Prop slip

Bigmuz7 · 05 October 2007, 22:15

Having searched a good number number of threads through the forum, I've found calculators to help me work out what mine is, but I dont understand the importance of what it means ? and how it is relevant ?, and how i can improve my set up.. Is slip bad or good, and when does it become a detriment to performance ? My 275 does 5800 rpm witha 1.85 ratio leg that achieves 62 mph tops ?

codprawn · 05 October 2007, 22:53

What pitch prop is that?

Obviously the lower the slip the better. 10% is considered pretty good.

Dirk Diggler · 05 October 2007, 22:56

Quote:

Originally Posted by codprawn

Obviously the lower the slip the better. 10% is considered pretty good.

Why's that then, I currently have 15% slip, but i bet mine is quicker than yours!

captnjack · 05 October 2007, 23:00

basically rpm x pitch = theoretical distance travelled in one revolution

distance actually travelled in one min (6.2 miles converted to inches) / theoretical distance = a percentage

85% or better is good. Slip is sorta good and bad, depending on what's going on at any given moment with the engine. You want a little when starting from a dead stop to avoid lugging the engine. While running consistently, you want as little slip as possible though.

Downhilldai · 05 October 2007, 23:09

Quote:

Originally Posted by captnjack

basically rpm x pitch = theoretical distance travelled in one revolution

distance actually travelled in one min (6.2 miles converted to inches) / theoretical distance = a percentage

You've been down the pub, haven't you Captn?

codprawn · 05 October 2007, 23:16

Quote:

Originally Posted by Dirk Diggler

Why's that then, I currently have 15% slip, but i bet mine is quicker than yours!

Doh I would never have guessed.......

Dirk Diggler · 05 October 2007, 23:17

Quote:

Originally Posted by codprawn

Doh I would never have guessed.......

Probably not as fast as your mates though!

codprawn · 05 October 2007, 23:21

Quote:

Originally Posted by Dirk Diggler

Probably not as fast as your mates though!

Ahh you know Jonathan Jones as well then........

Dirk Diggler · 05 October 2007, 23:22

Funny enough, yes, I do, rather well infact!

codprawn · 05 October 2007, 23:24

Thought you would somehow.

captnjack · 05 October 2007, 23:26

Quote:

Originally Posted by Downhilldai

You've been down the pub, haven't you Captn?

why yes I have

never been any good at maths either...

Bigmuz7 · 05 October 2007, 23:32

Steady guys its a 21..

now any chance of refocusing on the question you pirates

codprawn · 05 October 2007, 23:39

Quote:

Originally Posted by Bigmuz7

Steady guys its a 21..

now any chance of refocusing on the question you pirates

Well now we have the full story............

With a slip of only 0.005 either you need to recheck your figures or you have the most efficient boat in the world!!!

You can think of slip as like wheelspin in a way - the more wheelspin the less speed but sometimes you need a bit of wheelspin to keep the engine on the boil.

Bigmuz7 · 05 October 2007, 23:52

Quote:

Originally Posted by codprawn

Well now we have the full story............

With a slip of only 0.005 either you need to recheck your figures or you have the most efficient boat in the world!!!

You can think of slip as like wheelspin in a way - the more wheelspin the less speed but sometimes you need a bit of wheelspin to keep the engine on the boil.

Well thats surprising codders ... its a 4 blader .. and I have a narrow Osprey derived hull, perhaps the speed is a factor ? but why is slip an issue and what does it mean?

Downhilldai · 06 October 2007, 00:07

The maths would be:

5800 engine rpm / 1.85 = 3135 prop rpm
3135 prop rpm x 21 inch pitch = 65838 inches per minute forward travel speed
As there are 63360 inches in a mile (according to my grandfather!)
65838/63360 = 1.039 miles per minute
1.039 x 60 = 62.35 theoretical mph with your set up at 5800 RPM, assuming nil prop slip.

If you are getting true 62 mph, then your prop slip is 0.35/62.35 = 0.56% : very good

Just as a comparison, the other day I was on a boat running at 5300 rpm on a 1.87:1 gearbox, 21" pitch prop and getting 46kt (roughly 50mph) Theoretical speed on those figures would be 56.4 mph, so prop slip would be 11.35%

Do I need to go back to the maths class or was there perhaps a 10kt current running with you?

Dirk Diggler · 06 October 2007, 00:16

Bigmuz7 · 06 October 2007, 00:24

Quote:

Originally Posted by Downhilldai

The maths would be:

5800 engine rpm / 1.85 = 3135 prop rpm
3135 prop rpm x 21 inch pitch = 65838 inches per minute forward travel speed
As there are 63360 inches in a mile (according to my grandfather!)
65838/63360 = 1.039 miles per minute
1.039 x 60 = 62.35 theoretical mph with your set up at 5800 RPM, assuming nil prop slip.

If you are getting true 62 mph, then your prop slip is 0.35/62.35 = 0.56% : very good

Just as a comparison, the other day I was on a boat running at 5300 rpm on a 1.87:1 gearbox, 21" pitch prop and getting 46kt (roughly 50mph) Theoretical speed on those figures would be 56.4 mph, so prop slip would be 11.35%

Do I need to go back to the maths class or was there perhaps a 10kt current running with you?

Wow ...

I appear to be maxed out ... and no these are on various tides and runs N1 guys ta

Dirk Diggler · 06 October 2007, 00:26

or confused?

codprawn · 06 October 2007, 02:51

Quote:

Originally Posted by Bigmuz7

Well thats surprising codders ... its a 4 blader .. and I have a narrow Osprey derived hull, perhaps the speed is a factor ? but why is slip an issue and what does it mean?

I tried to explain it with the wheelspin analogy - wheels slipping - you are wasting power!!!

Nos4r2 · 06 October 2007, 09:29

Erk!

I just worked mine out and I'm getting about 24% slip. I think I need to invest in a decent prop.