'96 Sunbird Spirit 170, 88hp Evinrude, Prop slipping

wcheney

Cadet
Joined
Aug 11, 2012
Messages
6
First time I've run this boat with four adults (about 850 lbs worth, Usually run two adults and two kids). When water started getting choppy the prop started slipping before I could get on step. Motor appears to be mounted correctly. Prop is a 15.5" dia x 15" pitch Rubex 3. I'm thinking I may want to switch to a four blade prop. When running with me alone and about 10 gallons of fuel (1/2 tank) rpms max out at about 5100 and gives me a speed of about 34 mph (gps) on moderately calm water. I was thinking of going to a Rubex 4, 15 x 13 but I'm wondering if dropping 2" of pitch is too much of a drop? Any thoughts?
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Are you sure those are 15.5" diameter?Perhaps 13.5? I couldn't find props in any pitch near 15 only 14" was the max I found.
I think your tach is reading low 88hp with 2.00 gear ratio" 5100" @34 gets 6% slip thats very low and unlikely.
If we substitute a more reasonable number 12% we get 5440. Tachs are always suspect
Is the hole shot strong with a light load ?You should confirm your tach before any changes.Maybe borrow a shop tach.
Or get a induction tach.
 

wcheney

Cadet
Joined
Aug 11, 2012
Messages
6
You are absolutely right, 13.5" not 15.5" diameter. Damn auto-correct! ;)
I'll see if my strobe tach still works.
 

wcheney

Cadet
Joined
Aug 11, 2012
Messages
6
I had the opportunity to test rpm with an external tach today. Boat tach at 2000 rpm external tach at 1900 (+/- 20 rpm). Will try next time it's on water at higher rpm next week to see if I get similar results.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
A light wot run for rpm and gps speed.A 4 blade should help.The test will help figure what size,.
 
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