Causes of prop slip?
#1
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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!
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!
#2
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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.
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.
#3
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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?
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.
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Now how do I correct it?
#4
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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.
#5
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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.
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.
#6
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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.
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.