tunnel drive setup?

[Tom v.d Brink]

lately i was reading up on some tunnel facts to find to my interest the setup of the outdrive.

most, or can i say all, outboard tunnels will have a flat running pad of reasonable length. of course this is a good thing

but now when it comes to setting up the outdrive most setup sheets call for the tunnel to sit on a flat surface

thus this before mentioned flat surface will in my expectation also be flat to the setup surface

now the drive is adjusted to be neutral or minor degrees of negative aoa.

"right" you all must say, that is the way we setup our tunnel.

Now my interest is this. when on the water we all know what a tunnel runs like mostly nose high with a tendancy of the occasional blow over.

so in fact the tunnel will nearly never run at the angle that was setup on the table but effectivly with a much larger negative angle.

is there any loss caused by this or this an acceptable tunnel hull feature?

 

[HTV Boats]

You are correct. Lets say on the flat surface you propellor shaft measures 3 degrees negative and on the water the hull hangs at 2 degrees positive. Net result is your prop is 5 degrees negative to the surface of the water. So move the lower 5 degrees (+2 now) and you may see the hull go to 3 or 4 degrees and now blow off the water. It would be difficult to determine the actual hull angle of attack on the water, but the prop shaft angle on the flat out of the water is easy. Not going to be linear to when its running, and what your really doing is finding where the boat runs best, speed and cornering generally being the goal without comming off the water. Theoretically the prop would be most efficient at a parallel with the surface but reality is different hulls and angles will make the boat work best. In a light model you generally need negative trim to keep it on the water. In full size it may be the opposite as you may run a prop over positive to lift the hull. ( I use a long copper tube the ID the same as the bare prop shaft OD slipped on the shaft to get a more accurate reading)

 

[Jerry Wyss]

Well, this stuff is not simple or we'd all have perfect running boats. The major concern is to achieve the hulls

Aerodynamic AOA at the highest rate of speed and still be able to negotiate a turn somewhere. Neutral thrust

(I think) is chosen for a good starting point, but lead balancing would normally be exhausted first before

band-aid thust compensation takes place. $.02 of that is mine, could be worthless though