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Looking for prop options 70hp merc

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  • Looking for prop options 70hp merc

    hey gents

    I've got a 16 foot starcraft aluminum with a 1979 70hp mercury. The prop thats on it now is a michigan prop 12 1/2 x 8 and it came off a pontoon. I swapped it over and I feel like the speed isn't there. I am hitting 5800rpm on my new tach but going from. 5krpm to say 5800 its like no additional power at all. Also here a pitch change in tbe motor while running wot. Kind of a slight up and down hum. Don't know much about props but maybe its a torque or thrust prop? I run about 27mph but I feel I should be getting closer to 33-35. Tbe boat weighs 800lbs I'm guessing no more then 1000. Its about a 70 inch width. My old Chrysler 55hp pushed my 17 foot flat bottom at 30 so I'm scratching my head. Just looking to get a little more speed out of it. It has a hydrofoil on it and that solved my cavitation issues.

    Thanks guys

    Tony mac
    Last edited by Tmacular; June 19th, 2020, 11:32 AM.

  • #2
    The new noise is probably prop hum....blades beating the water. I have had that before when the prop was partially out of the water at 50+ mph at a high trim angle.

    Rule of thumb is 150-200 drop in rpms per inch of pitch INCREASE for the same approximate diameter (fractions of an inch variance due to prop design). 5800 vs 5250 (if your engine is of that ear otherwise 5500) is 2 inch increase in pitch of your available prop only comes in 2" variants. If you can do an increase of 3" I'd go for that.....try to stay in the same mfgr. and general prop type otherwise you open a new can of worms......performance can and will be different even if of the same dimensions as what you have.
    If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

    Comment


    • #3
      Think about it. From a heavy pontoon to a light 16" aluminum boat that will plane. You probably need a 13-15p prop. I wouldn't be trying to run that engine at wot with an 8p prop. It's going to fly apart if it doesn't have a rev limiter I don't know the gear ratio for that engine, but if it's 2.0 or higher, I'd start with a 15p prop. Under 2.0, maybe a 13-14p. One thing is certain, an 8p is way too low no matter what the GR.

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      • #4
        Originally posted by WesNewell View Post
        Think about it. From a heavy pontoon to a light 16" aluminum boat that will plane. You probably need a 13-15p prop. I wouldn't be trying to run that engine at wot with an 8p prop. It's going to fly apart if it doesn't have a rev limiter I don't know the gear ratio for that engine, but if it's 2.0 or higher, I'd start with a 15p prop. Under 2.0, maybe a 13-14p. One thing is certain, an 8p is way too low no matter what the GR.
        Exactly what I was looking for. Thr guy at marina (prop expert) told me thats oem proper prop for thst motor. He said i couldn't get any more performance. The thing that made me think he was wrong us when I asked if going to a same diameter and pitch stainless prop he told me no change will come and from what I know thats impossible.

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        • #5
          I get 38 mph with a 60 hp engine on my 1000# 17.5' aluminum boat. Assuming your boat is no more than 800#, you should get over 40 mph if the 70 hp engine is in good shape and mounted properly with the right prop. Actually about all a ss prop of the same pitch will do is increase rpm and speed just a little compared to aluminum. I use ss props simply because they don't damage as easily as aluminum. But if you hit something bad you sre more likely to bend a prop shaft than with an aluminum prop. But I've only bent one in my life. And they aren't that hard to change.

          Comment


          • #6
            Originally posted by WesNewell View Post
            I get 38 mph with a 60 hp engine on my 1000# 17.5' aluminum boat. Assuming your boat is no more than 800#, you should get over 40 mph if the 70 hp engine is in good shape and mounted properly with the right prop. Actually about all a ss prop of the same pitch will do is increase rpm and speed just a little compared to aluminum. I use ss props simply because they don't damage as easily as aluminum. But if you hit something bad you sre more likely to bend a prop shaft than with an aluminum prop. But I've only bent one in my life. And they aren't that hard to change.
            Right I was asking him about speed increase and he said none would come and that only 150hp motor and higher benefit from them. But I figured it should give mr 2mph increase. I know its not quite the place but am I correct in the 5500-6000 rpm max for my motor (ser# 5356950) I looked in book and it mentioned 5000-5500 under the timing adjustments section. If that's the case im definitely over revving at 58/5900 rpm right now and by standard number move up 3 inches of pitch possibly more. I just installed the tach and I assume its working. The only thing I wasn't sure of is the poles/cycles thing and how many I had. I left it where it was stock and I run 0rpm when key is on and idling saying 1k neutral 8-900 under min throttle forward gear. As for diameter of the prop i only have 3/4 clearance from the cavitation plate above right now with 12.5. Could I moved to 12 3/4 x 12 to start. Hate to blow money on props. Thanks again for the info

            Comment


            • #7
              Your "timing adjustment section" is telling you how to set your timing at "max rated" rpms. There is your number!

              Changing props for better top end can give you the benefit of less boat in the water meaning less drag and better ability to take on more pitch and still retain your rpms. My best guess for you is: http://boatpropellers.iboats.com/Mer...t_id=188040890. Otherwise go back to the top of this page, prop section put in your info and change pitch number to 10....solve for available props here on iboats. Price is right and with free shipping and prop and hub a real deal.
              If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

              Comment


              • #8
                Find your boat here, so we know what we are dealing with weight wise.
                http://boatspecs.iboats.com/Starcraf...ne/bp/20br1466
                Since you have the engine manual, post the gear ratio and recommended wot rpm range.
                With that info, a proper prop can be determined.
                If you have a whale tail attached to the av plate, remove it, unless it's above the water line at wot. Otherwise all they do is increase drag slowing you down. With the engine mounted at the proper height, they do very little accept on take off. With the engine low, they kill your power and speed.
                Go to your tach manufactures website. They should list the settings there. Make sure it's set properly. All the info about your engine should be in the manual. The wrong tach setting can put it off by up to 1000rpm.

                Comment


                • #9
                  Originally posted by Texasmark View Post
                  Your "timing adjustment section" is telling you how to set your timing at "max rated" rpms. There is your number!

                  Changing props for better top end can give you the benefit of less boat in the water meaning less drag and better ability to take on more pitch and still retain your rpms. My best guess for you is: http://boatpropellers.iboats.com/Mer...****=188040890. Otherwise go back to the top of this page, prop section put in your info and change pitch number to 10....solve for available props here on iboats. Price is right and with free shipping and prop and hub a real deal.
                  My previous motors were always propped correctly and I've never put a ding or bend in a prop ever or spun one so I've never done it. I went with a 10 6/8 x 12 prop also reccomended by another marine and its the pitch theu outfit all their aluminum runabout and fishing boats with. We shall see what happens.

                  If what you say about the timing is right then I'm over revving at 58/5900 right now.. the marina states 5500-6000 based off serial number.. the book is hard as hell to find the right ibfo bc it covers 100 motors lol I only found the 5-5500 in the timing adjustment section and now where else
                  maybe I'm blind lol I'm going to run it with this prop and see. The marina will swap them no problem if not right. By my numbers I should drop to sround 53-5500 rpm now which according to what I read in manual might be spot on

                  Comment


                  • #10
                    Originally posted by WesNewell View Post
                    Find your boat here, so we know what we are dealing with weight wise.
                    http://boatspecs.iboats.com/Starcraf...ne/bp/20br1466
                    Since you have the engine manual, post the gear ratio and recommended wot rpm range.
                    With that info, a proper prop can be determined.
                    If you have a whale tail attached to the av plate, remove it, unless it's above the water line at wot. Otherwise all they do is increase drag slowing you down. With the engine mounted at the proper height, they do very little accept on take off. With the engine low, they kill your power and speed.
                    Go to your tach manufactures website. They should list the settings there. Make sure it's set properly. All the info about your engine should be in the manual. The wrong tach setting can put it off by up to 1000rpm.
                    Nice website. Seems I have a 1973 super sport 1. Rated at actually 600 lbs however there has been some marine floor custom upgrades up front probably adding weight so 800 lbs. No load was damn near spot on im guessing. The tach guage told me to reference the motor but I have no clue the location to find the info. I'm pretty good with sound and pitch and I could say my tach seems to be right (I know a 1krpm at idle well) and thsfs what my tach says I'm idling at. I can't imagine it would be off at higher rpm ???? Anyways all solid info and leads. Ill report back with how this prop works. It's a mercury flo torq series

                    Comment


                    • #11
                      Originally posted by Tmacular View Post

                      My previous motors were always propped correctly and I've never put a ding or bend in a prop ever or spun one so I've never done it. I went with a 10 6/8 x 12 prop also reccomended by another marine and its the pitch theu outfit all their aluminum runabout and fishing boats with. We shall see what happens.

                      If what you say about the timing is right then I'm over revving at 58/5900 right now.. the marina states 5500-6000 based off serial number.. the book is hard as hell to find the right ibfo bc it covers 100 motors lol I only found the 5-5500 in the timing adjustment section and now where else
                      maybe I'm blind lol I'm going to run it with this prop and see. The marina will swap them no problem if not right. By my numbers I should drop to sround 53-5500 rpm now which according to what I read in manual might be spot on

                      With the diameter dropping down like it is the 12 probably will be spot on. The smaller, higher pitched prop will work better for a light fast boat. As you can surmise from what you got off a "toon"......fat prop with shallow pitch for pushing barges....er ah toons. The 11 I guessed is right in the middle.
                      If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

                      Comment


                      • #12
                        Originally posted by Texasmark View Post


                        With the diameter dropping down like it is the 12 probably will be spot on. The smaller, higher pitched prop will work better for a light fast boat. As you can surmise from what you got off a "toon"......fat prop with shallow pitch for pushing barges....er ah toons. The 11 I guessed is right in the middle.
                        You actually might be spot on. Luckily the dealer told me I could swap it if it doesn't work. I ran WOT at 5200 on the new prop with an improved speed of 31.8mph. S9 im thinking 10 or 11 might be perfect putting me at at a comfy 5500 WOT.

                        Comment


                        • #13
                          I also noticed stalling at idle speed in min forward all of a sudden? Maybe the increased push of my prop now needs a little adjustments?

                          Comment


                          • #14
                            Well, somethings not right here. 71-79 70 hp mercs had 2.0 gr. so 5200 rpm with a 12P prop makes the max theoretical speed 30 mph with 0 slip. And that speed can't be exceeded.

                            Comment


                            • #15
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                              Originally posted by Tmacular View Post

                              You actually might be spot on. Luckily the dealer told me I could swap it if it doesn't work. I ran WOT at 5200 on the new prop with an improved speed of 31.8mph. S9 im thinking 10 or 11 might be perfect putting me at at a comfy 5500 WOT.
                              You are running where you should be.

                              Now try it with the hydrofoil removed.

                              Different props will perform differently, and the foil may actually be scrubbing speed.

                              Another option would be to try a cupped version of the same prop if you are still getting some ventilation.

                              The 16SS is a light boat, but that hull was never really a fast one. You aren't gonna find 35 or 40 mph with that boat and motor, in my opinion.

                              She is over 40 years old, so be nice to that motor.
                              Last edited by roscoe; June 23rd, 2020, 01:59 AM.
                              Medford, WI


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