Who knows steel channel?

[colbyt]

I am getting ready to rework and older trailer. It was constructed in the old style with twin center beams about a foot apart and the rollers in the rear were supported with a piece of steel channel bolted perpendicular to the main runners ( think T shaped). This was done using 3/16" 4 x 2 steel channel. The 2" portion carries the load. It must have been custom work cause you can't buy that now. 44" total span from port to starboard.

My I hope simple questions are:

If the smaller, 2" dimension is carrying the vertical load does the width of the channel influence the carrying capacity at all? Would not a 3 x 2 be just as strong as a 4 x 2?

Two inch is not available; 1.72" and 1.5" are. How much would you down grade the capacity for each of those two sizes?

A second cross piece is being added for a total of 3 (the axle is one) so the total load on each will be less than it was before. Also the boat is just a 14' tinny at about 350 pounds max with motor though I would like some cushion for a motor upgrade in the future.

 

[minuteman62-64]

Re: Who knows steel channel?

At 350 lbs. you could almost fabricate roller brackets from bamboo

Seriously, not sure I have the total picture - but, based on my interpretation of you setup, the width of the channel would govern the roller length. For example, a 4 inch channel would accomodate a 4 inch roller.

The 4 inch rollers on my boat are supported on brackets made of about 1/8" thick steel. They are a little over 4" wide and the flanges are about 1.25". They easily accomodate the 800 or so pounds of my boat and motor.

Why not just go on-line and buy rollers and brackets from iboats or West?

 

[colbyt]

Re: Who knows steel channel?

I never seem to provide enough info.

If I do this I am converting from rollers to bunks as I have become convinced they offer more support for the boat. So while the old roller arrangement did require 4" wide channel to connect to, the bunks will only rest on the channel and be attacked to it.

Pardon the crudity of this sketch.

 

[UncleWillie]

Re: Who knows steel channel?

...This was done using 3/16" 4 x 2 steel channel. ... It must have been custom work cause you can't buy that now. 44" total span from port to starboard...

4" x 2" x 3/16" x 4 foot is easily available from many sources. HERE is one!

 

[minuteman62-64]

Re: Who knows steel channel?

And, bunk brackets are available at West and probably iboats. Mine are about 1/8" thick, 2 1/2""wide with 1 1/2" flanges (which support my 2x4 bunks that are mounted with the 4" dimension vertical.

 

[colbyt]

Re: Who knows steel channel?

4" x 2" x 3/16" x 4 foot is easily available from many sources. HERE is one!

Yep. They are about 15 miles from me. But that is not channel that is tubing. I may yet go that route.

 

[dingbat]

Re: Who knows steel channel?

They have never made an American standard C-shape with nominal dimensions. The closest you get to your 4? x 2? would be a C4 x #7.25. Whatt you have was most likely fabricated.

If the channel is loaded in the X axis, your looking at a ~50% decrease in strength by going from 4? x 2? to 3? x 2?. Loaded in the Y axis, your looking at ~38% decrease in strength.

Using tubing instead of channel would minimize the loss of strength when going to the smaller size

 

[colbyt]

Re: Who knows steel channel?

They have never made an American standard C-shape with nominal dimensions. The closest you get to your 4” x 2” would be a C4 x #7.25. Whatt you have was most likely fabricated.

If the channel is loaded in the X axis, your looking at a ~50% decrease in strength by going from 4” x 2” to 3” x 2”. Loaded in the Y axis, your looking at ~38% decrease in strength.

Using tubing instead of channel would minimize the loss of strength when going to the smaller size

Grady that is a great reply. I am a total newb to metal stuff. Could you define X and Y?

In this application the weight is bearing on the 2" vertical.

 

[matt167]

Re: Who knows steel channel?

I would just use 2"x3"x11ga tubing.. It will be stronger than the 4" channel, and is not that expensive. Not hard to get either, because that is what many hotrod and custom car chassis are built from

 

[minuteman62-64]

Re: Who knows steel channel?

Maybe I'm missing something, but, why screw around with trying to fabricate something when you can get these for about $9 each?

C E SMITH Bunk at West Marine

 

[UncleWillie]

Re: Who knows steel channel?

Maybe I'm missing something, but, why screw around with trying to fabricate something when you can get these for about $9 each?

He is looking to add the 4 foot cross support at the front of the trailer that those would mount to.
See post #3.

 

[minuteman62-64]

Re: Who knows steel channel?

He is looking to add the 4 foot cross support at the front of the trailer that those would mount to.
See post #3.

Well, duh! To paraphrase the instructors I had in the Army, I should have RFP!

OK, now that I finally get the picture, maybe I can offer something constructive. Your proposed new configuration is close to the way my Dilly is set up, with two cross members, each 4' long and each supporting one end of the bunks.

The Dilly cross members are 2 3/4" X 2" (2 3/4" dimension mounted vertical) X about 1/8" thick. As I mentioned, my trailer easily carries about 800# of boat, motor and load. So, in response to your original question, you could go at least as small as 2 3/4" X 2" X 1/8" using the "C" section and easily carry your less than 400# load.

 

[littlerayray]

Re: Who knows steel channel?

pictures would help

 

[colbyt]

Re: Who knows steel channel?

OK, now that I finally get the picture, maybe I can offer something constructive. Your proposed new configuration is close to the way my Dilly is set up, with two cross members, each 4' long and each supporting one end of the bunks.

The Dilly cross members are 2 3/4" X 2" (2 3/4" dimension mounted vertical) X about 1/8" thick. As I mentioned, my trailer easily carries about 800# of boat, motor and load. So, in response to your original question, you could go at least as small as 2 3/4" X 2" X 1/8" using the "C" section and easily carry your less than 400# load.

Your cross members are or are not supported at the ends? Are they tube or C channel.

Note in post 3 drawing that these cross members are cantilevered off the twin center beams.

Good reply. I wish we did the rep thing here.

 

[minuteman62-64]

Re: Who knows steel channel?

Arrgh! I still didn't get it right

OK, now I have (I hope) the full picture. I'll take one more stab at providing useful info for this post.

Yes, my cross members ("C" channel) are supported at the ends by the fore-aft structural members of the trailer. So, the loading on my cross members is different than the loading will be on yours (mine is similar to a simple beam and yours is a cantilever loading) - and my suggestion for using the dimensions on my Dilly was all wet.

I actually aced my structures courses, but that was back in the '60's, and I haven't used them since - but here's what you have, to the best of my recollection and ability to explain in layman's terms.

As pointed out above, you will have a cantilever loading on each cross member. On your original cross member both the 2" flanges and the 4" web are important in resisting this load. If you decrease the flange dimension or decrease the web dimension (or both) you decrease the load carrying capacity of the cross member (assuming the steel thickness remains the same). So, a 3" X 2" "C" section, as on my Dilly, might not be adequate in your case. You would also increase the load carrying capacity by increasing the steel thickness.

From my readings on this site, it appears there are some engineers who have more recent structural experience (or better memories) than me - maybe one of those folks can give your the dimensions that will work.

If not, here's a couple of suggestions. First, you know the original 4" X 2" section at the transom did the job. You can't go wrong using that (or bigger) for the new cross member. Since the load on the new configuration will be less, you could go slightly smaller in each dimension. I'd have to do some calculations to figure out how much smaller. I think the 4" dimension is a little more critical, but I'd have to locate and read some of my old texts to be sure.

Second, go to an experienced trailer guy who can probably give you good info just based on his (her) experience.

 

[The_Kid]

Re: Who knows steel channel?

4 X 1-1/2 X 3/16 (5.4#) is more than strong enough. A 4 foot length will hold 3170 pounds of concentrated load at the center when supported at each end. That's probably 3 to 4 times what your boat motor and trailer weigh and you are supporting it in two locations. Here's where I got that info.

Andy's Place

 

[loose rivet]

Re: Who knows steel channel?

For the weight your dealing with, you could certainly use 2x3 channel. They most likely laid the channel down to reduce overall height.
You will need to be sure that the connection at the two frame rails is secure, whether you use U bolts or weld it.
I had a similar set up a few years ago, the trailer had two 3x3" square tube main frame rails but it used 4 cross members which were made of 3.5"x3.5" angle iron which was notched and welded over the top of each frame rail. There were short roller bunks that traversed the rear two and front two cross members. It was super strong but not very adjustable. The boat was an old flat bottom skiff. There was also a row of keel rollers down the center.
I liked the trailer but it sat high and was a hassle to load at low tide on some ramps. I eventually found a better suited trailer for the boat and planked that trailer out and made a flat bed out of it.