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Props and Mako

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  • Props and Mako

    I have a '88 Mako 21B, '92 Johnson 225. This is gonna sound unbelievable, but....need input. WOT is 6000 RPM 42 MPH slinging a OMC SST 21" SS prop. Yes it is in fact a 14 X 21. Took it to the prop shop and had it measured. I cruise at 30 MPH at 4700 RPM @ 13.5 GPH. Tach is good, prop is good.

    Sounds crazy don't it.? Well I'm pretty sure the RPM range for this motor is WOT 5500 - 6000. This prop, in all likly hood has been on the rig for all it's years........should I risk trying a 23' cupped prop, or leave it alone, as it runs fine. I just don't like 4700 at cruise...........I would think 4000 at cruise would be better, 23 cupped should drop it back and that would drop WOT down to around 5400-5500. Thoughts ???

  • #2
    I don't have specs on that engine, but 6000 WOT sounds too high. I would think you should be 5400 to 5500 WOT.

    Just floundering around,

    Ole Joe
    Just floundering around
    Shrewsbury, Pennsylvania
    1978 Mako 25, Sold


    • #3
      You've got to be slipping like crazy. I don't know the gear ratio for your motor (I'll ask Dunk on FS), but I'm betting your getting about 50% slip on that prop. The theoretical speed at 6000rpm is like 66mph if your gear ratio is around 1.8.

      Go up to the next prop. Or you might try borrowing a Rapture 21P or 19P to see how those run on your boat. With my Merc 225 EFI I was running a Rapture 14.25" X 17P and having trouble getting above 5600 with a light load. The top speed was about 49-50mph. Stiletto and Power Tech are awesome props too. Several Mako dealers have told me that Stiletto props perform great on Mako hulls for some reason.
      Slidell, LA 1993 Mako 261B - Temperance


      • #4
        I just ran the numbers for your cruise... It looks like about 70% slippage. The theoretical speed for 4300rpm should be around 50mph.
        Slidell, LA 1993 Mako 261B - Temperance


        • #5
          Try the 23 and if that gets you into the RPM range you want 5500- 5600 that is the prop for the boat. One comment however, sometimes a higher speed cupped prop will not give you enough low end torque(out of the hole) OR in a large following sea it will break loose from the water cavating like mad. When this happens the RPM's will need to be dropped way down to get the prop to rebite the water again. Also another thing to double check - could be that there is something slipping getting the power to the water. Sometimes heavy load (when the motor is putting out max HP) will cause the blades of the prop not to go as fast as the motors shaft is going - that is the shaft will actually be going faster that the blades. When this happens, the shaft will be actually "slipping on the props rubber hub" (I think that's what it's called.) Anyway, if you look at the way a outboard prop is built, you will find that there is a rubber "spacer" (hub) between the blades of the prop and the center splined hub that you mount on the engine shaft. This is there for 2 reasons - one is to dampen the vibrations associated with a several steel blades moving through the water and the second (which we are all faimulare with )is when you hit something with the prop. If this rubber hub was not there, you would break a main motor drive shaft or blowup the motor. Have your prop guy take a look at this center rubber hub and if necessary have him put in a new one - they do this all the time when they are redoing a prop that has been dinged (which forces the prop to spin on the hub and once a prop has been "spun" like this it most likely will happen all the time). It's not much money and should / could do the trick.
          1973 22 CC Milford, CT USA[br]


          • #6
            Mike, check this thread out:

            Slidell, LA 1993 Mako 261B - Temperance


            • #7
              I think some of your problem is in the diameter of that wheel. Also, what did the prop shop say about the cup? I would say a 14 3/4 X 19 with alot of cup would bring your slip ratio into the proper range. A Laser II or a Turbo prop has the kind of cup I'm talking about, lots of cup on the tips and the trailing edge. Where are you located? I think I've got a 19P LaserII around here(Palm Harbor) you could try.
              \'87 261 200 yamis[br]\'86 Actioncraft 130 yami[br]Palm Harbor,Fl


              • #8
                OK, thanks for all the input. I ordered a SST II (double cupped) 14 1/2 x 21. It is all to obvious the old prop is just slipping. Perhaps from just being wore out....if there is such a thing. The prop shop said the hub was good. I think the tach is accurate.

                What is suspect is that I show 13 Gal per hour at 4800 rev and 30 MPH. I guess if the motor is doing no work (prop slipping) it could get 13.5 at that RPM.

       any rate we will see what happens with the new prop. Report to follow.

                I got the prop dirt cheap on ebay, fellow has a bunch of new , in the box OMC props at a great price. Got this one for 80.00 !!!!!!! I orginally bid on a 23, won it and he traded it out over e mail for a 21 for me.


                • #9
                  My dad has the same motor as you on his 1978 Boston Whaler Revenge 21. He is running a Power Tech 19P its a big diameter, but I'm not sure what it is. He gets about 51mph at 5800 rpms. He had a Rapture 14.25"x 21P and was getting about 51 mph at 5200rpm

                  Your boat is alot more boat than that flat bottomed Whaler. Hell, if that prop doesn't work out and you have to go down to a 19P or 17P you can sell it here for twice that!
                  Slidell, LA 1993 Mako 261B - Temperance


                  • #10
                    My thoughts exactly. []


                    • #11
                      OK folks, recieved a brand the box, OMC SST II 14 1/4 x 21. Put it on, test run yesterday. WOT = 5900 RPM @ 39 MPH. Now ain't that a kick in the butt. It's OK I needed a spare anyway. I am replacing the tach.