Porposing

[robbo1]

Hi guys, trying to get my head around porpoise, in particular a catamaran. What is the cause of it? I think I understand that the tunnel needs air to give the cat lift, but what is actually happening and meant to happen to that air once in the tunnel?. Is the air escaping or too much air causing porpoise or is porposing just controlled via cog (battery placement)? Little confused, hopefully someone can shine some light on the subject. Thanks in advance.

[fweasel]

Porpoising, or bouncing, has multiple causes. The most common are center of gravity (CoG) and drive trim angle. It would be best to know what boat you're attempting to adjust, but typically, start with a neutral drive trim angle and adjust the CoG more forward if you're still experiencing the bounce.

[robbo1]

Boat is an apparition (rivercat). Blueprinted sponsons, offset rudder etc.
Ran it on the weekend in slight chop, with strut 3mm above sponsons, 0.5mm negative trim angle (which converts to .028 degrees) and batteries about 31% cog?? boat ran well.
Next run fresh batteries with same setup but conditions had flattened off and noticed slight porpoise and wasn?t comfortable at speed (was concerned of blowover).
I did move batteries forward very slightly and all good but for the cost of 3mph (guessing more hull in water).
I suppose the question in all this: could I have moved the strut down some?, inturn giving more lift in transom and lowering the bow which theoretically (in my mind) would have been the same as moving batteries forward (or am I looking at it all wrong) and what is the actual cause of porpoise, in particular a catamaran?

[Jesse J]

What motor, kv and prop are you using? What elevation and water salinity? Does negative mean prop down or up?
I prefer a larger prop and higher strut - haven’t had porpoising in a while.
If all else fails you could try a small air dam.