Trim tabs all the way down (bow down) and drive up (not trailer up) helps on
some.
What actually helps a lot more is a lot of low speed experience and then you
will stop constantly over correcting.

"Peter Higgins" wrote in message
...
Hi,

I have a sterndrive on a 20' trailer boat and it zigzags a lot at slow
speeds which I understand is a generic problem for sterndrive boats, and
may
be to do with water flow around the back of the boat and the stern leg. I
have also heard of one manufacturer that uses some type of vortex
generator
around that area to combat the problem.

Does anyone out there know of any aftermarket add-ons that may help, or of
any information sources on the issue.

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Peter Higgins
Ph + 64 3 377 2522
Cell +64 25 20 50 341