Prop Slip

[NHGuy]

What is considered acceptable slip for a boat such as mine? 21 degree deadrise, 2700 lbs, 19' 6" LWL, 5.7 litre 260 hp Mercruiser with 1.47 gear Alpha One drive. I'm turning a once repaired 19" Michigan wheel aluminum 3 blade Vortex. I'm trying to find a GPS to borrow so I will know actual speed. Now that I have my engine tuned up well I get 45- 4600 RPM with an inch of throttle travel left. The original equipment pitot tube speedometer gives me 48 MPH at 4500 RPM. Who knows how fast I'm going but it feels pretty fast because the boat is kind of low & close to the water.
That calculates to .128... slip. I have it trimmed up to get that speed. How do you tell when the drive is raised enough? Should I raise the drive til it ventilates? I'm a little afraid to overrev my motor. What's the technique to determine best drive angle for top speed?

 

[usskydvr]

Re: Prop Slip

Don't fixate on slip. Sounds like you are propped pretty well. With a normal load what is your WOT RPM? Trim until you get at your best speed.(either using your MPH guage or a GPS, as long as you use the same one on measurements to keep things relative) (Do this on a calm day with good water)

 

[NHGuy]

Re: Prop Slip

I feel that the boat is under propped. It was just right until I put on new ignition wires and shazam, instant horsepower. On my 14.75 X 19" Michigan Vortex RHS I get 4600 RPM at 85% throttle. I can't decide between a 21 and a 23. I found some OE documentation that says the boat was shipped with a 21.
I am a happy guy to have found the HP I thought should have been there.
Plus with the bonus of improved fuel efficiency I get to spend a little more time on the water.
About the only negative is my home lake has a speed limit of 45 MPH. But since it is a big lake and was once a haven for go fast boats there are still quite a few GFBs on there. I see people doing 55 or 60 often. Maybe they will treat it like the interstate and be a little flexible with the limit! I need to find a way to ask.
And the original point, slip, is something I'm not sure of anyway since my speed has been monitored with a 22 year old pitot speedometer.

 

[usskydvr]

Re: Prop Slip

You "MAY" be able to go with the 23. If I remember correctly the max recommended RPM for your engine is 4400-4800 ( check your owner's manual)
If you are already turning 45-4600 and have more left the 23 may work, but
I bet it will be close, especially if you start adding more folks/gear.
Remember each 2" of pitch increase will lower your RPM by 300-400 RPM
Going from a 19 to a 23 will drop the RPM by 600-800..Do you have that much throttle left?

 

[xeddog]

Re: Prop Slip

What rpm do you get if you use ALL the throttle?

Wayne

 

[180shabah]

Re: Prop Slip

OK readers digest version:

12-15% prop slip is good/normal
10-12% prop slip is very good
less than 10% - rare and typically expensive (ie racing)
greater than 15% prop slip and you probably have some issues to work through.

BUT

This all applies to WOT performance so until you open it up and trim it properly this conversation remains theoretical..

 

[NHGuy]

Re: Prop Slip

Thanks to all for your input. I had effectively the same thoughts as many of you have put forth, but I thought I'd put my situation out there for discussion in case I had forgotten anything. Now I have to put it into the fudget, which is what I consider the boat budget. There's never enough money for upgrades, just repairs and maintenance. At least that's what the kids shoe & clothing buyer thinks!