Portable or built-in? My boat is aluminum and I mounted the 6 gal. portable in front of my console, which is just forward of mid-ship. Did it for better weight distribution. Only con I'm aware of was a few feet more of fuel line required and a few more pumps of the primer bulb to get started.
O portable tank. I too want to do it for weight distribution. My real concern is if the diaphragm fuel pump will be able to support the further tank and structurally in the fiberglass boat. I do think it will be ok
Normally the outboard fuel pump will handle the extra distance w/o issue. The only con was a possible safety issue, should you hit anything. I would recommend the tank be strapped down or secured in some other way, so any impact will not rupture it.
