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Hull drag calculation guidance

Juancorrea

New member
Joined
16 Sep 2020
Messages
1
Hi. I am looking for advice or help on a hull drag calculation. As we are on a conceptual design and this data will be so helpful to size the required propulsion horse power and related appendages. TKS Juan
 

Laminar Flow

Well-known member
Joined
14 Jan 2020
Messages
1,015
Location
West Coast
Hi Juan,
I've just been through the process myself to try and quantify the "improvements" I made to our hull. In it's simplest form, you separate frictional resistance and form or wave making resistance and calculate them separately.

For frictional resistance you will have to calculate the submerged surface area of your hull and appendages and factor in the correct Reynold's number for a particular speed.
Form resistance is invariably based on displacement.

I highly recommend "Principles of Yacht Design" by Larsson/Eliasson to help with this. The book also includes, in some detail, the necessary formulas, graphs with the appropriate Reynold's numbers and an overview of other types of resistance.

There is also a much simpler method that uses the frontal projection of a hull in sqm, multiplied by the speed in m/sec x a factor of 0.4 I have found this to be far too inaccurate an approach, in my case, by about by 20%.
Edit: last formula requires further component: specific gravity of (sea) water, of course.
 
Last edited:

TernVI

Well-known member
Joined
8 Jul 2020
Messages
3,045
Hi Juan,
I've just been through the process myself to try and quantify the "improvements" I made to our hull. In it's simplest form, you separate frictional resistance and form or wave making resistance and calculate them separately.

For frictional resistance you will have to calculate the submerged surface area of your hull and appendages and factor in the correct Reynold's number for a particular speed.
Form resistance is invariably based on displacement.

I highly recommend "Principles of Yacht Design" by Larsson/Eliasson to help with this. The book also includes, in some detail, the necessary formulas, graphs with the appropriate Reynold's numbers and an overview of other types of resistance.

There is also a much simpler method that uses the frontal projection of a hull in sqm, multiplied by the speed in m/sec x a factor of 0.4 I have found this to be far too inaccurate an approach, in my case, by about by 20%.
I second the book recommendation.
Any others people care to recommend?
 

Soton sailor

New member
Joined
19 Sep 2020
Messages
8
It’s a very complex issue as it will depend on a whole range of factors - including the heel angle, leeway angle, waves, hull form and appendages. Whilst the skin friction is relatively simple to predict (albeit with a lot of assumptions) the other components of drag are far more complex. There is a reason why naval architects still spend a lot of money on tank testing scale models!

You can use the Delft Series as explained in Principles of Yacht Design for an estimate, but depending on your hull the answers may be out by a good margin.

If you want to know to a higher level of accuracy, contact one of the Universities with a tank (Southampton, Solent, Strathclyde, Newcastle) and ask about the daily rate for testing. You would need to supply a suitable model (they may be able to assist with CNC cutting the model) and you would need at least a full day in the tank for basic data. You’d be looking at a couple of £k minimum for the data but at least you would know for certain!
 

Laminar Flow

Well-known member
Joined
14 Jan 2020
Messages
1,015
Location
West Coast
It’s a very complex issue as it will depend on a whole range of factors - including the heel angle, leeway angle, waves, hull form and appendages. Whilst the skin friction is relatively simple to predict (albeit with a lot of assumptions) the other components of drag are far more complex. There is a reason why naval architects still spend a lot of money on tank testing scale models!

You can use the Delft Series as explained in Principles of Yacht Design for an estimate, but depending on your hull the answers may be out by a good margin.

If you want to know to a higher level of accuracy, contact one of the Universities with a tank (Southampton, Solent, Strathclyde, Newcastle) and ask about the daily rate for testing. You would need to supply a suitable model (they may be able to assist with CNC cutting the model) and you would need at least a full day in the tank for basic data. You’d be looking at a couple of £k minimum for the data but at least you would know for certain!
You are right when it comes to absolutely precise predictive data for unusual design features, but the Delft series provides sufficiently accurate information for your average, garden variety of prospective performance evaluation. Furthermore, there has been plenty of data feedback from full scale craft to corroborate the Delft model.

In my case, I wanted to a quantify changes I made to my hull and to evaluate and predict the benefits of enlarging the rig. The predictions were, closely enough, confirmed by the outcome.
 
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