Viper/Yamaha 115 Four Stroke

Hello Randy,

Late last summer I purchased a 1973 Viper to be used primarily as a ski boat and got about 5 weeks use out of it. This spring I sold the 85 hp Merc that was on it and purchased a new 115 hp Yamaha Four Stroke.  The motor weighs 406 lbs. (dry?) according to the specs. This is quite a bit more than the Merc and I have some concerns about the added weight to the back. Right now the motor is bolted to the transom but not rigged.  I have several questions for you:

1. What would be the optimum set up with this motor for skiing?  Would a manual jack plate with minimum set back help with the set up?  How would the boat set up for skiing affect a high speed run?
2. What prop would you recommend for skiing?
3. I have a Quick Silver Laser II prop (that was on the Merc) with these numbers on it: EB Quick Silver 48 16990 A40 20P. This prop seems to fit the Yamaha. Could I use it and how would it perform?
4. I am planning on installing a fuel tank under the raised box that the seats sit on.  I want to cut out the back portion and extend it rearward about six inches.  This will allow the seat backs to sit completely on the platform and still have some storage under the rear seat cushions as well as give me some additional room for the tank. With a tank of this size I calculated it would hold 18 gal. I found someone to make a custom tank but need help with rigging it.  First, I would like to have the fill hose and vent line exit the tank under the driverís seat back, come out the side of the seat back and then out the boat. Does this seem reasonable?  Second, can I install a fuel gauge sending unit and where do I get one?

I have a couple reasons for moving the tank forward.  First, when I install the ski pylon it will be right in front of the cover that encloses the area under the deck.  I am going to split the cover into two pcs so I can still get in this area, but the pylon will make it harder to get the present fuel tank in and out to fill. Second, I would like to move some weight forward because of the added weight of the new motor. I found that it helped a lot to move some weight up to the bow when pulling up skiers with the Merc motor, however it did not have trim/tilt.

Thanks in advance for your help.

Kenn Gall



Interesting!  My dad has a 115 four stroke on his boat too. It's a Lund fishing boat, but the thing runs nice. I like your gas tank idea. Maybe you can have the tank made with the fill fitting on the side then route it to the outside below the windshield rail. For skiing and general boating I'd run a small hub Merc Hi Five prop. Yes, a HI Five. That prop doesn't lug the motor too much, which is a problem with four strokes getting on plane. Then get a cav plate device. I like the small cav plate extension that Merc sells for the inflatable boats and 15hp motors they sell. It's small and looks nice. Retails for about 20 bucks and attaches to the rear of the cav plate. Try to find one. Try a 20p Hi five and set the motor fairly deep. These things together should get rid of the low speed porpoise (with your gas tank) and pull skiers pretty good. I think it will be quite nice.

Let me know how it goes!




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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