

H I'm wondering how much the Force likes to rev.HP is developed at 5,250
6,000 may be pressing things a little.??Anyone know what the recommended range is?Obviously he could throttle back a little with a light load.
I don't think it revs like an Evinrude 15hp @ 5,500 with a max of 7,000.

 

[Old Gamefisher]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

The owner's manual says 15 hp at 6000 RPM, with a recommended operating range of 5500-6500. Yesterday it was turning just under 6000 at WOT. Today it was 5600-5700, and I couldn't figure out why at first. Wind? Fresh vs. salt water? Then I realized my girlfriend in the boat (I was alone yesterday); she insisted it had to be the wind!

I'm still wondering if the motor is making the power it's supposed to, since it's turning that close to the low end of the operating range with a-shallower-than-standard prop. It seems to be running fine.

 

[steelespike]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

They must have tweaked the motor a little cause the 89 is rated at 5,250.
Sounds like your right on.You might try raising the moor a little.add 1/4 inch sticks to the top of the transom.Raise it to a point just before venting(cavitation) becomes a problem,be sure it has a good grip and pumps water.
Your motor should be able to make about 25 mph with a light reasonably fast boat.It may not with the 6" prop Your gps may tell the story.

 

[hwsiii]

Re: Stock 8 1/4 or 6 inch pitch on Force 15 hp?

Steele you are right about most of them liking low RPM, my 1995 40HP only turns 5,000 RPM max. But change the carburetor and it turns 5,250 RPM as a 50 HP. LOL

H