To make them easy to launch, retrieve and transport by trailer, boats cannot have long fixed keels. Therefore, most trailer sailers have swing keel centreboards or daggerboards. Generally a trailer sailer should be towable behind a vehicle without special licences for oversized loads. This means that the beam is usually limited to approximately depending on jurisdiction. The practical limit for the length of such boats is , although some trailer sailers such as the Robb Legg 28 or the Gougeon 32 are longer. Weight limitations are determined by the towing vehicle. Smaller trailer sailers can weigh or less and can easily be towed behind average cars, while larger, heavily ballasted trailer sailers can weigh over require heavy vehicles with specialised towing equipment. Larger trailer sailers over 26 ft are difficult to tow and are sometimes kept in the water and only placed on their trailers for occasional towing or storage. Trailer sailer masts must be lowered for towing. This becomes difficult on larger boats with heavier, longer masts. Such boats require complex mast raising systems. Some sailboat designs for trailering incorporate special features. The MacGregor 26S, for example is a boat in length, but that weighs empty and dry and carries of flooding water ballast in tanks which are filled when the boat is launched and drained when the boat is removed from the water, thus making the boat lighter to transport than if it used more traditional iron or lead ballast. The design also has a pivoting centreboard, plus a pivoting rudder and easily rigged mast to make getting the boat from the trailer to the water easy to accomplish. It also has a beam of , under the width limit for highway trailers. On the design limitations of trailerable boats with flooding water ballast, Cruising World writer Bill Lee said in 1996: Rigging Most trailer sailers use a Bermuda rig with one stayed mast, a mainsail and a single foresail. Mast raising Masts can be raised by attaching them to the deck and walking them up. This is practical on smaller boats with lightweight masts, but as larger boats have longer and heavier masts. On such boats, raising the mast by brute force requires significant strength and is beyond the abilities of smaller, frail and short-handed crews. Therefore, many designs employ mast raising systems to make the task easier. Methods vary but can be classed into three groups.