Viper/Merc 115 Setup

Hi Randy,
I recently joined the IHR, however, I have been reading Tech Talk for about six months now, and I was hoping that you could help me with my rig.  I have 1981 Viper with a 1984 115 horse Merc. (stock), CMC Power Tilt, and a stainless 13" prop.  Right now the motor is set back 5" with the tilt, and the propshaft is about 4 1/2 inches below the pad.  With the present set up, I can get up to about 60-62 mph before the boat starts to chine walk really bad.  I realize that 4 1/2 inches is too low, but without a nose cone how high can I raise the motor and still pick up enough water to cool the motor? My goal is to go as fast, and stable, as possible given the limitations of a 115, a small budget, and the need to be able to water ski from the boat.  Would you recommend a new prop  (Hydromotive?), if so what size? , a Bob's nose cone, dual steering, torque tab, or something else?  I also need to keep the CMC Tilt because my lake has a lot of shallow areas.  Where should I start?  I have heard that a Viper with a 115 will only reach the mid to upper 60's.  Is that true?  Thank you for your assistance, and keep up the good work.  




Your boat has all the right stuff pretty much already. Dual steering would help but a nose cone is a big no-no. Your gearcase won't blow out 'till 80 plus mph. Running 4 1/2" propshaft depth is not all bad. If you saw 1/2 of a mph for every inch you went up I'd be surprised. Most props will work the best at about 3 1/2 to 4"of propshaft depth so you’re not that far off. A Yamaha drag prop or Merc laser 2 small hub will be good to start. The problem is nobody makes a decent prop for the inlines and OMC V4s. Every small non-V6 prop is always full of compromises. Some go fast but don't get on plane or vice versa. The dual steering will help you get to speeds you haven't got to yet with the set up you already have. Then just borrow some props that you find and try every one. The upper 60's are within your rig.




Please submit whatever questions you have to  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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