Interceptor plates on pockets/tunnels?

Would that notch in Pascals picture (post #34) be a buoyancy trim area?
Can be filled in to add buoyancy if needed.

We have done the same on smaller boats work boats when adding larger engines, equipment and my phat asp..

Not sure why my boats relevant But for the record it’s just lifts up and out like Pascals Lazzara ( if I understood what he writes correct?) without any trim tab use , they stay at zero .Pretty flat no fwd bow lift visibility issue thst you describe .18 T dry 48 ft .

Like Pascals Lazzara mine has no bolt on after thoughts to correct or better worded improve anything regarding the running attitude or indeed transition .
For those unfamiliar ( friends across the pond ) my boat brand Itama , old Amarti versions are mid engined the helm stands on one .Cabins are split forward and aft of the central engine room , that’s arguably the “ compromise “ ? One of them .

Along with a 23 degree deadrise this almost coindental CoG with CoLift means great longitudinal balance .
The tabs are just for adjustment of bow in either following sea or head sea in the trad way .

A permanent interceptor like on the SL 96 A pic .To quote JFM

“ It can only be to create a cleaner and more constant point where the laminar flow separation occurs, like a lip spoiler on the trunk lid of a fast saloon car (which is not there to create downforce and is only there to define and keep constant the point where laminar flow separates).”

^ quote ^ That’s one way of looking at it but missing a few vitals imho . .
It’s not inaccurate.

Consider this the car lip spoiler while not creating down force per se is reducing lift .So indirectly there is a net gain of rear end weight on the rear axle.
But not by a underbody diffuser or venture underbody adding down force .
How ever the net effect is more downforce.

Back to the topic the fixed interceptor works similarly in that it reduces suction at the stern .As I said earlier tunnels add suction compared to std P brackets on flatter hull sections .
So by reducing stern suction net benefit is more lift than there would have been without the fixed interceptor .

It’s just a play on words - lift , suction or bow rise , stern sinking .
The Net result is what matters , irrespective of which way round it can be described.



The max suction on a typical single hull motorboat is at the rear 1/3 rd near the stern .
Counterintuitively adding a bit of hull behind the props , filling in that step might add net suction , and move it back further away from the CoLift upsetting the balance .

Lazzara have thought long and hard with that hull design.

@ MapishM- I never said “ owners cabin “ just accommodation benefits by moving machinery aft .Which is good in terms of marketing and keeping pace with competition btw .

But one can’t help thinking it’s comes at a compromise re the distance between the CoLift and CoG increasing ? = permanent interceptors fitted .

It’s inconsequential to me but this forum is called “ General yachting discussion “
So thought it a good topic to talk about .
Interesting difference the Lazzara / San Lorenzo hull details .

Pardon? I just quoted what you wrote.

Yes my bad .Apologies. Speed reading skipping etc .

Not entirely sure what relevance the actual usage has on the interior vol usage Fwds of the machinery room bulkhead ? Perhaps you could expand on why it’s so important to accurately define this usage ?

Indeed in theory a Fwds upper level owners suit towards to bow area helps keep the CoG Fwds to kinda assist cross compensating for the most rearwards machinery bulkhead Vs interior vol conundrum.

But despite that they feel the need to fit permanent interceptor on the SL .

Just trying to discuss why ? Any thoughts yourself btw .

Regarding fixed “interceptor strips” around the tunnels, my 62’ Neptunus Yachts also has them, which I also found interesting and almost counterintuitive at first. But from a fluid dynamics perspective, what the specialized interceptor plate does, in layman’s terms is: create a pressure wave from the interceptor plate forward, effectively increasing the pressure in the tunnel between the plate and the propeller. The propeller, however, has so much more vertical surface area for the pressure wave to act upon pushing the boat forward compared to the interceptor’s drag. So that when properly designed, the net effect can be to increase lift and thrust in the tunnel.
.

You should rather ask yourself.
If you re-check this post in the nose cone thread, you will see that I only mentioned the master cabin placement in reply to one of your posts (which is actually now in this thread, after the two were splitted) speculating that they moved the engines astern to make room for the master cabin.
So, you actually seemed to consider the interior usage as relevant, not me.

I don't have the faintest idea, but I see that in the meantime Capt J already answered your question.
And as we all know, he knows better than Sanlorenzo - or than any other boatbuilder on planet Earth, for that matter.
So, if he says that it was added to fix a hull that performed improperly, there you are, that's a very good reason, 'innit?

Phew, lucky me, I just dodged a bullet.
After jfm ordered an SL96A, I was thinking to buy one also for myself, but now I'll reconsider.

This ^ .
That’s the inescapable conclusion . Obvious in my experience looking in yards like Pascal and watching / observing boats running .

I don’t buy MapishM s “ I don't have the faintest idea,“ answer either FWIW .


I find this book by Lindsay Lord useful in theses discussions and those interested in furthering there knowledge on the subject I recommend it .

Extracts from the book from pressure sensors in tank tests 1/8 th scale .


Solid black is lift .50/50 dot density neutral see the red line .Excess white see the blue areas are suction , stern sinking .
A vertical appendage efficiently ( interceptor ) decreases the size of the blue area I have marked esp the most rearwards .

This is old hat tried and “ tested “ stuff preached in every university naval architecture course , in the tank .First year stuff .



A prop tunnel increases suction in the already “ suction “ area .

Of course tunnels primarily are to lower the shaft angles ( with or without V drives ) meaning the machinery space can be pushed further aft , but they come with a price .

Hope this helps everyone.
esp the book tip .

Yeah, right.
I had in mind that both SL and Hatt were established around the same period, and google just confirmed that.
Actually, SL in '58 vs. '59 of Hatt, for sake of accuracy.
Now, after 60+ years, just look at where both companies are.
Obviously the former learned more than the latter from their trial and error.

PS: are you sure the boat you are "dealing with" (whatever that means) isn't a chinese copy?
The original SL is written in one word, not two.

'Salright.
I don't buy your and CJ's armchair speculations either - fair and square.

No “ arm chair “ here .
Ps don’t shoot the messenger if you don’t like what he ( they inc Capt J ) says .
Just passing on info .All grad students in NA have tank test results , they see practically what does and importantly what’s doesn’t work then write it up .

The professors publish papers + that book I referred to for general consumption.

Unless you can enlighten us differently on the effects of vertical interceptors and why they are used ?

I already told you I can't, for the very simple reason that commenting without knowing ain't my habit.
I appreciate that you might find this unusual, but you can save yourself the time to insist further.