Motor turning too many rpms, makes boat feel like flipping. Hoping for advice, Thanks

[Jeff2012]

Hello all, I now own a 15 ft Hydrostream Viper with a 140 hp Johnson and do not know what size prop to use The boat has trim tilt and last time I had it out for a ride, it was turning slightly over 6000 rpm at 54 mph. It felt like it would do alittle more but when I tried, the boat started rocking from side to side. I was told this is called "chine walking" (very dangerous). I was told this motor should only turn about 5500 rpm max and move the boat at a top speed of 60 mph. Any advise out there on what size prop I should try and how to eliminate the "chine walking" would be very much appreciated! Thanks.

 

[Toypirate]

Re: Motor turning too many rpms, makes boat feel like flipping. Hoping for advice, Th

WOW Killer boat you got there Jeff, a rule of thumb I was given is for every inch of pitch you increase in the prop-rpms will drop by 100, and speed will increase by 1mph until you reach the correct rpms. (what pitch prop was on the boat during that run?)

pitch is how far the prop will travel through the water per revolution, like threads on a bolt, this also allows you to calculate your next prop size using the gear ratio of the outboard and the rpms of your motor at wot (5500)

Its almost always trial and error as no 2 boats are the same weight/size/drag.

it sounds like your already too fast though, is that boat rated for 140hp?

 

[steelespike]

Re: Motor turning too many rpms, makes boat feel like flipping. Hoping for advice, Th

I also wonder about the boats horsepower rating. The boat weighs 570 dry weight.
Seems unlikely that it is rated for 140.
A boat handling normally at those speeds could kill in a second with the wrong move or wave action.
Hope you are wearing a pfd.

 

[craze1cars]

Re: Motor turning too many rpms, makes boat feel like flipping. Hoping for advice, Th

Good chinewalking info here...how to cure, what it is, etc...:
Chinewalking1

If this site is accurate your boat is rated to 120 hp max:
Viper Page

Have fun, but be careful. Do some internet searching about those hulls. They are known for chinewalking, flipping over, getting caught by the wind, and other wild behavior, and indeed in the wrong hands these little hotrods can actually be quite deadly. And the fact that you likely have more HP than the hull is designed for could add to your possibility of catastrophe. Might consider getting some professional boat racing and setup instruction before really pushing the limits.

 

[steelespike]

Re: Motor turning too many rpms, makes boat feel like flipping. Hoping for advice, Th

Wouldn't hurt to do a little search using something like bass boat crash etc.