HST Propshaft Height


    I'm the new owner of my first hydro, a 91 HST.  The boat carries a professionally rebuilt merc 2.4 bridgeport efi with all updates.  The lower unit is a merc high perf. with low water pickup  Per the builder, the motor is good for around 240 HP.  It is mounted with a 51/2" wide powerlift manual jackplate.  The prop shaft is about level with the pad.  The prop it came with was a 24p chopper.  Wide open it ran just over 70 trimmed out.  I had to feather it out of the hole because it would just spin out and had no bow rise. That prop was all or nothing.  Recently the TPI went bad and I had it in the shop to be replaced.  They gave me a 26p chopper to try and said it might get me out a little quicker and give me some low speed. It did get me out quicker, however it still spins out a bit, revving to 6100rpm.   Trimmed out it only turns about 5900rpm and close to 70mph.  When people hear this, they say "That motor.......that hull.......are you serious!"  Randy, is that it?  I thought at least 90!  Am I expecting too much out of this setup?  Also can you recommend a good ski prop for this setup, I really soaked a tuber last week!




    Set the motor so the propshaft is 1 1/2" below the pad. You will probably gain 15 - 20 mph because the boat will have lift and carry itself. Some guys run the propshaft even with the pad height, but have to run 14 plus inches of set back to make it work. This brings other problems. Run the gearcase at the 1 1/2" below height and you'll be just as fast without the shortcomings. There is a myth that gearcase drag is this almighty thing that must be avoided. Believe me, getting the boat to fly is a thousand times more important than that little drag. Your prop efficiency will be much better with it deeper and the boat will carry so much better.

    Give it a try and don't despair. The speed will come.   



Please submit whatever questions you have to IHRTechTalk@yahoo.com.  All questions and answers selected will be displayed on this page as they come in.  Note: though all boaters are welcome, priority may be given to I.H.R. members.  Randy can not be held responsible for any advice given.  Though his information and expertise is second to none, he has no control over what you do with your boat.  It is up to you to boat safely and act responsibly, and his advice is only to be used as a guidance for your high performance boat/motor of which you are the one responsible for the risks involved.

Mark C.


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