Jackplate question

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12-10-2016, 01:23 PM
KMcClure

Jackplate question

Question hopefully someone can add some input to. My boat is a 97 Nitro 884 savage with a merc optimax 135. 23p tempest plus prop. My boat came with an 8" jackplate and the p2p at 6 1/4. I changed it out and put a 5.5 jackplate on it and raised up my p2p to 3 1/2. I actually lost 3 mph doing this and I'm not sure if changing that small amount of set back could do that or not. I gained rpm with the new setup but lost speed. Is it possible that I lost bow lift from the motor not being back as far?
12-10-2016, 04:38 PM
OldTimer57

Setback didn't do that. I am not sure how you are measuring P2P distance, but it sounds like you are way too high if you see higher RPM but lower boat speed. I'd go back to the original setting and work your way up in 1/4" increments. At some point steering torque will climb, along with RPM, while boat speed will start to drop. You are now too high. You do have to take your time and measure speed carefully, always with/against the wind or current, i.e. not a run upriver, lower the plate, run downriver. That gives skewed GPS speeds.

The primary benefit for a jack plate comes from moving the center of gravity aft to reduce the trim level needed to get the bow up to a reasonable running attitude. Losing 2 1/2" off the original setting is not going to be huge. More likely it is the 3" higher setting p2p you mentioned.
12-10-2016, 06:50 PM
mdtritn21

http://www.bbcboards.net/showthread.php?t=62660 This will show you how to measure prop to pad. Just make sure what your measuring to is level.
12-10-2016, 07:07 PM
KMcClure

Before I was at 54.7 now I'm averaging about 51.6 with same load and relatively similar temperature. The pad area has a slight V in it.., I put a 6' level on the bottom of the v and measured from that to the center of the nose on the lower unit
01-10-2017, 02:36 PM
digthemup

Quote:

Originally Posted by KMcClure

Before I was at 54.7 now I'm averaging about 51.6 with same load and relatively similar temperature. The pad area has a slight V in it.., I put a 6' level on the bottom of the v and measured from that to the center of the nose on the lower unit

You are almost there on measuring the Pad to CENTER of PROP SHAFT, not the nose cone. BTW, by changing you set back distance, not only did you change the fulcrum geometry, but you may have moved your prop into the slip stream of the hall (dirty water) causing your prop to bite less and slip more, hence, loss of speed and increased RPMs. Oldtimer is correct to a point, if you had not made the JP swap. I would also make sure that you start with a good fresh prop and balanced preferably.