The springs look like the same ones on their mini trailer, which is rated for 1100 lbs so I figure that is realistically the max if you wanted to push it. I have a number of small boats, but the one that sits on the trailer the most is my PDRacer. Took me a while, but I finally figured out a simple and sturdy way to make a bow stop for the PD, since it has a transom bow. The winch post that comes with the kit is adequate for typical jon and small power boats, but I mostly carry small sailboats. A couple of pieces of angle iron make a diagonal support to hold the post upright -- those pieces are painted black. It fits between the fenders so all I have to do is remove the aft bunk board and drop the box on.
The trailer is rated for 600 lbs, but the springs look identical to the ones used on the HF trailer which is rated for 1100 lbs. I do a lot of projects in my yard that needs bags of concrete, and often haul 800 lbs of concrete per load. I have also carried up to 1200 lbs, but find it hard to push that weight thru my yard by hand, so backed down to 800 lbs.
I have put many, many highway miles on my HF trailer and am very impressed with it's performance for such a cheap and simple bolt together kit that it is. The best way I have found to assemble Harbor Freight trailers is to build them upside down, with all the parts propped up on 2x4's. This gets all of the parts off the ground so you can put a box wrench on the bolt head (which is now between the floor and the trailer frame), and use a socket wrench on the nyloc nuts, which is now facing the sky. As for whether to put the nut facing upward or down -- I consulted an aircraft enthusiast friend of mine, he said the rule of thumb is to always put the nuts on the bottom, so that if they fall off, you still hopefully have the bolt in the hole, holding stuff together. This is the mini trailer kit, it is dirt cheap at between $170 and $200 depending on if you catch it on sale. The tongue that comes with it is ridiculously small, so I purchased a long piece of square tube steel as a replacement tongue. I made a very simple set of bunk boards that bolted on top of the frame bed, and tried using it without a winch post. The only thing I didn't like about the trailer was one day when I was backing it down the boat ramp empty, it caught on a piling to the side of the ramp and the whole frame twisted sideways. Wasn't really a problem to fix, I just kicked it a few times and it was pretty much back to normal. I think the attraction is the initial cheap price, and if you can weld & have lots of scrap steel laying around, you can quickly fabricate a very nice, usable boat trailer. Possibly another big feature is that the trailer kits come with a manufacturer's statement of origin (MSO) so that when it is time to register the trailer, it is a lot simpler than a 100% home made trailer.

This is a bit ambitious, looks a bit on the top heavy side to me, but it shows a more extreme version of what you can do. This is their extra heavy duty model that comes with a metal bed, bigger tires, and heavier springs to carry a larger load.
Occasionally, they have a galvanized PWC trailer - the advantage is that it is galvanized, so if you dip your trailer in salt water often, the trailer frame won't take as much abuse. When it finally did, I put it together with a deck & several cross beams below the deck for support, and replaced the tongue.
Also I installed a set of fence post stakes along the perimeter and made the surrounding gate so that I could carry gravel, dirt and other messy stuff.
That was a lot of work, and on a later trailer I just made a plywood box that sits on top of the trailer - see the top of this page.
I used to be a boat dealer, and would acquire boats on trailers that did not have working lights.
I made a "light bar" from a 2x4, so I could strap it across any old boat and presto, have working lights. Some of the features which made it more handy was 2 of the "pull thru jaw" cargo straps so I could quickly tighten it down.
I also added a place to wind the wires on top of the 2x4, so when not using the wires were wound up and tidy.
I was able to replace Peerie Maa's trailer by reusing the rolling gear from an old trailer. If you are launching in salt water don't waste your money on a trailer thats not galvanized or aluminum.
Just imagine how much $$ and time is riding on that trailer - no place to cut corners if you value your time, your boat and other's lives and safety! Once you get below a certain weight, aerodynamic drag becomes more of an issue than weight.
I have seen some clever modifications jet ski trailers in the past, jet ski trailers can be had pretty cheap.
The jetski trailer is a great option even though the springs are a bit heavier than is needed for small craft.
I tow a 350lb boat on a heavy galvanized trailer, using my 5-speed stick 2003 Jetta TDI wagon which is rated to tow 1100 lbs. Don't know if you've ever towed with a horribly underpowered vehicle -- I have and it is frustrating, dangerous and expensive, the latter when you've paid for several trannys.
If you want to tow a 600lb boat on a 300lb trailer up mountains, you'll either want to get a stronger small car or buy a used diesel pickup for this towing. Don't go the Harbor Freight route -- I did and it ended up costing more than buying an American trailer used and fixing up.

I will be towing an Argie 15 (175 lbs), Lido 14 (225 lbs), and in the future -- a strip-built 15 ft Marsh Cat (600 lbs?).
Stock boat trailers usually don't fit our light, long, skinny, rockered keel pride and joys very well.
I found a custom aluminum trailer builder who built me a trailer to fit my 25' x 6' launch.
On my current 22' runabout project I found a used aluminum trailer meant for an 18' fg runabout. I live in SE Michigan where there are a zillion boats so it doesn't take long on Craigslist to find used trailers. I wouldn't worry too much about towing < 1000# especially if you don't load up the car at the same time. This is Product is an assembly put together of Stoltz products and Stainless Steel Hardware. To get WoodenBoat delivered to your door or computer, mobile device of choice, etc, click WB Subscriptions.
I tow with a small 4-cyl car (1.8 L Sentra) and must ascend steep mountain passes at high-altitude so low-weight is vital. Axle failed in one season, had to buy an entire new trailer to replace the non-standard axle.
Many come with 12 inch tires, which makes towing and even launching in certain angled ramps better.
You can use this product to replace and existing cracked and faded bow roller set-up, or to beef up your current one with the best option on the market. Use a walk board down the middle between the bunks to the back frame and you will not be dealing with walking in the water while not sinking the trailer either. This 3 inch Stoltz Roller Roller with Bell assembly would mount on the front of your trailer in the winch post or winch stand.
In many cases, having the addition of the bells significantly helps guide and direct the bow of the boat the last final inches, ensuring correct placement.
In addition, having the bells protects the boats bow front making contact with the winch post arms, which can damage the hull of the boat.

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