I don't know your gear ratio for that engine. I assumed a common gear ratio number for 90 hp engines on a prop slip calculator of 2.33 (engine turns 2.33 revs per 1 prop) and the numbers were impossible. So I plugged in a 2:1 that is commonly used on 100-150 bottom end engines and still didn't get to your 35, even with no slip and all props have some slip.
Something is dreadfully wrong with your numbers. Either your tach is off or your speedometer. Once you get that all figured out you can talk about changing props.
-------------------------
I just took a look at a performance pdf. for the 90 etec on a Alumacraft 185 published online by Evinrude. Engine specs were 5500 max recommended rpm, gear ratio 2:1 prop a 13 7/8 x 17. The boat is a bass boat style layout and hull is probably pretty flat without much V meaning it planes out easily for a given hp.
4500 was 34.6 mph, 5100 41.3. Picture was taken with a single person (driver) and probably not much gear. My last boat was that style and driven by a 90 Merc....pic in Avatar. Alone I could beat that number big time with a SS, high rake, cupped, Ballistic 24P with a 2.33 gear box running 5800 rpm. So I know 41 is a good number for that engine on the right boat.
If you have a glass boat (weight) and maybe a dirty hull, or pretty good dead rise at the transom, you will not get these numbers with one person and not much gear aboard. However, his numbers were with a 17P and you are running a 15 and only getting up to 4800.....pitiful.......but you said with a second person and fishing gear, both of which he didn't or probably didn't have.
Back to the prop slip calculator: Using the 2:1 gearing, your 4800 rpm, 15P prop says your max speed with no slip is 34. Slip is hard to get better than 10% so using that your actual would be 31 mph.
I ran etech's numbers through the calculator: 2:1, 17P, 4500 rpm = 36 with 10% slip is 32 mph. Plugged his numbers back in looking for slip and it said 4% which I don't believe. The boat pictured is obviously a fast hull butttt. Course test data was provided by etech on one of their engines...........................come to your own conclusions on that! One thing that could have changed the actuals would be the prop. If he is running a cupped, high rake, SS thin bladed prop, then yes, he could have improved his numbers as the cupping usually increases the effective pitch by another inch at WOT, the rake gives you bow lift which reduces hull drag allowing for increased rpms and speed and the thin blades possibly a little more rpms and speed....advertised as such and makes sense.
So let's rerun his data using 18" for pitch assuming what I said: 2:1, 18, 4500 rpm = 38 max with 34.6 actual = 9% slip. I'll buy that.
First thing you need to do is see which of your information sources is telling you a lie. I have established the gear ratio is 2 turns of the prop for 1 of the crankshaft.....2:1. You have to verify the other two.
Hope this helps you some. I'm retired and this is my coffee shop jabber in the mornings and maybe I can do somebody some good.
Mark
