Additional Minimum Draft when the Rudder is Down

This picture is from the Rudder Craft page, yet it is a picture of “Sassy Gaffer” herself (as confirmed by the license plate) on her trailer at the marina of the Nepean Sailing Club.

Now that “Sassy” is sailing in shallower waters (particularly since this season water levels in her “hunting grounds” seem to be on their way down) the draft determined by the position of the rudder blade, as well as that of the motor, will have to be taken into more careful consideration.

“Sassy” sports a Unifoil Rudder Craft rudder, which given the angle of the transom does rake forward a few inches under the boat. This is not necessarily bad, as it better aligns the center of force of the rudder with the rotation axis at the pintles and gudgeons.

With the rudder and motor out of the water, the draft of the SunCat with the center board up is 1 ft. 2 in. and with the board down it is 4 ft/ 6 in. However, from the above picture, even accounting for the parallax error, it would seem that the long blade of the rudder when down, adds abt. 1 ft. 6 in. to the boat draft with the centerboard up (see purple dotted line).

According to the Rudder Craft page, the Unifoil-A rudder blade “provides a maximum draft of 32 in. when fully deployed”, which corresponds to 2 ft. 8 in, this adding 1 ft. 6 in. to the boat draft with the center board all the way up, and fully agrees with the draft estimated from the picture above.

Roughly, then, it would be healthy to estimate a minimum draft of 3 ft. when this type of rudder is down, even if the center board inside the keel is up. Yes, the kick-up mechanism may spring the rudder up, although given its rake, the foil may fail to clear the bottom and the rudder contraption may be stressed. Hence, it would be more prudent to avoid this situation altogether by observing the new minimum draft.

The Tohatsu has a long shaft, and also will add to the draft if the motor bracket is in its lowest position. However, even in this position the tip of the shaft does not reach the ground when the boat is sitting on it trailer (the tip of the rudder foil does). However, with the motor bracket in the middle position the motor shaft may clear the level of the keel and it may be the preferred position for maneuvering in shallow waters with the blade of the rudder and the center board both up.

I will confirm these draft estimates next time Sassy hauls out of the water.