see this table for the XP you will loose puting low level sailors on a high level ship:

http://trainworld.us/NF2/sailor_LV/index_sailor_expchat.html

I had the same problem as U as I needed 2 more engineers so I started with lvl 1 on FF1 til lvl 12. Nationed them, then boosted them. They had to work on lower level ships. I put them on an FFY and went both blitzing and GB. Went Subhunting in GB. If you have crew space in dock/HQ you can make some Junk Neutrals that you can take up to high lvls with the Engs. Later you can trash the Neutrals. (Although sub hunting is fun as heck). I also had a DDX for sub hunting and used the Engs to help speed up the ship and to help SDE.

You will lvl the eng up faster on a ship that is near or above their lvl. If you are 10 lvls below the ship level you will only get 88% exp points. At 20 lvls below, you get 50% exp. The further below, the worse it gets. However you can be 24 lvls above and still get 90% experience points At 50 lvls above, you get 50% exp. At more then 27 lvls below ship lvl, you will get 1% exp.

So, it's much better to be above lvl then below lvl.

My rookie pilots as scouts on my BB got 35 points of experience in battles because they were so far below lvl. I don't care as they are permanent scouts.

The problem is solvable, but putting low lvl sailors on high lvl ships is not the way to do it.

A good point. I never considered the effect of AA on experience points. I was only considering gunships and not AA ships. I stand corrected.