Brian would you please look over these numbers for the fuel tank and the water tank. I want to make sure the balance is correct. the numbers are nominal by a inch or two.
Thanks
OK ... As mentioned in the thread (
link) above, I usually tell people to center fuel/water and other "consumable weight" around 60% of the way back from the bow to top of transom. 1) if you do this, then the boat stays reasonably balanced around the CG as the consumables (water, fuel) get used. 2) The 60% rule is a bit on the forward side ... it hedges the bet a bit because most boats 'gain weight in the stern' over time and/or are used with lots of weight in the stern (fish, ice, people, gear loaded in the boat). The CG in your drawing is close enough, so we'll use it.
What you want to do is calculate the moment-arm (lb-ft) of the various weights around the CG ... a moment-arm is effectively the torque (around the CG) applied by a weight in the boat. I'll let you do the hard math and figure out where the center of the fuel and water tanks are, but here goes ...
Fuel tank: Center of fuel tank is 2.42 feet behind the CG. At 6.1 pounds per gallon for gasoline, the full 120-gallon tank weighs 732 lbs.
Fuel tank moment-arm = 732 lbs x 2.42 ft = 1769 lb-ft (in the direction of bow-up trim)
Water tank: Center of water tank is 3 feet forward of the CG. At 8.3 pounds per gallon for water, the full 30-gallon tank weighs 249 lbs.
Water tank moment-arm = 249 lbs x 3 = 747 lb-ft (in the direction of bow-down trim)
In reality, you'd add the weight of the tanks themselves too but this is close enough. It looks like you need to add weight forward or slide the tanks forward, or add a second fuel tank in front of the original ... consider saddle tanks inside the aft end of the pilot house or low-box tanks beneath accommodations on either side ... or on the flip side, put less weight on the transom than required (no bracket or light/short bracket, direct-inject 2-stroke outboard etc). Or ... some combination of the above.
