There are a number of points to consider when buying a trailer sailer.
The size of your car will have a big effect on your choice of boat. If you own a mini then your choice of boat is going to be somewhat limited.
Each car has a stated maximum safe weight, as well as the maximum weight of braked trailer it can move away from rest up a hill with a 12% gradient. The Caravan Club of Great Britain recommends suggests that the trailer's weight should not exceed 85% of the tow vehicle.
Another point to consider is whether your vehicle is two or four wheel drive. Remember launch ramps are slippery and so a four wheel drive vehicle is desirable, especially one with a low ratio gear box, especially if a larger boat is to be launched and recovered. Two wheel drive vehicles can tow trailers on normal roads but may struggle on steep slippery slipways.
Good brakes are also necessary. Many people carry chocks, as back-ups to the hand-brakes of their cars.
If you are new to trailer sailing then buying a popular second-hand boat may be better than buying some newly released modern boat. That way after a couple of years when you have decided whether trailer sailing is for you or not and you have more experience the boat would be easier to sell . You will be able to find more support for more popular boats too in case you have problems. It should also be remembered that popular boats tend to keep their value better too.
Obviously the size of boat will be limited by the towing vehicle but there are other determining factors too.
Smaller boats are easy to tow, launch and recover, and so make frequent use easier and more desirable. They are, however dependant on good weather to make sailing enjoyable. Also a smaller sailing boat has less speed so you will need longer to get places.
Larger boats are able to cope with a larger variety of weather conditions, have more speed and have more space available for the crew. On the downside the are more difficult to tow, launch and recover and need a larger vehicle to tow them.
Another size consideration is the accommodation. If you only require your boats for day sailing and perhaps the odd overnight stop then no real accommodation is required except for a small cabin to make a cuppa and shelter from the elements. Boats in the sub 20ft range fit into this category. One disadvantage of boats below 20ft is that cabin headroom can become a problem for taller people.
Boats in the 20ft to 22ft range, (excluding those with water ballast), are probably the ideal size within the range of options that can be towed behind a large car or 4 wheel drive vehicle. They often offer more headroom and more comfortable sleeping accommodation.
Above 22ft, the beam (width) of the boat starts to become significant due to limits on the width of boat that can be moved on the public highway. Longer boats are available, but the limitation on beam means the hull shape has to be a compromise, and some modern solutions to the problem can look awkward and 'boxy'. Boats above 22ft can generally be more accurately described as 'trailerable' rather than being a true trailer sailer. This means they may be difficult and inconvenient to launch. In fact many may need specialist equipment to get them into the water.
There are however exceptions to this rule, particularly those boats that make use of water ballast. An example of such a boat is the MacGregor 26 which is just under 26' long. This boat is very light due to its use of water ballast (the boat takes on around 300lbs of water which acts as ballast). When the boats is recovered from the water, the ballast is released. This means that the boat is much lighter than an equivalent boat which uses conventional ballast making it easier to tow and launch.
In addition to size careful consideration must also be given to weight. When choosing a boat to suit the size of towing vehicle dont reply purely on the manufacturers stated weight. Dont forget all the other items that will be required, trailer, fuel, gas bottles, etc. These can soon as 20%+ onto the total overall weight of the trailer/boat. Obviously its better to place heavy items in the car whenever possible.
There are three main keel types, fin, twin bilge and lifting. A lifting keel is one that pivots or lifts upwards similar to the centre board on a sailing dinghy. We will look at these three types in more detail.
A fin keeled boat has a single large fixed fin. Fin keels are generally not suitable for trailer sailing. The boat has to stand very high on the trailer, where the unstable load is exposed to wind and the buffeting of passing vehicles. Launching requires the trailer to be submerged deeply, making access to the boat after de-coupling from the trailer very difficult. Alternatively a long rope can be used ! Submersion in water inevitably means that sea water can get into the trailer's wheel bearings causing rust. Likewise recovery is similarly difficult.
Fin keeled boats that go aground on a falling tide face real problems that can be made worse due to the light-weight construction that is needed for trailing. The forces acting on a fin keeled hull that has gone aground on a falling tide are much greater than on a similar sized hull with a lifting keel because the fin keeled boat sits level on the ground. In such situations a fin keeled boats will heel over to an alarming degree, and risk flooding by the incoming tide.
Few if any fin keeled boats make suitable trailer sailers.
A bilge keel boat has two fins. These boats sit lower on a trailer than a fin keel yacht, but the main mass is still quite high above the ground. Like a fin keeled boats launching means submerging the trailer in water or the use of a long rope between car and trailer in order to get the boat to float off the trailer.
Alternatively a 'launching trolley' can be used for the launching and recovering a bilge keel yacht. This is carried on top of the trailer and easily removed for boat launching and recovery. The trolley is designed to get submerged meaning the road trailer never needs to enter the water. Of course a launching trolley adds to the overall weight of the trailer.
If a bilge keeled boat runs aground, it simply has to wait for the tide to rise far enough to lift it off again.
A lifting keel is one which either swings or lifts up inside the boat. These types of boats are generally easier to trail as the boat (and hence the weight) sits lower on the trailer. They allow the boat to be launched into shallower water. If a break-back trailer is used then it may be possible to launch without even getting the trailer wheels wet.
A pivoted lifting keel has an added advantage in that if the boat hits an obstruction if, say the water is too shallow, then seldom is any damage done. Vertical lifting keels can easily get damaged when hitting something under water. Vertical lifting keels however have many of the advantages of fin keels but without the associated sailing and trailing problems. In fact boats with bulbed vertical lifting keels can make very effective trailer sailers.
The best boats usually have long production runs and when production stops prices often rise due to demand. To the prospective buyer these types of boats may seem expensive when compared to similar boats. In almost every case this is because the boat is good, if it wasnt then there would be less demand and hence lower prices.
Forget about reviews in magazines, look for advice from current owners and their experiences and thoughts. Journalists often like to keep on the right side of boat manufacturers (after all magazines make alot of money from advertising) and rarely write negative things about boats. Look in the yachting and sailing magazines and see for yourself !