setting engine height on a single engine catamaran

370Dancer

370Dancer

Well-Known Member
Oct 2, 2006
2,489
Suncoast of Florida
Boat Info
1998 370 Sundancer
Engines
380hp MAG MPI Gen VI with V drives
A bit off all topics, but there are some smart people here.
The easiest way is for me to attach my story with pictures. Wanted to post this at thehulltruth, but that site has so many posting restrictions for an infrequent user, that it is pissing me off.

I know how to do this in my sleep on a traditional hull and transom. This one has me perplexed.
What do you think?
https://www.dropbox.com/s/uvg2sa74fc1r4mu/should the engine go up or down.pdf?dl=0
 
Doesn't look like a cat but rule of thumb is keep raising the engine until it vents on hard turns.
 
You should probably put that all in the thread here. At least text.

But I think I have your point
One thing what do you mean by “skipping”?
If its cavitation in a straight run your to high
as @scoflaw is saying you may cavitation on a hard turn but never straight

the cavitation plate should be basically even with the bottom of the hull.

Traditional Vee
single engine is at the very bottom of the vee
Twin engines would be at the bottom boat where they mount

Now with the cat you need figure where the hulls run when on plane
Draw a line (string) across the hulls at that height
That is the starting point
 
hughespat57 said:
You should probably put that all in the thread here. At least text.

But I think I have your point
One thing what do you mean by “skipping”?
If its cavitation in a straight run your to high
as @scoflaw is saying you may cavitation on a hard turn but never straight

the cavitation plate should be basically even with the bottom of the hull.

Traditional Vee
single engine is at the very bottom of the vee
Twin engines would be at the bottom boat where they mount

Now with the cat you need figure where the hulls run when on plane
Draw a line (string) across the hulls at that height
That is the starting point
Click to expand...
Yeah, I tried to post it here, but with the pics, too big.
Skipping = rpm increases momentarily, assume I'm grabbing air and ventilating.
I've got one symptom telling me I'm too high, and another telling me I'm too low.
 
scoflaw said:
Doesn't look like a cat but rule of thumb is keep raising the engine until it vents on hard turns.
Click to expand...
It's a cat. Transom to the center lowest point = 21.5". I'm not talking about the bottom of the deck. There is a small transom right in front of the engine. Transom to the outer hull bottom of the vees = 25"
I would think it should be set to the outer hulls, but the original engine was the opposite, set as high as it would go.
I'm overthinking it.
IMG_3568.JPG InkedIMG_2594_LI.jpg InkedIMG_2601_LI.jpg
 
370Dancer said:
Yeah, I tried to post it here, but with the pics, too big.
Skipping = rpm increases momentarily, assume I'm grabbing air and ventilating.
I've got one symptom telling me I'm too high, and another telling me I'm too low.
Click to expand...

this seems like your a little high right now.
 
If it's to deep it will be porpoising a bit, bow up, catch air, bow down, catch water, repeat...
That could feel like ventilation, rpm surges.

keep in mind the hulls will throw a wave inward towards the engine just like they do on the outside away from the boat.
So the "middle water" may be "higher" than you expect. (the engine is "deeper" into that water than you think)

You're just going to have to try, start at the bottom (down one) if no good, go one higher than now, and move up more if needed.
 
More research says I am not low enough. Need to be at the outside transom bottoms. The sponson, skeg whatever in the middle is purely to push "clean" water towards the prop. It is not a height goal for the anti-ventilation plate. That's what the yard was shooting for. FAIL
So, down a bolt hole to the bottom when I get it back, and let's see. Still, why is this the opposite of where the Evinrude was? Fortunately for me, I am in a pretty tight knit circle of haul, launch, and A Frame. I just don't have this kind of unpaid time.
Must be a B-O-A-T.
 
hughespat57 said:
If it's to deep it will be porpoising a bit, bow up, catch air, bow down, catch water, repeat...
That could feel like ventilation, rpm surges.

keep in mind the hulls will throw a wave inward towards the engine just like they do on the outside away from the boat.
So the "middle water" may be "higher" than you expect. (the engine is "deeper" into that water than you think)

You're just going to have to try, start at the bottom (down one) if no good, go one higher than now, and move up more if needed.
Click to expand...
Well, hmmm.
Dropped to the top hole this morning (lowest setting) which should be 25" to the bottom of the hull transoms.
Worse. Had to trim up to ventilation just to keep the engine from filling up the transom well, which had the bow way too high, and bogging. Any trim down, and I have a swimming pool in the back of the boat. So, lower is not better.
Next is to lift to as high as possible, and start there. I've already been in the middle.
Another variable in this is the fact that I have an extension in the leg to turn this 20" engine into a 25" engine. It's not square, but it's not wafer thin either. Maybe that is the shovel for the water coming over the transom.
What I can't do is get the engine perpendicular to the water on plane. Maybe this is a jack plate solution. That's an expensive test. I do know that the original motor was rigged all the way up (bottom bolt holes), and never had these issues.
Gotta fly tomorrow for a few days, but when I get back, first order of business is video of the symptoms. My words may not be as accurate as they should be.
 
So, I raised the engine to the top hole, which is what the Evinrude was set to as a 25" motor. Things got much better, but I think it can still go higher. Had my son chase me in the Yamaha this morning, so I can see where the waterline is on plane. As I suspected, it doesn't appear to be the bottom of the outer hulls, or 25" from the top of the transom at the engine. I'm really starting to think that I could get away with the 20" foot and drive shaft, and start back at the lowest point and raise until I get rid of all the turbulence around the engine. This was at 3200 rpm, doing 20 knots, engine trimmed high enough that I wasn't throwing water into the engine well, but low enough that I wasn't ventilating, even on sharp turns. Definitely much better than when I had it at the bottom of the adjustment. This is probably going to turn into a Jack plate as I'll need to adjust up and down as conditions change.
https://www.dropbox.com/s/41vllifo8...ine all the way up - 3200rpm 20knots.mp4?dl=0
 
Finally got my Atlas after ordering in July. Installed one hole down from top, just in case.
Put her in the water, and the halfway lift mark is the sweet spot.
6" off the transom, and 3" up on the jack plate finally got the foot far enough out of the water to act like a proper outboard.
that one is in the books. Now for the heads on Beach House.
 
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