to much prop?

[glenn property of pam]

i have a 91 stratos 1135 max weight with 4 people on board. running with only 2 people using a 84 evinrude 70hp at 35.5 mph on gps my engine is maxing out at 5000 rpm. the prop is a 13-1/4 17p stainless 3 blade is real good condition the boat has trim and tilt and a oversized tail fin on the cavation plate. my only complaint is the engine takes a little while to wind up and im wondering if i should be on a 12-3/4 prop. the original prop on the boat was a 12-3/4 21p aluminum and that acted just about the same as this one (never got the readings before i changed it) engine max rpm is 5500. boat does plane out nicely and the trim does keep the boat stable on take off just wondering if that extra 500 rpm top range will free the little motor off a little at lower rpm

 

[Texasmark]

Re: to much prop?

Diameter makes little difference compared to prop characteristics and pitch. 2 cycle engines like the top end. I run, and have run all my engines at and above the ratings on the top end. Never lost one due to that in over 50 years of boating. At your numbers, if you are happy, leave it alone!!!!!!!!!! You are at the mid point and that is OK. One more time leave it alone, or spend who knows how much $$$$$ and time to play with it and possibly wind up no better off than you currently are. But it is your rig and your money.

HTH,
Mark

 

[glenn property of pam]

Re: to much prop?

i will take ya advise and just leave it unless something happens to this prop which i hope isnt for a very long time

 

[steelespike]

Re: to much prop?

As Mark says diameter is a secondary consideration.
But your numbers don't add up. a 17" pitch can't make 35.5 at 5000 I think I have the ratio right at 2.42.And your speed is on gps
So my thoughts are the tach is reading low. Using 10% slip you would need about 5900 to get 35.5.
My after market service manual says 70 hp motors before 86 the hp is rated at 5000. So 5500 may be a reasonable max rpm.
As far as the fin we'll deal with it if necessary after we get an accurate rpm.You may be able to borrow a shop tach.

 

[glenn property of pam]

Re: to much prop?

forgot until you mentioned this. bottom end is off a pre 78 90hp motor. the po had a ops when he decieded to shift while on ears and jammed the clutch dog out of the side of the lower. i will have to turn the prop to get a ratio and if its close i will have the tach checked

 

[trendsetter240]

Re: to much prop?

I've got a similar setup to you, my boat is a little heavier at about 1300 with 4 people. I run a 15p when the boat is fully loaded and a 17p when I am on my own.

previous poster was right, the gear ratio should be 2.41:1 (12:29). If you have an older 90hp gearcase it might be 2:1. If thats the case that is probably why you are noticing the slow acceleration. It could also be that the 84 motor is a little tired. If you have the chance try out a basic model 3 blade 15p prop. The evinrude aluminiums are a good value, so are the solas amita props.

 

[steelespike]

Re: to much prop?

It is likely you have a 2.00 ratio and this seems more normal with about 11% slip.
Does the boat respond to up trim?Is the fin out of water when at wot?

 

[glenn property of pam]

Re: to much prop?

lower is about 2.3 to 2.4 ratio when i turn it by hand so im going to get the tach checked. i have no history on the prop other than the numbers printed on the hub so i might try a diffrent prop to see if the numbers match up. i believe in physics so either the tach is off or the prop has been reworked. the blade tips look like they have been shaved as they are square compaired to other props i have.