Question about two test props 90hp johnson/16.5ft fibertech boat

[roonie]

Ive tried two different props on my 90hp johnson ..the boat is a 16.5 fibertech. The prop that came with the boat (14x17) gave me 23mph @3000rpm.....33mph @4100rpm...and 44mph @ 4900rpm trimmed. So i swapped the props and put on a 13 1/2 x 17 today and got only 35mph tops and was reving @ 6000rpm. I could tell it did not have the meaty pull out of the hole shot so i swapped back to the original prop. I could tell the 13 1/2 x17 physically had shorter blades as well.

My questions are...if i went the other way and put on a higher # prop (eg. 15x17) would i get even more improvement? What should i be max. revving this engine? Its a 1986 90hp. on a 16.5 ft. fibertech family boat.

Can someone explain the what the danger, if any, there is by going with too much torque and lower rpms as opposed to low mph and too high revs Thank you

 

[jakedaawg]

Either the 14 isn't 14 or the 13.5 isn't 13.5. You dont get 1k of rpm by shaving down .5"'s

 

[Faztbullet]

What condition are the props in?? The smaller diameter prop will add some RPM but not that much and don't think a 15" will fit as it will hit cavitation plate

 

[steelespike]

Your numbers don't work.You can't go 44 at 4900 with no slip.Is the speed by gps? Only accurate way.
Usually the diameter is a secondary consideration.A prop intended for your motor will have a diameter
that simply follows the design.I have a feeling that while the 13 1/2 physically fits it isn't intended for your 90.
A lightly loaded wot rpm and gps speed will help tell if your setup is working.

 

[emdsapmgr]

Agree- something is wrong with your numbers or one of the props. Generally, the diameter of the prop is meaningless. It's the pitch to be concerned about.
Theoretically, your 17" props should propel the boat forward exactly 17" each time the prop goes around one time. (there is a slip factor which enters in here) That's the critical measure. Sounds like something is off on one of the two props. They should be pretty much equal-but obviously are not. I run a 13.5 X 17 on my 90. Great holeshot prop. 5500 @ 35-36 mph.

 

[roonie]

perhaps my speedo is incorrect and/or the smaller prop or even my tach. I will do some more digging. The smaller prop is quite noticebly different. thanks for now. I will wait for my new raymarine dragonfly parcel to come ....that way i will have an accurate speed to go on. thanks for now.