I can think of a couple possibilities. If the mast should be resonant at some frequency it could affect the directional qualities of the antenna. If the antenna was too close to the mast it could affect the resonant frequency of the antenna , but with the antenna tuner this wouldn't make any real difference. As for mounting a short whip on top of the mast and feeding it with a co-axial able, you will probably have problems getting the antenna tuner to work properly. Then again it make a difference between what kind of an antenna tuner you use. Some are designed to feed the base of the antenna directly with a piece of wire. Others are basically designed to feed the antenna through a length of co-axial cable. The latter type requires the antenna to be more or less resonant at the operating frequency for proper operation. Then you might need a tunable antenna to make it work properly. I would use the first type of antenna tuner mentioned above, feeding the 23 foot antenna mounted on the side of the pilot house with as short of a ground connection as you can manage. You could mount the antenna tuner down inside the hull closest to whatever ground connection you can come up with. These automatic antenna tuners feeding the antenna directly generally work best with the short ground and will handle a few extra feet of wire connecting to the antenna mast rather than the other way around. Try whatever configuration you come up with and having the antenna mast just temporarily clamped in place to see how it works before drilling a bunch of mounting holes. When you speak of an "H" shaped mast, I am not quite sure what you mean. Is it two separate mast rising from either side of the pilot house? Is it a sailboat mast? Could the antenna get in the way of sail handling? Is the mast on a power boat? >From your description, I might also suggest using a simple wire antenna running up from the pilot house roof to an insulator at the top of the mast. This would be less expensive than the 23 foot antenna mast which serves mostly just as a structure to get a piece of wire up in the air. There is nothing sacred about the 23 foot mast beyond it being solid enough to stay up under all sorts of conditions and having a conductor inside of it. The requirement for a length of 23 feet usually has to do with the frequency range over which the antenna tuner will be able to work to make the antenna resonant and the system to operate with a low SWR. Some older marine radio mast may even have loading coils to effectively lengthen the antenna electrically. These also limit the highest frequency they can be used at. They were common with the 2-4 MHz marine bands and you see the same general type of antenna mast being used for those 530 KHZ and 1610 KHz traveler information radio stations you see installed along the nations highways. Of course these stations always operate on the same frequency all the time and once the antenna is adjusted to resonance, it just stays that way. Keep in mind that none of these suggestions are fool proof and it all depends on the many possible variables associated with the antenna. Anything might actually work, and at the same time some configurations may never work, and you won't know until you try it. It helps to know a little something about the subject to keep you out of trouble by steering you away from some obvious pitfalls.. Russ Hughes Slo-N-Easy Richland, Wa.