Ratio of lateral to sail area

[Andrea Wasserliebend]

Hello, I am in the process of constructing my own design for a catamaran and the question arises as to what the ratio of sail area to lateral area should be. I had previously assumed that the lateral area should be about 10% in front of the sail area. But a friend said that both should be congruent for good sailing characteristics. What are your opinions on this?

 

[Skyak]

Hello, I am in the process of constructing my own design for a catamaran and the question arises as to what the ratio of sail area to lateral area should be. I had previously assumed that the lateral area should be about 10% in front of the sail area. But a friend said that both should be congruent for good sailing characteristics. What are your opinions on this?

My opinion is that there is no consistent relation between the two. The right relationship is that all of the lateral force from the sail must be applied to the water and catamaran hulls have far less drag going straight through the water than sideways. To do this you start with an educated guess of the relationship between wind speed and water speed, then set the lateral force of the sail equal to the lateral force of the water on the appendage (keel or daggerboard) at an acceptable leeway angle.

 

[Andrea Wasserliebend]

My opinion is that there is no consistent relation between the two. The right relationship is that all of the lateral force from the sail must be applied to the water and catamaran hulls have far less drag going straight through the water than sideways. To do this you start with an educated guess of the relationship between wind speed and water speed, then set the lateral force of the sail equal to the lateral force of the water on the appendage (keel or daggerboard) at an acceptable leeway angle.

Thanks for the answer, but this confuses me. I'm more planning a motorhome than a racing car on the water. What should I do now? 0%?

 

[Skyak]

That is going to be low performance and have a hard time making headway upwind. If that is acceptable, you can scale the design and proportions of low aspect keels from the nearest comparable (similar waterline length) condo-maran.
What is the length, width, and weight of your design?

 

[Andrea Wasserliebend]

That is going to be low performance and have a hard time making headway upwind. If that is acceptable, you can scale the design and proportions of low aspect keels from the nearest comparable (similar waterline length) condo-maran.
What is the length, width, and weight of your design?

Yes, I am making comparisons. They are currently between 4.2 and 0% of the sail area. I plan with 10m LOA and 4.9m BOA. Waterline length is 9.86 m. Hull width 0.87 m at waterline. Total displacement 4.08 tons. I think that's enough for a caravan on the water?

 

[Skyak]

Yes, I am making comparisons. They are currently between 4.2 and 0% of the sail area. I plan with 10m LOA and 4.9m BOA. Waterline length is 9.86 m. Hull width 0.87 m at waterline. Total displacement 4.08 tons. I think that's enough for a caravan on the water?

That sounds very similar to the specs of a Gemini 105. ~64 sqm sail area, 2 X 1 sqm centerboards. ~1 sqm planform area for any low aspect keel.

SailboatData.com - GEMINI 105MC Sailboat

I am sure there are more opinions depending on your performance expectations.