# Interaction between hull and transom-hung rudders

Discussion in 'Hydrodynamics and Aerodynamics' started by erdben, Nov 27, 2016.

1. Joined: Oct 2005
Posts: 441
Likes: 25, Points: 28, Legacy Rep: 97
Location: France

### ErwanSenior Member

Erdeben,

Your Induced Drag seems alright , check if you need to consider Oswald coefficient.

I think it is appropriate to start with the righting moment and a polar point (boat speed, True wind speed, and True wind angle.
You get apparent wind velocity and angle
then you can project forces on the apparent wind direction for lift and drag

For the lift coef I use the Von MISES proxy:

Effective Cl(3D)= Cl(2D)(1+2/AR) with CL(2D) = Cl with infinite AR = Cl from XFOIL

the effective Cl(3D)is the lift coefficient corrected with AR

Hope it can help

Cheers

2. Joined: Oct 2005
Posts: 441
Likes: 25, Points: 28, Legacy Rep: 97
Location: France

### ErwanSenior Member

Sorry for my poor English, What does Weather Helm means Please

3. Joined: Nov 2016
Posts: 19
Likes: 0, Points: 0, Legacy Rep: 10
Location: USA

### erdbenJunior Member

Thanks Erwan. I'm not a native English speaker either, so I may not have used the term correctly, but by weather helm, I meant the angle of the rudder to the mid-line necessary to keep the boat from rounding up into the wind.

Generally, it is recommended that the rig is balanced so that you have about 3 degrees angle on the rudder when sailing upwind so that the rudder and the keel shares the lift properly.

I can't really set my boat up that way even though I already sail with a pretty noticeable mast rake. I found a vague statement in an article stating that on boats that sail with a lot of leeway upwind, the optimum weather helm or angle on the rudder is smaller. I don't know if it's just empirical evidence or if there are some calculations that show how the leeway angle affects keel and rudder lift balance. Maybe with more leeway, the centerboard or keel has a smaller effect on the flow that reaches the rudder?

It's also possible that I have a hard time getting the boat balanced, because not only the lateral area got reduced from the boats original daggerboard version, but now the centerboard is also positioned further aft as seen from the previously attached pic. Anyway, it's still a lot of fun to sail that boat, I just enjoy learning more about aero-hydrodynamics.

4. Joined: Oct 2005
Posts: 441
Likes: 25, Points: 28, Legacy Rep: 97
Location: France

### ErwanSenior Member

the more boat leeway, the more angle for both the keel & rudder, that is probably why boat with a lot of leeway have a smaller rudder angle (weather helm).

5. Joined: Jul 2009
Posts: 194
Likes: 10, Points: 18, Legacy Rep: 38
Location: New England, USA

### Tom.151Senior Member

Bill Gibbs did some neat stuff with rudders. He was kind enough to share. Attached is one of the 'testing' versions. By placing a pivot point on the dagger case you can get the rotation moved aft easiy (relative term )

With that accomplished, you can have a simpler blade profile.

Though the addition of a fence at the bottom of the case just below w/l -and- fairing the bottom of the case abobe the fence, still sounds v.good idea

#### Attached Files:

File size:
58.6 KB
Views:
401
• ###### Gibbs_rudder 5.jpg
File size:
284.5 KB
Views:
379

6. Joined: Nov 2016
Posts: 19
Likes: 0, Points: 0, Legacy Rep: 10
Location: USA

### erdbenJunior Member

Very interesting, thanks!

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.