# How to calculate midpoint draft depth of a keel/centerboard sailboat

Discussion in 'Sailboats' started by BGstar19, Aug 8, 2019.

1. Joined: Aug 2019
Posts: 6
Likes: 0, Points: 1
Location: USA

### BGstar19Junior Member

I am looking for suggestions on how to calculate midpoint draft depth of a keel/centerboard sailboat.

I am looking to use this to calculate theoretical PHRF base ratings using the Schell Regression Formula.

My sailboat has a stub keel that extends an average of 12" below the hull having a length of about 10 feet. The centerboard extends about 4.3 feet with a 15" width along the center line of the sailboat when fully deployed.

2. Joined: Sep 2011
Posts: 7,376
Likes: 706, Points: 123, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

Could you make a schematic drawing of the point you want to calculate?

3. Joined: Aug 2019
Posts: 6
Likes: 0, Points: 1
Location: USA

### BGstar19Junior Member

4. Joined: Sep 2011
Posts: 7,376
Likes: 706, Points: 123, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

Well, that seems to be a very complicated "point," no wonder you have difficulties.
I guess what you need to calculate is the position of the geometric center of the drift area. That is simpler but you will need to have a well-made drawing of that surface.

5. Joined: Aug 2019
Posts: 6
Likes: 0, Points: 1
Location: USA

### BGstar19Junior Member

Here is a quick excel doc attached I used to attempt to calculate the approximate midpoint draft depth of a keel/centerboard sailboat

What are your thoughts on this?

#### Attached Files:

• ###### Keel-centerboard average draft depth calculator.xlsx
File size:
12 KB
Views:
507
Last edited: Aug 10, 2019
6. Joined: Sep 2011
Posts: 7,376
Likes: 706, Points: 123, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

I don't quite understand how it is intended to be done. It is trying (I think) to calculate the geometric center of the submerged area, projected on the longitudinal plane, but it seems to me that the procedure is very inaccurate. It is reducing everything to trapezes and that is only approximate.
If you have any drawing software you can comfortably obtain much more correct results.

7. Joined: Aug 2019
Posts: 6
Likes: 0, Points: 1
Location: USA

### BGstar19Junior Member

Tansl, as much as I appreciate your input I'm guessing you may be overthinking it.

8. Joined: Sep 2011
Posts: 7,376
Likes: 706, Points: 123, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

You may be right, although, if that is what I believe, the solution is very simple. But the truth, I have nothing clear what can be the "midpoint draft depth of a keel/centerboard". Nor does your drawing in post # 3 help me a lot to calculate anything. Perhaps, so that I don't have to think so much, you should clearly express what you need and use not-so-simple methods to calculate it. If what you need is to find the point where the ship's drift resistance is applied, the matter is very simple and I would not have to think almost anything.

9. Joined: Aug 2019
Posts: 6
Likes: 0, Points: 1
Location: USA

### BGstar19Junior Member

Tansl,
My hope was that there was some mathematical formula using easily defined sailboat measurements to make a determination as to the approximate average draft depth of the keel/centerboard sailboat. Apparently there is not.

10. Joined: Sep 2011
Posts: 7,376
Likes: 706, Points: 123, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

Sorry for my insistence, please, this is the last time I intervened.
There are formulas and procedures to calculate almost everything but my problem with you is that I don't know what "average draft depth of ...." means.
Perhaps the fact that, for the moment, no one else has answered, in a forum as this one, with so many people willing to share their knowledge, it means something.
Good luck.

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.