Viper/Merc 90 Setup


  My name is Marco and I own a 1984 Viper with a stock 90hp Merc (20"shaft). I'm currently running the motor on the transom (no jackplate) at a height of approximately 27". Currently my prop is a Merc chopper (small ear) with a 24" pitch. Pictures of my boat are on the Hydrostream website under "members pictures". Best run to date is 61.2mph @ 6900rpm. I want to change the setup to lower my rpm's and if possible increase my top end. How do you feel about cleaver style props on a "padded" V-bottom boat like this? I just bought a "through hub exhaust" cleaver prop on eBay (haven't got it yet!) with a 28" pitch. How about jack plates, in particular the amount of setback? I get a lot of "chine walk" at speed, will the increased setback make the boat more stable? Thanks, Marco Dorn, Long Island, NY


 Where do I begin?  Luckily I used to race a Viper with a 90 hp; I went undefeated with it, so the combo is very good.  Your pre-set speed is about right for the current set up.  The gear ratio change would work.  To change to a 14 in. x 28 in. or 14 in. x 26 in. Chopper would work better.  Going to a 26 in. would probably get you to 65ish at 6700 plus—with good punch.  Going to the 28 in. would give you one or two more with lower rpm but you’d lose a lot of. punch.  I would not put a jack plate on the boat.  Yes it would carry a passenger slightly better but it would go slower.  Balance is everything.  The 28 in. you talked about, if it is a Merc Cleaver, part #48-74338A??, you will like it.  If it is not, don’t be too depressed, I’ve found that on e-bay you get what you pay for.  A Merc Cleaver of that part number is $800 new so go figure.  I don’t believe the set back would help chine walk.  Also, the faster you go the more it will walk, you will have to learn the boat.  But the following might help chine walk and add speed.  The new 90 hp (three cylinder vs. six cylinder) gearcases absolutely suck concerning the bullet shape.  They are very blunt in the nose.  Adding a small cone and keeping the side water inlet will do wonders.

In conclusion, if it were my boat I’d run a 26 in. Chopper with a cone gearcase at the transom height your currently at.  You would creep 70 mpg at the upper 6000 range and punch like crazy.  Just like my old race boat!  A Yamaha 26 in. Drag prop would be good too.


Viper/Merc 90 followup


    Thank you very much for the input. I received my "through hub" exhaust 28pitch cleaver (manufacturer unknown). This prop got me to 63.5mph @ 6400rpm (saltwater), first try! I mention saltwater, because the 24pitch chopper gave me a top end of 58.7mph on saltwater and 61.2mph on freshwater! I've been told that this difference in speed may be caused by greater surface tension on saltwater vs freshwater. If my calculations are correct I should be doing about 65mph on freshwater. I'll be making a run on the Connecticut River hopefully sometime early summer to verify this. That's where I turned the 61.2mph last year at the River Run. All speed measurements were made via GPS. My 90hp is a three cylinder. I will try your advice and experiment with a nose cone. I will stay away from cones with the low water pickup though, because of the amounts of seaweed I encounter where I normally boat, the South Shore of Long Island. Any suggestions on a manufacturer for the cone? One other thought concerning rpm's. Mercury recommends keeping this motor below 5500rpm. I've got lot's of experience with car race engines (four stroke) and what they can or can not take with respect to rpm. I'm sure Mercury's can rev higher than the "recommended" limits that they list. What are your experiences with respect to what a "stock" motor can safely handle? Are max rpm limits in 2 stroke outboards more a function of reed opening/closing cycles or reciprocating assembly (crank, rods, pistons) limits? 

Thanks again,

Marco Dorn

Long Island,NY



Your prop is one of the old Merc "VC" cleavers. A very good find. Contrary to popular belief in testing over the years we found ,because of salt water's added buoyancy, it was always faster. Remember the current on the river. It could add or subtract speed depending on the current direction. Mazco had a small nose cone called a "Mazcone" a while back that would work and I know Hydromotive makes one. Any cone is better than the "square" front on the 90 gearcase. In SST 90 class racing we ran your motor in excess of 7500 rpm so don't fret revving it to 6500. At 6000 plus it past it's powerpeak so try to keep it to 6500 or less. I think the nose cone will cause you to lose rpm and gain speed. The prop will just hook up better and the boat will need less positive trim. 

Give it a try 




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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