Foamy diesel coming out of vent MS 400

New to me 2007 Mainship 400. On filling the fuel tanks I have to go extremely slow (10-12 minutes for 50 gallons) otherwise diesel "foam" will come out the side vent on the hull. Even going slowly small amounts kind of "spit out" to spill out on the side of the hull.

The tanks are 2 years old. They are slightly oversized to hold 178 gallons aside rather than the standard 150 gallons each side. I looked and did not see any type of constriction, odd routing that would cause this.

Ideas? Thank you all in advance.

Ed

Welcome aboard and congrats on your new boat. First off, make sure you contain the diesel coming out of the vent, there can be serious fines for spilling it into the water. I would look at the routing of the vent hose and see if they are constricted anywhere in the run. You say the tanks are larger than normal, mabe that caused something to happen to the vents, maybe a kink or a dip in the hose. Good luck.

Sounds like the vent design is bad. The plumbing should go up several inches before turning toward the side vent. Several feet would be better. If the vent plumbing is too horizontal, rolling would be enough to spill fuel thru the vent.

It could be the fuel return is plumbed into the vent. I don't know what engines you have but they probably don't return enough fuel to agitate the tank.
My vents are 5-6' above the tank tops.

What Comodave said!
Trapping in the vent lines is way too common, the line must run only uphill, as high as possible, and then return downhill to the point of exit.
I’ve seen way too many installations where the installer included a 360 degree loop near the vent, wrong, wrong, wrong!

In addition to to a continuous downward slope to the tank, check the diameter of the line. Just as with holding tanks, bigger is better.

Ted

“A 360 degree loop near the vent”

What?

Some builders did put a 360 loop to stop water intrusion but it also stops the vent from venting. What could the problem be with that???

My 400 is similar, though not as bad. I have to be careful when filling, I hold a fuel diaper under the vent to catch the overflow. I have the standard 150 gallon tanks. Pretty cool that yours are bigger, I wish mine were.