Catamaran Draft – Keel

[APP]

Hi,

Several Cats as e.g. the Lagoon 560 and the NYX 565, with hulls both designed in France, declare a draft of 1.5 m (4.92 feet) but they do not say how much is the boat draft and the respective fixed keel (length – height). Any idea how the dimensions of the keel should be calculated?
In the picture below the keel of lagoon 450.

Regards
APP

[cavalier mk2]

With a tape measurer If you have a good photo you can scale the proportions. A keel with 40% of the total depth etc...is a easy calculation.

[APP]

Originally Posted by cavalier mk2

With a tape measurer If you have a good photo you can scale the proportions. A keel with 40% of the total depth etc...is a easy calculation.

OK, But a designer here could tell us something more, as the fixed keel dimensions may depend on the mast height, mainsail area and similar. Some Sail Cats have no keel. What is the discussion here?

Regards
APP

[cavalier mk2]

Good question. Keel dimensions often change in proportion to hull shape, V hulls need less, roundhulls more. Shoal waters may influence a design, performance range sought, then there is the influence of the marketing department. If you start checking keel areas you'll find a wide range of sizes. For a designer to specify a size and depth he must have those variables filled in. If you are looking for a rule of thumb, the shallower the keel the more area it needs to compensate for the reduced effectiveness So if the designer can't go down he must go long.

[APP]

Originally Posted by cavalier mk2

Good question. Keel dimensions often change in proportion to hull shape, V hulls need less, roundhulls more. Shoal waters may influence a design, performance range sought, then there is the influence of the marketing department. If you start checking keel areas you'll find a wide range of sizes. For a designer to specify a size and depth he must have those variables filled in. If you are looking for a rule of thumb, the shallower the keel the more area it needs to compensate for the reduced effectiveness So if the designer can't go down he must go long.

Good! Here is another picture. Keel related with center of gravity: stable on earth also.

[cavalier mk2]

Those keels look adequate for the pictured vessel. I should have answered the question of design and purpose differently. A cat may be designed with sufficient water plane when launched but later in the production run the sales department adds a flying bridge to keep up with other design offerings. So now the boat has more windage to overcome but if the keels are deepened it is more liable to tripping with a higher center of gravity in storm conditions. People talk of design spirals in terms of positive evolution but as pilots know the reverse can be true as aircraft have "augered" in. The charter trade who have a boat designed for a specific location may decide their customers will motor to windward and if there is a problem call for the rescue boat, services not available to the second hand buyer caught off a lee shore far from the designed location. Not the designers fault or the builders but a sales requirement because boats are usually designed "For Sale" rather than "To Sail". These people are in business to make money for the shareholders. The buyer does need to beware and be honest about intended use, most craft spend their life in a marina impressing guests not sailing beyond the horizon. The best solution for the buyer is to compare boats and read all the sail tests you can find being aware that the results are only valid for the conditions the boat was tested in with the gear used.
Most boats can be made suitable for most purposes, there are formula for lateral plane but they are subjective if everything from solar panels to Bimini tops, rounded deck edges etc... aren't taken into account. Here owner modifications also need to be considered. After observing many successful and unsuccessful craft you can develop a sense of proportion that will help you evaluate new designs. If you are buying test in all weathers if you can, if you can't contact owners who have. Just my opinion, I design for myself for the fun and as the end user I have to maintain a great client relationship.