measuring motor height

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Paul Ritter

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I have a 2002 Nitro NX882DC with an Optimax 150 on it and I'm trying to figure out how much to raise the engine. Russ and others on this site said it should be 3 inches below the pad to the center of the propshaft. My question is exactly what part of the stern constitutes "the pad"? My propshaft is 4.75 inches below the keel on the bottom of the boat but there is an indented section of hull at the back of the boat. If that is the pad then my engine is mounted really low! Do you measure from the lowest part of the keel at the back of the boat before it indents or at the indented section itself? It may sound like an idiot question because I should know my own boat but could someone help me with this? Thanks, Paul
 

gone bye bye

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Do a search on the board. Someone just posted this info...



Mini
 

Staci Matheis

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Paul -



It's the distance below the "keel", not the indented section.



Like Mini said, someone just posted an excellent, detailed explanation on how to measure this in just the last week or so.....



me!
 

Michael Snow

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Guys, I appreciate the "go pull a search", but I've tried and I can't find the thread in question.
 

Michael Snow

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I'm assuming on this that the straight edge would be angled downward slightly like the keel and running that plane vs trying to get just a 4' level, putting under the back of the keel and pulling level toward the motor.
 

Michael Snow

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I'm assuming on this that the straight edge would be angled downward slightly like the keel and running that plane vs trying to get just a 4' level, putting under the back of the keel and pulling level toward the motor.
 

Staci Matheis

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Here's the post.....





re: Question about NX882 performance with 150Merc by Russ, 11/24/2002 08:32 CT



Paul,



First you will have to raise the trailer axle slightly, and be on fairly flat ground. Pulling the trailer up on a couple of 2X4's should work as it only needs to be about 2" higher. Being on an inclined drive as long as it's flat is OK too.

Using a carpenters level held on the pad, level the boat surface using the trailer jack.

Next, lower the motor, and using the carpenters level on the cavitation plate, level the motor (the reason for raising the trailer is to let the skeg clear the ground with the motor in the level position).

Now the motor is in the same plane as the pad. Measure the distance from the ground to the pad and from the ground to the center of the shaft or the cavitation plate. The cavitation plate is exactly 8 inches above the prop shaft.

For the 882 the optimum position is the prop shaft 3 inches below the pad (this puts the cavitation plate 5 inches above the pad).

For the 150, the motor should use the lower of the 5 motor mount holes in the lower of the 2 mounting holes in the fatcory set back plate.

This setup has also been confirmed by the Richard at the Nitro test facility.

After a lot of testing with a CMC hydraulic plate and various props (About $1,500), I found the factory position (with 23 Tempest) described above to be the optimum.
 

Paul Ritter

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Thanks for the info everyone. It looks like I need to raise my engine 1.75 inches. Do you think 1.75 inches of height will change performance enkough to notice?
 

Staci Matheis

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Be absolutely sure to test the water pressure in a hard turn.

Absolutely sure.
 

inkslinginmofo77

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Here's the post.....





re: Question about NX882 performance with 150Merc by Russ, 11/24/2002 08:32 CT



Paul,



First you will have to raise the trailer axle slightly, and be on fairly flat ground. Pulling the trailer up on a couple of 2X4's should work as it only needs to be about 2" higher. Being on an inclined drive as long as it's flat is OK too.

Using a carpenters level held on the pad, level the boat surface using the trailer jack.

Next, lower the motor, and using the carpenters level on the cavitation plate, level the motor (the reason for raising the trailer is to let the skeg clear the ground with the motor in the level position).

Now the motor is in the same plane as the pad. Measure the distance from the ground to the pad and from the ground to the center of the shaft or the cavitation plate. The cavitation plate is exactly 8 inches above the prop shaft.

For the 882 the optimum position is the prop shaft 3 inches below the pad (this puts the cavitation plate 5 inches above the pad).

For the 150, the motor should use the lower of the 5 motor mount holes in the lower of the 2 mounting holes in the fatcory set back plate.

This setup has also been confirmed by the Richard at the Nitro test facility.

After a lot of testing with a CMC hydraulic plate and various props (About $1,500), I found the factory position (with 23 Tempest) described above to be the optimum.
Hey sorry I’m new to this forum but I desperately need a clarification on this post. I have a 1998 tracker nitro 700LX DC 17’ bass boat with a 2005 merc optimax 115 DFI and I need to know if this applies to my boat as well? It has a similar ledge/shelf on the back transom. There’s a 12-18” recessed area where the drain plug and live well inlet/outlets are mounted to a second shelf which is the actual bottom of the boat. So there’s two bottoms essentially. Can someone clarify if this applies to mine as well? Thanks!
 

Pat Dilling

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The "Pad" refers to the actually bottom of the boat. It is the primary running surface, the part in contact with the water, when the boat is trimmed up. The step at the back of the boat is not relevant to this discussion. One reason for the step is to put the running surface or pad further forward from the engine lower unit and prop. This gives the motor more leverage to lift the front of the boat. Additional setback can be achieved with a setback or jack plate. Your pictures show that you have a manual jack plate. One advantage of a jack plate, either manual or hydraulic, is the ability to adjust the engine height. When more of the front is lifted out of the water, there is less resistance and the boat can achieve higher speeds. Raising the motor too high can put the water intake out of the water and cause loss of water pressure. It can also lead to porpoising or other handling issues. Finding the "sweet spot" can take some experimentation. It usually depends on how the heavily the boat is loaded, so it can vary from boat to boat. If you are happy with the performance of your boat, you may just want to leave it alone. It would be good to measure it just so you know what it is.
 

poss0289

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Hey Paul try this, it seemed to work for me. My gap was 5", once I played around and found the "sweet spot" as someone mentioned above I was able to get 5mph more speed and on plane faster. My gap ended up being 3.25" when it was all said and done. This was done on a Z21 with a 250 Pro XS.
 

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