Some ideas here would be most appreciated as usual.

Where does the boat manufacturer say you should put the sender?

If you had issues with the old one, at speed, then it's not unreasonable to
imagine the same thing happening with a new one. Placement is key but some
boats just don't have a 'perfect' place to put it and have it work reliably
at speed.

I'd ask the boat maker or other owners where they've got it situated and try
it there.