How to dimension a sailing catamaran?

Discussion in 'Multihulls' started by terhohalme, Apr 27, 2008.

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FanieFanie

I have another stone to throw in the bush.

Normally the centre force of the sail and the daggerboards are more or less in the same place. I assume this is to correspond with the side force the wind will have on the boat to make the daggerboard the rotation point when the boat turns. You do have to move foreward or else you cannot turn the boat.

Now assume for a moment one move the daggerboard foreward quite a bit so that the centre force of the sail is way behind the rudderboard, and you make the rudder fast so it doesn't turn. Doesn't the rudder and the daggerboard now both become daggerboards ? The centre of sail force being between the rudder and daggerboard will still be balanced.

Logic tells me if the centre of sail force is midway between the rudder and the daggerboard then they should be the same size for balance and the turning circle would be relative big. Also, both would have the same side force from the wind.

I estimate that the rudder will now be quite large and difficult to steer with, however, if the rudder is turned to say 90 degrees (asume the boat is not moving), the aft part of the boat would now be pushed to leeward, hence a turning moment is achieved. (keep this in mind for tacking)

If you begin to move the daggerboard closer to the centre of sail force it will have to become bigger in order to maintain functionality (leverage) and the rudder can now be reduced some.

The rudder now begins to become smaller it becomes more easy to handle and less force is required to steer with, while the daggerboard is larger and now becomes the pivot point. When the daggerboard is the pivot point, the boat can only turn when it is moving, no matter how hard the wind's side force.

Now could it be that if you do move the daggerboard foreward past the centre of sail force, and compromise a bit with having a larger rudder (and part daggerboard in one), that when you turn the rudder the wind pushing the the aft part to leeward that you may be able to tack easier, and with less speed required ?

I know the tri's have their daggerboards slightly ahead of the centre of sail force... is it why they tack easier ? Why not on a cat as well ?

Something else I have been pondering for a while. Sport fish all have long thin slim fins and tails, but they do move fast (or else they won't be sport fish )
Other fish have larger wider fins, but they don't move a quickly, it doesn't mean they cannot be stronger swimmers if you have a tug of war between them.

I keep on thinking the long thin slim daggerboards are more suitable for speed while the bigger wider ones would work better for lower speed.

All fish's dorsal fins are ahead of their centre of sail force, and they use them for steering and braking assist. On boats the rudder cannot be in front (maybe someone tried this before) but works aft. A fish's rudder cannot be aft since the engine sits there

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catsketcherSenior Member

Hello Fanie

In the old days it was quite normal for mono designers to want neutral helm. It was good for self steering. In the sixties and seventies designers went for the fin keel and worked out that the rudder could be made to take some of the side force from the rig. They loaded the rudder - usually a balanced blade. This meant that the rudder acted at positive angles of attack and helped reduce leeway.

As materials improved designers could make the rudders bigger so they could make more lift - rudders are now very deep, very big and efficient on racing monos.

One problem is that the boats are more twitchy downwind - this also is because the keel chord is less. It also means that if the rudder fails the boats are very hard to jury rig. I was on a 31ft Dubios half tonner in 1987 that lost its rudder when we hit a sunfish. It was impossible to sail in any direction and only three of us could steer it back the 50miles to harbour under power.

For a cruiser I would not load the rudders highly. Planes have much the same issue - their tail plane is usually set an a negative angle of attack. This is so that the CG can be ahead of the CP of the main wing. So the plane naturally wants to dive slowly and improve flow over the wings - stay away from the dreaded stall. Planes that have the CG behind the CP of the main wing can go into a tail first stall that they can never get out of - very dangerous for a plane. Not so bad for us as we can alter sail trim.

BTW I have never designed a plane. Just was good friends with a 747 pilot who told me lots of stories and let me read his books.

cheers

Phil Thompson

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FanieFanie

This is more complicated and involved than I though.

I did some reading on the daggerboard and the lift it creates. I can see the logic behind it if the shape counter the leeward push the sails make, but on a cat does this mean one sail on only the windward daggerboard ?

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FanieFanie

Wikipedia -a swept wing is more suitable for high speeds, while an unswept wing is suitable for lower speeds. A swing-wing allows a pilot to select the correct wing configuration for the plane's intended speed. The swing-wing is most useful for those aircraft that are expected to function at both low and high speed, and for this reason it has been used primarily in military aircraft

Ok so this is a centre board in boats since one can swept (change) the angle for different speeds but then the sail centre position change (as in planes)

Is there a way one can win in this ? It seems everything is counter measured with something negative...

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tspeerSenior Member

Yes, that is what I meant.

With regard to stalling, you can always get all the lift you want just by making the board bigger! But the problem is the drag of the bigger board. So in the end, it all comes down to drag.

I suspect high aspect ratio foils have gotten a bad rap because the aspect ratio was obtained at the expense of area. True, a high aspect ratio foil will stall at a lower leeway angle, but the goal of most boards is to reduce the leeway angle in the first place!

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Yacht SkipperJunior Member

Hello,

When I had my parameters to the spreadsheet, the reefing wind speed is at 13 knots, is that a huge safety margin or I'm really oversailed ?

Mat

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terhohalmeBEng Boat Technology

It is quite a big safety marginal and it is based on ISO 12217-2 standard where the reefing wind speed is defined (it is the wind speed the designer have to put in the owners manual).

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Yacht SkipperJunior Member

Ok, what I don't understand is why the reef wind speed reduces when I add overall beam by reducing LBRC.
it's optimum at 2.2, but as soon as I try to get wider than this, the reef wind speed reduce.
Why is this if all I do is to increament the righting moment?

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terhohalmeBEng Boat Technology

Actually, you have reached the point where limiting moment in pitch comes lower than limiting moment in roll which makes catamaran more sensitive to pitch pole than capsize.

In formula, the limit is (LWL+LH)/BCB < 4 . If this is more then 4, only limiting moment in roll is used.

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Manie BSenior Member

Hi Terhohalme

is there any way that you could incorporate a dimension for the ideal placement of the mast on a cat

"E" and "J" is defined but "J" is not clearly defined as starting at the bow

or is it ??

the way i read it is that i can still get an I/J ratio of 3.10 and still not define the exact position of the mast fore and aft on the boat?

so ideally where should the mast be??

i am just guessing 60 / 40 so on a 12m cat that would be 4.8m from the bow and 7.2m from the stern??

what would be ideal for a 12m x 6m cat

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terhohalmeBEng Boat Technology

Have to let something to the designer too...

The lead of the catamaran as a lemma is 0 %, (longitudinal underwarer lateral center - sail area lateral center) but can be +2 ... -2 % of LWL. In the cat the lead is not as critical as in the mono.

And there are aft mast cats.

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rattusSeĆ±or Member

How do I calculate the longitudinal lateral center of my tighty whities? ;-)

Mike

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terhohalmeBEng Boat Technology

Try to hang them fixed in corners by nails, when you are inside...

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Nordic CatSenior Member

First you find the CoE of the brown spots, then you draw a line between this point and the wet spot on the opposite side. If you intersect this line and measure the distance, divide by the same distance and you can calculate the number of active braincells the wearer can have activated at any given time

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Nordic CatSenior Member

Terhohalmes spreadsheet gives some good guidelines, but there are a number of things you need to decide also.

I analysed plenty of cats, and found that the "normal" mast postion was between 38 and 42% from the forward waterline (Station 0)