Which Prop? - Crap Shoot - 1974 Mercury 650 on a 15' tri hull

Collapse
X
  • Filter
  • Time
  • Show
Clear All
new posts

  • Which Prop? - Crap Shoot - 1974 Mercury 650 on a 15' tri hull

    WOW! LOVE THIS SITE!

    I have spent the last 9 months restoring a 1970 ArrowGlass "Tarpon" Tri Hull. New deck, floatation, transom, paint...etc. Also just rebuilt a 1974 Mercury 650 (65HP) 3 cyl to power the old girl. 120lbs on each cylinder.

    I am running a ProPulse composite prop that my friend Robert gave me. It is adjusted all the way out (17p) and I am able to get 29mph at WOT with myself and one other person - loaded with gas, battery, dry storage and cooler. The engine has no problem getting on plane (Light boat for a fiberglass) and will stay at 5000 RPM no matter where I set the prop (not sure if the tach is functioning exactly right). Being in Virginia, I am doing a lot of time on the James and Chickahominy rivers - both have their share of obstacles. This makes me very leary of having a plastic prop as a full-time prop, so I am looking for an aluminum prop.

    This is the second boat of this kind I've had. The only difference is the first one was a 1973 ArrowGlass Tarpon with a 50hp (1973) Merc and it would fly. I should never have sold it - so much fun - but I was younger and at the time wanted to go to a bigger boat (family thing). When replacing the floor, stringers, floatation and transom I was careful not to add more weight than original.

    My question is.... available props for this motor range up to 11"X17 pitch, and I would like a little more speed, but I'm not sure if that would be too much and bog down the ol' Merc and don't want to impede handling. I'm thinking somewhere around 3 blade aluminum 11" x 15" pitch. Am I in the right ballpark? Lower? Higher?

    I am going to try to post a picture of the boat so you guys can get a better idea of the size (bear with me, I'm a Harley buff). Any suggestions are GREATLY appreciated.. Thanks for your time and thoughts.

    Click image for larger version  Name:	Arrow-1.jpg Views:	1 Size:	118.2 KB ID:	48351

    Click image for larger version  Name:	Arrow-2.jpg Views:	1 Size:	101.2 KB ID:	48350

  • #2
    Ok, trying to find the gear ratio, I believe your's is 1.64 and RPM range 4800-5300
    foun that here. https://forums.iboats.com/forum/engi...nterchangeable
    40 & 402 2 cylinder.......................................... ..2:1
    40/45/50/500 4 cylinder......................................2:1
    50/60/650/700 3 cylinder....................................1:64:1
    650/75/80/800/850 4 cylinder..............................2:3:1
    900/90/1150/115/1400/140/1500.........................2:1
    1970-71 1150 & 1350.........................................1:78: 1

    Found RPM range here. http://download.brunswick-marine.com...roke/66-72.pdf

    be interested to check the engine height here. https://www.bandofboaters.com/forum/...gs-to-look-for

    As for pitch, with a decent aluminum prop, I'd say you'd expect somewhere around 10-14% slip. kinda varies on brand. Solas would be lower then that.

    I'd say try a 13 pitch to start with and hope for mid 30's


    Ken Reeves
    KR@Propgods.com
    941-735-5808

    Comment


    • #3
      AAAHHHhhh....!!! Thanks Ken ! I hope you're right. I'm going to look for one now. Thanks again.

      Comment


      • #4
        Awesome looking boat !
        I'm sure that one turns a lot of heads on the water.

        Ken Reeves
        KR@Propgods.com
        941-735-5808

        Comment


        • #5
          Ha Ha !! Thanks Ken. I've only had it out the one time so far. Had to know what needed adjusting. Weekday on Lake Anna - not many people there. Most people say "What th...why?" But I never was one to stick with the norm. Do you think a 14" pitch would be too much and do y'all sell props?

          Comment


          • #6
            I would go 13.
            I do sell props, I don't do returns on aluminum, so you can probably get it on Amazon quicker and cheaper.

            Ken Reeves
            KR@Propgods.com
            941-735-5808

            Comment


            • #7
              Thanks again Ken. I ordered the 13. I'll let you know what happens.

              Comment


              • #8
                sounds good, keep us posted

                Ken Reeves
                KR@Propgods.com
                941-735-5808

                Comment


                • #9
                  Hi Ken,

                  I ordered and received the new prop and finally had a chance to get to Lake Anna yesterday. First let me say this - I ordered a 13p prop and received a 14p prop. Well, why not try it. I switched to a RELIABLE Teleflex tach. When I powered up the boat the difference was immediately noticeable. It would JUMP on plane (although I normally prefer to take it slower to avoid spinning the prop). The water was somewhat rough - no whitecaps, but not as smooth as I would have liked, so we fished a while until the wind backed down so we could test the prop. When we were able to get on smooth water the boat quickly got to about 31mph although just for a second because the tach was reading 6000 plus. Still looking for the right prop only now I have a good starting point. All said it was a good day at the lake right up until the downpour while putting the boat back on the trailer. All in good fun.

                  Comment


                  • #10
                    Hmm, I wonder if the new tach is reading right.
                    I've seen these Photo tach's before. Not sure how accurate they are, seems like it would be an easy test against your existing tach.
                    https://www.amazon.com/Neiko-20713A-...2%3A2661618011


                    Ken Reeves
                    KR@Propgods.com
                    941-735-5808

                    Comment


                    • #11
                      This was an older needle and dial Teleflex. Hooked it up as per information given by users of both the tach and the 1975 650 3 cylinder (mine is a 1974 - same). All pre-tests were positive.

                      Comment


                      • #12
                        If its right, the prop is just slipping a little more then I'd expect.
                        14 pitch, 1.64 gear ratio, 6000 RPM and 31 MPH is 36% slip which would be pretty awful.


                        Ken Reeves
                        KR@Propgods.com
                        941-735-5808

                        Comment


                        • #13
                          YES ! That's the figure I got this morning too. That would cut my gas mileage by around 25%. TERRIBLE!

                          So... Strange surprise. I received the prop. 10 X 17p. The 1" diameter difference between the 11x14p and this one makes this one look small. Anyway, I put it on and found a good excuse to head for the river. When I throttled up I DID NOT have the immediate jump onto plane, but a slightly struggled one - although it did plane out within about 15 seconds. All that was expected. What wasn't expected was the tach read around 5000 at WOT but the gps speed would not get past 28 mph. I thought it might be current or tide, but this happened in either direction (+ or -). Slippage? Only adjustment I have is to maybe set the tilt pin out one. So far, the best was the 11 x 14p. Maybe a 11 x 15p or 16p next? Curious. Now I'm puzzled again.

                          Comment


                          • #14
                            17 pitch at 5000 and 28 MPH is 43% slip. so again, Crazy high slip numbers.

                            So either every prop you put on it has the worse traction of any prop I've ever seen...... or that tach is off

                            I think the tach is off.

                            Ken Reeves
                            KR@Propgods.com
                            941-735-5808

                            Comment


                            • #15
                              Hmmm... So many things don't make sense. If the 11x14 sent the tach to 6000 and the 10x17 sent it to 5000 WOT (which is understandable), then why would my speed be so slow? The boat is very light. If the tach were reading half figures, then why is it reading only 6000/5000 (respectfully).. Can my slip really be that bad? Like it's pulling a parachute? Thanks again Ken for all your input. I may end up with a room FULL of props, but this has become an obsession and it WILL get right. lol... Dave

                              Comment

                              Working...
                              X