# How can I calculate total drag on a multi-hull?

Tags:
1. Mar 16, 2016

### raniero

I was given the task to create a hull which must be propelled by human and solar power; I have found it difficult to obtain existing hull designs with their respective coefficient of total resistance so I am looking for a way to calculate the total hull resistance for a hull of my own.

I require the hull to be simple in geometry as I should propose a way to manufacture it at the least possible price. I was thinking about a circular cross section hull, as shown below, as I think it would be quite simple to manufacture. I am aware of the 1957 ITTC line but do not know how to extract more data from the model to get the total drag resistance. I am aiming at a speed of about 1.5 to 2.5 m/s.

Also, is there any simple method to calculate the extra power required to propel the hull in slightly rough seas?

Any help would be greatly appreciated.

https://d2t1xqejof9utc.cloudfront.net/screenshots/pics/b116352c73e13dcea85289ee6504a3cd/large.png

2. Mar 16, 2016

### Baluncore

If the project goes ahead, how many hulls will be built in the initial run? What hull materials are being considered? Those answers will help decide the trade-off between hull shape complexity and hydrodynamic inefficiency.

2.57 m/s = 5 knots, is not fast for a displacement hull. Your picture shows two very long and thin catamaran hulls. We have not been given the required displacement or scale, which makes it difficult to identify the appropriate hydrodynamic regime. You might consider making the hulls shorter and fatter to reduce the wetted area for the same buoyancy. You need to minimise the total drag, (wave making + wetted area).

Without a better idea of the hull scale, shape and the meaning of “slightly rough seas”, it is hard to calculate the extra power required. Wind direction and the vessel profile above the waterline may be much more important than sea state.

3. Mar 17, 2016

### raniero

Hi, thanks for your reply. I was thinking of aluminium or E-glass as a hull material, and around 10 would be built in the first run. The scope of this project is not to design a hull but rather to work in a team and go through a design process, so certain technical details about hull design are not expected (I have only attended fluids classes - nothing specifically for hulls).

Our general dimensions for the boat are 4.2m length by 3m width, so maybe a 0.5m width by 4.2m would be appropriate (of course I would need to optimize this when I have more data about the chosen hull) for the hulls.

To get an idea of what I mean with "slightly rough seas", the boat is intended to be operated in harbors, lakes and in seas close to the shore.

I am more inclined at using existing data (coefficient of total resistance) for existing geometries of hulls but I can't seem to find any hull details available, not even for the most simplistic hulls such as a cylindrical profile. Of course, if a find a set of data for different hull dimensions / ratios I would optimize my dimensions to get the least possible total drag, but without having this data or without having the capability to analyse analytically I am finding this difficult.

Last edited: Mar 17, 2016
4. Mar 17, 2016

### Baluncore

There are two effects you should consider. The first is wetted area. You can calculate the wetted area geometrically and apply the square of the speed to compute drag. The second is wave generation. The length and speed of your hull are related by the formation of a wave crest near the stern. Those relationships can be applied generally to most real hulls.
See;
https://en.wikipedia.org/wiki/Wave-making_resistance
https://en.wikipedia.org/wiki/Hull_speed
https://en.wikipedia.org/wiki/Ship_resistance_and_propulsion