We measured the pressure drop in our existing exhaust system today. To my surprise it is linear with engine speed.
The maximum back pressure is 51 inches WG, which is a bit above Volvo's spec for our engine (43 HP Volvo 2003 turbo)

We plan to upgrade to a 53 Yanmar or a 60 HP Beta, so definitely need a new exhaust system.
We have a 60 mm system (2 3/8") at the moment. Yanmar wants 76 mm (3")

We will replace out Vetus waterlock with one for the new hose size.

We would like opinions on a gooseneck for the new exhaust. Presently we have a Vetus WLOCKLT gooseneck, installed vary close to the centreline, just under the deck, with the discharge sloping down to about a foot above the waterline.

We measured the backpressure it creates, and found that is it 25% of our total backpressure.

I am thinking of just looping the new 3" exhaust hose up to the underside of the deck, and skipping the gooseneck.
I also plan on a custom dry riser to 18" above the engine (12" above WL) before injecting the water. We will need a custom riser of some kind to resolve an engine access issue, so may as well make it high.

As usual, opinions from the experts on the board will be welcome