Bernhard Saneke asks:
<<what is the least size for the battery. Is a 32 Ah battery
sufficient? what about 24 Ah? I have an IO 320 engine, full IFR panel and
electrical gyros. What is the highest current in ampere the battery has to
supply to start the engine?>>

It depends on the battery, starter, wiring, engine size and temperature. The current needed to crank an engine can vary by a factor of 4 or better depending on temperature and the type of starter but you should plan on 200 amps. The batteries ability to produce high peak currents is also dependent on temperature which effects internal resistance if the battery. The theoretical maximum output current occurs as a terminal voltage of zero (dead short). A 12 V x 20 AH Panasonic LC-R series battery has a nominal internal resistance of ~11 milliohms so the theoretical peak current out of this battery is over 1000 amps! It is rated for 100 amps for 5 minutes. At a terminal voltage of 9 volts this battery can produce 272 amps. For high current wiring I would recommend #2 AWG for a firewall mounted battery (larger if the battery is aft mounted).

I would recommend you see if you can fit a Panasonic LC-X1220P (physical 181mm x 76mm x 167mm and 6.6Kg) spec sheet at http://www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_VRLA_LC-X1220P_AP.pdf. The Panasonic site lists the following distributor in Germany:
Hamburg Office
Winsbergring 15
22525 Hamburg
F.R. Germany
Telephone: 49-40-8549-0
Facsimile: 49-40-8549-2500

List price for this battery from Digikey is 41.13 USD.

Combined with a B&C geared starter and a B&C linear voltage regulator you shouldn't have any starting problems.

Remember that this series of batteries can be mounted in any orientation. On my airplane I mounted it flat against the firewall, held in place by a 4 sided, 5 BID, E-glass box and a 5 BID cover plate. For heat resistance I covered the box with Newtex Inc. Zetex A600 aluminized cloth. Less weight, works great.

Regards
Brent Regan

PS. A boat displaces the mass of the boat and it's cargo (Eureka!).  If you unload the cargo (spit) the boat will rise in the water. That rise can be estimated by dividing the mass of the cargo expressed as a volume of water (1 Lbm = 27cuin seawater) divided by the area of the boat at it's waterline. If the change is large you will need to integrate the waterline area as it changes.

Forgive me Marv, I am an engineer and, like the scorpion, it is my nature.

Brent