A-frame thickness?

[mwb.wright]

Hi guys,

Just a quick one...

What thickness is recommended for an 2" diameter a-frame?

I have the option on both 2mm and 3mm thickness.

I'm planning on having it similar to nicks on his SR4 and potentially tow from it also.

Fitted to a SR4 with the transom mounts and floor mounts.

Thanks.

[beerbelly]

if your going to tow off it go thicker cant understand why your asking

[mwb.wright]

Because the weight difference between the two is pretty big so I suppose my question is, is the 3mm necessary?

What thickness are other a-frames?

[Pikey Dave]

Is it a single or double "A" frame? Either way 2mm will be plenty strong enough, the weak point will be the attachment point to the boat.

[matata]

Typically, what weight would a double A frame be in 2mm stainless? 5.5 to 6 m rib. Just wondering. Nik

[beerbelly]

Originally Posted by mwb.wright

Because the weight difference between the two is pretty big so I suppose my question is, is the 3mm necessary?

What thickness are other a-frames?

ah sorry

[jeffstevens763@g]

8m x 50mm x 2mm = 19.11 kg
8m x 50mm x 3mm = 28.17 kg

strength will be down to design & bracing my A frame was 2mm 60mm dia assumed length for calcs off google hope that helps

cheers

[mwb.wright]

Thanks for the replies.

The link below is what I'm wanting to have made.

http://www.rib.net/forum/f8/new-aframe-for-my-sr4-42705.html

I'm guessing 2mm would be fine but just thought I'd double check first.

[jwalker]

Originally Posted by mwb.wright

I'm guessing 2mm would be fine but just thought I'd double check first.

Save yourself some more weight and money and go with 1.6mm. 50mm dia with 1.6mm wall will be more that strong enough. Something to bear in mind though, stainless tube is surprisingly springy and stainless welds are prone to cracking if they are flexed - good design is essential.

[willk]

Originally Posted by jwalker

stainless tube is surprisingly springy and stainless welds are prone to cracking if they are flexed .

Yer! Watch the video:

http://www.rib.net/forum/f16/a-frame-41400.html

[matt h]

Originally Posted by Pikey Dave

Is it a single or double "A" frame? Either way 2mm will be plenty strong enough, the weak point will be the attachment point to the boat.

Pikey D is the total expert so listen to him. I'm surprised his mrs isn't made from marine grade stainless

[Pikey Dave]

Originally Posted by willk

Yer! Watch the video:

http://www.rib.net/forum/f16/a-frame-41400.html

I've never been convinced with the A frames that sweep out over the tubes, the footprint is too small for the mass & forces, all imho of course[emoji57]

[Chris Caton]

Originally Posted by mwb.wright

Thanks for the replies.



The link below is what I'm wanting to have made.



http://www.rib.net/forum/f8/new-afra...sr4-42705.html



I'm guessing 2mm would be fine but just thought I'd double check first.

I used Nick's info for my SR4 A-Frame and went to Marine Fabrication to get it made, very pleased with the result, the frame is plenty strong enough for anything I'm ever likely to use it for and it looks great [emoji16]

[kerny]

[kerny]

Originally Posted by kerny

Attachment 117184

Marine fabrications did a good job for me as well 3mm too 👍

[Chris Caton]

Originally Posted by kerny

Marine fabrications did a good job for me as well 3mm too 👍

Imogen looking good Kerny

[kerny]

Originally Posted by Chris Caton

Imogen looking good Kerny

Cheers Mr C......[emoji106]

[GuyC]

If you are happy with larger sections, you could consider a change of material to aluminium.

A 65mm aluminium tube with a 5mm wall thickness has a 15% greater resistance to bending as a 50mm stainless tube with a 3mm wall thickness.

You could hang around 15% more weight off the aluminium tube before it started to fail.

The aluminium tube is 70% of the weight of the stainless tube.

I know you've had one made but it may be food for thought if someone else is thinking about it.

[mwb.wright]

Thanks for all the replies guys.

I think I'm going to go with the 2mm tubing.

I can pick up 316 2"x2mm for £11.50 a meter which is much cheaper than I was expecting which is a pleasant surprise.

Judging from the comments and internet searching, the 2mm is plenty strong enough and as some have said, the welds or fittings to the boat would fail well before the tubing breaks/bends.

Had the boat been bigger I wouldn't of worried about the weight issue so much but I suppose with a small 4 meter rib, the weight is pretty sensitive and wouldn't want to jepodise the current performance of the boat as much as I could help it.

[Pikey Dave]

Originally Posted by mwb.wright

Thanks for all the replies guys.



I think I'm going to go with the 2mm tubing.



I can pick up 316 2"x2mm for £11.50 a meter which is much cheaper than I was expecting which is a pleasant surprise.



Judging from the comments and internet searching, the 2mm is plenty strong enough and as some have said, the welds or fittings to the boat would fail well before the tubing breaks/bends.



Had the boat been bigger I wouldn't of worried about the weight issue so much but I suppose with a small 4 meter rib, the weight is pretty sensitive and wouldn't want to jepodise the current performance of the boat as much as I could help it.

If it's not a personal question[emoji6] why are you going for 2"? If you are going for a double A frame 1.5 will be plenty & look more in proportion on a smaller boat. It will still be strong enough for anything you are likely to tow with that rig. You will pull the transom off before you snap a well made 1.5" double A frame.