17' ski boat with Mercury 115

[iboatWayne]

Okay finally got this thing out into the water and I came up with the following:
GPS 43 mph @ 6000 rpm
Boat specs: hull wt 800 lbs, motor wt 300, had approx 400 lbs passenger weight and stuff and 8 gallons of fuel.
Using a Mercury Vengeance 16P SS prop
Boat model: witchcraft banshee
Motor: 1978 Mercury 1150

The recommended rpm range for this motor is 5000-5500 so I'm not sure how much power it's making at 6K. I'd really like to see this thing get up passed 50mph, which would require at least a 21P prop.

 

[steelespike]

Re: 17' ski boat with Mercury 115

Shooting from the hip if the boat has a nice fast bottom ,"Banshee" sure sounds fast,
50 may be possible.It does appear you could go to an 18 or 19 in the same brand/model to get more predictable results.a 19 would put you at ABOUT 5,400.
The 21 would drop you to ABOUT 5,000 a little low for todays doctored fuels.
With the new prop raise the motor to a point just before ventilation becomes a problem.Of course be sure the motor is in excellent tune and throttle plates open all the way,bottom clean.I've also seen some pretty amazing results adding or removing cup.I've heard that timing is critical on that type motor both for motor health and performance.You might ask at Johns Old Mercury Web Site about the timing issue.

 

[iboatWayne]

Re: 17' ski boat with Mercury 115

Always good to have a second opinion.
Update 5/23:
I just purchased a Solus HR Titan 3 19P and will post the results in a week or so when the prop arrives. I'm expecting 46mph @5400-5500 rpm

update: FINALLY got the boat out today.
* old prop - 16P SS Merc Vengeance (43 mph gps) @ 6000rpm
* new prop - 19P SS Solus HR Titan 3 (47.9 mph gps)
At first I wasn't sure what to expect with the higher pitch prop. The solus blades looked much larger than the vengeance!
I got an average top speed of 47 mph @ 5500-5700 rpm. Raising the motor slightly gave me approx the same top speed but raised the rpm from 5500-5700. Dropping the motor down lower in the water results in much lower top speed. With the 19P, I noticed the motor takes a second more to rev up but will wind out to 6000, at about which time the prop grabs hard and I'm up on plane very quickly. Somewhere around 35 mph, the rpm will drop and it feels like it's shifting gears. Very cool prop! Makes me wonder if I can get away with a 21P!