[susan53]

Hi Michèle,

I think you mean have someone do something rather than done -?? Are you confusing it with have something done?

Taking these two first, just in case it was a confusion and not a typing mistake, compare :

I'm going to have the electrician look at that wiring.
and
I'm going to have that wiring looked at.

These two express the idea that someone else will do the action for you, as in the more common : I'm going to have my hair cut or I'm going to have my shoes repaired.

If you use "get", the parallel expressions are get someone to do something - I'm going to get the electrician to look at that wiring - and get something done - I'm going to get that wiring looked at.

get here has the meaning "cause something to happen", and the idea of causation is uppermost. So I'm going to get David to repair the car, might mean I'm going to persuade him to repair it, or I'm going to pay him to do it, but there's no indication in the sentence that he might volunteer to do it (that he might well do so in real life is immaterial). The action is in some way caused by you. That's why you can say I couldn't get the boat to move (correct) but not I couldn't have the boat move (incorrect). Boats can't move voluntarily, so it doesn't make sense. In essence you are saying "Whatever we did it didn't cause the boat to move" and so "get" is necessary.