Ballasted Rudder

A Sandpiper S565 is a micro-cruiser 5.65m LOA. It has a retractable daggerboard keel on a lead screw. The keel rectracts completely up into the table inside the tiny cabin, so the depth of the keel has a practical limit of 2'1" below the hull. Extending the keel length would require increasing the table height, etc. To leave the keel protruding below the hull when fully retracted would take from the little boats ability to be beached or trailer-launched in shallow water.

The keel includes 300 lbs of lead ballast. A few Sandpiper skippers have added weight into the keel to increase the righting moment. But, this is a significant project in the backyard without a boat hoist.

The winter project for this off-season is to redesign and construct a new, higher aspect rudder.

Why can't ballast be added into a rudder. It reaches just as deep as the keel. I have heard the dialogue about keeping weight out of the ends of a boat to prevent hobby-horsing. But, could a wing on the rudder not damp the pitching? And reduce the drag?

The rudder's design spiral is tightening so I need some thoughts fairly soon.

 

It's not impossible, but here are some issues you will have to contend with:
1. Your boat will be trimmed stern down. Whilst it is fairly straightforward to calculate the change in trim, it will be pretty tough to predict the impact on boats speed, handling and motion in a seaway - so you will be taking a risk that the result is acceptable. A wing may reduce pitching, but won't do anything for yaw.
2. You will have much higher loads in the gudgeons, pintles and transom and will almost certainly need to strengthen these parts.
3. A ballasted rudder will magnify the effect of any imbalance in the system.
4. If the rudder is detachable, it will be fairly difficult to handle and lug about.
5. The aspect ratio of the rudder and dagger board should be roughly matched.
How much ballast are you thinking of adding?

 

bad idea

Not to rain on your parade but...
That is a transom hung rudder? Ballasting the rudder would likely add minimal stability unless you put significant weight in it. Then you would:
-add extra load to the rig for which it was not designed
-add extra load to the transom, pintles, gudgeons for which they were not designed
-add load to the tiller, making it harder to steer
-trim the boat by the stern
-tend to twist the hull, the structure might not bear it well.
-make the rudder a pain to deal with, ie removal.

 

A Sandpiper dressed in cruising mode typically has a 5hp outboard hanging off the transom, and has a full tank of gas and a 12-volt battery in the lazarette. So in that context, if the ballasted rudder weighed approximately the same as an outboard motor (50 lbs) and the transom was reinforced to suit, the trim of the boat would only be marginally worse than it already is.

How do I add 50 lbs at the bottom of a rudder?

 

50 pounds is not that much lead. You could dig out a hollow in the rudder and cast it in. You could also cast a 50 pound lead shoe that bolts to the rudder, it could act as an end plate to reduce tip losses and it would be removable if you don't like the way it changes the boat.

The bigger question in my mind is what gain would there be? 50 pounds at the bottom of a rudder won't have a very large effect on the VCG of the boat or it's righting moment.

Changing the area and or the AR of the rudder may have a great effect on how the boat sails. How have you evaluated the effect of both changes? How does the change effect CLR? How does the change effect RM?

 

Good points Randy. Your post will keep mine short. Such an insignificant amount of weight (compared to the main ballast), will have minimal effect on stability, in spite of it's distance below the CG. One additional point should be considered, the turning moment formed by the additional ballast being so far from the CLP. In a heeling situation, the new CB (now shifted aft slightly) will create a longitudinal couple between the main ballast and the weighted rudder, attempting to lever the stern. I would suspect ill handling as a result without careful study and implementation.

 

All the responses had the typical negative bias one would expect from this forum. But, there were enough valid points that I will not attempt to ballast the rudder. I just had to ask.

You see, a great deal of effort and/or expense will go into keeping the rudder as light as possible. When on the other hand, with little effort or expense, the rudder could have been made much heavier. I even considered ferro-cement construction for the foil. The additional 50 lbs would have been a 17% increase in ballast and that could be considered significant on this fat little boat.

Instead, I will keep a couple cases of beer on the cabin sole next to the keel.

 

Typical negative bias?
The beer idea sounds good!

 

Sorry for dragging this topic up again.

In a discussion about light, high aspect ratio rudders with full NACA sections, an experienced sailor commented that if they are too light, they are hard to sink onto their pintles. They want to float off unless rigidly attached.

So, will my transom become more bouyant? Will the rudder affect the trim of the boat? Will the bouyant rudder act to reduce the boat's righting moment? Are there examples of rudders that have been modified for neutral bouyancy? Is positive bouyancy prefered? Is this a real problem or a perceived problem?

 

I think you will want near neutral positive buoyancy in the blade, so with headstock attached it has slight negative buoyancy. This minimizes the weight while holding the assembly in the pintles and allows a kick-up rudder to rotate up from it's own buoyancy.

I think you would be hard pressed to feel much difference at the tiller unless the rudder was very heavy (solid SS) or very buoyant.

 

So, no need for high-tech foam and carbon fibre. The new rudder should weigh at least the same as a bucket of water.

Ideally, the new rudder should be as light as possible near the top and heavier near the bottom. Oh, oh. Isn't that where this thread started?

 

what about beefing up the hardware that supports the rudder? added wieght = added strain on your facetts?longliner yes I know you want to keep it the same ,,weight ,but transfer or extend the weight to a further point.