posted 12-31-2006 11:03 AM ET (US)        

Recently it was suggested that a gear case could contribute to a tendency to porpoise if the anti-ventilation (AV) plate were positioned such that it was not running above the water when the boat was on plane. This seems like a reasonable observation. Consider how the AV plate acts in a manner similar to a diving plane. As a motor is trimmed out to obtain bow lift, the angle of the AV plate does become much like a diving plane intended to sink the gear case deeper into the water. If the leading edge of the AV plate hydrofoil engages the water flow it will act like a diving plane, driving the motor deeper into the water. This pushes the stern of the boat down, and raises the bow. These forces may be responsible for inducing some of the porpoising motion that is often seen.

Previously, there have been anecdotal reports that raising the engine mounting height resulted in a decrease in the tendency for the boat to porpoise. However, there was never a particularly good explanation for why such a shift in mounting contributed directly to a decrease in the tendency for the boat to porpoise. I believe the concept of the AV plate acting like a diving plane explains this. These results are consistent with the suggested behavior of the AV plate as described above. As the engine mounting height is raised, the AV plate will tend to run above the water, and in that position it does not act like a diving plane.

In additional, there is another benefit from the higher AV plate location. When the AV plate is positioned so that its leading edge does not engage the water flow, this not only removes the diving plane effect causing the stern to be lowered, but it may produce a new effect in which the stern in raised. The mechanism for this new force comes from the deflection of spray in a downward direction. Water and spray coming off the boat and the leading edge of the gear case itself are directed upward. The water and spray hits the AV plate, and is re-directed downward. This imparts some upward lift to the boat, raising the stern of the boat. This is the exact opposite effect of the AV plate as diving plane, and this is again consistent with the observed effect that raising the engine mounting height helps to reduce porpoising.

The as yet unsolved element of our inquiry is how the design of the gear case could help contribute to a reduction in porpoising. Such a claim is made for the gear case on the new Honda BF90 motor. Unfortunately, Honda does not specify what element of the new design provides this improvement, so it is left to speculation to provide an answer.

One possible element of the AV plate design which could affect the tendency of a boat to porpoise would be the leading edge of the AV plate foil. On the BF90 the foil seems to be quite tapered and to slowly grow out from the gear case. There is also a secondary foil located immediately above the AV plate whose shape follows an inverse patter, that is, this foil has a wide leading edge and then tapers into the gear case as it grows aft. And above this secondary foil is the usual large spray deflector. The introduction of this secondary and reverse-shaped foil may indeed be the new design element to which Honda makes reference as being a contributor to reduced porpoising.

The secondary AV foil is several inches above the main AV foil. It is unlikely its leading edge would engage the water flow and act as a diving plane. However, the secondary AV foil can deflect more spray, adding some lift. Perhaps closer observation of the new gear case design and comparison to existing designs will yield more insight into this behavior.

In summary, I believe that the notion of the AV plate acting as a diving plane if the engine is mounted too low is a good explanation for the general observation that increased engine mounting height tends to help reduce the tendency to porpoise.

posted 12-31-2006 11:36 AM ET (US)            

Now let's look at the new Honda design:

posted 01-04-2007 03:36 PM ET (US)            

If the anti-ventilation plate were doing so, it would divert water up and over the plate (or dive plane), which when running at speed, would shoot water along the plate and outwards from gearcase behind the boat. This does not happen.

I think that this is better explained in that when the engine is trimmed just so, the thrust is applied in such a way as to push the bow up. When the force of the propeller thrust is overcome by the weight of the boat (levered out from the transom), the bow drops down and the process begins again, gaining momentum which evenually devolves into porpoising.

This can be overcome by greater throttle, which in turn, delays the process, or rather, increases the amount of leveraged boat weight that is needed to cause the bow to drop. For instance: I can run my boat WOT with the engines trimmed pretty far out. As I reduce throttle, if I do not reduce the amount of trim, the boat will begin to porpoise.

Similarly, when the motor is mounted too low, the boat becomes like Pamela Anderson riding a unicycle. Since the proplusion is applied low and off-center, the top becomes unstable. Again, the faster you go, the more "stable" the ride becomes, it would be most unstable at idle or slow speeds, but the effect is negated by the bouyancy of the hull in the water when operating at speeds below plane.

DISCLAIMER: Author not responsible for mental images of Pamela Anderson riding a Unicycle, nor for any marital strife that may result from your inability to get said image out of your head for several days.

:)

Dave

posted 01-04-2007 05:03 PM ET (US)            

I think you missed the part of Jim's hypothesis that suggested the problem occurs when the motor is mounted low enough (too low) that the AV plate is below the water line at planing speeds. When that is the case, water flowing over the top of the AV plate *is* deflected upwards and forms the signature rooster tale that usually tells us the motor needs to be raised.

posted 01-04-2007 05:13 PM ET (US)            

What's scarier still is that reading/comprehension was one of the higher scores on my ACT...

Although one is wont to wonder if the AV plate is the problem when the motor is mounted too low, why wouldn't Honda just recommend that the dealer mount the motor higher, rather than design a special gearcase?!

posted 01-06-2007 10:40 PM ET (US)            

In slower planing speeds, viewed from the side, the center of thrust is somewhere in the propeller while the axis of rotation about which the hull is porpoising lies somewhere forward of the stern and along the keel. A line from the axis of rotation of porpoising to the center of thrust lies at a downward angle from forward to aft. With wallowing, the axis about which the hull porpoises can oscillate fore and aft, complicating the problem.

Generally, as the direction of thrust of the propeller lies more parallel to the water's surface the tendency to porpoise is reduced. As the line between the axis of rotation and the center of thrust approach being parallel to the surface of the water, the lever available to lift the bow becomes nil. In practice with transom mounted engines, this desirable condition is impossible to attain. But to approach it, the center of thrust may be raised by raising the engine. (The hydrofoil effect of the horizontal surfaces on the mid section and gear case are not discounted and add to the controls and causes.) Another way to help accomplish this is through the use of a jack place with setback. Of course, it is possible to have a hull that might porpoise due to other problems with its shape. Other subtle effects from shape of the horizontal “bullet” itself may contribute. An example is the up turned trailing edge of the gear housing itself just ahead of the propeller on some OMC gear cases as an example.

With a surface piercing propeller the effect of the horizontal surfaces and the gear housing shape become almost nil because they are not in the water at all. All portions of the gear housing are clear of the water except for the skeg. It is possible to achieve a center of thrust at the water's surface and the direction of thrust may be directed up or down by tilting the engine. Steering is accomplished by an interplay between the skeg and the single blade of the propeller in the water at any given instant. With the direction of thrust parallel with the surface of the water and the line between the axis of rotation of the porpoising moment lying together, the center of rotation of the porpoising should move aft to allow all parts of the hull to be above the water. In this condition, and with sufficient speed, the ground effects of the air under the hull takes over as the major lifting force to keep the hull out of the water. Porpoising can then occur again with the entire hull rotating about the point of thrust. When this occurs, the thrust angle can be adjusted downward which tips the bow down and allow allowing the entire hull to run clear of the surface, occasionally touching the tops of ripples or waves in order to retain control.

posted 01-07-2007 10:33 PM ET (US)            

I like your comment. Most any planing hull can be made to porpoise. What do you think about Jersey Speed Skiffs?

See: http://www.speedskiffs.com/Video/darbyandlyohle.wmv

posted 01-08-2007 05:50 PM ET (US)            

Tom, your right, there is a lot to porpoising or the lack of porpoising, its called balance, and sometimes its hard to obtain.