A lot of single engine Velocitys had them mounted horizontally and inboard above the knotch in the transom.

Typhoon,
What do you think about Spyder's set up? Thats how I was thinking of mounting them......

You are looking at probably the original "K Planes"

http://imageshack.us/a/img208/3255/unne.jpg

From the people that I have had help me set my boat up and i might add they have many world championships under their belts so I sure they know what they're doing, parallel with the water because when running at full speed and on the outsides of the transom they will be much higher out of the water as typhoon said. and the water is not coming off the boat at an angle so you don't want your tabs pushing at an angle (side to side) slowing you down. The red line shows where the boat rides and that's how the water exits the bottom of the boat when running so that how the tab needs to sit. but you have to run longer tabs as well.

I plan on installing mine horizontal to the water and in on the lifting strake.
I'm using 280's on a 24-7 pantera.

Parallel to the hull and you may as well be throwing a parachute off the back when you actuate them downward.

Glad I'm not the only person who drives like that! I trim my boats the exact same way. Once the drive is trimmed neutral (or close to it) I use the tabs to manipulate the attitude of the boat given the water conditions more so than the drive angle.. In a following sea I'll even run a little more positive drive trim and control the bow with my tabs. Less barking of the drives, much easier on the equipment, much more controlled exit and re-entry.

To answer your question. The K-Planes on my Scout are mounted traditionally parallel to the V. Those tabs make a huge difference in the ride of that boat. Personally, I couldn't imagine not having them. I'm not sure if it would make as big a difference as an outboard Scout but on my I/O Scout I highly recommend them!

In my situation, if I need tabs at higher speeds it is to keep the boat running flat in rough water while still using neutral or positive trim. Negative trim does not have the same effect and can get you in trouble. If I am running in water rough enough to require tabs then I will not be running flat out so a little drag does not matter. This is how my little Donzi 22 came from the factory and it works great. I do not need anything bigger.
http://i763.photobucket.com/albums/x...s086Medium.jpg

I cant see for sure from the pic but I bet it has an extension box other wise you most likely couldn't turn the drive with out hitting the tabs. You might run the risk of the tab hitting the water being that low in the transom. I also like the idea of them being further apart with the concept of them also working like training wheels if you are chine walking from side to side. These are just my opinions and Im sure there different than many others.

We boat in the south of England in fairly rough waters, prevailing wind is normally south westerly which means i usually have to run with one tab partially down to make the boat run level and i have found that if the tabs are parallel with the V as you extend the starboard tab it creates so much drag you have to steer to port to maintain your course, also the speed drops of dramatically. With them set parallel to the water they don't cause anywhere near as much drag or unwanted steering, i have also found the longer the tab the better as you don't need as much angle on a long tab.The tabs in the pictures are carbon fibre and are 8" longer than mec 380's with a slight upturn on the end to reduce drag they will have a carbon fibre reinforcement bonded to the top and the cylinders will operate almost vertically so the leverage isn't trying to pull them of the transom.

Peter