I'm going to build my own 'strongback' (is that what it's called?) to hold my boat on to the trailer, as the strap and clamp I usually use are just rubbish.

Any top tips for a DIY version? Feel free to email me your photos and I will add them to the website.

Here's a photo of the professional version. Are the straps really needed? Surely some rope would do?

I have a 2" wide rachet strap wrapped across a plank of wood with wedges on both ends to fit the slope of the deck.  Works well, I wouldn't use rope really, more likely to slip and stretch.  Top tip, if you put a single twist in the strap between side deck and trailer the strap doesn't vibrate when you are trailing.

Having used just a strap and two bits of carpet for years, after 2988's recent revarnish I decided to build a strongback, exactly as Andrew suggests to avoid any possibility of damage to side decks or gunwale. The result of holding the boat down with this  in ONE trip to our lake was that one of the side decks opened up an old split, NOT where the pressure is applied,  but some distance between there are the transom. The pressure is applied over a purpose built fitting shaped exactly to the sidedeck, and at a point directly over a knee to the thwart, so you wouldn't think this possible, but I can see that even moderate tightening of the strap makes that split move, in a way that it doesn't move using the strap alone. This happens even if I add packing so that I'm sure the pressure is not being applied excessily to the carlin, and my tentative conclusion is that the problem is that the strongback is applying pressure downwards but not maintaining equal pressure inwards at the gunwale. I'm going back to the strap, possibly with thicker bits of carpet or closed cell foam. Obviously, my hull is older than yours, and not epoxied. A twist in the strap certainly works, we have always done this.

Flat plank with carpeted/padded ends to sit on deck plus chiseled out rounded corner at end for ratchet strap to flow over and sides of cut out keep strap from sliding off plank.

You describe exactly what I did,- but didn't stop the deck splitting!

Rod, I suspect the strongback enabled you to get more tension in the strap, or the tension pulling the gunwhales together somehow helped hold that particular joint closed.
There are two problems that I can see. 1) you need to apply enough force downwards to stop the boat moving on the trailer and 2) ropes and straps tend to move around finding a position where they are slacker than at the start of the journey. 
You don't need ever so much force to hold the boat down, but in general pulling the gunwhales together is not good, so the strongback should prevent this. I should also keep the strap in a fixed postion so it does not go slack after a couple of miles. On my RS400, I gently tighten the ratchet from the strongback to the cradle, then add a couple of light ropes pulling the strongback forwards a little (tied down to a different point on the trailer)so that it does not move. This tends to hold a constant tension over a long drive. I need to sort something similar for 'Armed Forces'. My aim will be to put the downforce from the strongback onto the deck close to the gunwhale, so that the load goes into the hull structure as directly as possible. I'm interested to hear other points of view.

I agree 100% with this.  After a couple of miles just stop and check the strap/strongback and retighten a notch if it is loose.  I use a board about 1/2" thick and 5" wide and when you tension it up it curves upwards - you are pulling the ends downwards after all.  I have used this now for 252 and 111 - both nailed up hulls and have travelled thousands of miles with them no problem.  Use the big industrial straps - the cheapos fray and are rubbish

Hi Mags,

I laminated my own pads for the decks with glass and polyester resin, after taping polythene to the decks to prevent it sticking.  To take up the angle of the deck, I acuired some insulating foam from a friendly builder and stuckj this onto the pad (in situ on the boat)let the resin bond cure then very easily sanded the top surface flat to provide the surface for the wooden 'strongback' to be bonded to prior to over glassing the whole thing.

I have done a couple like this (my wife also has a boat, as you may know...) and produced durable and effective pieces of kit.

The tie down is webbing with a 2:1 tackle and cleat.

All v good.  Personally hate the big ratchet jobs, but that may just be me!

GGGGGG

Excellent replies all, and thanks. The deck has not opened a glued joint(which are fragile enough on boats of this age, but in the middle of the side deck, for & aft, on an old & repaired crack. I don't believe I had more tension in the strap, yes, my strongback curved up in the middle(as it should, and yes this has a 'damping' function when you hit a bump;- I had attached the middle to the thwart with strong elastic; Yes I had stabilizing lines to the trailor to stop the strongback moving fore & aft, and I also built in a feature on the moulded part that locks onto the spinnaker guy cleats on the inner sloping face of the sidedeck. All made sense and I thought would improve on the strap alone which we have used since 2003 and towed all over Europe without any trouble other than slight chafing of the gunwale which I was trying to avoid. I'm sure that the problem is that it does the job of holding the boat down, by pressing downwards and as the hull flexes, fatally, outwards, since unlike the strap alone there is nothing to restrain it.  Don't forget, my boat is much more flexible than yours, and the split was an old wound waiting to reopen.

No matter; the repair is a cinch, but I will go back to a suitably padded strap which holds the boat together,- as indeed we have on all out other boats.

I think it was John Stokes who told me the first Merlins came with a towing 'box' included in the price. ANyone else remember? Sounds like a much better arrangment than a modern trailer and undercover!

Thanks for all your replies.

Andy Leigh at Burghfield SC (trailor maker amongst other things) made a full box trailer for his Fireball. Saved on B&Bs at events .... and certainly did away with covers. Sort of a cross between a horse box and a glider trailer.

Brings a new meaning to "sleeping in a wooden box". My concern would be the weight.

It was a bit of a pain when he parked it in the BSC boat park ...

Did this on 3441 when it was new. Flat plank with carpeted/padded ends to sit on deck plus chiseled out rounded corner at end. Then jib sheet rope going from plank to the trailer fastened back on the plank with two jib sheet cleats. If intrested I can mail a photo.

Horses for courses. I agree that one doesn't want to squeeze the boat laterally, and I should have mentioned that my strap alone goes over the deck and down around the full width traveller  Perhaps a boat builder could tell us, but at a guess the best way to apply force to the gunwale/deck area is in a direction downwards parallel to the planking, pretty much the vector resultant of the forces my strap alone applies. 

At all events, I fitted the strongback thinking it would better protect the boat, but it had the opposite effect.