Benefit of trim tabs on semi-d boat?

[Kawasaki]

Spot on Bandit, ref the CI22
And you are right about semi displacement hulls ref their characteristics
Normally , as you say they don't need trimming much at all
Correct as well, your 'Seeker' would have needed trimming under certain conditions
'Chine Walking' is a snag some planing hulls have and 'tabs' can help there
Went for a 'test' / jolly on a CI 32 last week
An aquaintence/ kinda friend just bought one
Jammy git!
The tabs did help to trim the vessel under certain conditions
Didn't want to show Him really
I was that jealous!
:ambivalence:

 

[MapisM]

semi displacement hulls don’t seem to need to level side to side in the same way as planing boats.

Actually, I believe it's more a matter of speed, rather than hull type.
I mean, at steady P speed (let's say 20+ kts), the portion of hull which sits in the water becomes lower, raising the hull sides+superstructure and hence creating more windage.
That does happen also when raising the speed from say 7/8 kts to 12/14, but to a much lesser extent.
And while the latter can be a "normal" speed for a SD boat, you rarely cruise at that speed with a P boat, which typically is either used at one digit speed or at 20+.
I understand that in most SD hulls there's a deep(ish) keel which you don't find in P hulls, but that can do something for rolling (if that), not for listing, imho.

 

[PCUK]

Gained three knots at no extra revs by fitting tabs.