Circumnavigator
Senior Member
In ordinary coastal cruising, the comments above are entirely valid. However, if you intend to go offshore or out in conditions that will cause significant roll, then you must worry about free surface. You want to have only one fuel tank (and one water tank) partially full to minimize free surface -- that when a boat rolls, all the liquid in a partially full tank will go to the low side and make the roll worse.
The best solution to this, as well as other issues, is to have a day tank that holds 24 hours of fuel. The return goes to that. It also has the advantage that you have fuel that has been well filtered going into the day tank and then again between the day tank and the on-engine filter. This means that a clogged filter will almost always happen while filling the day tank and not unexpectedly while drawing from a main tank. Even with parallel filters, an unscheduled engine stop can be a serious problem.
If you want to add a valved return line to the second tank, either install a new port at the top or tee into a vent (as suggested above) if the vent goes far enough up so there is no chance of fuel spraying out. Don't tee into the draw -- part of the reason for the return is to help cool the injectors and putting hot fuel back into the draw won't do that.
Jim
The best solution to this, as well as other issues, is to have a day tank that holds 24 hours of fuel. The return goes to that. It also has the advantage that you have fuel that has been well filtered going into the day tank and then again between the day tank and the on-engine filter. This means that a clogged filter will almost always happen while filling the day tank and not unexpectedly while drawing from a main tank. Even with parallel filters, an unscheduled engine stop can be a serious problem.
If you want to add a valved return line to the second tank, either install a new port at the top or tee into a vent (as suggested above) if the vent goes far enough up so there is no chance of fuel spraying out. Don't tee into the draw -- part of the reason for the return is to help cool the injectors and putting hot fuel back into the draw won't do that.
Jim