Rudder depth

[warren631]

I want to sail on a back bay from Ocean City MD but the depth there is only about four feet and the rudder will probably keep hitting bottom. Can I sail with the rudder lowered only half way down and tied? My rudder is home built so maybe I can shorten it to about the same depth as the outboard motor? Not into competition racing.

What do you guys think?

[NiceAft]

I don’t recommend lowering the rudders halfway; you will be leveraging too much stress on the rudder mounts.

Being a “D”, you have a daggerboard; how deep does that extend How far does it need to extend to significantly affect side slip?

Sailing in four feet of water, wow too shallow for me.

[warren631]

With four feet if I sink I just get out and walk home. A little side drift don't bother me. I like drifting in no hurry. Not going anywhere anyway.

[NiceAft]

Looks like you have a plan.


Do you take the boat with you, or a nice salvage for someone.

[dlandersson]

You have a "D".

I think you should be prepared to break it.

I want to sail on a back bay from Ocean City MD but the depth there is only about four feet and the rudder will probably keep hitting bottom. Can I sail with the rudder lowered only half way down and tied? My rudder is home built so maybe I can shorten it to about the same depth as the outboard motor? Not into competition racing.

What do you guys think?

[Jimmyt]

Shorten the rudder or build a new, shorter one. As said before, it won't behave well when angled back. Mark your dagger board rig so you don't over extend it, use a shorter rudder and go enjoy some thin water.

[Tomfoolery]

Shorten the rudder or build a new, shorter one. As said before, it won't behave well when angled back. Mark your dagger board rig so you don't over extend it, use a shorter rudder and go enjoy some thin water.

Beat me to it. Since the OP made his own rudder, he should make another one. Just don’t heel the boat as far, or rather, it won’t heel as far anyway as it will round up with so little rudder in the water, assuming the rig isn’t perfectly balanced. Which is something to strive for - steering through sail balance.

[Stickinthemud57]

My first though is, how many miles is you shallow water section? I'm calling anything less than 8 feet shallow. Depending on the distance, you might want to consider raising the daggerboard so you only have 2 feet or so below the bottom of the boat, raising the rudder completely, and using your auxiliary motor rather than attempting to sail in such shallow water.

[Jimmyt]

I'm calling anything less than 8 feet shallow.

You're fortunate that you have water like that.

I'm calling anything 8 feet or less, a lot of the places I sail. I'd be doing a lot more motoring if 8 feet was my cutoff. I think my draft with the board fully down is 5'-9" if I recall correctly. Which means I'm good in 6 feet as long as I know the area. However, I don't have my board all the way down except when I'm well out in the bay. You learn to be careful (or you at least learn its expensive when you aren't). Rudders are a good bit shorter than the dagger board on my boat. I've thought about building some shallow draft rudders, also, so I'm curious for someone else to try it first. I have a lot of very thin water around, and I'd rather sail than motor, so I understand what he's after.

Your boat, your rules. You shouldn't ever break anything from grounding, if 8 feet is your sailing boundary. I certainly respect that.

[Starscream]

On our X, we sail in skinny waters a lot. Rudders full down, but not cleated, centerboard at 50%, and as much heel as the sails will give. (The rudders MUST be full-down, otherwise steering is next to impossible and something may break).

Video evidence on the link below . Depth is shown top left on the screen, speed is bottom middle but hard to see. We were flirting with 5mph.

https://1drv.ms/v/s!Ag0w1ybOLvphk4cYDnIC_4FJh-lGwQ

(This is a link to the video on OneDrive...not sure how to embed it into the post).

[Jimmyt]

On our X, we sail in skinny waters a lot. Rudders full down, but not cleated, centerboard at 50%, and as much heel as the sails will give. (The rudders MUST be full-down, otherwise steering is next to impossible and something may break).

Video evidence on the link below . Depth is shown top left on the screen, speed is bottom middle but hard to see. We were flirting with 5mph.

https://1drv.ms/v/s!Ag0w1ybOLvphk4cYDnIC_4FJh-lGwQ

(This is a link to the video on OneDrive...not sure how to embed it into the post).

Those depth numbers look strikingly familiar! Thanks for posting this!

[warren631]

Who is an expert on rudder design?
1. If I shorten my rudder but make it wider to give the same number of square inches of rudder in the water would that work the same as the standard rudder?
2. Do I have to have the same number of square inches on both sides of the vertical 'axis' so the rudder is balanced (same force to turn in both directions)?

[warren631]

3. Also, do I really need an airfoil shape?

[Jimmyt]

Who is an expert on rudder design?
1. If I shorten my rudder but make it wider to give the same number of square inches of rudder in the water would that work the same as the standard rudder?
2. Do I have to have the same number of square inches on both sides of the vertical 'axis' so the rudder is balanced (same force to turn in both directions)?

Not me.

1. Not likely. The depth is a factor. There may be a shallow rudder design that will work nearly as well, but it is very unlikely the design will be as simple as duplicating the surface area. But, since you don't have a test tank and a degree in similitude and model design, using the same wetted area might be a good starting point - if practicable.

2. No. In fact, you don't want to split the rudder foil in half. Think 15-20% in front of the rudder pivot as a likely start point to give you a decent rudder feel and limit effort to an acceptable level. Some "experts" say 18% is the sweet spot for sailing yachts, but I don't have the expertise to confirm or refute this. Upper limit for power boats seems to be around 40% in front of the pivot.

Not sure where you got the idea that "balancing" the rudder means the force is the same to turn in both directions. Putting area in front of the pivot basically reduces the steering effort because water hitting the rudder in front of the pivot creates a force that partially offsets the force of the water behind the pivot. If you make the areas equal, there will be virtually no rudder feel because the forces would be virtually equal. This is true regardless of which direction you turn.

[Jimmyt]

3. Also, do I really need an airfoil shape?

3. No

You can see rudder shapes that vary from almost pure NACA foils to plain rectangles. The rudders on the 26M are close to a NACA 0012. A foil shape is likely better, but a thin rectangle will steer the boat.

I'm assuming that peak performance is not a goal of your exercise here. It appears that your primary goal is thin water sailing. Your rudder draft is only part of your problem. Since your dagger board will be retracted (significantly?), you will increase your leeway under sail - possibly a lot (depending on how much you retract obviously). You could fabricate a dagger board with a longer shallower look to give more lateral resistance if it is a real issue. Unfortunately, this would limit your ability to retract the dagger into the boat - the expanded portion won't fit in the slot...

I would start with some sacrificial prototype rudder shapes and test them. I might round the edges, but I wouldn't waste time trying to give it a foil shape until you find one that gives you decent sailing performance. I might slap a couple of coats of varnish or paint on, so they last long enough to give decent data.

I would go as deep as you feel you can. Maybe go out and get on a broad reach; pull up the dagger until you get excessive leeway; then drop back to a draft you can stand. Mark the minimum acceptable dagger draft. Make that your design draft. Keeping your rudder area equal to the original, and using a smaller dagger area, is likely going to move your center of lateral resistance. You may wind up having to reduce your rudder area to get a decent sailing feel. Think several cheap prototypes before you're done.

Good luck! Please post your progress and results! This is a very interesting topic for those of us who have a lot of thin water around.