Ackerman angle

[Mark Hamill]

Ihave a Tangaroa 4 and want to add the ackerman angle feature on the rudders--could somebody give me the formula or a place to look?? Thanks, MarkH

[kim whitmyre]

Mark,

This info is from a fellow named Tom Speer on BoatDesign.Net:

"The easiest way to achieve it is to use tillers to control the two rudders, linked together with a bar connecting the ends of the tillers. If you angle the tillers toward the centerline, the inside rudder will be turned more than the outside rudder.

How much to angle the tillers depends on what you are trying to achieve. The tighter the turning radius, the more Ackermann angle to use. As a quick approximation, draw lines perpendicular to the keels/boards at their center of lateral resistance (say, the quarter chord) and perpendicular to the plane of each rudder and board. You want all three lines to meet at the center of the turning radius. This sets the amount of differential deflection of the rudders. " Speer says between 10 to 15 degrees is typical.

[Mark Hamill]

Thanks--I am hoping this will solve a few problems in coming about.

[kim whitmyre]

Do you have the lashing system on your rudders? More experienced folks than me say that the smaller the gap between the rudders and the hulls, the better the tacking. I originally had gudgeons and pintles on my boat that created a large (1 1/4 to 1 1/2") gap. I recently built new rudders and hung them using the lashing system: much reduced gap!

[Mark Hamill]

HI Kim: I have the designed gudgeons and pintles and withe filler pieces the gap is under 1/2" . I am not aware of this lashing system. I have plates welded to the bottom strap that is supposed to help in increasing the apparent rudder area. They are about 8" wide> I used to have some really wide plywood plates but they kept catching "stuff" floating. The fellow I got the large plate Idea from faired out his rudder and all surfaces such as the skeg with scuptured curved shapes. He said it really made a difference. I didn't go that far. Some people claim that filling the gap betweeen rudder and hull is supposed to help however I have heard completely the opposite and have a book by Brent Swain that explains why. I'll look for it and maybe scan it if you want.
MarkH

[kim whitmyre]

That sounds like a reasonable gap, Mark; mine was really excessive! It was Steve Turner, a long time Wharram builder and sailor in England who made the remark about more positive tacking. I wouldn't mind hearing what Brent Swain has to say about it, nonetheless!

[Tom]

Been thinking about this Akerman linkage a lot lately. Very interesting stuff.

The Pahi plans I'm looking at have me mount the tillers canted 3 degrees inboard in relation to the rudder. The plate that receives the tiller is welded to the stock with 3 degrees of difference from the rudder.

Given the 3 degrees, I made my tillers 5 feet long. And in order to have fun with this idea, I laminated them up on an arc, so that they have 2 inches extra on the for and aft ends from where they are mounted to the stock, like a flattened out C.

Unfortuanately, I don't have a good grasp of the Ackerman principle as applied to hulls skidding through the water, which seems to be what we would like to avoid. Conceptually, it seems easiest to treat the water as a solid surface here, despite the obvious flaws. The produciton cats that my friends have don't even try to address this - they have tillers mounted inline with their rudders...skidding through their tacks every time.

Perhaps one fun way to play with this is to connect the two tillers with two cables, one forward of the stock and one aft. Each cable has a turnbuckle, so that we may adjust the angle via the forward and aft turnbuckles. Given the initial 3 degree tiller/stock offset, and after determining how far forward on the tiller to place the forward cable, the turnbuckles should allow for fine tuning/error correction.

[kim whitmyre]

Here's an old photo of Vaea's aft end that shows the starboard tiller fairly well:

As I recall from the plans, when building these tillers, you hang weights(or some other method) on the crossbar end until you obtain the proper amount of deflection, and then you let the epoxy set.

[sam]

RE tacking difficulty --what worked for me on a tiki 21 was fitting end plates or small vortex generators to the rudder ends it seemed to increase the turning effect . the other thing i noticed time and again in 14 years of sailing this boat was that people whom i took sailing often total sailing virgins never seemed to have a problem putting the boat about -they never knew or imagined it might be a problem

kim whitmyre said:

Here's an old photo of Vaea's aft end that shows the starboard tiller fairly well:

As I recall from the plans, when building these tillers, you hang weights(or some other method) on the crossbar end until you obtain the proper amount of deflection, and then you let the epoxy set.

[kim whitmyre]

I've never sailed a tangaroa, but here are some comments on tacking a tiki:

Tammy wrote:
> You CANNOT tack a cat like a monohull, it won't turn on a dime.
>
> What experience we have on the Tiki bears this out. When you begin the tack, throw
> the helm gently over. Don't throw hard over. Sail thru the tack, as it were, make
> a big U-turn; as you begin the turn, sheet harder as you are essentially sailing
> closer to the wind. As you reach head-to-wind, let out the main, LEAVE THE JIB IN.
> The jib will help put you on the new tack. As the bows cross the eye of the wind,
> let the jib backwind and blow the bows on over to the new tack. Once across the
> wind, then cut the jib and sheet it first, on the new tack, then harden the main.
>
This is EXACTLY how it works and in doing so you won't miss a tack.
some minor comments from my experience with a 26:
1,how fast you 'throw' your helm over, depends very much on wind & the boats
speed and the waves. The magic word is slow and as said before sail
your boat to the new tack.
2,Once on the new tack, don't push hard upwind in the beginning,
get speed an then sheet in.
3,If you have missed (e.g. wind change during manouvere), do it like the old square riggers, reverse
tiller, sail backward to your new tack
4, In case you have a traveller rigged for you main sheet, you can
even sheet the main to windward and skip the jib backwinding job.
Gerald
....................
Experienced keel boat sailors have also expressed the gradual arc to weather as the way to go.

[Mark Hamill]

You are right about non sailors not having trouble tacking--thats hilarious come to think on it. I am going to build a traveller--thanks for the push. Somebody just told me about a newer book written by a catamarran designer and he solved the ackerman anble thing rather cleverly but not a prettily as the bent rudders by usingpieces that exteded sideways on both rudders with holes in them so he could experiment with differnt angles. Will find out name of book. MarkH

[Mark Hamill]

Kim: I have copied your advise and will put it in the ships binder so others can read it when we sail. MarkH

[Mark Hamill]

Thanks for the ideas to everyone. Pardon the 2 year delay in reply. Sincerely, Mark Hamill

kim whitmyre said:

Mark,

This info is from a fellow named Tom Speer on BoatDesign.Net:

"The easiest way to achieve it is to use tillers to control the two rudders, linked together with a bar connecting the ends of the tillers. If you angle the tillers toward the centerline, the inside rudder will be turned more than the outside rudder.

How much to angle the tillers depends on what you are trying to achieve. The tighter the turning radius, the more Ackermann angle to use. As a quick approximation, draw lines perpendicular to the keels/boards at their center of lateral resistance (say, the quarter chord) and perpendicular to the plane of each rudder and board. You want all three lines to meet at the center of the turning radius. This sets the amount of differential deflection of the rudders. " Speer says between 10 to 15 degrees is typical.