How to prevent diesel theft by syphoning?

[Hippocampus]

When possible leave your tank near empty. After few times of that they probably will move on to another more productive boat. Alternatively wire an alarm. One contact at the center of the lid. The other on its inside edge. When the circuit is broken the alarm goes off. When you’re on the boat hit the breaker to turn the system off. You can even add a camera and a light appropriately placed. If they know a picture is being taken when they unscrew the lid doubt they would come back.

Don’t know how you unscrew it for acces. Other thought is to fill the holes or slit used to unscrew it. Then set it up to be unscrewed with something unusual such as a star, square, hex etc. or make your own design as many people carry those designs in their tool kits (I do). Still it’s less likely a thief will come to the boat with those in hand. Would be reluctant to modify the fill hose or vent line. But by modifying just the cap you could go back to a standard one once the threat is gone. Personally prefer that to other possibilities I and others have mentioned. KISS.
Have had sailboats where you used a winch handle to unscrew. That might be simplest. Even though you’re power it’s should be easy buying a cheap winch handle and a new lid which might be just the thing.
View this the same as locking dinghies and their engines when using a public dock. I just need to be more difficult than my neighbors not perfect. Thieves will usually hit the easiest mark.

 

[sjnssn]

If there is room for a valve then that would be the way to go. I don’t like the idea of stuffing something down the filler hose. If it impedes fuel flow into the tank it could end up being a PITA getting it out. You could turn a half hour fill up into hours due to slow flow rate.

Installing a valve seems the best solution so far

 

[luna]

Similarly, a large check valve should let fluid in and not a hose.. if they could snake the hose in it would likely become yours!

I don’t know of a check valve the would do this job.

Any spring loaded check would have difficulty opening under whatever pressure you could apply (while filling your tank) prior to the fill nozzle shutting off.
A swing check would simply be pushed out of the way by the siphon hose end as they are generally a straight through flow path.

Maybe you have a design in mind that would work.

 

[rgano]

At this point the thief and their friends also know which one is the fuel fill.

You may be inviting someone to put something in your fuel tank in retaliation.

Much smarter to keep them out.

That is true, AND the lettering for Waste or Diesel is stamped on the outer ring NOT the caps themselves on many (maybe most or all) fills to avoid accidental switching by an innocent person.

 

[captain69]

Install prominent CAMERAS. They may steal, but you'll have their mugshots for the cops. Then sue for treble damages plus legal fees.