um i just got a nitro hammer and i was wondering how fast it goes also could i put a .21 engin in it

A lot faster but I don't recommend it. There would have to be a lot of other work involved to make it handle the power of a good .21 engine. Number one would be to replace the driveline to a 3/16" flex cable setup. The hull is a little thin for a serious engine, the torque would crack it big time.
The boat could use a few mods to get more out of it. The tuned pipe length is way too long to take advantage of even having it on there.

why couldent i put a .21 in there if i cant put a .21 in i wanna know what is the smalest boat for sale with a .21

I didn't say that you couldn't but I didn't recommend it. There is a lot of difference in power between what you have in there now and a decent .21 .
With a good .21 motor I'd go at around 29" as the shortest. There are lots of .21 hulls out there, it just depends on how deep your pockets are. My sons Speedmaster 21 has a Rossi Niagara, Irwin quiet pipe, Airtronics radio system and Speedmaster hardware. I've got probably close to $800.00 into it. You can go low Dollar and do it for several Hundred less. Picking out the right boat for you (and me) is a long process. With a .21 hull you will get a better handling and faster boat.

if i put one in how long would the hull last BC this guy said his friend had a 20 inch hull with a .21

I'll bet that he has a handful there! I have a 24" hull with a .12 in it and that gets interesting to drive at times.
I can't give you a time frame on how long the hull will last. It could last hours or years.

this is a 24 inch with a .15

how fast does yours go

Mine was radared at over 32 MPH before I had the pipe and other things dialed in. The hull was an electric that the manufacturer made into a nitro hull. He only made 2 this way that I know of and mine is the only one that got finished. A friend bought the other and he never finished it. I built it using a lot of old R/C truck parts that I had laying around and put a Trinity/Picco/NR motor in it.