"A" Frame

[TB Minnow]

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

[Ian M]

Lots of threads on these, I put some pics of mine here http://www.rib.net/forum/showthread....ighlight=frame
Hope there are some ideas you can use.

[Limey Linda]

Originally Posted by TB Minnow

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.

[jyasaki]

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

[9D280]

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.....

[Tomas]

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)

[Downhilldai]

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.

[Hightower]

Like this

[Pete7]

Originally Posted by Hightower

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

[TB Minnow]

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

Jeff

[TB Minnow]

Spoke with a fabricator today.

I will be going with a single post with steps. Suitable for a white light on top and a step (braket) to mount an antenna.

I agree with the comment of having my brain fried by radio waves.

The post will only be tall enough for the light to clear our heads. He was worried about vibrations on too long of a post causing stress on the transom. He explained that the longer the post the more wiggle like a tuning fork an could make the effects of a sudden slap of a wave last much longer through transferance.

Makes sense to me.

Jeff

[9D280]

TB,

Yes, a longer pole will "whip" slightly more if you slam a wave, but vibration wise you may find that a longer pole helps - remember the most likely constant vibration will be from your engine - a longer pole will have a lower natural frequency, so is less likely to "sing in tune" with it.......

Bigger diameter will be stiffer and not hugely more heavy.

How are you planning on fixing it on? Could you get the fabricator to make an over length version, and if you're clever with a couple of temporary clamps (e.g. u- bolts at the same spacing as your fixing plates), do a trial run, check it for resonsance, can then cut it down a bit if it sings, and when you're happy, weld on the "proper" mount brackets? Remember three mount points will help a LOT to stiffen it up.

For what it;s worth my prototype SR antenna mast (holding up a Vtronix Raider) was made form a length of 40mm exterior grade black plastic waste pipe form the local plumber's merchant! It survived half a season with no misdemeanours. (then I sold the boat).

[Hightower]

So what diameter tube is it made from?

[Downhilldai]

Originally Posted by Pete7

Ah yes the Fareham design ...

I hope you Fareham boys aren't claiming rights to the original Llanelli design

Some pics which may help TB Minnow when specifying bracketry etc. Originally, it only had the VHF antenna bracket, then I added the GPS antenna by simply using a flat plate sandwiched between the VHF antenna and its mounting plate

[BruceB]

I have designed my pole to be a T shape, it means the VHF and GPS have clear fields of view rather than being mounted lower with some shading caused by the pole itself. The disadvantage is the anchor light is slightly shaded by the aerials from the side. Will post a pic when it arrives from the fabricators.

[Jon Brooks]

Originally Posted by 9D280

Jeff,


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.

Ok before we all go running for the hills and stop using our radio's lets add some perspective here.

UHF and Cell Phones are far far more dangerous than marine VHF and to the best of my knowledge there are no reported case of anyone getting fried whilst using their marine radio. Also if you look at how often you actually transmit the danger is, if at all, very small.

As mentioned by getting the VHF antenna up on a frame or on pole it will increase your range But consol mounted antenna's do work well.

There seems to be some train of thought that you need to get the GPS antenna up high to, this is not the case. Many, me incluled, have the GPS antenna consol mounted and it works fine. There is no proof that they work better higher up so save yourself cable run and agro and flush mount on the consol.

IMHO an A Frame even on a smaller boat is a god send ans it gives you more place to stow bit in polly bottles etc. If they are frame mounted this then frees up the deck space etc.

Hope that helps.

[BruceB]

As long as the GPS has a clear field of view and is horizontal it should work just as well mounted low as high.
Unfortunately my console has only one place I could do this and it isn't ideal for several reasons.
Since the boat NMEA network needs to go back the outboard anyway it is only a few feet further to put it on the pole beside it.
I didn't go for an A frame this time for reasons of cost and usability. It was far too expensive to buy or have one made and I had loads of space for storage anyway. It also helps access to the aux which was always a bit of squeeze on the old boat.
The only thing I am missing I would have likd is a bit radar reflector and I would have needed a large double tubed frame to carry it at a cost of around £700 more than my pole is costing!