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Force 40HP Prop Selection

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  • Force 40HP Prop Selection

    I have a 40hp Force outboard on a 14ft Bayliner Capri 1401 LS. I use it at high elevation (5000ft). Motor runs well but boat won’t get up on plane. Current prop is Mercury Black Max 10 x 17 which seems to be overpropped. I ran the Prop Wizard at Turning Point Props and it is recommending a Hustler 12.5 x 8. That seems like a really big jump down in pitch so I’m a little nervous to buy. Just out of curiosity I ran the Wizard again for sea level and it recommended the same prop. That seemed odd so talked to Turning Point rep and he confirmed that 12.5 x 8 should work well but also said prop could be exchanged if it didn’t operate at proper WOT rpms. I’d rather get it right the first time so thought I’d see if anyone has experience with this setup at high elevation and could confirm that such a low pitch makes sense.

  • #2
    Again and again we tell folks you cannot possibly get a realistic recommendation unless you have a baseline with the prop you currently have. In other words, splash the boat and run the existing prop at wide open throttle with what you consider an average load and note the engine rpm and gps speed. If the engine rpm doesn't reach the upper end of the maximum rpm band or very close to it, the engine is over-propped and you need to go down in pitch. If the engine over revs at WOT you go up in pitch. I suspect the 17P prop you have is over-pitched even for sea level. At 5000 feet an engine loses a bunch of power so the 8P may indeed be what you need. But make darned sure you know where you are at with the current prop before spending money on a "guessing game". And just so you know, all of this is a waste of time if the engine is not making full power because of a poor state of tune, wear, or on a boat that is water logged and greatly over-weight. If you do not have a tach, get one.

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    • #3
      Thanks for the reply. I realize I don’t have all the information needed for someone to make a really technical recommendation but I’m providing what I do have. All I’m really looking for at this point is if someone out there has successful experience with this type of setup at elevation then I would be grateful if they could share their prop diameter and pitch. Boat did not have a tachometer but I have installed a temporary one now. Won’t be going to the lake till spring so just trying to do what I can in the meantime to hopefully make that first spring trip a little more successful. New prop is only $80 and they have an exchange program so it’s an easy thing to try to have ready in advance. If I can get up on plane then I can start dealing with the more technical fine tuning of things.

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      • #4
        No less than 12P or more than 14P according to solas prop finder for mercury 40hp 2s at 5000'. had no recommendation for force 40hp, but they should be about the same. an 8P is ridiculous. BTW, at sea level it recommends a 16P. 12-14p according to heavy, medium, or light load would be my choice to start with.

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        • #5
          Thanks, Wes. That’s really helpful.

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          • #6
            If you know what the lower unit gear ratio is on that engine I can provide a little better guess as I'm very familiar with OMC 40 & 50's but they have 2.34:1 gear cases and can spin props with more pitch. On those motors 13, 15, and 17P as well as stainless, cupped and non-cupped were tested extensively on a 16 foot aluminum walleye type boat. I always came back to the 12-1/4 x 15P cupped aluminum. On a 40 HP engine there is no real advantage to spending bucks on stainless. Here in MN we are at about 1100 ft. A 13P stainless would blow out. A full load of gear and two adults the boat would pop out of the hole and run about 37 MPH on the boat speedo. Using your gear ratio, you can extrapolate your approximate pitch for sea level and then add compensation for altitude. Do make sure the engine is tuned properly for altitude (jet changes as required).

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            • #7
              I like turning point props but there prop tool is not good. I would suggest a 10x13 prop for that boat at your elevation. I've had several force engines and the gear ratios are too tall usually.
              2003 Malibu Sunsetter LXI, Acme 525, Radar P6 ski

              (I still like to help with Force outboards !)

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              • #8
                Thanks for everybody’s input on this. I ended up ordering the Turning Point Hustler 10.5x13. Got a great price on it through BoatID website ($73.61 for prop, hub kit, shipping, etc). Also ordered service manual for my 40hp Force through eBay ($23) so I can make sure everything is ready to go this spring. Apparently Turning Point does a prop exchange program for $25 to cover shipping so if I’m able to get up on plane and want to do further fine tuning then it shouldn’t cost me too much. Thanks again!

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                • #9
                  you never said what year your motor was - but I hope this works out for you I do know TP props have pretty good cupping so sometimes they run like a higher pitch prop
                  2003 Malibu Sunsetter LXI, Acme 525, Radar P6 ski

                  (I still like to help with Force outboards !)

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                  • #10
                    It’s a 1995 Force 40HP.

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                    • #11
                      the 95 has a 2:1 gear ratio. Seems like you should be good - fingers crossed. Let us know how it goes
                      2003 Malibu Sunsetter LXI, Acme 525, Radar P6 ski

                      (I still like to help with Force outboards !)

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                      • #12
                        Recommend pitch for 700# runabout with 470# people and fuel with 40hp 2s and 2.0 GR is 17P.
                        With 1.83 GR, it's 15P @5000 ft. altitude, so somethings wrong here. Either your engine is crap or you've got a lot more weight in the boat or your GR isn't 2.0. With 2.0 GR you shouldn't have a problem getting on plane with 2 or less people with a 17P prop. Calculated speed is 37 mph with both gr's.

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                        • #13
                          Okay, WesNewell is on the right track. My engine appears to be having problems. I tried out the new 10.5x13 prop today and it still wouldn't get up on plane. This time I did have a tach attached to the spark plug wire and I wasn't getting above 3500rpm. It runs fine at idle and has good takeoff but it won't move into high rpms. I confirmed from the manual that it is a 2.0:1 GR. It doesn't appear to be fuel starved because I was squeezing the fuel bulb while at WOT and it didn't change anything. A couple things I've noticed: 1 - I looked at the spark plugs after coming home and they were all oily and gunked up (see pictures of plugs). The junk cleaned off of them just fine but something seems extra oily. Is this a sign of things being wrong with my carb? I haven't done anything to it yet but I was maybe going to take it apart and clean it and see what it is jetted for (high altitude or not). 2 - It seemed odd to me that the throttle handle has to travel so far before actually moving anything in the engine (see pictures of throttle by driver's seat). It makes me wonder if I'm not getting to max rpm simply because the throttle cable isn't adjusted properly. Also, I took pictures of the position of the Throttle Cam both for neutral position and full throttle. Does it appear that the Throttle Cam is opening up enough or should I dig into figuring out how to make the throttle cam travel further at WOT due to some sort of throttle cable issue?

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                          • #14
                            A follow-up note: While I was running on the lake, I noticed that there was quite a bit of oily stuff in the carb intake area that appeared to be dripping into the lower cowl from time to time. After looking more closely at the lower cowl, there's quite a bit of old oily stuff in there that I assumed was probably just from gas spilling from the carb when the motor was tipped up and collecting over time but now I'm thinking there's way too much oil for that to be the reason. Does this pinpoint the carb as needing to be worked on?

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                            • #15
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                              That carb is so easy to work on and simple, that's where I would start. From there, further troubleshooting, knowing you're working with a clean carb, will be much simpler. In other words, do the easy stuff first....

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