Sterndrives in a CAT

Hi,

By placing two sterndrives (In-Outboard) e.g. 170-200 HP in a power planing Catamaran of let' s say 9-10 meters, what precautions should we take against hobby horsing and/or porpoising? Cruising Speed 20 Kn and Max Speed 30 Kn. I understand one should know the boat details (LCG, LCF, LCB) but let's start a general discussion, if possible of course.

Regards
APP

 

No precautions whatsoever because the trim range of I/O's is large enough to accommodate even wrongly designed hulls or layouts.

A dangerous situation arises if one of the units fails and the other one tries to capsize the boat. I would construct an rpm monitor that shuts down both engines if the rpm difference exceeds say 500 rpm while at least one engine is spinning at more than 3000 rpm.

 

Very Good!
Thanks
APP

 

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

 

CDK
can you explain how one engine can capsize a cat, i'm intrigued?

 

Me to You got me bet !!
So all the thousands of cats and tunnels included can capsize ??
what kind of books do you read ??

 

The only precautions you need is to go find a cat and go for a ride and see and look and learn . understand every thing there is to know go get em tiger !!

 

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.

 

i am not sure what you mean here, i have never heard of this happening. do you mean if you lost one engine surfing down a quartering swell.

 

Bulit tunnels with twin out boards for long time and once on the plane its possible to actually raise one motor to full tilt right ou of the water and the other motor would keep the boat on the plane !! Dangerous!! no way was safe as !!

 

I find they usually have tons of bow lift and if you use std props ( which are all designed to create bow lift) they are a bit ugly

 

how about turning them at a high rate to lift the inside hull out of the water??
some will do that

 

I s'pose with deep sponsons it wouldn't be too hard to have a flush floor hatch for the sterndrives. Nice gain in deck space.

 

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 :idea:.

 

You clearly think it is impossible and I do admit it is unlikely to happen on a quiet afternoon on the lake. But even with a mono-hull the propulsion loss of one engine causes a very sharp direction change when it happens at high power settings.

It once happened to me when I attempted to get to the lee of an island in malicious weather. Instead of riding the crest, the boat suddenly banked, dove down the slope and the bow dug into the next wall of water. By the time I realized what had happened the boat had completed a 180 turn and the next wave went over the stern, dropping over a ton of water in the boat.
The stern drives in my boat were only 3 ft. apart and already the loss of power on one side was devastating.

I can imagine that with a cat asymmetrical propulsion causes an even wilder change of course where the hull without power digs in and the powered hull tries to overtake it.