Trimming off AV plate

Hi fellow boaters

First let me thank you all for this great forum. I've been "lurking" arround this place for more than a year and have found some realy good solutions to my problems.
As all DIYers I'm getting my 2005. BF90A to a best possible condition and been trough repairing all common issues on those engines.
Right now it's power trim/tilt time ... (that might end as separate thread).

Now I'm thinking of one, possibly, extreme job ... trimming off AV plate.

I'm running 16ft (5m) RIB with deep V hull and have raised my OB all the way up. Bracket is 2" above stern.
Since BF90 hight (shaft length) is more than 20" my AV plate is just 1/2" above keel at parallel.
At cruising trim angle (80% out) it's front tip is 1" above keel, back end tip is 3" above keel. That corresponds to 10" setback 1" up.(?)
This position is still deep enough not to blow the prop at this trim angle but if I go max out I get +400 rpm (no speed gain).
At max out trim angle I still get front quarter of AV plate scrapping the surface of water.
I realy don't want to go any higher with OB because at that trim and hight I can blow the prop in turns. If I have it on 80% trim all is good.

So, I would like to avoid that spray from AV plate with max trim out.
Another thing that came to mind is at that condition front portion of AV plate is "digging" a hole that propeller meets further down the line. Right?

I've been searching on the web but couldn't find anyone did that.
Looking at newer OBs all have front part of AV plate much narrower (Suzuki beeing the most extreme in that sence).

Any thoughts or experience is very wellcome.

Thanx in advance
Marko

I'll try to upload some pics (noticed that is problem here ).

OK, looks like I got it

the idea ... somewhere along this line



... transom position ... just FYI - I had to re-drill upper holes to new position already becouse rigger messed it
at most up position there was 1/2" more available space at bracket slots.


... parallel to keel


... at cruising trim out (80%)



spray at max trim out

It sounds like you've thought the issue out well regarding top speed. I believe there may be an additional concern when the water is NOT being force fed into the prop area. With that large section of the AV plate missing, have you considered how the missing sections might affect your low speed performance? It would seem to me that sucking air from the surface, rather than water from in front of the prop, might be an issue....

Hi Alan
Thanks for reply.
That's exactly why I started this thread. I have good technical background but am not naval engineer. :/
Meaning I do understand things but can't predict all the aspects of changing this important feature.

Again, looking at Yamaha and Suzuki design, that part is shaved off. Even on newer Honda OBs although not so much.
I was thinking engineers figured out something better in last 30+ years since design of this lower unit was originally made.
Is it a trade off and for what? Is it "covered" with some other feature of newly designed lower units? I don't know for sure. Thinking out loud here and hopping someone will chime in.

The "issue" is V-hull. Once you get AV plate out on surface that lowest point of hull is deep enough to vent/blow the prop. So I guess surface pierceing prop or AV plate just below the surface are only options. Or?

How did you manage to solve the problem in the end? Of course, if you succeeded, I'll wait for your answer. I'm also interested in what kind of tool you used. You can't always find something that could maintain its quality over many years. For example, being a gardener, when I realized that my trees had to be taken care of, I had nothing to do with it. They could see that they were choking and that they needed to cut some dry twigs. Thanks to the advice of a friend, I found this site https://www.hausandgarten.com/collections/garden-tools. There I found some instruments of high quality, which until now are of great use to me. That's why I'm still recommending them to everyone. It's like telling everyone the best secret.