Best way to fit an A frame?

davis · 03 March 2007, 21:04

Hi , has anyone any advice on how to fit an A frame, should it be inside or outside the transom? , bolted through or a plate on the tube ? any pictures appreciated

Ian M · 03 March 2007, 21:39

Most heavy duty 'A' frames seem to be bolted to the outside of the transom, altough I have seen some lighter weight ones bolted to the inside of the transom. Personally, I prefer them bolted to the exterior. In general, the tube used is relatively thin walled, and personally I have never seen one with the fixing bolts fitted directly through the tube - the torque required on the fixing bolts to adequately fix the tube would also crush it. Mounting plates welded to the the frame are the way to go. Double 'A' frames with one tube fixed to the transom and one fixed further forward to the deck or some other part of the structure of the RIB provide a more robust solution than the single tube variety. Depends on your intended use of the frame.
Personally, I have always gone for a good heavy duty double 'A' frame, using something like 2" diameter tubing, which gives a few more options such as a high tow point, but for this use it does need to be very firmly attached. If you want pics I'll try to post some tomorrow, if the predicted rain is not too heavy.

Richard B · 03 March 2007, 22:05

Ian, I don't see any logic in your suggestion that fixing the A-frame to the outside of the transom is better. I do agree with your thoughts about welded plate aginst bolting through, however.... with one caveat... it depends on the quality of engineering as welding can dramatically reduce the strength of stainless steel. So a good bolt-through solution may be better than a poor welded solution.

Ian M · 03 March 2007, 22:43

Richard,
Didn't say that fixing the 'A' frame to the outside of the transom is better, I said that I prefer it! Aesthetics old chap!
There is the minor aspect that you do get a little more space for steering gear, and fuel seperators/filters etc. if you want to mount them on the transom. Very much depends on the detail of the installation.
Cannot agree about bolt through, crushed tubes, nasty! Unless you were to somehow insert a spacer inside the tube of the 'A' frame at the point of bolt through - that might work.

Nos4r2 · 03 March 2007, 22:48

Bad stainless welding looks awful and I wouldn't want it on a boat.

Richard B · 03 March 2007, 22:53

Quote:

Originally Posted by Ian M

...Unless you were to somehow insert a spacer inside the tube of the 'A' frame at the point of bolt through - that might work.

Exactly... that would be a reasonably well engineered solution.

Sure... the aestheticts thing is down to personal opinion. And space for all the bits and pieces is a good point.

Hightower · 04 March 2007, 00:35

Don't like the Idea of fixing the A frame tube direct to the transom even if a solid spacer was inserted up the end of the Tube. The bolt wouldn't sit right against the diameter of the tube and would look cheap and nasty. Also that same diameter would crush the gel coat when you tightened the bolt due to a small surface contact area.

Go for a plate welded to the tube, looks much better, will be stronger (subject to getting the welding right), will sit against the transom much better and be kinder to it as well.

BTW, I think that A frame tubes sitting outside the Transom spoils the looks of a RIB and looks a tad commercial to me. But if needs must for space reasons then who am I to argue.

red · 04 March 2007, 17:13

There is no general view, A frame mounting will vary from boat to boat, as will the type of frame you are fitting - for example, a simple light construction A frame intended to carry a couple of lightweight lights needs alot less support than an A frame intended to carry a radar scanar, multitudes of antennae, reflectors or self righting apparatus. Give alot of thought to its purpose, you have the choice of aluminium or stainless, beware of off the peg A frames constructed from low grade Stainless. Whatever you do contact the manufacturer of your rib so that you do not damage it.

Cheers

Jizm · 04 March 2007, 17:49

I have just bought this double a frame which is intended to be mounted to the deck. However I have no way of doing this with substantial fittings. I am going to cut off the base plate and get it modified using a bend and a 4mm thick stainless plate to mount it to the transom on the inside.

Ian M · 04 March 2007, 17:52

Davis,
The rain has been lashing down all day, but I have ducked under the cover to get some shots for you. Not suggesting that this is the only way an 'A' frame can be fitted, but you did ask for pictures

The first one shows the general layout (this was taken over a year ago!). The next the detail looking forward from the rear of the boat, with the rear 'A' frame leg, welded mounting plate and the integral towing point. The next shows the internal fitting, and finally the last one shows the under side of the hull and probably needs some explaining. The forward leg of the 'A' frame is bolted through the deck and then via a deck to hull insert, through the hull. This gives a very strong fitting of the 'A' frame. The plate, in the position it is fitted does not interfere with the flow of water over the hull, as when we are on the plane it is out of the water.
Hope this helps.

davis · 04 March 2007, 18:18

Thanks- everyone for the advice, just wanted to be sure before I decided to drill holes, great photos IANM I can see clearly what you have done, though I am not comfortable bolting through the hull , maybe expxy something to the floor and bolt to that?, just a thought.

Ian M · 04 March 2007, 18:49

As i said, I am not necessarily advocating this style of fitting, it depends on the use you want to put the frame to, as others have said on this thread. Any 'A' frame is by its nature, is going to take some abuse during its life, accidental or otherwise! I would be wary of it depending on an epoxy bonded fitting.