Trailer Design for Displacement Smallcraft

If you go shopping for a boat trailer, the offerings from the stock manufacturers are almost exclusively designed to carry a planing hull. By that I mean a boat (or jetski) which has buttock lines which run parallel from the midsection aft. This type of hull is frequently referred to as a "monohedron" hull.

The Planing Hull/Displacement Hull Trailer Problem

Monohedron hulls, and the related type commonly know as a "warped-vee" sit well on a trailer which has longitudinal skids (or bunks), for lateral support and centreline rollers to support the keel. Because in most planing hulls the keel and buttock lines are parallel, or close to parallel, these stock trailers work well with the limited adjustment built into their roller and bunk mountings.

This is a Phil Bolger-designed Diablo built by Patrick Querengasser. You can see how the conventional trailer arrangement works very well with a monohedron hull such as this.

To get an idea of the difference in hull-form to which I refer, have a look at my Flint and Fleet designs in profile. Both boats are very similar with the exception that Flint is a displacement hull with curved buttock lines, and Fleet is a planing hull with something close to a monohedron hull. Fleet is not quite a true monohedron, as she has a slightly "warped-vee" hull-form, but this illustrates the matter quite well.

Fleet (top) and Flint (bottom) compared in profile

Now, a conventional commercially-manufactured trailer can be made to work very well with longitudinal bunks (or slides) as long as they are carefully positioned so that the curved, or "rockered" bottom of the boat runs along the bunk touching tangentially. This is very well illustrated in this photo of the Bolger Micro I built for Dr. Paul Truscott about thirteen years ago. In the case of Micro, the installation was made easier because the bottom of the keel runs aft in a line parallel with the waterline, just like the keel of a planing hull - so the centreline rollers towards the rear of the trailer are basically in a straight line, and the boat rolls off the trailer easily as she is launched.

Micro sitting comfortably on her trailer, with the longitudinal side bunks just touching the rockered bottom tangentially,
and the centreline keel rollers running back in a straightline from about the midsection. The side bunks are not carrying any load, but are simply giving side support to stop the boat from tipping sideways.

The situation becomes more complicated when you have a displacement hull with a rockered bottom, and no keel to straighten things out. What I do when designing these sorts of boats is to incorporate a skeg (which is usually beneficial hydro-dynamically anyway) and I usually draw the bottom of the skeg parallel with the waterline. This is a clear case of making a practical compromise, with the skeg performing two functions - a hydro-dynamic function, and a very practical trailering function.

Note how the skeg allows the centreline rollers to run aft in a straight line.

Here you can clearly see that although the keel and skeg design allows for a straight set of centreline rollers, the longitudinal bunks must still be very carefully arranged so that they just kiss the hull tangentially.

No Skeg, No Keel Batten, and Flat Bottom

But what happens if the designer of the boat does not incorporate an external keel or a skeg? A few years back I built a Jim Michalak-designed Mayfly 14 for a customer. I really like the Mayfly 14, but she does not have a skeg, does not have an external keel batten, and does have very marked rocker in the bottom.

Mayfly 14 - beautifully adapted to the water, but a real problem when it comes to trailer design.

Because Mayfly 14 has a flat bottom, and no keel batten to engage in the normal "cotton reel" centreline rollers on a production trailer, a bit of lateral thinking was required.

Here is the sort of external keel batten I normally design. This one is still very rough, but you get the idea - just a tapered piece of 3/4" stock glued and screwed on the flat.

To have a commercially-built, hot-dip galvanised trailer custom-made would have been far too expensive, so what I did was to select a suitably sized trailer frame from the manufacturer's standard line, and purchase it without any of the rollers and bunks fitted. I then padded the straight cross pieces with lengths of UHMW polyethylene (or at least that is what I think it is! At any rate, "slippery plastic") and placed an extra cross piece at the forward end supported by stock-standard adjustable forks.

The rear and mid cross-beams complete with slippery plastic cap-strips. The two longitudinal lengths of wood are guides which bear against the chines of the boat to keep the hull centred as it is winched onto the trailer. Before the boat was put on the trailer, I covered the inner edges of the guides with strips of carpet.

Here is the boat loaded on the trailer, showing the adjustable, elevated forward cross-piece.

Looking aft along the port side of the boat, showing how the wooden longitudinals guide the boat onto the trailer and prevent sideways movement during transport. This photo was taken before I had padded the longitudinals with carpet.

The Mayfly 14 trailer turned out to be exceptionally successful, and has made me consider a developed arrangement for boats other than those with wide, flat bottoms such as Mayfly 14.

The Bending Trailer Problem

The commercial trailer frames that we use for light-weight sailing, rowing, and motoring boats are built to a minimum standard in order that the manufacturers can compete effectively in the market. The box-section steel (RHS) used in the majority of these light trailers is quite springy, and the trailers are usually supported by a single axle. In most cases the light-weight trailers have a keel roller at the rear and front cross-pieces, and one on a cross-piece near the middle - which is normally in the same sort of longitudinal location as the trailer axle.

On this Periwinkle trailer you can see the three centreline keel rollers

The problem is that when the trailer hits a bump at road speed, the axle and springs push upwards violently, and the trailer frame works like a leaf-spring, driving the middle roller up into the keel of the boat. If the boat is tied-down at the bow and stern, the tendency is to break the back of the boat.

I have three approaches to solving this problem. Firstly, I try to tie down the boat using a single strapping arrangement (usually with a strongback incorporated) located fairly close to the longitudinal location of the axle and middle trailer roller. It doesn't have to be exact, but just in the general area. Secondly, I arrange things so that the bow and stern are free to move upwards a little under load.

Lastly, and far and away most importantly, I position the centreline rollers such that the middle one is below the level of the forward and aft rollers. This way, as the boat is winched onto the trailer, the bow rides up on the rear roller, moves forward onto the middle roller, and then finally rolls upwards onto the forward roller. So, the middle roller supports the boat as she comes forward on the trailer during retrieval, but as the boat is winched into the final location with the bow against the winch-post, the keel lifts clear of the middle roller. Ideally, the forward and rear rollers should be positioned under a frame or bulkhead so as to distribute loads throughout the hull.

The reason for the vertical location of the rollers which I have just described is to allow the trailer to bend upwards due to road shocks without the middle roller actually touching the hull, and tending to break the back of the hull.

A Better Trailer for Lightweight Boats of Traditional Design?

Currently I'm designing an experimental trailer to carry boats such as Phoenix III, First Mate, and Periwinkle. This trailer will incorporate transverse supports instead of the longitudinally positioned keel rollers of the standard commercially-built trailers. Remember how well this worked for the Mayfly 14 trailer problem?

There will be a total of three transverse supports, but the middle one will only support the boat during loading (refer to the previous few paragraphs), and when the boat is fully loaded on the the trailer, she will be supported vertically at only two places - both of which will coincide with frames or bulkheads in the boat.

The trailer design is not yet finalised, and I'll have a couple of different frame designs, depending on the frame material (they will not always be made of steel, nor even aluminium....). However, in order to illustrate the basic idea, I have included some rough sketches for your information and to aid visualisation.

Click on the image to expand it, and you will note that the boat (First Mate in this case just as an example) is supported at only two locations when fully loaded, and that these locations correspond with internal bulkheads. You can see that there is a crossbeam in the trailer frame between the two supports, but that the boat is sitting well above that beam. In practise, the two transverse supports, and the middle crossbeam, will be padded with carpet. The boat can rest on the middle crossbeam as she is winched forward, but will rise above the middle beam as the bow reaches the forward support. This will protect the boat from trailer bending due to road shocks - no more boats with broken backs!

With boats as light as First Mate and Phoenix III etc., there should be no problem lifting the bow up onto each support as the boat moves forward. In fact on one boat I currently load twice a week, I don't even use the winch - I just pull her onto the trailer using muscle-power. Anyway, supplementary rollers or guides could be added.

This is a very rough isometric sketch I did to show the above trailer frame with out any clutter. Axle location will be somewhere in the vicinity of the middle crossbeam, which will be padded with carpet in the same way as the two main transverse supports.

The Catch

There always has to be a catch, and this case is no different. Unless you are lucky enough to find a commercial trailer frame which will adapt to the dimensions of your boat (as I did with Mayfly 14), you will have to pay a lot of money to have one built as a custom project, or you will have to build it yourself. Rules will be different in different places, but where I come from there are regulations and standards that have to be met before a home-made trailer can be registered for use - so do your homework. As you can see from what I wrote at the beginning of the article, for most boats a commercial product can be used as long as it is correctly adjusted. The single most important element as far as I'm concerned is making sure that the middle support or roller is not going to damage your boat due to road shocks.