Aerotec 420 - strange occasional "jerkiness"

[daiquiri]

I've been testing the Aerotec 420 I bought recently, it's in great shape (transom and bottom of boat professionally reglued, tubes/floor holding air nicely) and mostly I'm quite happy with it.
However today I finally had the chance to test it with my new 20hp outboard (until now I have been testing with my old 9.9 which felt quite sluggish) finally getting some proper performance, but when on the plane the boat occasionally makes a sudden and strong "jerky" motion to the right side, and at some speeds it also had a continuous slight jerkiness horizontally, feeling like the stern trying to move left and right like an ecstatic dog shaking its butt while wagging its tail Hard to describe.
The water was very cold so I stopped to top up the floor after 10-15 minutes or so, as it noticeably dropped pressure. This made the issue less prominent but it was still happening. This made me think it was a floor-related issue, but it was topped up to 100% pressure and the issue still didn't go away completely so I'm not sure what else to try.

[tinker]

Some ideas........How are you measuring the pressure? Is the engine short or long shaft? What sort of tilt have you got on the engine? If you turn the boat upside down have you got any excess floor material, is the floor stretched?

[Fenlander]

Another thought are you running bow down/heavy? If I am one up with an over-long tiller extension placing my weight too far forward my 380 feels twitchy like this and in the extreme feels like you are fighting a veer off to one side or the other.

I wonder too if you were looking at the floor panels if either shown any flex/movement when this happens.

[Peter_C]

As long as pressure is up, I too would say motor trim is causing it to pivot off the bow. Can be a scary feeling. Trim it up and run it again.

[Brinormeg]

Not long back from a short trip on a 3.9 air floor sib and Suzuki 20hp.
While away we trialed a different prop and also all the manual trim settings.

When we hit around 20 mph you could definitely get a few cheek clenching experiences at certain trim . so the guys could well be correct with trim being the issue.

[daiquiri]

Thanks for the tips everyone!

Originally Posted by tinker

Some ideas........How are you measuring the pressure? Is the engine short or long shaft? What sort of tilt have you got on the engine? If you turn the boat upside down have you got any excess floor material, is the floor stretched?

I'm using both a decathlon electric pump/compressor with a digital display, and an oldschool analog gauge on the foot pump, depending on the situation (the latter was used for top up this time after the first 15 minutes). The engine is short shaft, the transom has been modified to accommodate a short shaft (fairly common mod with 420 Aerotecs) it was tuned in so the cavitation plate is level with the bottom of the transom when the trim is set to the second hole from the transom (out of 6 holes) so that's where I've been testing it so far. It might ventillate if I trim it up more, but it will probably splash a lot more (based on the experience of others) if the transom is cut deeper.
Yes, the floor is stretched, no excess material

Originally Posted by Fenlander

Another thought are you running bow down/heavy? If I am one up with an over-long tiller extension placing my weight too far forward my 380 feels twitchy like this and in the extreme feels like you are fighting a veer off to one side or the other.

I wonder too if you were looking at the floor panels if either shown any flex/movement when this happens.

I don't think we were bow heavy, if anything, we were stern-heavy (me sitting on the bench closer to the transom (custom install), 3gal tank near the transom, girlfriend sitting on the front bench, minimal gear/food/water around the front bench, light anchor bucket around the bow area.

I think it's there was some very minor floor flex going on when it happened, although not even close to the dramatic waves my previous Cadet fastroller was producing on occasion.