90 HP Merc on 17' tracker speed

[bojango541]

I have a late 1980s tracker tx 17. The boat originally had a 45 hp merc classic 50 and the boat did around 30 mph if i remember correctly. In the fall of 2015 I put a 1986 75 HP Merc on the back and she topped out around 40 mph. I just got a good deal on a 2000 90 HP Merc, so I decided to swap out the motors again. The 90 HP has good compression on all 3 cylinders 110-120 psi and has spark on all three cylinders. I took it out for the first time today. It does seem to idle a little rough but will not stall. When I accelerate there seems to be a hesitation before taking off. Motor seems to run pretty smooth otherwise. It had been sitting for 2-3 years when I bought it. I was disappointed to only max out at 38 MPH. Any suggestions? I was thinking I should get upper 40s, if not 50 mph with this motor on this boat and definitely not less speed than the 75 HP

 

[ondarvr]

You will need to get it running correctly and then use a tach to verify the RPMs to achieve the best performance.

 

[roscoe]

Yep, a 75 turning 6300 rpm, will push a boat faster than a 90 that is only turning 5000 rpm.

 

[GA_Boater]

The boat is rated for 70 Hp max, isn't it? A more powerful motor than it's designed for doesn't always mean more speed. The hull can only be pushed to it's limits.

 

[yzf125]

Might not have the right prop causing low speed... But that motor is to big for that boat !

 

[Chris1956]

Speed increases are not linear with increase in HP... Firstly, doubling HP may not change speed very much. Hull shape and trim angle determine performance at different speeds. Generally, if you have a vee hull that flattens aft, and you can use power trim to trim the motor, and added HP will usually add speed, at least to a point. Your results may vary....

 

[Faztbullet]

Boat should run in the low 50's with correct set up. Propshaft about 2" under bottom of boat and start with a 21p prop.

 

[jimmbo]

In theory you should get about 3 mph more from the 90 than you got with the 75

 

[bojango541]

ordered a 23P prop. going to try in out in a couple days

 

[jimmbo]

23? Be prepared to be underwhelmed

 

[ondarvr]

ordered a 23P prop. going to try in out in a couple days

What makes you think a 23 is the correct prop?

 

[bojango541]

I used the "Prop Wizard" from turning point propellers, and that suggested a 23p. It sounds as if you guys disagree??

 

[ondarvr]

You need to know your current RPMs before you can make any guesstimate on what prop may be needed.

 

[jimmbo]

Ok I was thinking 2:1 gear ratio, my mistake, your motor has 2.3:1 ratio, the same ratio as in your 75hp. Both have 5000 - 5000 as their WOT RPM range. the 90 has 71.1 cu.in. block, loop charged, and the 75, 66.6 cu.in. direct charged. What prop and rpm were you running on the 75? And again, what prop and rpm on the 90?

 

[bojango541]

Learned a lot reading up on props over the past few weeks. I do not know the pitch of the prop that was on the 75 hp. It was whatever prop was on the motor when I bought it. There was a 17p on the merc 90, so I realize its a significant change to 23. The tach is currently not working on the boat, and I need to get that working to see if I will be getting the recommended RPMs for the this motor with the 23 pitch prop. Probably launching the boat tomorrow now that we finally got some nice weather up here in Massachusetts.

 

[jimmbo]

You are looking at more than 1000 rpm drop. usually 200rpm/inch, but since you are dropping that much, I feel the engine is really going to struggle. Which prop as in brand and model? Aluminium or Stainless?

Lake still frozen up here, but might break early due to very mild winter