Aft Beam Location

It looks very stylish to have the aft beam of a cat as far back as possible.  Its kind of like skinny tires on a lowered car.  It does move the CG back a bit which helps if sailing deep in strong wind.  In the most common conditions, it causes the transom to drag.  To prevent that at least one ultra famous designer packs more displacement back there in the form of rocker.  That rocker levels it out at rest but can cause squatting at speed.  In the case of ultra famous, the cats don’t  squat when sailing; the center of effort it way up there.  In his powercat modification of that design, it squats so much that he added trim tabs.   The pic below is not that designer but another example of too much rocker aft.

badcat 3g

The new Gunboat G4(accidentally typed G$ which may be better), has aft beam way back there also.  It however is all carbon so it is very light.  It drags transom at rest but with so much power it is not a concern.  As I always say, with enough money and enough carbon, you can do almost anything.

On normal cats I find it best to have the aft beam far enough forward that the hulls can have a fairly flat run aft.  Related, the goal is to have as skinny and as rockerless a hull as possible, to carry the weight.  The weight/trim spreadsheet helps with this location also.


3 thoughts on “Aft Beam Location”

  1. I read somewhere “no more than 9 degrees or you loose laminar flow.” This one has an over all look of about 25 degrees and ends up way more than that. You may as well fill your dingy with water and drag it behind you!

  2. Trim tabs and power trim in power boats introduce a large safety and comfort margin. There is more maintenance and complexity in operation. The dynamic equilibriums in speeds greater than 15knt is well worth it. If you watch the old power boat ocean racing you will see one guy working only the trims.

  3. Hello,

    Maybe that’s due to the difficulty to reduce main sail surface when “sailing deep in strong wind” to avoid “front capsize” .

    Soft sail wing should solve the problem by almost instantly release propulsion.



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