Boat Motor Cavitation Problem

knottynavigator

I think I’m chasing a cavitation/ventilation gremlin and could use some beginner-friendly guidance on how to diagnose it without just throwing money at props and engine height.

Setup and symptoms:

19’ glass bay-style hull with a mid-size outboard on a manual jack plate
3-blade stainless prop (moderate pitch), no hydrofoil, small trim tabs
I get a quick RPM flare and loss of bite when trimming out past “neutral,” in hard right turns, and when running down-swell or over patchy chop. It’s worse with people or a full livewell aft.
Water pressure blips down a few PSI right when it happens, then comes back when I trim in
Prop has a light “frosted” look just behind the leading edge on one blade and some shiny burnished areas near the tips
I have a fairly large transducer on the starboard side of the transom and a bracket for a shallow-water anchor near the prop’s path

Questions:

Does this sound like actual cavitation (blade surface vapor) or ventilation (ingesting aerated surface water)? Any telltale signs from the prop marks I described?
Could the starboard transducer/bracket be feeding bubbles into the prop wash in right-hand turns? If so, is lowering it, moving it inboard/outboard, or adding a small fairing plate the usual fix?
Engine height: is there a simple rule-of-thumb for a non-pad V hull? I’ve heard everything from “AV plate even with the keel” to “prop shaft 2-3 inches below.” What’s a good starting point before I start moving holes on the bracket?
Would adding cup to my current prop or trying a 4-blade be the smarter first move to keep bite in turns? I don’t want to kill hole shot or top end if I don’t have to.
Minimum safe water pressure at cruise/WOT while testing height and trim? I’m trying not to cook anything while I experiment.
Do small trim tabs change the flow enough near the prop to matter here, or are they mostly a separate thing?
Are hydrofoils on the anti-ventilation plate a legit band-aid for this specific scenario, or do they just hide a setup issue?

Planned tests to isolate it (open to better ideas):

Tape over the transducer face and/or temporarily remove the anchor bracket to see if the problem disappears
Run with different jack plate heights in ¼” steps and note rpm/pressure/trim angle where it lets go
Try tabs full down vs up in the same chop and same trim
Chalk or Sharpie on the prop to see where it’s getting scrubbed, and video the cav plate with an action cam at safe speed to look for aerated water
Borrow a similar-pitch 4-blade with more cup for a quick A/B

If you’ve solved something like this on a bay boat or similar transom layout, what ended up being the big culprit? Engine height, prop cup, or “that one innocent-looking transducer” sending a bubble train straight into the wheel?

CruiseControl

I’ve chased plenty of these gremlins on my old 20′ center console-bay boats love to bite you with setup quirks. Sounds more like ventilation than true cavitation to me; that RPM flare and loss in turns/down-swell screams air ingestion, especially with the water pressure dip and those frosted/shiny prop marks (ventilation often leaves uneven polishing from sporadic air hits, while cav’s more uniform pitting). The transducer on starboard could absolutely be the villain in right turns-bubbles trailing off it right into the prop arc. I’ve seen that exact issue; try zip-tying some foam or a small deflector upstream of it first before drilling new holes.

For height on a non-pad V, forget the AV plate myths-aim for prop shaft 1-2″ below the bottom at rest (hull-dependent, but start there and watch for porpoising). Safe pressure? At WOT cruise, you want 10-15 PSI steady; dipping under 8 risks overheating the impeller, so back off if it blips low.

Trim tabs? They help bow lift but won’t fix prop flow much-yours are probably irrelevant here. Hydrofoils? Band-aid at best; they mask height issues but don’t solve ventilation from accessories.

Your tests are solid-do the transducer tape job first, it’ll tell you quick. On my boat, it was the damn jack plate slop letting the prop kiss the surface in chop. Cupping the prop or going 4-blade helped grip, but only after nailing height. Don’t chase props yet; fix the setup. What pitch are you running now?