What i am envisioning is a very micro system. similar in scale to a windmill, using, for example, this generator
http://www.windbluepower.com/Permanent_Magnet_Alternator_Wind_Blue_Low_Wind_p/dc-540.htm
if you look at projects on that website, there is one where they used the pumping action of cars driving across a road and translated that tiny up and down movement through a gear system to get enough rpms.
If i calculated right, that particular dynamo (dc generator) will have a max output of 236 watts. 18 amps at 12 volts at 2000 rpm. it produces 1 amp at 12 volts at 130 rpm. i'm trying to figure out the math to see how much weight and buoyancy would be needed supply the thing with enough torque at max capacity. Here, maximum potential would be midtide with a 30 foot tide difference from low to high. Thats 30 feet in 6 hours, and using rule of 12/ths we get 1, 2, 3, 3, ,2 1; so the 3rd and 4th hour of that tide cycle there would be 3/12ths of 30 feet or 30/4= 7.5 feet.
so 7.5 feet/hour is .125 feet per minute
(for gear/pulley ratios, I'm using
http://www.blocklayer.com/pulley-belt.aspx)
if that is set up with a gear rack turning a 2 inch gear, one circumference of the gear is 6.28 inches, or .523 feet, so that would give .239 rpms. if there is a 24 inch gear on the same shaft turning another 2 inch gear, the next 2 inch gear will have 2.868 rpms, now if we do it again, a 24 inch gear on that shaft turning a 2 inch gear, we are up to 34.4 rpms, now we do it again, and we get 413 rpms, and one last time, but this time the small gear we turn is attached to the generator and has a diameter of four inches and will spin at 2478 rpms as our approximate maximum speed. (it will be a little higher exactly at midtide) (the generator has no upper speed limit)
so, for the system so far we have a gear rack with a very long straight piece, then a 2 inch gear mounted on the same axle as a 24 inch gear, then another, then another,then another then the generator with 4 inch gear, so we have 4 axles plus the generator to get us up to proper rpms.
now lets go in reverse and see what it takes to get the minimum functional output of 130 rpms.
we go in reverse, that 4 inch gear will need the 24 inch one before it turning at about 22 rpms, so the 2 inch gear on the same axle will need its 24 inch driver turning at 1.8 rpms, then the 2 inch gear on that axle will need the 24 inch gear powering it turning at 0.15 rpms, which will require the initial axle to spin at .0125 rpms. if we divide 2478 by .239, we get 10,368
and if we divide 130 by.0125 we get 10,400 so that confirms the math for a potential gear scenario and rpms. now this .0125 rpms translates to about .39 feet per hour.
our smallest 'hold up' tides are around 10 feet in difference, so that gives us approximately one foot per hour during the slower part of the cycle, so we would still be operational.
now i think the torque will translate at the same ratio as the rpms, so for every foot pound of torque at the generator, we will need 10,400 at the initial gear, except that at the generator our gear is double the radius, so we divide by 2 and get a multiple of 5,200.
Now we know the generator has a max output of 236 watts at 2000 rpms. Since watts measures power, we should be able to calculate the torque, so how much torque is required at 2000 rpms to produce 236 watts of power?
using this calculator,
http://planetcalc.com/1908/
236 watts at 2000 rpms requires about 1.13 Nm of torque. Now we convert Nm to foot lbs (
http://www.convertunits.com/from/newton+meters/to/foot+pounds) and we get 0.83 foot lbs. so we multiply this by 5,200 and get 4,316 foot lbs of torque needed at original driver. that seems like quite a lot. now, the radius of the 2 inch gear is 1 inch, so we have 1/12th of a foot fulcrum so now we have to multiply by 12 and get over 50,000 pounds of force required, so if my math is right, the concept is a no go, as it would be really impractical to have a gear that could withstand that force and enough weight and flotation to drive it. : ( I'm not sure but it would seem however you gear it, the initial flotation to final watts ratio is going to be the same. anyone please correct me if i'm wrong, as i'm not an engineer.