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Hi Folks,
What a great site I just recently discovered. I've read many posts and have learned a lot.
I just purchased a 1995 Robalo 1820. With a 150 HP outboard it's listed as 2800 lbs. Add in batteries, fuel, gear etc. and I'm in the 3500 lb range. It currently sits on a single axle trailer with 15" tires likely rated 3500 lbs. The brakes are junk but otherwise the trailer is fine.
I'm pulling with a 09 Pilot rated for 4500 lbs so I'm close to max load between the weight of the boat and the trailer.
Trying to decide if I replace the trailer should I stay with a single axle which is approx. 350 lbs lighter than a similar tandem. Tandem would tow nicer and is perhaps safer but do I want the additional 350 lbs. I will tow approx. 600-800 miles per year. Don't worry, I'm not planning to tow this without brakes, no matter what I do I will have brakes on the trailer. Thanks for any advice.
The single axle should meet your need fine if you keep the bearings clean and packed and maintain a good set of tires. A second axle typically weighs 200# or less with tires (no brakes), but will give you lots of security if you make long trips. However, if you mostly trailer locally and do good maintenance, tandem is probably overkill for an 18 footer.
Single aluminum weighs about 707lbs, tandem weighs about 905lbs. I like the tandem, plus at a 3500lbs you are maxing out that single.
Tommy, I own one of your trailers and I know you are trying to give good advice but, since his vehicle is only rated for 4500, I don't see the "relevance" of "maxing out" his trailer. His boat can always lose weight (junk stored on boat) but his trailer can't.
The standard trailer axle is 3500lbs capacity, but some people seem to think that this is all that is available.
I have a 26-foot heavy aluminum boat on a single-axle trailer - the axle is rated for 5400lbs. I believe you can get them rated even higher. This boat/trailer gets towed around 3-4000 miles every summer, no problems.
If your boat weighs over 2000lbs, I would definitely go with a heavier axle. When you hit bumps, potholes, etc., you could easily have more than 2000lbs of force coming down on the axle, especially if you are carrying any gear in the boat.
I like the single axle because I park this thing in a shed w/ concrete floor and it is easy to manhandle the rig around in the shed with the single axle - can't do that with a tandem axle.
Is there no plate on the trailer giving you the actual ratings? Many 15" tire axles are 4100#, 5200# or more, mine is 2x3 rect.5200. Round or rectangular? Most axles have the rating stamped on them somewhere. Go Kodiak disc. on the brakes if you can and though the car may be rated 4500, make sure the hitch, ball mount and ball are each rated at least that high as well.
If the trailer is in good shape and worth it, you might be able to have a shop add a second axle. If your axle is 3500, you are likely well over with the trailer weight included, even after subtracting your tongue weight.
Start saving towards the bigger tow vehicle you are going to want after you eventually have to make an emergency maneuver.
Tandem vs. single - weight of total rig. 
