My advice to you- and this is advice I had to follow myself when I tried my first AU...
Some people will say 'don't throw out the baby with the bath water.'
I say this...throw out the bath water, then throw out the baby. Follow that by throwing out the parents, grandparents, the high school biology teacher that failed to explain genetics properly, and the clerk that issued the marriage license. Then finish up by burning down the house and salting the ashes.
Your first idea for an Alternate Universe is never really good enough to chase.
Generally the second or third isn't, either.
By the fourth, you've started looking at things logically.
I'm now working on my 6th AU... and though I've tried to post here before, I won't post any more for a long time. I'm only now beginning to take things like population available for military service and budgetary constraints into account. I'm just starting to understand that I have to look at the number of available harbors and the size of the ships they can take by draft and length. Just this week- while working on my latest AU and a few RL Japanese drawings- I realized that just because a certain ship can be built or converted doesn't mean that it's a good idea... even if it is a desperation measure that appears to fit in with sound tactical doctrine (like the Japanese escort carriers- they could have built a couple of fleet carriers for that effort and come out better).
For the New Year, start a new AU... make it your present to yourself.
Don't design a ship for it though, not even a converted trawler.
Design a nation- choose it's geographic location first, then pick a language, a predominate religion, and a general moral outlook. Pick out your main harbors, noting the maximum depth of each, and any rivers large enough to warrant inland ports. Then decide where your point of diversion from OTL is; start with something which might not have world-changing consequences! If the Confederacy had won the War Between the States then North America might have looked very different, but then again it might not... and WWI would have almost certainly occurred anyway. If the Orange Free State had defeated the British in South Africa the local map might have changed a bit and the Dutch might have had another ally... but the British Empire's history wouldn't have been irreparably altered.
Redhorse's Republic of Texas is among the best examples of an AU which is designed along these lines. Study this as though it was a university entrance exam- the man has basically written a small book on the proper way to design an AU in this thread; it should be stickied and listed as required reading for anyone that is going to try an AU.
After you have done that, then design the smallest ships in your fleet first. Work your way up from trawlers and minesweepers to gunboats and coast defense ships. If your fleet is large enough, design a few armored cruisers. If it is large enough for a battleship... well, it's probably too large for a first AU, but design a few never-were's or almost-were's anyway just for fun. Then you can design the destroyers and light cruisers and/or battlecruisers, because the displacement/armament/speed balance of these types of ships is the hardest to master.
If you want to know how to make better ships, this is the best advice I think you can be given.