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Hello
I have just purchased a 15.5 ft viper with a 115 mariner on it. Everything works great , but I have a little problem . Once I hit around 30 my boat starts to bounce ( up and down ) I have tried trim up and down nothing works , I have tried starting different ways with different Trim and nothing really changes , I'm thinking its just to light In the front and really heavy in the back because of tower of power motor , gas tank , battery .
I was told to get a aluminum gas tank and put that up in the front of the boat to get more weight up there , I was just wondering if you guys had any other opinions.
Thanks any replies would help

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From what Ive read on here, gas tank up front is a big no no. I would try the battery up front, and maybe figure out how to trim down more. adding some weight in the nose might help. I would guess the jacking plate has something to do with the weight out back being too much. How much set back is that? The smaller the better on a viper(from what Ive read here)

Just mount a whaletail or dolphin on your cav plate hop will go away and will not hurt performance at all as it will be out of the water at speed.

The bouncing is porpoising, your boat and motor are fighting each other. The hull is trying to produce lift, but can't, it's either being let down by not enough speed or forced back down by the motor. A drag plate, as mentioned above will help, but it's still going to happen a bit at lower speeds. Play with trim and throttle through it. My boat is only a foot longer, with a 200 Merc on the back, I've had porpoising issues for years. The drag plate nearly eliminated it. It used to porpoise from 30 to almost 60 mph, now it only does it between 40 and 50ish, but now I can drive through it much easier, before I was timing the bouce and nailing the throttle to get out of it lol. Now with a little throttle and trim it settles down.

ABSOLUTELY DO NOT mount a fuel tank in the bow. You can add ballast, I use sand bags, but not a tank. The behaviour of the boat will change as you use fuel, you don't want that. Static ballast is what you would want if you go that route.

You're in Toronto?? You should check out fast boats canada, and high performance boats canada, both websites have a lot of great guys that can help out. We also plan meets throughout the year, I just posted pictures from last weekend's run from Bewdley to Peterorough. The best thing you can do is come out, check out some boats, talk to the owners, and get an idea of what you may need. Good luck!

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welcome.... and nice ride

all suggestions above are a good place to start.

Welcome to the site and congratulations on finding a clean Stream.

I don't have a Viper, but...

Viper's tend to porpoise. There are a lot of things you can try to rectify it.
As far as the gas tank, since it's weight will vary between full and empty, it's not a viable fix. Moving the battery might work, but where to put it is just a guess. I would try sand bags. You can move them around to find what works best.

I don't think your motor is too heavy and your set back is minimal too, but I suspect that your prop is losing and regaining bite as the boat porpoises. Try lowering the motor a bit and see if that helps.

Try different props at different motor heghts.

Back in the day, transom wedges were available. They installed between the motor and plate for more negative trim.

A cavitation plate or stabilizer fin may help too.

Testing can sure be fun. Enjoy tweaking your set up.

first time in a hydrostream ?
you got power trim on that ?
your manual jack is set high ,
learn to "drive" it the way it is,
or lower the motor till you learn how,
trimmed in , also with that prop i'd lower it ,
your boat looks mint by the way

I'm sure that I'm pointing out the obvious, but make sure you are getting the motor fully tucked in at low speed. I see you have the older twin piston ram, I'm not sure if there is a trim stop bar toward the low setting. A series of holes where you can limit the "in" trim setting. Just thought I would mention it.

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I don't like any setback with an inline.

Hey guys thanks for the help and comments .
This past weekend ( thanksgiving ) I put about 3 sand bags in the front , it seemed to help a little bit but there is deffinetly still porpoising. So I think I'm gonna drop my motor a few inches on the jack plate because my friend/great mechanic told me its too high and that is most likely part of the problem . Also I'm gonna add a fin on the motor hopefully that will help . Thanks again for pointing me in the right direction ( away from gas tank relocated) .
Cheers and ill let you know what happens
I bought this boat for 2k , I'm pretty happy with that price considering how much some of them go for . And mine came with a spare parts motor.
But mine is pretty plane on the inside .
Doesn't have a sound system , back bench, many gauges .
Just have a few questions
I'm only a 15 years old kid so I can't spend 1000$ on a sound system , so what would you guys suggest I try and find used ? I know how I instal just need to know what's good .
Also about a bench , should I try and find a universal bench or get one custom fabricated?
Any answer will help out. Thanks again !

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