Sterndrives in a CAT

[powerabout]

that was a giant bow steer into a wave
can happen with a single engine

[Mr Efficiency]

I have had motors suddenly lose power on one side of a cat without any significant or dangerous deviations, typically they are slow to turn anyway.

[powerabout]

Originally Posted by tunnels

What movies you been looking at !you do stupid things you suffer the results of you brainless actions ! dont blame the boat for your dumbness !!
I spent hours in the seat ,hours in rough water that would scare you short and curlys straight and ages working on boats since 1972 ,i never ever had one single frightening moment either in a cat or a tunnel boat !! even jumping surf like a jet ski or running parallel along the top of waves with a hull either side of the rolling wave Tunnels will go places you would never dare to even think about in a mono hull .
Have built ,set up, and fitted motors, never had a problem !. I dont have any secrets ,just i make sure i draw things out full size on plywood !! if its a boat i never worked on before!! guessing at measurements is not good ,real size takes any guessing out check check and check again . usually set the under side of a cav plate 6 mm above the keel line of the hull . if a hull has any sort of obstructions this has to be taken into account naturally . Out boards are a breeze to set up and get right . Some of the boats are in the high 70 to 80 mph mark and have never exsperianced any of sort of problems !!.So many people get carried away with the trim buttons and everything goes up the whop in seconds and they never seen to get it right just once in a while its part way there but they dont know what they did to get there .
Nothing takes the place of seat time and knowing the beast you are playing with ,intermitly , every boat is differant, because of set up in the very begining .weight in the wrong place ,ballance have a lot to do with trim and what the boats likes and dislikes !!even to the point where one motor can be trimmed sighly highter than the other to stop rock and roll .
Get the feel of whats good and whats not good and know what causes what to happen .

Sure been there done that, we do have waves in Australia and plenty fo bar crossings. I worked with several builders to actually test their boats, sounds like you only went in a straight line?
Fast cats have killed plenty of people so to think they are fundamentaly safe is not true, they have plenty of vices

[powerabout]

Originally Posted by Mr Efficiency

I have had motors suddenly lose power on one side of a cat without any significant or dangerous deviations, typically they are slow to turn anyway.

yes I agree
slow to turn normally because no-one builds any ackerman into the steering so very heavy to steer as well
No off the shelf parts as far as I know so you have the manufacture the stuff yourself

[APP]

Originally Posted by Jimboat

There are lots of offshore power cats of this length with i/o drives. an important consideration to pay design attention to is the height of the drive or "x-dimension". All I/O's have lots of trim adjustment, this can be controlled in operation. However, unlike outboards, I/O's don't have height adjustment available, so getting the x-dimension right can help to preclude handling problems like porpoising or other dynamic instabilities.

Thanks. For vertical height I undersand as mostly used the Z-axis. I believe then that the best alternative is to give to the engine supplier (Volvo, Yanmar, etc.) the boat drawings and ask them to define the best height. I attach a picture of how others have installed the sterndrive in a planing Cat.

Regards
APP

[APP]

Originally Posted by whitepointer23

twin sterndrives on cats that size are common here in australia.

It seems, there must be a limitation in Cat dimensions in order to use the Sterndrive with adequate efficiency. Such limitation might be caused by the max available Diameter of Sterndrives propeller (I think usually it is max D=16" and P=23") or by the fixed gear ratio of this system. So, for a Cat that would exceed let us say 9 metric tonnes a D=16" only, may not work properly. Anyway what could be the heaviest Cat in Australia that uses Sterndrives (Inboard/Outboard)? Any idea?
Thanks
APP