Ignoramus26696 wrote:
I made this boat trailer guide base:

http://igor.chudov.com/projects/Trailer-Guide/

To it, I planned to attach a short 1 foot 1.5" pipe nipple, and to it,
another 5-6 or so feet of same pipe using a rigid union.

A person mentioned that pipe is seriously weakened by threading, and
that I should reconsider. I am now seriously considering NOT using the
1 foot section, and instead to use a solid piece of pipe. The nice
thing about using the union is that the big pipe sections could be
removed for storage easily, and the thing could be a little more
easily assembled and drilled in a small basement shop.

Anyway, now I think that the latter consideration is BS as I could
always unbolt them etc.

My question is, is this an important issue? The objective for the
guides is to hold the boat in relatively strong (to me) current.

I can't put a figure on it, but my instinct is that the pipe will be
weakened quite a lot by this joint. The threaded collar is pretty short
and will therefore not offer much support when the pipe is flexed. It
might fail suddenly under load if the thread is stripped, or if the iron
coupler isn't ductile enough and cracks. As someone else here pointed
out, a plain mild steel pipe wouldn't fail in this manner. You could
strengthen the joint by using a longer threaded coupler, or even better
a slightly larger mild steel pipe which is bolted to your trailer which
the long mild steel pipe slots into (secure it with a pin). The most
important question is how serious the consequences of a failure will be.

Hope this helps,

Chris