What are the advantages and disadvantages of using a 6 or 7.2 volt system vs a 12 volt system in what are basically scale models? Cheers.

I would be a good Idea to show the boat you wish to setup.

Generally speaking, 6-volt systems are most often seen in smaller models (36'' or less), since smaller motors will draw fewer amps at that voltage than large motors will. Large 6-volt systems will draw significantly more amps than a comparable 12-volt system, which is not only inefficient, but requires larger wire sizes to prevent overheating.
Additionally, many optional features that you may choose to add, like smoke generators, water pumps, and incandescent lighting operate much better with 12-volts instead of 6. In the same vein, you may recall that automobiles were originally equipped with 6-volt systems as well; but as electrical loads and accessories became greater, a shift to 12-volts was seen.
So the bottom line: More numerous and larger loads = more amps from a given voltage source. If you're planning a big, decked-out model, 12 volts is the only way to go.

6 volts was very convenient way back - you got the same voltage whether running off 3 lead acid cells, 4 dry cells or 5 NiCads, and most model radio gear - radios, servos and some extras - was designed around the idea of using 6 volts. 7.2 volt 6 cell packs gave the go-faster crowd a bit more go. Some modern extras use PIC chips that are designed to run on a very regulated 5 volts, and with BECs running off higher voltages, a 12 volt supply makes a lot of sense.
Otherwise, what mariner02 said.

I think there would be a few questions to be asked before locking into what voltage to use in the model. First and probably the most inportant is how big is the model and were are the batteries to be located in the hull. Second would be what type of battery will you be using, NI-CA, NI-MH, or LI-PO battery. I my self have not yet graduated to using LI-PO batteries and their associated chargers and balancers, but still using NI-MH batteries. Third would be, in addition to main power use in the model, what other drains will be placed on the batteries, such as lights, sound effects, other accessories. Are you also going to be running the receiver off the main power batteries thru the receiver BEC circuit or a separate, stand alone BEC unit, or a separate receiver battery pack.

Some models will thru the power plant used to power them, will automatically determine the battery voltage used in the model. Anyone who uses the EP 1 outboard motor knows from the start that the battery will be either a 7.2 or 8.4 volt battery, usually with a speed control with a BEC circuit to power up the receiver.

For scale models of size, usually 30” or more I like to go up with the voltage to either 12 or 14.4 volts, so I can use two 7.2 volts packs of batteries in series. With this arrangement I can reduce the wire gage to the components, have a longer run time due to the increased efficiency of the motors wound for higher voltages. It also opens the door for running different voltages in the model for different applications thru the use of voltage regulators for the different voltages.

As mentioned before on the bigger models with the interior room, I opt to run a separate receiver battery pack to completely isolate the receiver from the other power systems in the model.

I guess I was showing my ignorance by thinking that a 12 volt system might give me more speed than a 7.2 volt system. My issue is that I built these two boats for my grandsons and both have indicated that they wish the boats would go faster. Both are using a MACK #3600 motor, Dumas three-bladed 1 1/2", 2" pitch prop, and 7.2 volt NI-MH battery. Can't, at this time, do much for their boat but I had enough mahogany left over from the two kits to build another CC Runabout and am looking for more speed. Even thinking about going brushless but would need a recommendation as to the correct size. BTW, the third boat most likely be about 8 oz heavier that the original kit. Thanks for your initial input. Cheers. WAM 168

All I can say is lucky grandsons!

Speed is determined by RPMs at the prop end, which is determined by the windings of the motor. You could install a 'hot' motor that turns faster on the same voltage without having to change your battery or controller, but the amp draw will be higher, and not in a linear fashion.

I'm curious as to why your grandsons feel their boats aren't fast enough. The Mack 3600 motor turns at 11900 RPM on 7.2 volts, which is already over the maximum effective speed for that size propeller, so either they're just being overly optimistic or their boats are experiencing lots of cavitation and power wasting at full throttle.

the third boat most likely be about 8 oz heavier that the original kit.

Find a way to lose at least 8oz. In the words of the late Colin Chapman (of Lotus cars) "Add more lightness".
More voltage into the same motor will make it spin faster at the cost of faster depletion of the battery due to the increased current drawn. To get the run time back it needs a battery that holds more energy, which either means more weight or a different technology, or, as we say, more money. To convert the increased power into more water moving faster, a change of prop might be needed. For more speed, this usually means a SMALLER diameter prop, unless the original was too small anyway or there was some other problem.

The easiest way to obtain more speed on your 7.2 volt system is to just move up to 8.4 volts with the battery pack . You will not have to change out any of the components since they are all rated to go to 8.4 volts. It will be like super charging the set up you now have in the boat.

Okay, so increased voltage may increase speed. I guess I asked the original question incorrectly. Thanks to all who have responded. I will look to try to lose some weight and will try an 8.4 volt battery. I'm not sure why the grandkids think the runabout is too slow but I believe the boats do not seem to get up on a plane. Also, I don't think they understand "scale" speed. One boat is in Ohio and one is in Colorado so I have not actually seen them run. Hopefully I will get a chance this summer when I visit. Again, thanks to all. Cheers.