Advise on twin rudder sailboat design

Hi, I recently acquired an Gilbert Caroff design (Chatam 330) aluminum sailboat. The boat is a center board keel, flat bottom, twin rudder that is capable of beaching.

The twin rudders pose a challenge for me as a solo sailor. When i maneuver in tight place or docking/launching from a berth, the boat tends to do what it wants as there is no flow over the rudder until I get some speed. it has led to some tense moments when docking. I feel a bow thruster would not work as the bow section has very little wetted area

I would like to change the tiller to wheel and have one rudder behind the prop. I have been trying to figure out if this is possible, I read there is a correlation between the rudder area and the wetted area of the boat, for the life of me I cannot figure it out.

I have the original plans for the boat along with a scanned copy, I can supply the plan or numbers as needed. If I could get some input, I would really appreciate it. I do not want to make any mistakes.

I'm based in the East coast of Canada (Nova Scotia), if anyone could recommend a navel architect or a yard close by, that would really help me out.

 

Some pictures would be helpful.
I don’t know why you couldn’t add a rudder behind the prop, it would have to be well tucked up if the boat is to be beached.
It would definitely add to the wetted area with resulting drag, but unless you’re a racer, it probably wouldn’t be significant.

 

Hi KapnD,
thanks for your comment, much appreciated. I'm definitely not a racer, I'll be using the boat for some high latitude sailing.
I'll try to put some pictures up, I sail better than I am at computers. I wondered about having a rudder behind the prop. I believe the design was drawn around 1996, the rudders are absolute vertical, there is no angle when heel was considered. My other thought was part of the rudder kicking up for clearance if beaching, sort of like the Ovni's. One thing I do not like about the Ovni design is the use of hydraulics for the kick up part. Being up North, I'm limited to services, I would be up for doing something mechanical.




This image is when I took the boat out in Ireland for a bottom job. The rudders are the same distance above ground as the skeg to allow for beaching. If the rudder was moved to the middle, i believe the length of the rudder would need to remain the same. There is space in the back to widen the rudder if necessary.

 

I don't really have very good pictures, the boat was purchased in Ireland, I was quick to get going. This is the photo of the stern, a very basic setup two rudders connected to two tillers. where the tubes exit the sugar scoop, the tillers are connected together via linkage inside the boat. The rectangular "box" where the tubes exit is the inside of the boat, basically two horns on each shaft are connected together, along with a linear actuator. Easy enough to replace with a wheel as there is a large amount of space to access everything.

 

Tom, Welcome to the forums...
That is a very strange layout, and I can see why you have trouble slow maneuvering under power. As you implied there are requirements on rudder area/location for good course keeping and control authority. It seems that Gilbert went for a fairly effectevive shape, but split the area between two short span appendages rather than one directly behind the rudder. A little digging shows that the original Chatam 33 was also offerend in plans (all were built from plans in different material) as a single rudder fin keel....so I think this is an inherited trait caused by the swing keel beaching requirement.

Is it your intent to keep the swing keel?

 

Hi Jehardiman,

Thanks for the reply and your insight. I would like to keep the swing keel, I'm based on the Bay of Fundy, very high tides with lots of sand along the coast. I like the idea of drying the boat out to inspect/maintain her bottom. In your opinion, would I need to lengthen a single rudder to equal the length of the two rudders combined? If so, the only work around I see is building a rudder like the Ovni's. This is somethin I would leave to a pro. One interesting fact, the rudder stock from the top of the rectangular box to the rudder is solid aluminum, I think 3 inch thick. After rectangular box to the tillers is aluminum tube.

 

Attainable Adventures talks about rudder options,
Rudder Options, Staying In Control https://www.morganscloud.com/2010/10/17/rudder-options-2/

My boat was next to an aluminum Boreal 44 with a swing keel and single rudder, one neat feature was a retractable dagger board on each side of the rudder, one would be deployed based on tack, I believe.

Also I have noticed a long dagger board directly behind the prop and in front of a single rudder. I do not know of the advantage for this design, but I find it intriguing. I'm wondering if this could be an option, would like to hear thoughts on this.

 

Can't say I've ever seen a configuration like that before.
Twin tillers!

I would be very resistant to changing anything.
How far north are you planning on going and how much docking do you plan to do?
How much have you sailed it?

 

I don't think I have ever seen a seagoing cruiser with such a small amount of rudder area,but plainly the beaching aspect dictates that.Might I suggest tat you consider the concept shown in the photograph that I hope to attach,if not the precise proportions.Each rudder is mounted in a tube that is braced by a trapezoidal frame that can pivot and when lowered,it is additionally braced by the cutout in the hull surfaces.The short tillers are linked to a central projection of the steering mechanism,which may be wheel,tiller or more likely autopilot.Since the hull form dictates that the windward rudder is likely to be lifted clear of the water when sailing in any breeze,it isn't vital that the geometry is perfect at any point of deployment,which ought not deter efforts to achieve such a state of affairs.An additional advantage is that with the rudder(s) lifted at sea there is little difficulty in clearing kelp or ropes that have become snagged and no need to wait for low tide,or face a chilly swim.

 

I would buy myself a Yamaha 9.9 or 25 High Thrust and stick it on the sugar scoop with a remote control kit in the cockpit. No need for rudders while docking, lock them and turn the outboard like on a motorboat. The added redundancy of having a second engine if the inboard craps out is just a bonus.

You can still convert to wheel steering if you like, but most sailing is done under autopilot/windvane anyway, so why spend the money. Rig a bungee to keep the tillers up and out of the way if cockpit space is important.

 

I really like @Rumars approach but worry about adding a gasoline powered thing to a long voyaging craft. Maybe a conventional thruster in the stern either in a faired enclosure or a drop down configuration?

 

You could gain a bit more authority by adding a fixed skeg in front of the rudders. It will improve handling both with the prop and without. The skeg will load up when you turn the rudder. The combo is nearly as effective as all rudder, and it doesn't change the power draw of your autopilot.

A second trouble is that you have a gigantic prop and you can't slip it enough to gain handling without the boat taking off like a cog railroad. If you don't cover a lot of distance under power, you could also try hanging a little 14 x 14 two-blader under there and you can keep some water moving over the rudders you have. You'll also get a whole bunch of prop walk.

 

99% of the time a yacht is going to have gasoline on board, and plenty of it. After all few people can get by without a gasoline outboard powered dinghy. For me, dinghy, outboard and gasoline storage areas that are already baked in by the designer are a hallmark of a serious expedition yacht.
There are of course better mousetraps available, like electric or hydraulic outboards, adding two small folding electric driven props in front of the rudders, steerable pod drives, etc. The good old bow thruster also works, his wouldn't be the first unit of this design to be retrofitted with one.

The primary problem with the Caroff design is it's high windage, newer DI's try to minimize it. For his bigger boats the designer actually recommended twin engines.
The twin rudders also act as beaching legs, keeping the boat stable and horizontal, it's one of the reasons this solution was chosen, despite the obvious low speed maneuvering problems.

For a single rudder conversion the new rudder surface area has to be approximately the same as the combined area of the rudders. Approximately because it's a function of aspect ratio, a deeper higher aspect ratio one will need slightly less, one that matches the existing draft and therefore has a much lower aspect ratio needs more surface area (judging by eye it could potentially have to extend past the edge of the sugar scoop).

 

After seeing the pictures, I’d be very cautious about beaching this craft!
Any small wave action or settlement of the hull will bring the rudders in contact with the bottom, and I seriously doubt that they are designed to carry that kind of stress.
A single rudder behind the propeller could be strengthened and protected by a shoe from the keel.

 

Hi Wet feet,
Thanks for your comments, much appreciated. That is a very interesting idea you presented. I will look more into this.
Cheers