NX 750 Motor Height

[Ron Himmelreich]

I've had the below listed boat for two years now and I have a question...



Boat specs

2004 Nitro NX750,

2004 Mercury 115 2 stroke,

Laser II 22P

52mph (GPS) at 5150RPM





I was installing an All-Aboard ladder, when I noticed that the motor used to be mounted in the middle hole (3rd from the top) but is currently in the top hole. The prop is 6.5" below pad. In doing a little research it seems the factory setting is the middle hole.



I am happy with it's current performance (speed/hole shot etc) but I do want to make sure I am getting the most I can. I know it is not hard or expensive to raise the motor. What would I gain and what are possible pitfalls if I had it raised.



Thanks!

 

[Bob Bourget2]

Correct mounting height, 1.5" above transom.......or...third hole.

Benefits:

1. Less drag, increased speed

2. Less steering torque

3. Less draft {less damage!}

4. Better motor performance {response} due to exhaust outlets {prop and vents} closer to water surface.



Cons:

1. MAYBE some additional ventilation. The closer to the surface, the sharper the prop needs to be.

2. Lets' review the benefits....

 

[Chris Owenby]

Hey ron bob is right. Last year i went through the same thing. Mine was in second hole from top. Horrible hole shot.i moved it down one to middle hole. Huge difference with that one hole. Mine has the 125 elpto.

 

[Ron Himmelreich]

Thanks guys! I am having it put back into factory specs tomorrow. My local Merc dealer is going to do it while I wait.



Chris, you do not see to many 750s with 125s (I have seen 2 w/150s though!). How does she run?

 

[Bob Bourget2]

The 115 and 125 are IDENTICAL motors, the difference being....carb Venturi size. MAYBE 1 / 2 mph....

 

[Ron Himmelreich]

Bob, I know each boat is its own entity but do you think I will have to change props once the motor is set to the proper height?



Ron

 

[Bob Bourget2]

You have a hundred or so r's left before we need to think about changing wheels. Let's test it first.....

 

[Ron Himmelreich]

I had the motor lifted and reset to factory settings this morning. I had a chance to get it out on the lake but was unable to get a good run for WOT speeds and RPM due to boat traffic. Hole shot was a ton better though. With the motor in the lower position, I could trim the motor up and have zero steering torque from 4000rpm up to WOT. Now, the boat consistently pulls right but not too bad. I think I'll have to play with the tab to lessen the torque.



On a side note, my son and I boated 12 pounds worth of bass (3 fish) in 90 minutes this evening. One of the best evenings I've had in a long time!

 

[Ron Himmelreich]

Got out today in smooth water. My holeshot has improved substantially. I did not get any gain in speed. With the throttle all the way, the boat accerates nicely to 52mph (GPS)as I am trimming up. At a hair before 5200, the boat begins to surge mildly both in sound and feel. I still have some trim left to get the motor level and the boat wants to pull right. One click down on the trim makes the surge go away so I think it is prop/trim related. Is this cavitation? What can I do to fix this? Would going to a 24P Laser II, be the right answer? The steering torque remains and I was able to take some of it out by adjusting the tab.



If I trimmed it all the way out, I would get 5300rpm with the surge.



THANKS for the help!

 

[Scott Hammer TOXIC]

Sounds like that is the "sweet spot". Go more and it is over trimmed and hard to steer. There not a boat that I know of that you can just keep trimming to the max. There is a sweet spot where everything is as it should be. Bump trim down for rough water or high speed turns and return to the sweet spot for running.