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21 Superboat Porpoising

21 Superboat Porpoising

Old 04-06-2019, 01:28 PM
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YC4
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Default 21 Superboat Porpoising

I have an ‘81 21 Superboat with an ‘07 Mercury 250 Pro XS on the back. I’ve been having an issue with porpoising anywhere from about 45 MPH up to 76ish MPH (current top speed).

I’ve tried different amounts of setback. I can post pics later. I initially had an 8” setback plate + a 6” Atlas jackplate totaling 14” of setback. I swapped the 8” plate for a 4” plate and now with the jackplate I’m at a total of 10” of setback. This seemed to improve the drive quality a bit without sacrificing top end speed.

With the motor buried on the plate, the propshaft is approximately 1” below the pad. I’m currently spinning a 25 Trophy Plus but it is too small. I have run the boat with a 28 Promax and the larger diameter seemed to help the porpoising a little bit but no completely.

I have tried moving the motor up and down at all different heights and nothing has seemed to really help.

Any advice on setup, props or anything else would be very helpful.
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Old 04-06-2019, 07:29 PM
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I’m not an expert on 21’s by any means but I bet the right prop will be the key. Hope someone with a 21 can chime in with their expertise. Perhaps Dropping down the engine height on the jack plate one notch
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Old 04-10-2019, 08:41 PM
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S&F forum may also be a good place to visit as a bunch of owners hang out there....
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