Causes of prop slip?
 

Decided to take one last trip out tody before the winter layup. I hadn't been back in the water since I changed out the valves and the carb so today's short journey would also be a test of sorts to see how she was running after being torn down to the short block.

The 383 ran well and with my cheapo-SS 21" prop I was able to turn 5200 RPM at WOT (I believe that max timing was set at 30*... TADA... no audible detonation; could probably use some more timing though). I think there are some funny things going on with the cam (as in it's probably way advanced {according to coversations with the builder}... going to fix that over the winter) so there maybe some more left in it that I haven't quite gotten to yet. The engine was very responsive though and I attribute that to the Holley carb. I still have a squirter/pump issue to work on but it still did very well.

Now for the topic... I ran my speed through BAM's prop slip calculator and got (speed was 50 MPH... lost some speed... probably needs some more timing) 38% slip. How and why is what I want to know as well as how to fix it. I'd really like to see slip in the 10-15% range. Is it just the old prop? If I'm able to run a Mirage 3-blade 17P within that range, does that mean that I can run a Mirage 21P and expect the same slip?

BTW... I had one of those fin-thingies on the drive... gone now. The boat handles and moves so much better.

Thanks!

If you haven't already, I would get another tach wired up to check the factory one. Is the hull clean? What kind of prop and how heavy is the boat.

I once tried a Laser II at my friends urging and the thing slipped like mad compared to a mirage +. Didn't compute the slip, but didn't have to .........It was obvious. Could rev the motor at 4k at will. Didn't bother to check top end. The props don't look much different.

Bottom is/was clean (it never ever stays in the water). Prop is an older dog-ear Quicksilver 21P (closest thing to it is a Renegade I believe... I think that's what the P/N crosses to). Boat weight I would have to guestimate at 4600 lbs... tops.

The hull is not a performance design but is basically the same as a 4Winns Liberator... only mine has a different cap.

I've checked out the tach already. Though it's a little slow to react, it is accurate.

I already have a 17P large diameter Mirage, but I think the pitch is way too low. I put the 17 back on it earlier in the season because I couldn't turn the 21 for some reason. Well now I don't have that problem based on the RPM I was able to get out of it yesterday (with it still not being in proper tune). If I could get the prop to bite more I think I'd get to 60, which right now is the Holy Grail for my configuration (after all, it is a family boat. ;) ). I know the old gil has it in her, I guess I need to find the right prop to get there.

Now how do I correct it?

Get a couple of known reference props, like a mirage and a bravo, at that weight. That buddy of mine that bragged so much about the Laser was shocked at the result of a mirage. After switching over to one, he said his boat was faster at a much lower rpm and felt "hooked up". That's the definition of slip.....or lack of.

I posted the same question over on BoaterEd.com...

More than likely, the huge amount of slip is being caused by a lack of blade surface area and/or small diameter (the 21P is noticeably smaller than the 17P Mirage).

I guess I'll be testing some props next season... among other things.

i had a 211 liberator with a 350. went 52mph on gps at 4800. perfect setup for the smallblock. the prop was a 21 pitch quicksilver with diameter = ???

there's a reference for you. with 5200 and my prop, you should see

chang/bing/blonk..petoooy = 56.3 mph.

since you have more power than my 260hp did, and if you can pull a 23pitch to 5,000 then you might see....... just under 60........

just for reference. it was a 1.5 ratio drive. you do have the SAME drive don't you......as before.......

good luck.