Originally Posted by GordyP
Going out tomorrow night with the usual load of divers and kit to see how she goes, good point on the vent plate, I'll have a check and see how she runs with weight on board. The engines have been set up so that the vent plate is in line with the bottom of the transom. Props that are currently on are ali, I want to make sure that I order the right stainless props!!
To be honest I dont think you will see much more speed by dropping down in prop size however you are currently lugging the engine at 5000RPM so you do need to drop the prop size.
If your plates are currently level with the hull you may well be able to raise the engines at least 2 holes which could gain you 100-200RPM. A lot depends upon your hull and what props you end up getting. I would be tempted to go for 20" Vipers and see if you can also gain some RPMs by raising a couple of holes. Its a big decision to get right as the props are not cheap. If you wanted to borrow a 19" RH prop to try you are welcome to borrow mine then you just need to get hold of a LH one from someone else
(assuming you are running contra rotating engines of course)
I have posted a question on the US based E-tec owners forum to see if any of the US or Australian guys who rig hundreds of twin installs of these engines can add any ideas.