Haven't played SH5 yet, but in SH3 I've had good luck with blowing holes in the bottom of the hull. Sunk a Nelson very quickly with three right under the keel, but only needed two. Able to repeat the sequence on all manner of warships (DD and larger). If you have good weather try using magnetic pistols and set the run depth to 1-1.5 meters below keel depth and see how that works for you. Hopefully that will still work in SH5 and you won't have to use as many eels.
