Average speed ???

[chopper_man]

Hi<br /><br />I recently mounted a 1975 70HP Johnson on a 17'1978 Bayliner Mutiny.I'm currently reaching speeds around 30 Mph WOT with 2 people and all gear does this seem about the average speed??? While pulling a skier etc.it drops down to about 25 Mph.Just curious to know if this a good set-up or purchase a bigger motor.<br />Thanks in Advance. Chopper

 

[Silvertip]

Re: Average speed ???

Speed is is a "non-factor" unless we have some RPM readings to go with it. 30 MPH and wide open throttle RPMs in 5800 bracket is as good as it gets. 30 MPH at 6500 RPM is overreving. 30 MPH at 4500 is lugging. Help us out here.

 

[chopper_man]

Re: Average speed ???

Ok sorry the rpm's are around 5200 WOT.

 

[Solittle]

Re: Average speed ???

You probably want to get the rpms up a bit. Where is the cavitation plate relative to the bottom of the hull?

 

[chopper_man]

Re: Average speed ???

Originally posted by SoLittle:<br /> You probably want to get the rpms up a bit. Where is the cavitation plate relative to the bottom of the hull?

It's pretty much inline with the bottom of the hull.

 

[Dhadley]

Re: Average speed ???

If you have a decent prop it's time to go up, up and away!! <br /><br />What brand & style prop do you have?

 

[chopper_man]

Re: Average speed ???

Originally posted by Dhadley:<br /> If you have a decent prop it's time to go up, up and away!! <br /><br />What brand & style prop do you have?

I have a brand new Vortex XHS 13 1/4 x 17 prop.

 

[waterinthefuel]

Re: Average speed ???

Oh wow, my boat with a 28hp will do AT LEAST a gps speed of 27.5, and probably more since it's seems to scream bloody murder now that my father worked on the carb.<br /><br />A 70 is a good engine though. Powerful. I'd want higher than 5200 though. An engine that old has a tach that works? I'm impressed.

 

[Boatist]

Re: Average speed ???

If your motor running smooth and not missing and you plan to do a lot of skiing then I think you need more power. With you current power you are going to be running at or near WOT all the time. That not good for the motor life or fuel mileage.<br /><br />If you go to a lower pitch prop your top speed will go down but your skiing speed could go up some or stay the same and you would have a better hole shot.

 

[chopper_man]

Re: Average speed ???

What size prop would you suggest i use for a lower pitch??

 

[rickdb1boat]

Re: Average speed ???

I'd raise the motor one hole and see how it does before you buy another prop....Otherwise, a 15" pitch prop would be in order....BTW-Does it pop the skier up pretty good right now??

 

[Silvertip]

Re: Average speed ???

Before spending bucks on a new prop, carefully check the AV plate position. If its even with or below the buttom of the boat, try raising the engine one or two holes. Less engine in the water = less drag = better performance. Just don't go so high you lose water pressure. Then think about props.

 

[chopper_man]

Re: Average speed ???

Originally posted by rickdb1boat:<br /> I'd raise the motor one hole and see how it does before you buy another prop....Otherwise, a 15" pitch prop would be in order....BTW-Does it pop the skier up pretty good right now??

At the moment it has a slow hole shot but it does get a skier up,hesitates for a moment then takes off up on plane.I will try raising the motor one hole see how it goes then consider a new prop.<br />Thanks for all your responses.Chopper