Moving Rudder Position

[CMNZ]

I am completing the building of a Simpson 11m which has been lengthened to 11.6m by adding sugar scoops. I want to move the rudders further aft from the original design but it will mean that the tip of the rudders will (using existing design - lengthened to achieve same depth) extend above the waterline.

Is this likely to cause any deleterious effects and if so should I modify the top of the rudder design so that it is entirely below the waterline and how far? This will mean the rudder is no longer "hugging" the bottom of the hull.

Below is the original plan showing the original waterline and I have drawn on where the new waterline will be (approximately) when the rudder is moved aft. the circle indicates the amount of rudder likely to be above the WL.

[psk125]

Won't moving the rudders aft change the center of lateral resistance of the boat and mess up the balance with the CE of the sails? Having rudder area out "in the air" above the waterline should not be a problem. Traditional catboats, for example, have huge areas of their rudders up out of the water. Leaving a huge gap between the bottom of the hull and the top of a skeg-hung rudder WILL be a problem, however. Flow over the top of the rudder (in addition to the flow that already happens over the bottom end) will reduce the rudder's effectiveness considerably. This is why rudders are designed to hug the bottom of the hull in the first place. The problem moving your rudders back will be having enough planar area in the water for them to be effective. You will need to make them bigger (deeper) in order to work as well as they were originally designed. This will mean changing the skegs accordingly too. Besides increasing drag, this is going to change the center of lateral resistance of the hulls and possibly mess up the boat's sailing characteristics. This is the sort of thing you pay a Naval Architect to find out about before attempting, in order to avoid expensive mistakes. If you're only adding sugar scoops, why are you wanting to move the rudders and create all these problems for yourself? The scoops are not going to change the center of lateral resistance because they're out of the water. You should be able to leave the rudders where and how they were originally designed.

[Franziska]

Yes it risks throwing your boat out of balance to some extent, but not extremely, if your rudder surface is not very big compared of the rest of the remaining aft underwater lateral surface.
Don't move it to far.

Best would be to draw two sail plans with underwater to check how far the areas centers move.
If these area(!) centers do not move much than it won't be an issue.

You can take care of this by furling the Genoa a little later or letting her have a tad more pressure than before.
Providing your scoops are to the very large extent above DWL when slightly heeled.

You can reduce the effect of having a surface piercing foil with a "fence" at the upper end.

[CMNZ]

Originally Posted by psk125

You will need to make them bigger (deeper) in order to work as well as they were originally designed. This will mean changing the skegs accordingly too. Besides increasing drag, this is going to change the center of lateral resistance of the hulls and possibly mess up the boat's sailing characteristics. This is the sort of thing you pay a Naval Architect to find out about before attempting, in order to avoid expensive mistakes. If you're only adding sugar scoops, why are you wanting to move the rudders and create all these problems for yourself? The scoops are not going to change the center of lateral resistance because they're out of the water. You should be able to leave the rudders where and how they were originally designed.

Thank you for the responses.

The reason for moving the rudders aft is that they will be much easier to mount and fit hydraulic rams to without compromising internal space. As noted I will need to lengthen the rudder and skeg to achieve the same depth. I could probably slightly shorten the rudder width to achieve the same underwater surface area. The new rudder position is 500mm further aft and about 130mm higher.

I inherited the hull and decks finished to this stage and the previous builder had intended to move the rudders making it very difficult to put them where originally designed. Additionally if I was to try and put them where originally designed I would not be able to fit them until the boat was completed as there is insufficient space below the boat for the rudder and extended stock. I don't want to be fitting rudders when it is hanging from a crane before launching and find there is a problem with fitting.

[GRIT]

We moved our rudders aft about 2 feet, 60cm, to allow for kickups. Our designer didn't mind at all, and thought it was a good idea. One of his similar designs has similarly placed rudders, with the daggers in the exact same place as ours.

I really don't think moving them aft has any deleterious effect, as you get more leverage. Our kickups are a foot deeper in the water than the original rudders, and do not have an end plate effect with the hulls, as they do not end at the hulls.

I can tell you they work fine, steering the boat with the autopilot at up to 22 knots so far, while surfing against the current in the gulf stream.


If you're going to modify the rudders at all, install some small winglets on the rudders near the waterline to keep the flow attached to the foil going in a horizontal direction.

Cheers.
Paul.

[Fore and Aft]

CMNZ I have done transom extensions on a number of cats and mono hulls and never moved the rudders. The owners never reported any difficulty steering.
Grit I think you missed the . in your post. 2.2 knots sounds more believable when you own a floating condo!
Cheers

[GRIT]

Originally Posted by Fore and Aft

CMNZ I have done transom extensions on a number of cats and mono hulls and never moved the rudders. The owners never reported any difficulty steering.
Grit I think you missed the . in your post. 2.2 knots sounds more believable when you own a floating condo!
Cheers

An interesting comment. That's the first time GRIT's been called that. Should I feel offended?

Horses for courses.

I think you have missed the fact that he needs to move the rudders due to construction choices made by the builder, and is asking if it will cause issues with balance.

Cheers.
Paul.

[CMNZ]

Originally Posted by Fore and Aft

CMNZ I have done transom extensions on a number of cats and mono hulls and never moved the rudders. The owners never reported any difficulty steering.

Cheers

Actually that was not the point. I want to move the rudder.

Thanks
Craig

[CMNZ]

Originally Posted by Franziska

You can reduce the effect of having a surface piercing foil with a "fence" at the upper end.

Franziska , do you mean winglets like Grit suggested?

If so where would you put them and what orientation i.e. parallel to surface or parallel to hull and how far below WL?

[GRIT]

I hadn't noticed Franziska's post. I said the same as she did.

You want them parallel to the water, maybe 15cm below the surface. By the time you're sailing at any speed, the stern will sink enough to make it so they're another 5-10 cm deeper.

The thing about rudders, and balance; rudders have a variable effect on balance, since the angle of attack varies.

Cheers.
Paul.

[wingssail]

Your concern is whether or not adding area to the rudder so that it remains close to the hull and it was brought up that moving the rudders aft will effectively change the center of lateral resistance, moving it aft, and possibly affecting the sailing balance.

If I was doing this I would draw the sail plan and calculate the change to the center of lateral resistance. I doubt if it will be significant. But additionally, since the rudder will have more area you could make the rudders have a higher aspect ratio (by reducing the width) and keep the rudder the same size (in square footage).

I don't know how the skeg affects it, but normally a higher aspect ratio rudder will be more effective and take less effort to turn it. All pluses.

And if you did upset the sailing balance then it seems like a slight additional rake to the mast to move the CE correspondingly aft would bring this right again.

[Matt Johnson]

Ours are also 50cm aft from the normal position of the fixed rudder. The aft kick-up rudders are longer, but I thinks it's more to do with no end plate than CE.

Matt

[CMNZ]

Wingsail, sounds sensible, thanks,

Grit/Franziska

If I were to put winglets on would you suggest something along the lines as below? A or B or something else?

Thanks
Craig

[GRIT]

Originally Posted by CMNZ

Wingsail, sounds sensible, thanks,

Grit/Franziska

If I were to put winglets on would you suggest something along the lines as below? A or B or something else?

Thanks
Craig

Not like that. Franziska used a better word. It's an endplate you need, just a flat piece from front to back, about 5-8cm wide.

You want to slow the upward flow of water, and keep it flowing horizontaly. As Matt said earlier, a deeper rudder will also help to compensate for the upward flow of water. Upward flow doesn't produce lift, and rudders work on lift. Our designer also chose to make the rudders deeper, when we chose a rudder without an endplate/hull on top.

[Fore and Aft]

CMNZ I see your fitting hydraulic steering so you are not going to get any helm feed back compared to a cable system. So I would think that any extra load on the rudders is not going to be noticed by you if you move them aft.
Grit I was referring to the other thread "Don't believe everything you read"
Cheers