[Renoir]

Your insulator can facilitate a dipole drive. A balun is installed to hang below the insulator, the coax is installed so as to angle back towards the mizzen to clear the main roach, barely. The compromise is to not have the coax downlead perpinducular to the beginning of the dipole yet, as has been pointed out, all sailboat antenna installations are compromises. The proof is whether or not the final installation works well or not. The dipole length consists of the spring stay plus the mizzen and main masts (assuming both are aluminum).

For 35 to 50 ft boats the installation may yield dipole dimensions which happen to work well on the 20 and 40 meter bands. If the dimensions are too small for these useful bands then small air inductors are placed in series with the balun output wires to the insulated leads tuned to give the correct performance. If the dimensions are too large capacitors are placed (selected to give the correct response) in series with the balun output leads. Obvouosly it takes two people using a noise bridge and an SSB receiver to achieve the tuning in place (who know what they are doing).

When done correctly the performance can be outstanding. Brion is correct in that most fixed receive long distance (DX) antennas are horizontally polarized and, therefore, for direct radiation from a transmit antenna, a horizontally polarized signal helps to maximize receive signal to noise ratios. Such compromise dipole installations typically have side lobes almost equal to the two direct lobes giving you four quadrants of directional transmission such that the lobes essentially touch each other yielding an overall omni-directional capability.