adjusting CE and CLR to balance steering

Working on a 42 foot cruzing Cat (16,000# disp). The rig is a biplane with 2 jibs (mast are placed aft). The jibs will use furling gear for ease of handling.
As the jib is reefed for high winds, the center of effort can move forward
several feet. To maintain a balanced helm, the rudders are designed to be raised or lowered to change the wetted area which will move the center
of lateral resistance fore or aft. Attached image shows the rudders which can be raised about 3 ft.
Is this approach a common practice and what would the problems be?

 

Decreasing rudder efficiency as wind strengths build seems counter productive to say the least. Maybe some thought on balancing boards, little center or dagger boards, to regain balance, while still offering good steering response.

 

it seems to me with that much of the keel that far forward, there would be a lot of lateral force on the rudders as well.

I would consider moving the keel further aft, and than add a dagger board aft of that to trim sails, as PAR suggested.

 

That looks like a design that will broach. The centers are way off too. Keels are normally aft of the leading edge of the sail.

 

Me thinks you should rethink a bit how to manage the sails. Like reefing first the weather side jib and leave the lee side as it were (or just a few rounds in) should prevent most of the lee helm problem you encounter.

 

Is this a joke? If not then you need to do some serious study.

IMHO one should never design a boat until you have built one or two and sailed them lots. This design is full of problems and the CE stuff is really worrying. Do lots of study and then find a designer who can draw you something you like.

I am being nasty now to save you huge time and money later on.

cheers

Phil

 

Glad you did it Phil, I'm usually the one jumping all over a novice, with a set of concepts well out on a design limb.

 

Rudder depth adjustment

Thanks for the good feedback. Good learning experience for me.
sounds like adjusting the depth of the rudders will probably create more problems then good.
Rudder lateral area is 8-9 sq ft with a long moment arm to the CLR.
This present design has the CE 19 ft (aft of the bow) and the CLR 18.4 ft
which gives 7 inches of weather helm moment arm.
As suggested in the feedback, reefing down one sail (in a bi-plane rig)
will not move the CE. So adjusting the rudder depth makes no sense.

 

There are good reasons you do not see sailboats designed the way this one is.

 

CE and CLR balance

Good feedback needs to be supported with solid physics .
Feedback that is just a cavalier statement provides no usefull
information.

 

John Hitch’s X-IT is similar to your design in that it just uses head sails. It may be for sale here:

http://www.boatpoint.com.au/boats-for-sale/boatdetails.aspx?R=10687843

By buying his cat you could save yourself a lot of design woes.

 

Ou yes it does, transversally. To lee means more weather helm which is the opposite to what happens when you furl in..

 

CE effect on steering

TeddyDiver,
Thank you, Your reply makes good sense.
"Ou yes it does, transversally. To lee means more weather helm which is the opposite to what happens when you furl in.."

So i need to analyse CE effects longitudal and transversally. Thanks again.