I am running a 21 sized Prather deep vee with an OS 21 engine and a nitro pipe. Can you give me the approximate lenghth for the pipe? I know the distance is measured from the glow plug to the rise in the pipe. Also has anyone done any experimenting with quieting down nitro pipes? If you use a quiet pipe do you still have to cut the pipe to get maximum rpm from it?

Start it at 8-1/2", that should get it close.
There are a lot of ways to quiet a loud unmuffled pipe from afermarket mufflers to home-made ones and no, you don't have to make any other changes afterwards except for the correct pipe length if I'm reading it right.

Thanks Ron. Have you done any experimenting with using brass tubing that is the same ID as the outlet pipe ,placing it about 1/2 way into the tuned pipe to lower the noise level. I was talking with a boater from Grand Rapids that told me that this works but I haven,t tried it yet.He told me you could change the db level by sliding the tubing back and forth, just like playing a trombone.

I've heard of that trick but haven't tried it. That is also supposed to be an IMPBA legal "muffler".
If it was from a boater in G.R., chances are pretty good that I know him and most of them are very smart.

Mac's Products has a good start for finding the right length, here's the link to that page:

http://www.macspro.com/tuning.asp