Mmmmm... good topic.

It might pay to have your service agent do a electolysis test to see if the anodes already on engine is enough. I have been told too many anode blocks can have the reverse effect and actually create rather than prevent electrolysis or something like this, dont really understand the effects.
Just had my boat tested as Im now leaving it over weeknds on a berth and although what was on the leg & transom sheild was just within paremeters we added couple small anodes opposed to one large one.

Fiz, there is an excellent page by Nautech Marine with links to the scary phot album.

It is important to keep the inside dry between trips.

I also look for dark grey spots underneath the hull up to 5 mm diameter usually accompanied by some pitting or porous looking metal. When I pull up the floor board I find much bigger pitting across an area up to 20mm diameter or more.

Does everyone wash out the sealed underfloor chamber (as best you can) and leave bung out to drain or just leave the bung in and check occasionally for water? I wondered whether leaving a small amount of salt away in there to slosh around would be a good idea?

Garry 23041 can you post a couple of photos? Thanks.

Garry 23041 - photos of outside of boat and floor - like to see rig and compare outside and inside - also wondering if you are right and no issues - seems like a bit of care and no worries?

Wish I had realised this with my trailer - plastic boat is fine but trailer is getting sad - only really started looking after trailer once it was to late - has anyone tried fitting anodes to boat and trailer and does it help?

Trailers...
Im pretty sure the open end box section trailer we have is the original for the old sea nymph commander... and just about every other SN C has the same trailer.Other than replaced mud gaurd brackets that get the surface rust kill, seal attention about 2 times a yr  its and most the other mint.
Im sure not all have been well looked after over the decades.
So what is the difference between these and all those rusty trailers around it at the ramp?

Even going on trade me.. look up S N C  and check out the pics/ trailers....most same trailer, old and in damn good knick.

Also late sixties/ early 70s  American Chev muscle cars had a sacrificial anode mount up in the rear back passengers pillar frame between the frame and outer skin....Outside this period or muscle car I dont know if did or not.

It's important to differentiate between electrolysis and galvanic corrosion.

Galvanic corrosion happens when two metals/alloys with different reactivity are in contact in a liquid that conducts electricity well (e.g. seawater).  This causes the most reactive metal to oxidise.  Aluminium and it's alloys are quite reactive.  Screw and stainless bolt into ally and you will get galvanic corrosion of the adjacent alloy.  To avoid you use plastic screws/bolts if you can or use an isolating compound like duralac.

Electrolysis occurs when electric potential exists between dissimilar metals.  Electrolysis can occur spontaneously, generating a current or it can be forced by an electrical current.  In an alloy boat it can be forced by current from the DC system finding it's way into the hull.  Things that help in trailer boats include: -
- Fitting isolating switches to batteries,
- Fitting rubber/plastic mats under batteries,
- Checking bus bars, fuse blocks, switches cables to ensure nothing is leaking into the hull. 
- Removing batteries when the boat is not being used.
- Keeping the boat dry when not in use. 

In moored boats, you need anodes.