Hmmm, 25/30 hp on a 3m sib? I must admit that it sounds a bit too much. As you may have seen from previous threads I've had to take special measures to keep the nose down on my 3.4 when I'm going solo and I have the maximum allowed 15hp. However if the plate says 25 hp.....
The main problem is the shaft length. The sib will most certainly have been designed for a SHORT shaft engine. The longer shaft will alter the handling and tend to lift the bow. Even an engine well within the hp limit but with a LONG shaft would cause problems. A combination of over power/extra rear weight/shaft too long would not be good
Keith (been busy with BIG plans) Hart