Boat Trailers: A Naval Architect's Viewpoint

From the Glen-L Newsletter

Trailering to and from the water could be the hardest use your boat will ever get, especially if you don't select the correct trailer or use your trailer properly. Why is this so? Simple. When a boat is in the water at rest, its weight pushes down on the water through the force of gravity which, in turn, is counteracted by the equal, but opposite, upward force of buoyancy from the water. These two opposing forces are distributed more or less uniformly over the entire area of the boat's immersed underbody.

To show how minimal these forces are over the bottom of a boat at rest, a typical 24' trailerable deep vee of 4000 pounds displacement may have an immersed bottom area of about 135 square foot. Dividing the displacement (4000 lbs.) by the bottom area (135 sq. ft.) results in slightly less than 30 lbs. of boat weight supported by each square FOOT of immersed bottom area; a minuscule load in any engineer’s lexicon and a figure fairly comparable for a wide range of trailerable boats.

Of course, bottom loads can increase substantially once the boat is underway, partly because supporting bottom area is decreased as a result of planing, and also due to dynamic forces caused by motions such as slamming. However, dynamic loads in boats used in normal conditions seldom reach the intensity and concentration possible during trailering. Moreover, the types of forces or loadings a boat is subjected to on the trailer can be completely different than those experienced in use on the water.

For example, one of the most important is point loads or localized stress concentrations. A common cause is boat trailers which support the boat on rollers. Although rollers may ease the process of launching and landing the boat, and may have particular merit in this regard especially for larger boats, they are suspect from the designer's point of view. Why?

In looking over a brochure for such trailers, I began counting rollers. On one trailer I counted 104 rollers; surely a generous amount at first glance. But considering that this trailer was rated at 9500 lbs., this means that each roller must support about 95 lbs. of load, assuming EACH roller takes the load uniformly. The troublesome part is not the load per roller, but the fact that this 95 lb. load is concentrated in only a few square inches of roller area. Instead of the 30 lb. per square FOOT load experienced in the water, the boat now is subjected to a 30 lb. per square INCH load (144 times greater), and this does not consider any additional loading set up by motion, bumps, dips, etc.

Thus if you insist on using roller supports, several points should be kept in mind. First, point loads are virtually inevitable so make sure each roller is properly adjusted and taking as much force as all the others. Second, get as many rollers on your trailer as possible; you can't have too many. Third, the location of the rollers can be critical. More rollers should be located near the boat's center of gravity or location of greatest weight concentration; this is usually in the area of the engine. Stern drive and outboard boats should have plenty of rollers aft, and many of these should be right at the transom/bottom junction. The forward halves of most boats usually need much less support. 

If your boat has lift strakes, locate the rollers under these since they tend to be stronger than the flat hull surface areas between. Similarly, but perhaps more difficult to do, locate rollers under portions of the bottom directly under reinforcing members inside such as motor stringers, frames, bulkheads, etc.

In my opinion, roller supports should NOT be used for certain types of boats. These include most aluminum hulls, competition racing boats, and a type of fiberglass boat becoming more common, often called a "high-tech" hull. These boats are made from sandwich cores of foam or balsa with fiberglass or composite skins that can be quite thin in the trailerable sized boat, yet with more than adequate strength to meet service conditions on the water. However, these thin skins combined with the relatively soft core will deflect (literally bend) quite easily under point loads and could conceivably ruin the bottom of the boat.

Keep in mind that rollers can deteriorate in sun, air, and water, and they can get hard, making matters worse. If the rollers freeze up and don't roll anymore, they can scuff and mark your hull and should be replaced. If you store your boat for long periods on roller supports, consider putting wide wood shims or wedges between the roller and bottom to spread out the load.

Bunk supports covered with carpeting or padding are less problematic than rollers, but worthy of some comment. First, bunks should mate firmly to the hull at all points, but not protrude against the hull, especially at the ends. Second, let the bunks extend beyond the transom on powerboats rather than stopping them short; this will tend to keep the bottom true to form and performance up to par.

In choosing a trailer, keep in mind that anything that will contribute to a better ride will make things easier on your boat. Bigger wheels absorb potholes better while bigger tires give a softer ride with lower air pressure. Given similar capacities, tandem axles have an articulated linkage for a "rocking" motion that allows one wheel to drop into a hole at a time thereby evening out road irregularities better than a single axle.

And while on this subject, it seems time for trailer manufacturers to try new undercarriage systems. While the rigid axle/leaf spring approach is proven, simple, and inexpensive, it's not the most compliant of systems. New independent and semi-independent suspension undercarriage systems are available and seem desirable. Some have torsion spring systems, while others use air bags or metalastic-type suspensions that promise a smoother, easier ride, but may be more costly.

In any case, don't get a trailer with much more capacity than you need. The stiffer springs will give too hard a ride and transfer too many jolts to your boat's hull. Finally, tie your boat down adequately to the trailer, but only enough to keep it in place and prevent it from moving or shifting. Tiedowns that are too tight can overload the boat's structure.

If your boat is an outboard, I/O, or surface-piercing drive (Arneson, Kaama, etc.), think about supporting these appendages to the trailer, such as with removable brackets to take the load off the transom. Trailering with the outboard tilted may be necessary to clear the road, but another important reason is that this position puts the motor's center of gravity more directly over the transom to relieve some of the stress due to leverage.

While most trailers manufactured are quality products, keep in mind that the makers must attempt to fit as many sizes, types, and weights of boats and launching conditions as possible. Then too, most manufacturers offer models in varying degrees of equipment, number of crossmembers, bunks, rollers, etc. Your choice can depend on how often you will use the trailer, how far you will travel, what the road and ramp conditions will be, and the size and type of boat to be carried.

In all cases, the trailer must be stiff and rigid enough so that trailer flexing is not transferred to the boat. While a lightweight trailer is desirable for fuel economy, adequate and proper hull support, either by rollers or bunks, means that a sufficient number of cross members and other supporting members must be provided which adds weight; beware of a boat trailer that seems too light unless made from aluminum.

Finally, it's possible that you can't find the right trailer to suit your needs. There are firms who will make custom trailers or customize their stock units to suit. Or if you are so inclined, you can build your own if you have welding skills or have a welding shop that will do the work for you. I have seen many do-it-yourself trailers and the savings can be substantial with more than acceptable results if done properly. Glen-L Marine has Boat Trailer Plans for the do-it-yourselfer, and their book, "How to Build Boat Trailers", gives the necessary instructions. •SCA•