87MagnumII

23p laser II 13 3/4 diameter, 4900 rpm 1.47 drive. What does my slip equal out to?

Twin O/B Sonic

your speed to finish the equation.
Here's a link to a pretty good prop calculator. Just plug in the rest of your numbers.
http://www.rbbi.com/folders/prop/propcalc.htm

87MagnumII

Thanks a bunch, now I don't have to keeping asking silly questions lol!

Twin O/B Sonic

That calculator is a great tool for educating yourself on what pitch, gear ratio, rpm and slip do and how they effect each other. Great tool.
It's also a great lie detector One of your bud's makes huge claims......, all you need are his #'s to see how fast he really is.
Almost as good as a small, hand held GPS in your pocket w/o their knowledge
Then we you find out their boat is 10 mph slower than they claim you can just blame it on how heavy the GPS is and all that extra weight is slowing them down.
Watched a buddy of mine do that. Came back from a ride in a friends boat and asked the driver how fast they had gone (already knowing). After the drivers reply my buddy pulls his GPS out of his pocket (in front of the driver), looks at it and says "huh?" and walks away

87MagnumII

According to the calc my slip is like 17-18%. Its a little faster with a 21p but the slip equals about the same. Would a lab job on my prop get the slip down enough to worry about or should I just leave it?

Twin O/B Sonic

consider if you got your slip down to the 10% range that would give you an additional 5 mph w/o touching the motor I'd say that's pretty good.
Try those numbers in the calculator yourself and you appreciate what you are seeing.
"Labbing" is a very broad term and it's definition typically depends on who's responding. I would say no for your speed.
A lot of times they thin the blades which makes them weaker and they become a wear item. A good prop should be close as is for your boat. Might try a Mirage if you can find one. Heck, anything close in size that you can borrow, try. Take a buddy to back the trailer and you should be able to get through a 1/2 dozen wheels in an hr. You'll be shocked at what you see.
10% is kind of the bench mark for good numbers on a light/fast rig and you should be able to get close to that w/an off the shelf wheel.
That's assuming proper trim setting and as much bottom out of the water as possible. Typically a heavier boat like yours likes a lot of cup which is the curled up lip around the trailing edges. The farther it runs around the rear of the blades and more pronounced, the better.
The other thing I'd look for is "rake" which is the angle the blades are angled away from the boat. In theory, the more positive rake the more effect you power trim will have on the boat and less trim you'll actually have to run. Remember, rooster tails are wasted thrust. You want as much thrust pushing you straight ahead not up. A long, low rooster tale is way better than short and tall.
W/everything right, @ WFO you want your prop shaft parallel w/the water regardless of angle of attack of the hull. Over trimming to fly the bow means you need more set back (which is tough to add on an I/O), more rake, bigger blades, more blades, more cup, different weight distribution in the boat or any combination of the above.
A good picture or video of your boat @ speed is a great tuning tool.

87MagnumII

I just posted a video in the general discussion section of my boat running, my cousin is driving and has it a little over trimmed but it's pretty close to where I would run it.

Here is the link

http://www.offshoreonly.com/forums/g...d-wv-lake.html


There are actually two videos in the thread, first one is last year with old motor. Seco d one was last night with new motor

87MagnumII

I've tried a mirage it is not as responsive as the laser. Acceleration was slower as well as top end, couldn't get the rpm up with the big diameter of the mirage