We sail a Taswell 43 monohull sailboat, with a Yanmar 55hp engine turning a 20" diameter Autoprop. After getting the engine rebuilt, she now gives us full RPM (3600)on demand. But we have discovered a problem in the process. At normal cruising RPMs(1800-2400RPM), the stern "tucks" approx. 7" putting the engine exhaust port (transom mounted) just underwater, and coating the lower transom with a black sooty smudge that's difficult to get off. But...as we increase power, the stern tucks more and more... up to 22" from the still water line, putting the transom almost half way under water! And that coats the whole transom area with the smudge.

The smudge/blackened transom is one issue, but the excessive "tucking" is something else. We're stumped!

Autoprop says the prop is the proper size! I can't believe that a 22" tuck (under full power) is normal....but I don't know what the issue could be. We've unloaded and reloaded the aft end of the boat-without change. Any ideas????

Thanks

The maximum speed of a full displacement hull is determined by the length of the wave created as the hull passes through the water, with a crest at the bow, another at the stern and the trough midway between them. The yacht is essentially "pushing" this wave across the surface of the water. In relatively deep water where the effect of depth can be ignored, the maximum speed that a wave can travel is a function of its length, the distance between crests and can be calculated as v = (gL/2Pi)^.5. (g= the acceleration of gravity and L=wave length)

This formula roughly translates into the familiar equation for maximum boat speed of 1.34 x (Square Root of the Water Line Length). The water line wave simply cannot move any faster, regardless of the amount of energy expended. What the energy in excess of the amount used to reach the maximum wave speed can do is increase the wave amplitude, the height between the bottom of the trough and the top of a crest. About the maximum wave amplitude that can be supported is about 1/7th of the wave length or a peak angle of about 120º from face to face of the wave.

On your boat, the waterline length is reportedly 35.33'. The maximum speed a wave of that length can make is about 7.98 knots. Beyond the energy needed to get there, you just increase wave amplitude as noted about. And, at 1/7th the wave length, the maximum wave amplitude that can be supported is about 5.05 feet or about 30 inches above and below the mid-point between the trough and the crest or, in general, the level of your water line. Push the boat a little harder and, depending upon the shape of the forefoot, the boat might "climb" up the back of the bow wave somewhat, dropping the stern down the face of the stern wave, or squatting, by an equal amount, giving the yacht a bow up attitude. Pumping more energy into the sea will just exacerbate the situation but not increase your speed unless you have so much energy that the boat "planes" up and over the bow wave (which a Taswell 43 isn't likely to do) in which case wave length ceases to be a determining factor to speed.

Others have given you a means of determining the amount of energy necessary to move the yacht through still air and water-RPM verses speed. With RPM and propeller characteristics, one can determine the amount of thrust the engine is developing but the speed the yacht can make under such conditions is not the determinant of how much thrust you might need or be wise to have in your back pocket as sea and wind conditions may have to be overcome by the thrust of your power train before you even begin to think about limiting speeds.

In short, based upon the foregoing, it doesn't seem like you have a problem...eh?