How to calculate the sail area to displacement ratio!

Comparing boats is not that simple of a task. There are a lot of design parameters involved when you look at a boat and you try to compare them in order to make an informed decision. Displacement ratios are the main parameters to be involved in the calculation. As such, the sail area to displacement ratio, also referred to as SA/D is a non dimensional calculation intended for sail boats. What this calculation does is measure the relationship between the sail area of a boat and the amount of water it displaces. The sail area is usually calculated in square feet, a system used internationally, while the displacement of the boat is calculated in cubic feet to two-thirds of the power.

The formula for sail area to displacement:

ratio = sail area (ft2) / (displacement) 2/3

Generally, if the displacement is measured in pounds, than it can be converted to cubic feet by dividing it by 64, which is how much the cubic foot of a seawater weighs. 

A look into the sail area-displacement ratio

The displacement element in the formula mentioned above is used mainly for comparative purposes. The standard way that most designers use to calculate the displacement is with a half load. This means the vessel is ready to sail with the crew and half of the provisions, water, supplies and fuel on board. 

The sail area calculation is nominal and can be calculated as the foretriangle area plus the mainsail area. The foretriangle area is the height multiplied by one half of the base. The mainsail area on the other hand is calculated as the dimension of the hoist multiplied by one half of the foot. Sometimes however, the nominal area does not include the big roaches which are usually flowing on modern rigs. As a result, most people dealing with boats use the real sail areas for mainsails.  

The sail area displacement ratio is a nondimensional number because as a result, the division in the formula is from square feet to square feet. Consequently, the number you get is not related to the size of the boat. 

How to understand the sail area displacement ratio

The sail area displacement ratio is used to compare the performance of a sail boat. If the result from the calculation is over 20, that means the boat has a very high performance. If the number is from 15 to 20 than the boat has a good performance, while if the number is under 15, it means the boat is under performing. The sail area displacement ratio however is not the only parameter to show that a boat is good or not. The displacement to length ratio should also be taken into account. Before making a purchase it is important to compare these parameters. The database of TheBoatDB can help you compare all these features in no time.