Foaming diesel coming out fill cap

Diesel foams, it just does. Knowing this you must be slow and deliberate so not to get any in the water. I listen closely as I fill my tank. I often put my ear right nearby the fill port as fuel is going in. I find I can hear a change in the sound as the fuel level gets higher. I then slow the fill and stop it periodically to allow the foam to decrease. It’s similar to filling a glass with a carbonated drink. Let the foam go down and then fill some more. While you can’t see the foam you can hear gurgling in the tank and know the fuel level is getting close. Don’t try to top it off.
 
I got one of these a few years ago . https://cleanwayfuelfill.shop/collections/marine-products It works great.

As for the vent…..I take some oil absorbent pad, about a foot square, and put masking tape on three sides before sticking it over the vent. Listen to the filling and you can’t go wrong.
 
There's also the Clean-Way device that's like a fancy milk jug that catches over fill. I found it awkward to use and just decided I'd only fill to the top once or twice a year to check the Garmin fuel reading.
 
I emphatically endorse use of the absorbent pads Chimo mentioned. Most marinas have them readily available. I hold one at the vent since on my C30 the foam will bubble out there before coming out the top fill pipe - when filling slowly. If filling too fast, foam will blow out the fill pipe. I learned this very early on with this boat and have had no issues since when using the slow and stop method previously mentioned. The foam issue is common on most boats with diesel engines, it is not unique.
 
Diesel just foams when filling a tank. It’s not just boats either. Look at the area on the ground at the diesel pump at your gas station. It’s probably a mess.
I only fill up to the top with the boat on the trailer. Before filling up I read my Garmin fuel gauge to get a rough idea how much diesel is needed to fill the tank completely. When I get within 10 gallons of full I slow the pump way down and give it a few seconds break every 1/2 gallon or so. I also have pads ready to clean up the inevitable foam spill.
On the water, I never fill to full. Always fill to 90% max. Again using the Garmin fuel level readout to anticipate how much I need to get to 90% full before pumping. Still keeping absorbent pads at the ready just in case.
This method only works if you are diligent about updating the Garmin fuel level every time you add fuel. Going slow with the pump as you get close to 90% full with pads handy is the best solution I’ve found.
 
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Rule number 1: don't talk to anyone when fueling boat. Listen to the fuel gurgles.
 
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