What's going on here?

[badgermat]

After the (relative) triumph of my first project (red trousers for my Grandson, pics on my thread in Hi I'm New), I thought I'd have a go at turning some ill-fitting trousers of my own into shorts.

However, when fixed the hems, attempting to replicate the original folded-over style, I was left with the weird stitching shown below.

The front/top side seems fine, but the back/bottom is really odd and very messy.

The only setting I changed on my machine was the stitch length, so I'm at a bit of a loss.

Obviously I'm going to have to re-do it, but how can I avoid the same thing happening again?

bm

[Ploshkin]

It looks as though the tension of your top thread was too loose so it has been pulled down through the bottom layer of fabric - you can see the loops on the underside.  The bottom thread also looks wavy/ floppy because it is not being pulled up by the top thread.  The 2 threads should meet up and link in the middle of your fabric sandwich.

You must have a cut off bit of fabric - try out the stitching on the same number of layers as you have on your hem and adjust the top tension a bit at a time and keep checking the front and back if the stitching until it looks good on both sides.

[Vezelay]

Another vote for tension. As Ploshkin says, turn the tension knob to a higher number and try a test.

[Lachica]

Make sure the presser lever is up when you thread the machine. This releases the tension, allowing the thread to sit properly between the discs. It's s very easy error to make so try rethreading before tweaking tension dials.

[Acorn]

I can't describe to you the relief of an Admin when she sees a new thread entitled 'What's going on here?' in Technical Help, and then realises that it's technical help for sewing, and not technical help for the forum...   

[badgermat]

Many thanks to all for the help.

Tension then. I'll give it a try.

As a general rule, is there any relationship between stitch length and tension? Should I increase for shorter stitches and decrease for longer, or is it just a case of fiddling 'til it works?

bm

[Ploshkin]

Tension should not need to be varied for stitch length.  It may need to be varied for fabric type / thickness / number of layers or type of thread.
Usually there will be a middling setting that suits most things.
Whenever you start a new thing it's always good to do a sample bit of stitching on a spare bit of fabric if you can - it's always the first thing I do after threading the machine.