Force prop help

[Adam989]

I have a 1984 85hp force on a 86 bay liner trophy bass. I can't seem to figure out what kind of prop to use with it. I first bought a 13" X 15 and it runs about 5300rpm at 25mph. The next was a 13 1/4 X 17 and it ran 4500rpm and 25mph. Next I went to a 13 3/8 X 15 4 blade it ran 4500 rpm at 29mph. All these props are Michigan wheel. I'm getting tired of this trial and error with these props. Any help is greatly appreciated. Should I try the 13 7/8 X 13 and see if that helps top speed and holeshot? It seems to run a lot better with the 4 blade

 

[Texasmark]

go to go-fast.com and look at their prop slip calculator. Plug in everything except the answer you seek and leave that box open for the calculator to make the calculation and fill that block. Gear ratio would be just the number, like probably 2.33 for that engine. I just ran a quick check on your first prop and the max theoretical speed (prop measured as a screw turning in a solid medium) at 2.33, 15" pitch, and 5300 rpm is 32 mph. Plugging in your 25 mph yields a slip of 22%. That's about twice what it should be. I am pretty sure of the 2.33 as Merc used that ratio on several engines in that hp range including the last Merc I had a 90 hp. If it were closer to 2:1 which the next engines up the tier are, your results would be worse slip.

That engine is probably rated at 5500 max so your rpms are good. Problem is your speedometer or you are trying to drive a barge! 1984 spells possibly soaked foam in the boat and it weighs a lot more than it did when it was built, or barnacles or slime on the hull, or the wrong trim angle................

 

[Frank Acampora]

That year engine is most likely a 1.76 to 1 ratio lower unit although it could be 2 to 1. To check, put the engine in forward gear and turn the prop backwards until it locks against the drive dogs. Mark one blade and turn the flywheel exactly 2 turns. If the prop turns slightly more than one turn (about 1 1/4 ) the ratio is 1.76 if it turns exactly one turn the ratio is 2 to 1.

 

[Texasmark]

That year engine is most likely a 1.76 to 1 ratio lower unit although it could be 2 to 1. To check, put the engine in forward gear and turn the prop backwards until it locks against the drive dogs. Mark one blade and turn the flywheel exactly 2 turns. If the prop turns slightly more than one turn (about 1 1/4 ) the ratio is 1.76 if it turns exactly one turn the ratio is 2 to 1.

Frank, got me confused man.

I know Merc ran a 1.64 on midrange triples when they came out; the 50, and 60 triple loopers which had a 10" prop. The 75-90's ran a 2.33, the 115 thru 125 and the low end 150 a 2:1 and above that you could have a 1.87 on the lower hp engines or 1.64 on 225 and up.

Guess this is back before Force got to using a lot of Merc parts.
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Ok Adam: Revisiting the prop slip calculator with 1.76 for a gear ratio: 1.76 gearbox, 15 pitch, 5k rpm = 40 theoretical with your actual 25 means your slip is now up to 38%.....really a barge.

 

[SkiDad]

The force 85 lower is 2.0 ratio. I know bc I have 2 of them. Anyhow you are in rpm range with any of these props but I think something is going on as you should be about 35 mph. Are you testing speed with your cell phone or GPS ? Are you sure the carbs are opening all the way ? I bet the 4 blade is grabbing water better. See Franks video on timing and sync at the top of the forum. And 13p is not the answer. Normally you would see a 17 or 19 with this setup.