Even with this design, the original wing needed to be reduced in
area.
Sometimes there is too much lift and spoilers are needed to
kill it. Mike Bontoft’s sport hydro blew off at around 75 mph in
its original configuration. Adding spoilers allowed over 100
mph speeds.
The solution to wing stability in full size hydroplanes has
been a tail wing. This should lift the transom by increasing rear
wing lift as the boat’s angle of attack increases. Even with a rear
wing, blow overs can still happen. In models, our experiments
with a large wing didn’t show as much effect as forward
sponsons. Forward sponsons are loaded by the wings lift.
Bouncing the sponsons up increases the boat’s angle of attack,
increasing the wing’s lift. Lifting the wing drops the sponsons
back into the water.
Outrigger hydros are designed to reduce the wing size as
much as possible. Any extra lift can make the stability issue
worse at their speeds. There is little air lift on the center
section, so blow overs usually don’t happen.
Now the moment has decreased a little to -.071, but the lift has
increased to 1.211. This means that the lift has increased and its
center has moved forward. This is the start of the classic blow over.
You can see a strong negative pressure at the leading edge coupled
with the strong positive pressure on the bottom of the airfoil.
We can gain a lot by putting a wing close to the water. Not only
does the airfoil’s lift increase, but the drag also decreases. Even
more important, the airfoil’s lift reduces the lift carried by the
planing surfaces. Remember, even if these surfaces had as good lift
to drag ratio as the wing, the drag in water to carry the same weight
would be 800 times as much.
The problem with wings is stability, especially in model boats
where we have lots of power compared to full size boats. You can
have too much lift in light, high power boats. It requires a careful
balance between the lift from the planing surfaces and the wing.
Mike Bontoft’s outboard hydro is an example of a large wing
balanced by forward sponson planing surfaces. The wing really
helps carry the weight of the outboard.
PROPWASH
16
April 2017
Increased angle in ground effect
Flow field with increased angle
Mike Bontoft’s record holding outboard hydro
Mike Bontoft’s record holding sport hydro with spoilers
