Well...Let me give you a little education about trailers, as most people don't really have a clue.
First .. Up in the NE area where the tides run extremely High or Low, Roller trailers are preferred.
Toward the South....Bunk Trailers are preferred. I'm going to talk about Buck trailers, because that is all I have a good knowledge of.
You have a 21ft boat..... Your going to want a trailer designed for a 25ft boat.
Why? Because you want a FULL length trailer.
This is a full length trailer. Note that the actual frame of the trailer comes to the transom.
That is a 23ft hull.
Now lets look at that boat on a trailer designed for a 21 -23ft hull.
Note how the bunks are way cantilevered past the end of the frame. This doesn't give you as solid of a trailer and limits your adjust ability to move the boat around to the Sweet Spot.
I also have the manufacture add 18in over the normal length of the tounge.
Why? This is what keeps the tow vehicle out of the water at different tides.
Axles - Torsion bar w/ Kodiak disc brakes. The spindles will be Posi-Lube. You do NOT want Buddy Bearings. There is a difference.
Frame material. - Alum is the leading material.
I have seen boxed galvanized frames. These are the strongest due to the frame rails being a box...... BUT there is a No- No about that.
How are you going to flush the inside of that frame rail? Answer: Your not and so they rust from the inside out.
Lights - My preference is....and have installed many of these. Optronix brand LED's. I've never had a failure for anyone that I have installed them for or my own.
But let me tell you....New boat trailer wiring SUCK's! and I usually rewire them BEFORE a boat goes on the trailer. There is one manufacture that I know of , that for additional money they will do it like I do it. I'll mention them later.
The 2 inherent problems with wiring are.....A] Using the trailer frame as a ground return path B] Using a electrical component called a Scotchlock.
These allow water intrusion and thus corrosion into the wiring. Something YOU Do NOT want. Crimp/Shrink with a coat of liquid electric tape waterproofs the wiring. Just like you would do for bilge pump wiring.
Bunks - Specifically Forward Keel Bunks.
This is what you want. It helps guide the boat straight up to the center of the winch stand.
This is what you do NOT want. It's a pain and I have seen many a Stem / keel get screwed up on this setup.
And then after that......It will take some practice and experimenting as to how deep and on what ramp that the trailer should be submerged to get the easiest load and unload.
The tendinsy is to have the trailer to deep. Then the ass end floats sideways and doesn't load properly.
Hi - Tech Marine is who I recommend. They are out of Panama City, Fl. Tell Brad the owner Bobby from Pensacola sent you. They build a nice piece and will work with you anyway they can. But it really makes a difference if you have a clue, Most people don't and a trailer is a after thought.
A Good trailer and set up right makes for a MUCH more enjoyable day. One that is not, makes it a PITA all the time.
http://www.aluminumboattrailer.com/contact.html