‘91 merc 60hp hitting wot rpm??

[Evinrudej]

Hi all so last weekend I took my 16ft deep v aluminum boat with a 60hp mercury 2 stroke 1991 out on the water. With a full tank of gas and two people boat was hitting 33mph according to my gps on my phone and the rpm gauge was reading 4500rpm at wot. If I’m correct rpm range should be 5000-5500 for this model? Could the gauge be off??

 

[jimmbo]

Maybe, or you are about 4" or so, Over-Propped
What Pitch is your Prop?
Is this boat and motor, new to you, or have you had it several Yrs? If you have had it for a while, what was the performance like last year?

 

[Evinrudej]

Yes it’s newish to me. Had it about a year now and last season was the same result. Prop is a 13p prop stainless prop. Was thinking about dropping down to an 11p??

 

[jimmbo]

Before you go spending any money on Props, what condition is the Engine in? Have you done a Compression Test? If yes, what are the Numbers?
Is the Spark strong enough to jump a 3/8" Gap?
Is the Tach set correctly? Which for an Alternator driven Tach should be 6P

 

[Evinrudej]

Cylinder top: 131
Cylinder middle: 135
Bottom: 137

Spark is fine this outboard uses the ngk non resistor spark

Tachometer and outboard are original to the boat when it was bought brand new so I’m assuming it’s a 6p tachometer

 

[jimmbo]

Spark is fine this outboard uses the ngk non resistor spark

So you are saying the Spark does indeed jump a 3/8" Gap?

 

[Texasmark]

https://www.go-fast.com/Knowledge-base/What-is-Prop-Slip/Prop-slip-calculator:

Plugging in
1.64 gear ratio
13P prop
4500 RPM
Zero slip = 34 MPH
Add estimated 15% slip considering boat and load: 29 MPH

Changing pitch to 11:
27 MPH with same slip at 5000 if engine came up to that RPM

30 MPH with same slip if the engine were able to come up to 5500 RPM

I'd say that you would be doing your engine a favor by going to an 11" pitch and you wouldn't be loosing all that much speed. Question up for grabs here is that 5000 is probably achievable but really doubt 5500. If the boat had a flat transom, rather than a deep V which usually has "dead rise" at the transom, you have too much boat in the water (drag) to do much performing.
Your tach seems to be close enough to believe. Too many variables in boat shape, weight, this and that to pin it exactly with calculations.....only way you really know about any performance of any boat is to RUN IT!

 

[airshot]

Before spending money, is the motor performing at its peak ? Make sure carbs are wide open. Buy a cheap portable tech and see if your tech reads correctly. If you spend money on props and later find the motor was not operating correctly, you will need to start over. What boat, how much weight, my 16' deep V weight s about 1000# and has a 40 hp Merc two stroke....she does just 31 mph by GPS with just me, at 5600 rpm Just a comparison for you.