Hiya. I've recently built an aluminum catamaran 4.5m and had a new 60hp Yamaha 4 stroke tiller steer on it. I'm having problems with ventilation and sluggish speeds. The boat shop that fitted the motor said they had no idea on where to fit the motor on the transom but said that I would have to test it. So that's what I've done over and over again trying to get the optimum height! It was way too low to begin with and kicked up way too much water at the back into the back of the boat and up into the cowling. So I moved it up a couple of holes and still too low I felt with more water kicking up into the cowling and still not able to have WOT without ventilation. I had no more adjustment so I extended the transom to have the motor sit higher. I've experimented with sandbags in the centre, up the front, had others in the boat, made transom wedges to get more negative trim. Ive also added more flotation to the pontoons at the back which has helped level the boat out as it was rear heavy. It seems that the motor feels very bogged down. I have though had it going very well a few times into the wind and waves, quite fast indeed and skipping over the top of waves. But with the wind behind and in anything slightly choppy it will ventilate. I know catamarans hull create much more disturbed aerated water exiting the hulls which affects the prop. I have made an aftermarket cavitation plate. I'm unsure of what to do to eliminate the problem. When planing the cavitation plate is still well below the water. If I trim up it even to where it's still just below the water it ventillates. Do I need to raise the engine further or lower it? Or is the something else I'm missing? Appreciate any help/advice on this especially from someone who has an aluminium cat and has had similar problems. Boat is similar to a Blackdog Cat. Cheers

Yeah...Bountys right. Cats are a bastard for cavitating.

Update: So I've welded in a centre sponson, more like a wedge that directs water away from the middle where the motor is located
It has helped quite a bit. It has especially helped with buoyancy at the stern meaning that I had to lower the motor down.
Another issue is that it is running amazing into the wind allowing me to run full tit. into the wind. Downwind not great at all. The motor seems to bog down even when it's quite flat. Very strange. I have created a new thread on the subject.