What size Prop??

[georgiaboy]

I have a Triton 17.5 ft. aluminum bassboat with a 90HP Johnson motor. At full throttle it runs about 45 MPH and turns about 4500 RPM's. The cables are adjusted properly. Shouldn't it turn about 5500 like all the other outboards I have owned. The prop is 14" with a 19" pitch. What size would I need if it were to turn 5500?

 

[georgiaboy]

Re: What size Prop??

Also would I gain any speed with the higher RPM's resulting from pitch change?

 

[steelespike]

Re: What size Prop??

delete

 

[steelespike]

Re: What size Prop??

I'm not sure what your gear ratio is but guessing 2.00 You can't go 45 at 4500 rpm.
Is your speed by gps?
Assuming 2.00 gear ratio and 10% slip rpm is about 5500 at 45 mph.
Do you know the gear ratio?I have a feeling either the tach is wrong or the speedometer os wrong.

 

[georgiaboy]

Re: What size Prop??

I do not know the ratio. But the rpm gauge used according to the mechanic that worked on the motor once was not the recomended gauge to use with a Johnson motor. Also I know I am old but when I first got the boat I could have sworn that it said 5500 rpm's and speed was 55. And at 4000 it was 40 and at 4500 it was 45 and so on. I have been around and worked on auto engines forever and I know it is not the same but the motor certainly does not sound like past outboard motors that have turned 5500. It does not reach that kind of high pitched smooth whine like it is on top of it's game. So I do not know maybe I am making a mountain out of a mole hill and should just except what it is.

 

[steelespike]

Re: What size Prop??

For the health of your motor it would be better to know the actual rpm and speed.
Some cell phones have a gps app. And an induction tach like the Tinytach is easy to install is reasonable,
and will work on any motor except a diesel.

 

[dan02gt]

Re: What size Prop??

I'm not sure what your gear ratio is but guessing 2.00 You can't go 45 at 4500 rpm.
Is your speed by gps?
Assuming 2.00 gear ratio and 10% slip rpm is about 5500 at 45 mph.
Do you know the gear ratio?I have a feeling either the tach is wrong or the speedometer os wrong.

+1 and 45 is about what I would expect from that setup.