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Old 27 March 2008, 14:36   #1
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"A" Frame

RIBs are not as popular here in the US as elsewhere so accessories are a bit tricky to come by. I have the ability to build my own A frame and wish to have one. But I need some help with design.

I have recently acquired a 4.5m Achilles with a 50hp Johnson two stroke engine. The boat has a bench seat a mid ship with an offset consol to the starboard side. I plan on using this boat for pleasure and diving.

 I am purchasing a Lowrance 522c IGPS so I will not have to mount the GPS antenna on it.
 I am installing a VHF radio and purchased a rail mount for the antenna to mount on the consol.
 I plan on being able to carry four divers on the boat with two riding on the tubes.

With what I have explained do I really need an A frame?

What are their purpose / uses?

 Support for my two rear passengers and while moving about?
 Mounting an ensign?
 Mounting lights?
 Mount a life ring?

What is practical?

I will have less than $100 US dollars in this. $50 in materials and $40 in welding. 38mm aluminum electrical conduit and four aluminum plates being bent on a custom exhaust pipe bender. Much cheaper than fabricator shop.

Can any post some pics of various designs and how they are mounted and what is mounted on them?

Eager to hear from you,

Jeff
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Old 27 March 2008, 16:14   #2
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'A' Frames

Lots of threads on these, I put some pics of mine here Another A Frame question
Hope there are some ideas you can use.
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Old 27 March 2008, 16:40   #3
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Quote:
Originally Posted by TB Minnow View Post
RIBs are not as popular here in the US as elsewhere so accessories are a bit tricky to come by. I have the ability to build my own A frame and wish to have one. But I need some help with design.

I have recently acquired a 4.5m Achilles with a 50hp Johnson two stroke engine. The boat has a bench seat a mid ship with an offset consol to the starboard side. I plan on using this boat for pleasure and diving.

 I am purchasing a Lowrance 522c IGPS so I will not have to mount the GPS antenna on it.
 I am installing a VHF radio and purchased a rail mount for the antenna to mount on the consol.
 I plan on being able to carry four divers on the boat with two riding on the tubes.

With what I have explained do I really need an A frame?

What are their purpose / uses?

 Support for my two rear passengers and while moving about?
 Mounting an ensign?
 Mounting lights?
 Mount a life ring?

What is practical?

I will have less than $100 US dollars in this. $50 in materials and $40 in welding. 38mm aluminum electrical conduit and four aluminum plates being bent on a custom exhaust pipe bender. Much cheaper than fabricator shop.

Can any post some pics of various designs and how they are mounted and what is mounted on them?

Eager to hear from you,

Jeff
I have the same dilema; is it worth it. I asked this question a year ago and the only sensible answer was " something to hold onto while taking a piss".
Having said that I am still considering one, they do look good. I think 38mm Al. tubing may be a bit light unless it is very heavy wall thickness. Do not weld brackets to it you will inneal the tubing and create a week point. Lot's of stories about fractured Al. A frames. If I get one I will use 2 inch stainless. Thanks for the reminder: my local exhaust shop does excellent work very reasonably. Where else good you get a cracked manifold on a truck welded up while you wait for $25.00.
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Old 27 March 2008, 19:40   #4
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Hi Jeff;

Your boat falls below the 16' where you are required by the USCG to carry a throwable flotation aid, so you need no life ring (that said, if you have room, a foam cushion meeting Type IV requirements is a good thing to have.)

You also fall below the length where lights *have* to be in certain places; you can mount both side lights and an all-around white on the console, providing they meet the viewable angle criteria. White is supposed to be one meter above the sidelights, but I have yet to see that enforced in my area.

From what you describe, you do not need an A-Frame; it simply makes for a handy place to mount those things that you've already mounted on your console (and, as LL says, a handhold while using the facilities.)

One more thing: in lieu of an A-frame (should you need something mounted up high), you might consider a simple post; cheaper than a complete frame.


jky
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Old 28 March 2008, 07:40   #5
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Jeff,

I'm not 100% certian of the rules in your area, but as Jyaski says, do you really need a frame? It's going to be no use holding your passangers in unless they are sitting on the engine, so that just leaves Lights and antenna.

I know you are talking about antenna on the console, but do you really want 25W of RF power blasting through your brain at such point blank range? Also if it's on the console, a good chunk of it's "view" will be obscured by you & your passengers, reducing its effectiveness. Putting it on a pole will also give it a few more feet altitude, giving much better range.

If you just use a pole for antenna / light then you don't need to weld anything - two or three suitable sized U- bolts in your transom (console?) and job done!

I had my antenna on a pole (no lights) on my old SR4, worked a treat, and by getting the U- bolts with "welded washers" I had a "drop in" and could remove the pole for towing & storage leaving the bolts sealed and fixed. (with a retaining string so it didn't work its way out with use in heavy seas)

Let us know how you get on.....
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Old 28 March 2008, 20:23   #6
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Tubing

I second the caution that electrical conduit does not have sufficient wall thickness for your intended purpose. Conduit is designed to exist in a static world protecting wires. The sea, or any sizable body of water will test the integrity of any structure. It is a kinetic world gone mad.

Go to a supply house and bust for the extra cash to purchase proper structural tubing with a wall thickness of at least .125, preferably .187 (1/8 - 3/16). If you're going to weld it, alloy selection is important and welding aluminum, especially lighter tubing is tricky. Aluminum just loves to form hot cracks in the puddle where the weld terminates. I really question whether or not you would even be able to weld conduit. It's thin thin thin!

Good luck and try to incorporate a lattice design into your frame. Single vertical tubes with mounting plates are almost guaranteed to fracture at a point just above the plates that fasten them to the transom. (Voice of Experience)
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Old 29 March 2008, 07:29   #7
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If funds are tight, just get a length of 1.5" 316 grade stainless tube and weld a couple of mounting plates (4"x2" ish) for fixing it to the transom and a plate at the top for fixing the antenna. Your all-round white light can fix to a round plate closing off the top of the tube and fit the side lights to your console. About an hour's work for a decent welder/fabricator and another hour for you to polish back the materials afterwards

Now if you want to be fussy, the all round white light should be along the centreline of the boat, but you could argue that this is 'not practical' for your boat.
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Old 29 March 2008, 08:14   #8
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Old 30 March 2008, 05:02   #9
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Like this
Ah yes the Fareham design you don't need an A Frame and the costs are now getting rediculous. One of the less obvious advantages is that you have lots of room to remove the engine cover without the frame getting in the way.

Pete
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Old 30 March 2008, 07:33   #10
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Hmmmmmm

I am liking the post concept. Early this week I will be speaking to a Fab. shop this week.

Jeff
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