another what size prop question

kevincave

Petty Officer 3rd Class
Joined
Apr 25, 2009
Messages
87
i tried the search and reading other posts. looking for some help on selection. my specif data, 14 1/2 "x 19" on an 89 seaswirl, 5.0 motor omc cobra outdrive. holeshot is kind of slow. at 30mph it runs 4000rpm, 40 mph is just over 5000 rpm. would like to slow down the rpms a little. not concerned about the hole shot too much. would like to run 40 mph when i need to..thanks in advance..
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
The numbers don't quite work.40 @ 5000 rpm indicates about 26% slip.very high. either the speedo is off or the tach possibly both.
​ Very common for both to be off the older they get. Unless the Sea Swirl is a real tub I would think a 5.0 would be around 50 wot.
 

kevincave

Petty Officer 3rd Class
Joined
Apr 25, 2009
Messages
87
The speed was taken from the GPS. It actually matches the speedometer. I'm thinking the tach maybe off. 5500 is screaming for an older V8. And that's what it was reading at a little over 40
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
My calculations using 1.66 gear ratio and guessing 12% slip with a 19" pitch we get about 4200 rpm at 40 mph
My info says 4200-4600 So your barely within range. Going to a 17 or 18" pitch will gain (about) 200 or 400 rpm
Speed will be a wash but with better hole shot and responce to heavy loads.
I'm thinking the motor is a little tired.
 

kevincave

Petty Officer 3rd Class
Joined
Apr 25, 2009
Messages
87
I am trying to lower the Rpms at cruising speed. It will turn plenty or Rpms. What makes you think the motor is a little tired?
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
We are assuming your tach is not accurate.We now know your speed is accurate.(gps)
Your actual rpm is about 4200.
It is highly unlikely your rpm is at 5000 and only 40 mph
At 5000 your slip (prop slip) would be about 26%.Like pushing a really big pontoon boat.
Slip is the actual distance the prop moves forward in one revolution.
Slip varies based on speed, hull design/weight,prop design, and condition,rpm,
Typical slip on your cuddy is probably in the low double digits.
The calculator uses your gear ratio 1.66,prop pitch 19,speed(40) and rpm. to figure slip.
You can also use any 4 of these 5 numbers to figure the 5th.
We know the ratio 1.66 is accurate from a chart, we know the prop is accurate you see the number,, we know the speed (gps)
We don't know the "true" rpm except by the calculations. We start with your 5000 and we calculate for slip.
When slip is wacky, 26%, we go to the only remaining number that is unknown" rpm".
Do a search for" prop calculator". Put in your numbers and calculate for slip.
I feel your motor may be a little tired because typical speeds with a 5.0 are usually close to 50 mph.
Your cuddy may be a little less. If your 5000 was accurate speed would calculate at about 47 mph.
A typical healthy 3.0 I/O in a 19 ft cuddy would run close to 40.
Lost in the jumble I created is the need to varify your tach. Perhaps borrow a shop tach or use an induction tach like a tiny tach.
just follow the directions carefully.
 
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