Rudder longer than keel - too risky?

I have two rudders for my Freedom 21 because I bought a replacement only to realize that the replacement rudder is 6 inches longer than the old one. My guess is that the replacement comes from the fin keel variant; mine is a shoal draft.

Now I have to make the decision to either install the longer rudder or repair the one I've got. I figure a longer rudder is actually not all bad - it would perform better, right? The boat is definitely prone to weather helm in heavy air; I wouldn't mind more control.

But the longer rudder would extend down past the keel, and I'm worried about running aground - it seems like an accident waiting to happen. Maybe I should stick with the shorter rudder but try to improve it? I'm not sure how I would do that - maybe by adding on to the trailing edge to try and increase area and improve the trailing profile?

What would you do?

 

Welcome to the Forum Ian.

What is 'wrong' as such (re the repair work required) with your existing rudder?
Is it that hole that is visible in the second photo above?

Are both rudders effectively the same section shape?

Is your Freedom this one in the link below, with only 2' draft?
If yes, is the rudder the same depth as the keel?
SailboatData.com - FREEDOM 21 SLP SD Sailboat https://sailboatdata.com/sailboat/freedom-21-slp-sd

The 'standard' draft for a Freedom 21 appears to be 3.75' -
SailboatData.com - FREEDOM 21 Sailboat https://sailboatdata.com/sailboat/freedom-21

 

You could just fill the hole in the old rudder with an epoxy/microballoon mix when its dry and go sailing after painting it.For extra protection in future a metal capping would prevent a recurrence.I also have a feeling that a bigger rudder might give a little more control and it might equally exert more force on the tiller because of the larger area.Maybe earlier reefing would be the answer.In short,your concerns about a deeper rudder are justified and if you do a repair to the old one,it will give you the time to prepare the deeper one for a trim and eventual substitution.

 

Yeah, the hole is the problem with the old rudder, plus the pitting in the gelcoat but the replacement rudder is also pitted.

They do both have the same cross section.

And yes, mine is the 2' draft and the current rudder extends to the keel depth.

I think I'm leaning towards just filling the hole on the old rudder and hoping the shop I bought the replacement from will allow me to return it. I'll lose $200 in shipping but I guess that's the cost of learning.

Any thoughts on improving the profile? Or would that probably not be worth the effort?

 

I'd just fill the pits before anti-fouling,but thats just my way.Unless the refund is substantially more than the cost of shipping,it might be worth hanging on to the replacement-ready for next time.

 

I had a boat that needed nearly 30 degrees of rudder when pressed enough to heel 30 degrees. I felt that meant a rudder with not enough authority, but for other reasons chose not to keep the boat before deciding which of the possible fixes to implement. Therefore the following is based on theory, not personal experience.

For your rudder, four options for increasing control come to mind, which could be combined.

1) Make it an inverted T-foil. Bolger wrote that once he had put fences on the bottoms of rudders (the examples he showed had much lower aspect ratios than yours, intended for boats with very shallow draft), he included them routinely because they made rudders that much more effective. I have seen the rule of thumb that putting a fence at the end of the wing has about the same effect as extending the span by the same distance. I suspect that rule of thumb works for modest additions, and that there would be diminishing returns at some point. You may get similar increase in effectiveness as the longer rudder without increasing draft. The potential drawbacks are that either the horizontal part stalls when the boat pitches in a seaway and adds more drag under those conditions than the longer rudder would. You could make the horizontal part long enough to dampen pitching so much that the foil never stalls, but that might be big enough to add so much thrust load along the rudder axis to the gudgeons, pintles, and transom that you would need to reinforce the lot.

2) Add a flap to the trailing edge of the rudder to make an articulated rudder like this (note that it also has a bit of a fence at both top and bottom):


I think the flap could be a slotted flap. That would need no modification of the rudder's trailing edge. However, I have not found an example of a slotted flap articulated rudder, so I may have overlooked something.

The drawback would be that the centre of effort moves towards the rudder's trailing edge, partly because you add more area behind the rudder axis and partly because of suction when the flap deflects. You would have a heavier helm, unless you moved the rudder axis further back. That would require cutting a strip off the upper part of the rudder, then reinforcing the rudder not just to get back to the original strength, but more, because the greater area and greater maximum lift coefficient will increase the total load.

From: MIRIAM gets a new rudder! https://ptshipwrights.com/2018/miriam-gets-a-new-rudder/

3) Reading about windvane self-steering, I came across the concept of control volume, which is the area of a control surface multiplied by its distance from the turning point. So if you built a gantry off the transom like the International Moths use, the same force on the rudder would create more torque thanks to the greater lever arm. The boat might then exceed the length allowed in its berth or on the trailer.

4) Cut off the new rudder to the same length as the old, and mount them side by side as twin rudders. That might well be the simplest solution.

 

What angle of heel are you sailing at? Many (probably most IMHO) "weather helm problems" are caused by the boat being heeled too far with the mainsail oversheeted. Look at a Laser fleet with widely different abilities in strong winds and one will see the leaders carrying very little weather helm, because they are sailing flat, while the back of the pack is struggling with weather helm as they heel to 30 degrees.

I appreciate this may not be the case with a Freedom 25 because catboats are prone to weather helm.

 

A Laser is not a catboat? (I’m glad I got to ask this - is there a nomenclatural bright line?)