Two things about Hancor pipe for a sail box:



Because the pipe is ribbed the 15" i.d. pipe has a 17.7" o.d. Make sure you have clearance under your dolphin striker.



Hancor pipe is black. On a hot sunny day inside temps can get over 180 degrees F. This could be high enough to delaminate rudders or dagger boards.



Other than that it works great.

The style you would want is Hi-Q if I remember correctly.

Website - http://www.hancor.com/product/hiq.html