I refer to what Rick said but would advice difference measurements.

Arguably the Taipan 4.9 has one of the bendiest mast of all catamarans if not the most (I guess A-cat mast are more flexible).

For several years now crew are running the spi hounds at 7.5 mtr on a 8.5 mtr mast with the stay hounds at 6 mtr. This means that the spi hound is at (7.5-6.0)/(8.5-6.0) = 1.5/2.5 = 60 % above the hounds. F18's and sorts use the same ratio.

I know of only one mast breakage on a Taipan while under spinnaker. That was in Singpore in 20 knots wind with gusts and, get a load of this, 205 kg on the wires, double trapezing on a shy beam reach. This was putting alot of compression on the mast causing buckling failure. However things must be seen in perspective, in the past 15 years masts were broken on the Taipans without even a spi being on it. In the last 3.5 years only one mast did with a spi onboard. This doesn't seem to suggest that adding a spi hound at 60 % increases the risk of breaking the mast much. At least not on the Taipans with their wingmasts (arguably weaker from side to side than the H17 teardrop shaped mast).

Ricks suggestion for 33 % makes the mast almost twice as strong and stiff as putting the hounds at 60 %. The dependence is just that strong. Such a safety margin seems to me to be a bit to much. If you are really afraid than moveing the hounds from 60 % to 50 % will make the mast 21 % stronger and stiffer with respect to the spi.

Now a question for Rick. I heared some sailors comment that the hooter puts msignificantly ore compression load on the mast as a spinnaker. I that true. If that is the case than you must limit the area that goes into the hooter if you choose that option. Lowering the hounds on a hooter will not reduce its effect on compression laods if the area is not changed. It is my believe, shared by a few boat builders, that mast fail almost always under compression load. The sides of the mast buckle and the mast denting then causes the major failure of breakage. This is very much linked to compression laods and less to bending loads.

Wouter