John,
Since you want the volume in yards (really cubic yards) I think it is easiest to change all the dimensions to yards immediately. There are 3 feet in a yard so 21 feet is ^{21}/_{3} = 7 yards. There are 36 inches in a yard so 4 inches is ^{4}/_{36} = ^{1}/_{9} yards.
The volume of a circular cylinder is
r^{2} h
where r is the radius and h is the height. You want half a cylinder so the volume is
^{1}/_{2} 7^{2} ^{1}/_{9} = 8.6 cubic yards.
Penny
