Correct speed

Genjem

Cadet
Joined
Jul 26, 2013
Messages
22
I just purchased a 1993 celebrity 180 status. 4.3 v6 lx 21 pitch mirage ss prop 3 adults, half a tank of fuel GPS confirmed 43 mph. Is it just me or is this a little slow? By the way that was at 48 or 4900 rpm.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Re: Correct speed

Was that speed achieved at wot with stated load ? Were revs at near or max rpm factory wot parameters ? If so, seems a good speed, someone who has same combo could confirm better.

Happy Boating
 
Last edited:

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Correct speed

What hp is the motor or do you know the gear ratio(usually on the outdrive)
At 4900 rpm and a 1.62 ratio21" prop 12% slip your speed would be about 52, 1.81 ratio 47.
Assuming the gps is working with mph either your slip is 20-30% or the tach is wrong,too high.
Is the prop in good shape?Are you trimming up for best speed/rpm.You should verify the tach is accurate
You may want to check the compression, be sure the motor is in excellent tune be sure the timing is set right and advances as it should.
 

Genjem

Cadet
Joined
Jul 26, 2013
Messages
22
Re: Correct speed

I am not sure what the ratio is on the lower unit. I will have to check. We may not have been perfectly trimmed but close. What exactly is the slip and how do I reduce it? Sorry if that is a newbie question but I am a bit of a newb.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Correct speed

Props work in a liquid so there is some slip. A 10" pitch will theoretically move 10" forward in one revolution.Thus if there is 10%
slip It will actually move about 9". Slip varies depending on rpm ,weight, prop design, and hull type.
Reasonable slip numbers of typical planing hulls would be about 5% to about 12% Could be more but not likely less.
A tach that errors on the high side will give a false indication of high slip.
A tach that reads low could indicate exceptionally low slip.A prop calculator will figure performance the
5 variables that help identify prop performance. Rpm,gear ratio, prop size,slip and speed.
Any 4 accurately measured will give the 5th variable.Rpm should be easy but many times is suspect. Prop size is easy,speed is easy with a gps,usually the ratio can be found on the net.
with these we can get the slip.Slip is the trickiest to get. You can best guess some results based on logic of how a boat should perform. And you develop a feel for what numbers don't make sense.
 
Last edited by a moderator:

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Re: Correct speed

Great post, Spike! ^^^^^^
 
Top