Rigid boat, flexible trailer

[cliffordpope]

The long rebuild and restoration of my 21' gaff cutter is nearing the end, and I am beginning to think about the business of moving it from my barn to the nearest water.
It is sitting on a 4-wheel trailer, whose axles are quite close together.
The boat has a long straight keel with a heavy lump of cast iron running practically its whole length. At present the trailer is supported on blocks, and the boat is securely chocked along its length. When I move the trailer, it inevitably flexes a few inches. As the boat stays rigid, this means that the keel sometimes lifts clear of its rollers at front or back, and also from the foremost or rearmost of the metal uprights.
My question is, should I strap the boat down so hard that the trailer is forced to conform to the boat, or should I remove all but the central cradle and accept that the other trailer supports will flex and not always be in contact?
Which puts least stress on the boat as the trailer goes over humps and potholes?

 

[Peterduck]

If the trailer is strong enough for the boat it shouldn't flex. If it is actually flexing, then it might be too lightly built for the boat that it is to carry. If the ends are coming away from the ballast casting, two related questions arise for me; is the underside of the casting curved very slightly [sight along it] and are the blocks which have been supporting the trailer been holding it against the keel un-naturally? When the blocks are removed the trailer becomes straight. One of my greatest complaints about boat trailers is that they are under-designed in the number of rollers provided. It is an immutable law of mathematics that a load spread over a large number of supports or roller will bear less on each roller, making movement much easier. If there is a slight curve in the casting, it may pay to fit a tapered shoe to the aft end of the keel so that it rests more of its length on the rollers. The trailer really needs to be able to support the full wieght of the boat on its after end during the launching/retrieving procedure. The only alternative to this is to drown the trailer and float the boat off. If you choose to go this way, you can protect the wheel bearings by filling them with grease, but the brakes, wheels and springs will always be exposed to seawater, and you'll kiss them goodbye very quickly. You would normally only tie the boat down tightly enough so that it cannot move off its trailer during normal driving. Howewver, remember that you may be looking at two tons of boat moving at 50 mph, which is a considerable amount of momentum. When the safety space in front of your car disappears, you really want those trailer brakes to be top-notch. You also want the boat to stay on the trailer, not join you in the car. [Always looks very bad]
Peter

 

[cliffordpope]

The trailer was bought new many years ago, and was allegedly suitable for the boat. Certainly it is within its weight carrying capacity of 5 tons. It has been used once, when I towed the boat from Beccles to West Wales.
The flexing I notice is in the fore/aft direction, and occurs when the towing vehicle goes over a bump, but the 4 trailer wheels are still on level road. A similar effect occurs when the jockey wheel is wound to either take up, or release, the weight. It seems to me to be inevitable that the front of the trailer will flex a little - even half an inch is enough to leave the bow of the boat unsupported. A trailer that did not flex in those circumstances would have to be about ten times more strongly built than mine, or most I have seen.
Bearing in mind that boats are commonly propped up temporarily resting just on their keels, and two legs, I wondered whether that was the better loading to try to reproduce on the trailer, rather than all-over support. The cradle formed by the wheels is virtually under the boat's centre of gravity, with a small down-weight at the tow coupling.
Whatever its shortcomings, the trailer will have to suffice, because that is what the boat is now sitting on.
I'm not worried about the launching. That will be a one-off, and probably by flotation. I wasn't going to try and roll it down the trailer.
The keel casting is not quite dead-straight, but I put wedges underneath to even out the load during storage.

 

[Lakesailor]

I would leave the boat to sit where it touches. There is always movement in a wooden boat anyway and wood itself can flex.
The trailer will flex as a result of the loads it encounters during a journey and if you try to strap the boat that tightly to it you will
a) transfer the distorting forces into the hull
b) risk squashing the hull with your lashings.

I sold a dinghy to someone who lashed it down so tightly to the trailer that the shock loads cracked the ply on the bottom. It would have been safer with a bit of controlled bounce.

 

[Peterduck]

It seems as though you have done everything that you can to spread the load out, and as the trailer has already survived the trip from The Broads to West Wales you shouldn't have too many problems.
Peter.

 

[cliffordpope]

Thanks everyone.
My point really was that it is one thing to spread the load out while the boat is static, under restoration, but another to try to do so on a moving trailer under the influence of road unevenness and humps. The question seems more relevant to a four wheel trailer than a two, because the latter can always pivot over the single axle, so there is hardly any longitudinal flexing potential.
But imagine a four wheel trailer approaching the brow of a hump-back bridge, when the towing vehicle (a hefty 4WD with hard suspension) has already passed the summit. The V-shaped stem support near the front of the trailer's backbone is bound to flex away from the stem, surely? Even in the strongest trailer, even to the extent of only half an inch, or less? Is it better to let this happen, and rub on the stem, move the support out of the way entirely, or strap the stem down so that it stops the trailer movement?
I like best the advice to alloy a certain amount of joggle, which would seem to be best achieved by strapping down the centre of the boat to the cradle formed by the four trailer wheels, but allowing a certain amount of movement at the extremities. I certainly don't want a support upright pushed through the counter when the trailer goes down a steep dip.

 

[Peterduck]

With a tandem axle setup, the trailer pivots longitudinally around a point in the linkage between the front and rear springs. It is instructive to watch such a trailer [even if it behind a heavy articulated truck] negotiating a crossover such as the approach to a service station. There is [or should be] a linkage which allows the axles to operate independently. Certainly do not remove the stem support bar, as that stops the boat joining you in the car under sudden deceleration [eg. hitting the back of the car in front]. I also would not recommend a support upright under a counter. Most Australian caravans have rollers under the rear edge of the body in case the road dips away under the van and leaves it supprted by the towing vehicle and the rear edge. Pretty unlikely, but possible. It happened to me when boarding a river-ferry.
Peter.

 

[philhopson]

Hmm yes - you will want something with a little bit of weight and umph to it to be able to safely move something of that size.
My Morris Minor has recently had to be towed, and my Landrover is currently off the road, so a friends 1litre Corsa was inlisted..... things didnt go too well, even 1/2 ton of rusty Morris defeated it...
So try to makesure the vehicle you are towing with is ingood order, with good brakes, suspension and plenty of power!

FAIR WINDS

Phil