On something like a DF, as long as the working bit of the aerial is on the centerline and taped to the underside of the deck there should be no problems unless there are enormous waves and troughs involved.
A 40MHz Aerial is usually something over a foot long, and, while designed as a tuned length, for our purposes, is not too fussy. If a few inches are rendered unusable because of their siting, it is likely that it will go un-noticed. On the other hand, lose half an inch off a one inch tuned aerial, that is serious, as would be cutting the level of the signal by mounting the aerial inside a radio shield. Metal, carbon fibre and wood with a moisture content spring to mind.
Some sets have 2 aerials. If these are properly mounted, there is a stronger likelyhood that one of them will get a valid signal.
Reading vendors parts lists, some receivers come with a longer lead, some have a longer lead listed as a spare part. These are usually higher end products. I would avoid modding an existing one since there is no way of sorting out what went wrong with easily available test gear.
