Consequences of shortening rudder

[MGRodems]

We have a newly acquired 1988 Irwin 43 MkIII that has a 4"11 draft (wing keel) and a partially-skegged rudder that also has a 4'11 draft. Hopefully the attached image helps show the rudder well enough to help with any advice you may have.

This is our 5th boat and we are retiring next year to spend a substantial amount of time on her. Currently we are in the Great Lakes where draft has not been much of an issue, and running aground is not so common. We intend to head south and cruise the intercoastal and bahamas where running aground is basically inevitable.

We lived in Annapolis for 10 years and sailed the Chesapeake quite extensively. Then, I had a fin keel and always was able to maneuver off a shallows by pivoting the boat, heeling her, or both. With the Irwin, the wing keel is going to sit on the bottom or dig in, and if the rudder will also be in the sand or mud. I will not only be unable to reduce draft by heeling the boat, but I will not be able to turn and pivot the boat very easily if the rudder is also stuck on the bottom.

This brings me to the idea that if I shorten the rudder by about 6" and round the leading and trailing edges, I would be able to more easily pivot the boat to turn towards safety. I calculated the effect of making this alteration on the surface area of the rudder to be about 7 to 8 percent of the lateral area of the rudder. The rudder is about 18" fore to aft at the bottom, but up to 30" fore to aft higher up. I realize the steering effect at the bottom of the rudder is a bit more effective than higher up, but I am hoping the reduced size of the rudder will be offset by the apparent benefit of not running the rudder into the same shallow bottom that the keel is sitting on.

Any thoughts would be appreciated.

[Snore]

My bar napkin engineering says this would move the center of lateral resistance forward. To avoid having issues with the boat rounding up, you would need to go with a smaller headsail. This would move the center of effort forward.

[Bill O]

Would discuss the idea w/a naval architect who has done a fair amount of work w/under bodies, specifically rudders.
Could also do a search for rudder design papers to see if your idea is feasible.

We did both of the above before redesigning our rudder, but we went longer not shorter. (still wasn't as deep as the keel)

[MGRodems]

Bill:

Thnks for the reply. Maybe I'm off, but wouldn't a larger headsail move the center of force forward?

[Jim Cate]

I'm no NA, but wouldn't such shortening of the rudder simply mean slightly larger deflection (ie angle of attack) to achieve the same turning moment? Does not seem like a situation where one is concerned with changes in CLR or such.

One thing that might change is the amount of helpful balance area being reduced, with some increment in effort in steering. Offhand, I'd not think it a big deal, though.

And I'd also think that no change in sailplan would be required. Perhaps minor changes in how you trim, but that'd be about all.

Jim

[Sherpa17]

Considering the bowsprit, an anchor and a snatch block or two, you can probably pivot without modifying the rudder.

[dfelsent]

I’d also be concerned about degradation of rudder authority in reverse.

[wingssail]

Originally Posted by MGRodems

We have a newly acquired 1988 Irwin 43 MkIII that has a 4"11 draft (wing keel) and a partially-skegged rudder that also has a 4'11 draft. Hopefully the attached image helps show the rudder well enough to help with any advice you may have.

This is our 5th boat and we are retiring next year to spend a substantial amount of time on her. Currently we are in the Great Lakes where draft has not been much of an issue, and running aground is not so common. We intend to head south and cruise the intercoastal and bahamas where running aground is basically inevitable.

We lived in Annapolis for 10 years and sailed the Chesapeake quite extensively. Then, I had a fin keel and always was able to maneuver off a shallows by pivoting the boat, heeling her, or both. With the Irwin, the wing keel is going to sit on the bottom or dig in, and if the rudder will also be in the sand or mud. I will not only be unable to reduce draft by heeling the boat, but I will not be able to turn and pivot the boat very easily if the rudder is also stuck on the bottom.

This brings me to the idea that if I shorten the rudder by about 6" and round the leading and trailing edges, I would be able to more easily pivot the boat to turn towards safety. I calculated the effect of making this alteration on the surface area of the rudder to be about 7 to 8 percent of the lateral area of the rudder. The rudder is about 18" fore to aft at the bottom, but up to 30" fore to aft higher up. I realize the steering effect at the bottom of the rudder is a bit more effective than higher up, but I am hoping the reduced size of the rudder will be offset by the apparent benefit of not running the rudder into the same shallow bottom that the keel is sitting on.

Any thoughts would be appreciated.

I would be surprised if the 6" is going to make a lot of difference in your capability for getting away from a grounding. You are not, after all, sitting on a flat table top when aground, and then there is the balance, fore and aft, of the boat and wave action, but maybe it will be helpful.

As for the effect on the boat's handling I think you will only notice it in higher wind speeds where there is significant weather helm issues. Then you may notice reduced power of the rudder. The rudder was designed to be efficient in all conditions and particularly when more power is needed, I.E. in stronger conditions. In normal conditions I think you wouldn't notice the difference.

If you find that you just don't have enough rudder to overcome weather helm than a further modification might be needed: add area without going deeper. I mean add some extension to the forward edge and the aft edge.

Of course the most effective rudder (and keel) is a deep one with high aspect ratio, so you will be going the wrong way.

But this is all guessing from an uniformed source.

The advice to consult a naval architect is good advice.

[AKA-None]

It also moves the rudder out of the cleaner stream of water surrounding the hull

[knot smart]

I'm no expert. But just shooting from the hip, I'd say when you're heeled over the boat will "round up" sooner. . My basic understanding of the physics of sailing is : when the boat is heeled over so much that the rudder isn't deep enough in the water, the boat rounds up. So., Extrapolating that concept, the boat will sail just fine, as long as you're not heeling.. however, what's sailing if you're not heeling?

The juice isn't worth the squeeze.. I wouldn't sacrifice the performance.

It wouldn't be the first or last time that the dingy is deployed to push, pull , shove, swing myself off a sandbar

Just my humble opinion

I look forward tohearing other opinions on this

[jtsailjt]

I think it will be either an expensive or time consuming project with little benefit. I found as long as I paid attention in the Bahamas so didn’t go real hard aground, the sandy bottom is soft enough so I could simply back off or in some cases power through it to deeper water. Water is clear so it’s common to see “boat tracks “ in shallow harbor entrances. Your winged keel is somewhat worrisome but it’s so flat in the Bahamas that as long as you don’t go aground at absolute high tide, you’ll soon float off and be on your way, on the next tide at the latest.

[MGRodems]

Thanks for all of the good points. One thing I forgot to mention. I apparently bumped the leading edge of the rudder on something and it has a small repair needed already. That being the case, I would not be adverse to adding a little more effort to get what I want.

I also have rebuilt the rudder on a previous boat (adding material to the front edge to provide a balanced rudder when it was originally designed to pivot on the front edge). The result was much better handling.

Finally, I would like to know if the impact on the AutoPilot would be affected. I assume there would be a little less effort to move the blade, but maybe more often? Not sure.

[Shrew]

You are attempting to solve a fictitious or, at best, a POSSIBLE problem by introducing a potentially real and permanent one. Why would you want to potentially deal with any reduction in rudder responsiveness and pointing and weather helm just to avoid potentially sailing into skinny water??

Updated electronic and paper charts, an accurate depth sounder and conservative navigation will resolve the proposed issue with no potential impact on overall boat handling.

[Bill O]

Originally Posted by Shrew

You are attempting to solve a fictitious or, at best, a POSSIBLE problem by introducing a potentially real and permanent one. Why would you want to potentially deal with any reduction in rudder responsiveness and pointing and weather helm just to avoid potentially sailing into skinny water??

Updated electronic and paper charts, an accurate depth sounder and conservative navigation will resolve the proposed issue with no potential impact on overall boat handling.

+1. If the boat is a somewhat new to you, would sail it until you have an issue w/the rudder if you go aground. Totally agree you could be creating more of an issue by proactively shortening the rudder before you know it will be an issue.

Think you will have more difficulty getting off w/the wing keel after a grounding than any addition the rudder will add to the situation.

Can always wait for the tide to lift you off if you do go aground. This will be much easier on the boat and you than trying to use xs force to get it off.

We look at the charts more/don't take as many chances w/our 6.5' draft as many do w/shoal drafts. While we do occasionally go aground going up small creeks, we have seen many more shoal draft boats hit hard (presumably because they don't look at the charts as often).

[Tetepare]

Our rudder is also the same draft as the keel, and when I grounded on rock it took a beating. I’ve often thought about shortening it, but I’m sure it would negatively impact high-heel sailing and already iffy reverse. Each boat will respond differently.