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Roll Tide!
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Discussion Starter · #1 ·
Louis Baumann has a good article in the August GCC about how to determine the best jackplate and trim settings for optimal efficiency.
  1. Lower the jackplate all the way and trim the motor all the way down (neg. trim)
  2. Bring the boat up on plane at WOT.
  3. Record the RPMs and speed.
  4. Trim the motor up to the first mark on the trim guage (don't raise the jackplate)
  5. Record RPMs and speed
  6. Trim up to the middle mark
  7. Record RPMs and speed
  8. Trim up to the 3rd mark
  9. Record RPMs and speed.
Stop the boat
Raise the jackplate 1"
Repeat the process above.

Keep raising the JP 1" and repeating the process all the way to the top.

Once you're done, look for the top speed and this is the most efficient jackplate and trim setting for your setup.

As the jackplate and trim go up, RPMs and speed will go up. Once you past the optimal setting, RPMs will continue to go up, but speed will start to decrease as the prop will be "slipping" in the water.

He said that if you don't think you are getting the best performance out of your boat, to do this excercise before running to the prop shop. If you still aren't satisfied, take this data to the prop shop, and they will be able to better help you.

Makes a lot of sense to me. I'm going to give it a try next time I get the chance.
 
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Roll Tide!
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15,787 Posts
Discussion Starter · #3 ·
He didn't say, but I would guess you should test it with your normal load. I know on my boat, you can add 300lbs and it only changes the draft about 3/8" at rest. I don't know what this would translate to in terms of jackplate height while on plane, but I don't suspect it would make a noticeable difference.

I figure if I can keep it to within 5% of optimal, I'm probably doing okay. This would allow for any differences caused by the weight of fuel and a fishing bud or two.
 

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I am haunted by still waters.
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1,660 Posts
One thing to be mindful of.

In some cases, when you start getting to 4" and above on your jackplate, you will most likely blow out and not get on plane unless it is a tunnel of course.

GCB
 

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Roll Tide!
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15,787 Posts
Discussion Starter · #7 ·
SeaCraft said:
...keep going up until you blow your engine, then start over...
Somebody will probably do that and then sue me. LOL! Thanks GCB for covering my arse.
 

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Getting shallower means chewing mud
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1,643 Posts
And also, on tunnel boats, raising the jack plate will not always increase speed.

My boat runs most efficient (read fastest) with the motor trimmed out, but the jack plate buried down. As I raise the jack plate the prop gets into the areated water in the tunnel and starts to slip, thus less efficient.

Tim
 

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I am haunted by still waters.
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1,660 Posts
Gottagofishin said:
Somebody will probably do that and then sue me. LOL! Thanks GCB for covering my arse.
No problem. I think SeasCraft was hinting at the same thing.

I assume he meant blow out and not blow up (boom). That would be some extreme testing if he was....LMAO. He wouldn't be lowering that motor and starting over....LOL.

GCB
 

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Give it hell Remy!
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42,848 Posts
My 21 Shoalwater ran most efficient at a little less than half jack and trimmed out... just depends on the jackplate and transom angles. There is no exact formula for this, but if you start out with the j-plate all the way down and the motor trimmed and adjust from there, you should be able to determine maximum efficiency before your prop blows doing this. As soon as you lose speed, control and the boat starts acting funky, you will know where your optimum is.
 
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