Ackermann on Cats


After the recent discussion about the ackermann I did some math on it and ran into a problem.
Many of you are better engineers and I, so maybe you know a solution:

My Cat features rudders at the front crossbeam and at the stern of each hull. since the two rudders of one hull turn exactly against each other to make a turn, the pivot lies right in between them, off to the side. Distance between rudders is ~75cm. If I tilt the rudders on the inside hull +/- 45 degrees, the pivot is between them, 37.5 cm off to the inside of the turn.
The distance between the axis of the inside and the outside hull is 101cm.
If I did my math right, that means that the outside ruders should only turn about 14 degrees to have the same pivot point as the inside rudders.
Now here’s my problem:
Maybe i am just plain dumb, but I couldn’t figure out an ackerman configuration that allows the rudders to turn 45 degrees to one side and 14 to the other. I tried it with math and be drawing it geometrically, but the difference in angle seems just a little bit too much for a simple lever arm adjustment…
Especially since the arm should not rotate so far that the lever arm and the connection are in a straight line. (-> Zero rotation force)

Any help highy appreciated.

Thanks a lot,

Hey Marcus,

I do not race RC cats, but I used to race full size cats so I do have a little experience here… Maybe someone with RC multihull experience can correct me if I am wrong here.

I think your example is just about the limit of how tight you would ever want to turn. Most of the time you would want to turn with a much more gradual turn. Turning this sharply is going to bring the boat to a stop. For example, on my monohull, the LIMIT for my rudder deflection is about 20 degrees off of centerline. This is more than enought travel for every maneuver I need to do - even tight maneuvers on the starting line…

Having said that, I will see if I can whack together a spreadsheet that calculates the arm positions for this Ackerman. Can you send me an email with your email address so I can mail it to you when I am done? Do you have microsoft Excel?

  • Will

Will Gorgen


Of course I will not turn that sharp, but I assumed, that 45 degrees might be the maximum that makes sense and I also assumed that a Setup that works in this extremes would deliver the correct angles for less rotation too. Am I wrong here? [:-bigeyes2]

If my excel sheet is right, you would need about 11 degree rudder rotation on the outside when rotating the inside rudder 20 degrees with a turnradius of approx. 2.1m measured from the outside hull.

It might be interesting for all Cat builders to have access to an excel sheet that helps them to adjust their ackermann. Even for Trimaran builders who want to add rudders to their floats… Just a thought. [:-basketball]

thanks a lot,


I checked my big beach cat and found that the maximum degree of turn is about 12-15 degrees (as close as I can measure) - more than that - I use for braking at starting line if a bit early or to slow down to require a competitor’s boat to have to tack or duck instead of cross me.

The cat is wider than a typical beach cat (12 foot instead of 7 1/2 feet) thus the smaller turning rudder angle and greater turning radius. A narrower boat can use more turning radius and rudder angle before the rudders stall out or cavitate. My big cat is 18 feet long x 12 feet wide, so perhaps you can set up a ratio for scaling down to model size width and length. Not sure if this will help or not. From the model cats that I have seen - it seems builders go wider than normal in an effort to reduce side tip-over — but then run into tacking problems due to the excessive width.

For a typical cat of 8 foot beam by 18 feet long, the size of it scaled to an F-48 size cat would indicate a finished cat 48 inches long by about 22 inches wide to remain in same ratio of length to beam. Of course you can always squeeze/stretch those parameters, but the smaller the size model multihull - the less you can play and get expected results. It is a given for multihulls (2 or 3 hulls) that the bigger the model - the more realistic the sailing and handling properties. This is the reason for movement toward the big 2 Meter class by many in Europe. The F-48/Mini40 is a hard creature to sail by comparison.

I would be interested in knowing the dimensions of your cat - for informational purposes…

length - beam - mast height - hull maximum width - hull maximum height (if you care to share).


Its a Team Philips concept cat. I have posted some pics in the “New Member/ New Boat” thread.

Its 1.3 Meters Long, 1.05 Meters, Masteight is 1.78 Meters (Twinrigg), Hull width is 6 Centimeters, Hull height is 12 Centimeters.
weight is 2.3 -2.6 Kilograms depending on setup, but I will try to bring it down to 2.2kg.

I like to have little bit more elbowroom on rudder rotation in case it gets stuck in a tack. A little wiggle sometimes just makes the difference.

Will sent me an amazing excel sheet for calculating servo and tiller arm lengthes and rotations. If someone needs it too, just drop me a note.

Thanks again, Will



Marcus -

if you use the Team Phillips boat at 120 feet long and 70 feet wide, your model at 1.3 meters in length should really only be .758 meters wide - not 1.05 meters.

A scaled reduction in size seems to show your boat is .292 meters wider in relationship to the original Phillips cat. This wider scaled size may also be a factor in your tacking problems - remember that wider boats tack harder/slower than narrow ones. (mono vs multi)

It would be interesting to determine if extra width has a direct scale effect or not when it comes to steering performance spread across two hulls. Obvioulsy it must, since we agree/know that a single hull tends to tack faster than two hulls - and tacking requires a larger circle radius the wider the hulls are apart on a cat.

Also - if you want to keep mast scaled correctly (135 feet for Team Phillips) - your masts should be 1.426 meters high instead of 1.78 meters. Considering most r/c boats will raise mast and deepen keel this wouldn’t seem to be too critical - unless you are seeing overpower in medium winds.

Just some additional thoughts and observations.

Quite often scaling multihull’s don’t work. The 60’ tri’s scale down to mini40 quite well but they aren’t easy to sail. Seems to me that the Team Philips cat will be like all cat’s --very fast in a straight line but will never tack quickly.

The original Team Philips boat was never designed to tack fast.
So don’t get to annoyed and downhearted trying to make the cat tack quickly. Just go out and enjoy your boat.



No worries, I wont get dishearted. Its just like always: If something doesn’t work exactly the way you want, the challenge is to bring at least as close to what you want as you are able to.
The Ackermann configuration of the rudders is pretty sure a big leap.
The boat is good to sail, has little if no nosediving tendency and sails like on rails (in the light wind conditions that I’ve tested it in so far).
If the tacks are slow, I will be happy to live with that, but I will nevertheless do my best to optimize it.

I’ve never sailed a Mini 40 Tri. What do you exactly mean with ‘sailing them is not easy’.


Mini40 Tri’s and cat’s are difficult to sail when you first start with them for several reasons, but the main ones are.
The sheer speed of the things
These boats surprise many people with how fast they excellerate, this can be a problem when you first start to sail the things because until you learn how to control the speed you find yourself in the dinghy picking them up.

Setting them up properly
This is vitally important. The boats can be rocket ships or slugs. If your set up is wrong, you don’t realise just how slow your boat is until you get up against a fast boat.

Controlling the boats right
Sail Control

Never forget these boat weigh around 2kg, you don’t have lead to help you like the monohull guys do. Gaining knowledge from sailing them is the only way to go. Some people say sailing r/c multi’s is easy, and I suppose it is, but racing them is awhole different kettle of fish. Firstly you have two or three hulls. You are controlling a boat that is somewhere between 600mm and 1200mm wide. This means that when you are going to a windward mark you are basically sailing three (minimum) monohulls, when you consider the water that your boat is occupying.
The other point is that you must concentrate on your boat the whole time. Again the reason for this is the weight, no lead to right you if you aren’t paying attention. Being able to concentrate for the time that you are sailing takes time. Unlike monohull’s or real boat’s, you have a chance to let your mind wonder, but multi’s usually don’t give you that luxury.



dont tell me , that soon I’ll need to think and get some aspirine [:)] when sailing…


_/ if it isn’t broken, don’t fix it! _