For cruising sailors who actually go to sea I would not recommend the use of hydrocaps. Hydrocaps work using a calalytic reaction which generates heat, sufficient enough to melt the plastic housing if they get wet due to rolling and pitching that causes slopping of the electrolyte into the bottom of the caps. I have seen them melt so badly that they actually closed off the air to the cell and that could be very bad. Hydrocaps were intended for stationary battery bank use only.
Rolls batteries are, in general, not the best solution for modern cruising sailors who use heavy loads such as power inverters, and windlasses and desire to have minimum space and weight for the batteries installed. The main reason is that they have just about the highest internal resistance that I have measured for a battery, pound for pound. As a result a user suffers a lower terminal voltage under heavy loads and a wider fluctuation of operating voltages in general. I'll take a Trojan any time and for those who design their systems with good regulators and charge sources the AGM
and GEL batteries are even better yet. Why spend extra money
and have to dedicate more space and weight unnecessarily?
I do advocate using a multiplicity of charging
sources on one battery bank. One difficulty is that if one desires to have what is called "source current
or power sharing" it is necessary to provide some means of controlling each source with a master (one method). There are new power contolling techniques in the technology which allows independent power contribution yet I have not seen those methods used in the solar
charger, alternator regulator markets.
Without such regulation controlls the various charge sources will put out varying percentages of total power available when maximum power is needed depending upon their regulator settings (in the case of needing a bulk charge, for example).
During float conditions it will not matter as long as a proper float voltage is achieved. For cruisers who use multiple alternators or multiple chargers with a generator
to achieve minimum engine run times it is impossible to set independent regulators and have power source sharing equally. At least in the case of multiple alternators it is sometimes possible to use one regulator to control more than one field winding if the belting ratios and alternators are identical.