How much does trim tabs regulation affect your cruise speed

[baylabayla3288]

I believe that this is related to how much variable the deadrise is along the hull length, which is what I suppose you mean by "deep" fwd section. Not sure about "full", though.
Regardless, it surely ain't specific of BO design: my DP has exactly the same static attitude, but while under way she rides almost as flat as she is at rest.
Another relevant factor in BO hulls (though I'm not sure if ALL of their hulls are built like that) are the semi-tunnels, which have their advantages obviously, but significantly reduce the stern lift, AOTBE.

With “fullness” I mead that the hull is wide down to the waterline to gain cabin space and this in combination with a steep V shape gives a deep foreship. Almost so that at rest the deepest point of the boat is in the bow. Also what I tried to say is that the boat might appear to run more bow up than it actually is due to the shape of the windows, hull - deck line angle and how the antifouling is painted along the chine. Anyhow the deep V in the front gives the famous good ride with very litle broaching tendency and balanced natural hull trim. BO clearly got things right in my view despite the apparent bow up attitude.

 

[Rocksteadee]

I have a Fairline Turbo 36 and at 11 t laden is quite heavy for the size
The original Bennett trim tabs made no difference to speed and only slight change in attitude
So they got modified, nearly doubled the plate size, thinking if the extra load breaks something I have not lost anything.
They now work and in 5 years are still going.

She is just planning at 12k and the boat trimmed bow down gives an increase of just under a knot rising to about 1.5 up to 17k, then decreases and finds equilibrium at 20 then bow down reduces up to her max of 28 where bow down losses a knot

I assume reasons being:
The Turbo at low planning speed is very stern down and dragging, by dropping bow lifts the stern out;
20k is equilibrium;
Over 20 the full length of hull is wetted and now dragging, bows up reduces the wetting length.

I only tend to use full up and full down
Have never needed to adjust laterally, probably due to her weight and Bernard’s hull design
Have played with the setting irrespective of lost or gained speed but for changing attitude at different speed in different seas looking for ride comfort. This too makes a difference

 

[MapisM]

I assume reasons being...

The reasons you envisage sound reasonable to me.
But whatever the reasons, what you are reporting strikes me as a clear cut case of builder mistake in fitting too small tabs.
The improvements you got are well worth the extension you made! :encouragement: