1997 75hp Force outboard prop size

[polishsaint]

I'm having difficulty trying to figure out the best prop for my 1997 75 hp Force by Mercury. It came with a 10-7/8 x 11 prop. The max rpm for the motor is 5250 rpm. At WOT it was running at 5850 rpm. At 5250 I was getting 28-29 mph (GPS). I ordered a 10 3/8 x 14 prop with the hopes of getting that rpm down. I ran it today and it is still running at 5850 rpm and with the current light load setup I get between 30-31 mph (GPS at 5250 rpm). I have it sitting on a 1993 17'6" aluminum Starcraft SS170. I don't want to waste anymore money buying props that might work. I would like to know what other people are running with this kind of setup or what the recommendation is. I like the 14 pitch prop, great acceleration which I will keep if I go out loaded down, but I want to get as much top end as possible when it's just me and a light load. I want the motor to run between 5000-5250 rpm. The motor has a 13 spline shaft if that helps anyone.

 

[walleyehed]

You'll be close to the right prop at 5500 with a light load. we can only adjust speed by engine height adjustments, at least major speed gain. pitch is loading or unloading the engine rpm, maybe 2-3mph from highest-speed to lowest speed in the proper rpm range. Might want to be cautious of the tach as well...I just sold a 90hp Johnson Looper and the owner reported the numbers the next day as 3000rpm top and 37mph. Obviously this points to tach. You indicated "still at 5850" with the 14P but 30-31mph at 5250...we need to know the previous speed at wide open throttle-5850 with 11P. A 14 and an 11 can not both indicate 5850 if truly at WOT.

 

[steelespike]

Do you know the gear ratio? Sometimes on the leg I'm guessing somewhere around 1.64.
I too suspect your tach. Its nearly impossible for the rpm to be the same with a 3" difference in pitch.
You need to verify your tach before any more changes.Maybe you can borrow a shop tach.

 

[polishsaint]

With both props (11 and 14), my speed at WOT was approximately 3 mph over the speed at 5250 rpm. With the 11p I was getting about 31 mph at 5850 rpm, and now with the 14p I can get about 33 mph at 5850 rpm. This motor used to be on a pontoon and I'm wondering if it is still not enough prop to load the engine correctly for this new configuration. It feels like the 5850 rpm is an engine mechanical limit rather than being limited by load and prop. I can try testing the tach, but it was tested by the dealer when it was installed. There has been no change in engine noise at WOT since the prop was switched. How much of a height change would be needed to adjust the speed? If I adjust the height will that mess with the rpm at all? I'd rather get the rpm in the range before I adjust anything else. I understand its only a 75hp, just want to get the most top end possible. The gear ration from what I found online is 2.33. Not sure how accurate that is. It was the only option that would come up for that motor when selecting a prop.

 

[steelespike]

Here's how they calculate.Based on the diameter and pitch of your props a 2.33 ratio is un likely.
For our calculations we'll use 1.64. 5850,1.64,11" 13% slip =32.3 mphSlightly more than your 31 mph. Thats very likely because
your actual rpm wasn't 5850.our your ratio is slightly different.
5850,1.64,14", 13% slip = 41.1 mph.
if we go another way 1.64,14",13% 33mph we get 4692 rpm.
If it turns out you have a different ratio the comparison will still be the same just different numbers.
11" 2.33, = 22.8 14" 2.33 = 29 mph. both at 5850. I think you can see 2.33 ratio isn't likely.
Most outboard tachs have settings for the number of poles in the stator, some times mistaken to be for number of cylinders.
Tachs aren't lineal so accurate at idle might be way off at speed.

 

[polishsaint]

So I was able to get my hands on a 1997 Force service manual and did confirm the gear ratio is 1.64:1. I will test my tach and post the results. I am currently in the process of moving to another state so it might be a few days. The service manual also called for a 10 3/8 x 14 prop (installed), with a target range of 4750-5250.

 

[steelespike]

Your results will depend on the wot rpm your motor can make with the 14.

 

[polishsaint]

I finally have been able to test the tach and it is reading accurate at idle, top end and everything in between. I verified the rpm at the engine with another device and compared the reading on the tach. Looking to see if anyone else has anything they can input. I might just have to buy another prop and throw it on there and see what happens, which I really don't want to do.

 

[Frank Acampora]

Mercury really did some good engineering with the 70/75 horsepower Force engines. They kept the original block design but changed to single carb feeding three cylinders. The lower unit is Mercury design and I believe any 10 inch prop off a Mercury 40-50 HP engine will fit.

The engine is surprisingly strong, especially considering it only swings a 10 inch prop. If your props are not cupped, consider having them cupped--this usually drops RPM by about 200 and speed may increase by 1 MPH or so.

Look on the auctions for a used prop with 15-16 pitch. If it works, fine. If it doesn't work you are not out too much money with a used prop If it does improve top speed but the hole shot suffers, vent the prop by drilling small holes behind each blade.

After maximizing prop performance try raising the engine until the anti-ventilation plate is about 1 inch above the hull bottom. This will reduce drag a little and give a slightly higher top speed. Once everything is maximized, consider getting a stainless prop--unless you boat by braille in skinny water, LOL

Do not expect too much more speed--31 is respectable on a 17 footer--even a light tin boat. After all, it is only 75 HP and you can only do so much.

 

[polishsaint]

Update almost 2 years later, I bought another prop, 10x17, Part # QA2088x. Did a test run and everything is good to go. 5050-5100 rpm and 32 mph gps. Boat weighs 960lb dry, 244lb motor, 2 persons and light gear.