Catamaran Balance

Hi Red Dwarf,
You are raising some very interesting questions!
The reason for all this variation is that the water and air flows constantly change their high and low points and their pressure.
For example there is a low pressure bubble behind the mast which spreads aft then shrinks back with the movement of the boat and mast on each wave. The mast and sails are acceleration and decelerating changing the apparent angle to the wind.
As she works to windward the course varies, maybe 5 degrees off course, either way on the waves.
Which is enough to have some of the fancy new sections and sails go in and out of stall all the time. So I feel the best way is to use a reasonable average lead and adjust your sails to the conditions so that you maintain a reasonable amount of average weather helm which is what most of us do instinctively.
I have measured this on my 40’ tri with long wool tell tales. I know…bit sad..
So inshort, ‘It aint just mathematics
Cheers
ChrisSR’

 

Gooday Chris Thanks foir your in-put. really appreciated.

R-D - Chris says - you raised some interesting points - - What were they ???

I missed what was interesting or valid - to this discussion, Please help me understand ???

Chris - as the wind vears through a 15* arc - back & forth - back & forth as well as lifts & breaks & accelerates up & down in a constant oscilation of at least 15* - I don't understand your 5* comment - Please assist me ??? Thanks

Chris - which of the "fancy new sections and sails - etc" - go in & out of flow - I'm at a loss to think of 1 designer or sailmaker that is doing this today - could you please give us some examples ???

Chris - there is - will always be & has - since back in the mid 60's been much said & written about - 'tell-tails' & many who claim (some falsely claimed to have the right to 'copywrite' said info) - so as a little player from back then - I'd like to ask why - 'long' - 'tell-tails' would be the way to go - as I would have thought that several lengths - in several different positions - might have shown a most informative wind flo information - - look back to - Rudders Mag - in the mid to last 60's - as there were several issues - with articles written c/w drawings of 'tell-tails' & their placement - who - where - why - length & lots of reasons behind the practice. I don't recall any prominent sailmaker - saying - the info presented - was wrong nor did they have anything to say to the contrary - about the articles.

You seem - much more experienced than I am & I'd sure like to learn everything I can. I'd like to sail faster & spend less money & mistakes getting to that point - so I'd sure like to learn all I can from you. Thanks so much. I'm not trying to challenge you but I sure am trying to learn all I can, Thanks again, james

 

Just the smallest mistake in the model will set all testing way out of truth. Models are not toys for back yard hobbyists.

 

Hi James,
I hope I can help, at little anyway. Please note, I don’t claim to be “right” and am happy to learn from anyone. This is just as I have come to see things.
1. Red Dwarf raised the interesting question as to why there were so many uncertainties in selecting the position for his C/B.
2. The wind may veer 15 deg. Naturally OK, but that takes a few minutes to happen so you have time to respond.
3. I was referring to the boat wandering off course due to effect of the waves. The waves push a light displacement boat off course and so you are constantly correcting maybe 5 deg. Each way on each wave.
4. So, unless you have some very contentious sheet hands working the sheets at every wave, your sails and C/B are not going to be set at optimal angles all the time.
5. You will see the short tell-tales in constant movement. But the aim is to have them correct most of the time.
6. For C/B sections I prefer the older NACA 4 digit series over the NACA 5 digit series because though while they may have more drag in 2-3deg of lee-way they have less at bigger angles and are then more tolerant of varying angles of lee-way.
7. With the older Dacron cloth, the shape was also more forgiving and less inclined to lose drive as it pumped back and forth. With a metal hard surface, you have to have the sail set right all the time.
8. You can plot the flow of air over your sails with short tell-tales. But that only shows you what is happening very close to the sail. With a longer tell-tail, say 2 meters, on the end of a 2 metre wand, you can explore the movement of air further away from the cloth. You can map the flow of the air as it approaches the boat right through to how it leaves the boat
9. James, I found the most useful text was the North Sails manual, really worth tracking down a copy. My current favourite is Lars Larsson and Rolf Eliasson’s Principles of Yacht Design. I do wish they had been more inclusive of multihulls, Oh well, maybe their next book……
Cheers,
ChrisSR

 

Yes, it is a bit of an art, simply because we cannot easily measure every conceivable factor that might be influencing balance. Ultimately, it comes down to feedback feel on the tiller (or wheel) and the boat's ability to maximize VMG (speed made good to windward). VMG is the ultimate measure, and the boat with the least drag will have the best VMG. Drag can come from too much heel (prevalent at model scale, particularly with a monohull) or too much rudder--the biggest adjustable factor in trying to balance the boat. You want just a little bit of rudder for the best balance. How big the rudder is, how it is shaped, and where it is located are all factors that influence balance.

Doug's reply is good, effects can be seen over time, or between two boats sailing side by side with different features--see which boat sails better (faster VMG). Also, bigger models are better than smaller models.

ChrisSR mentions the effects of waves. Most of our number-crunching analysis that we do as designers is for steady-state, smooth water performance. But actual sailing is never steady-state and almost never smooth water. The boats move around so much that the power from the rig and the drag on the hull and appendages are always changing moment to moment. We can't really calculate what these momentary effects are, but they constantly change the angle of attack and resolution of forces on the rig and hull. Therefore, we end up with a "moving average" of how the boat performs moment to moment, and we steer by feel accordingly.

In the end, the best place to start for balance is, as we have said above, put the center of the sailplan directly over the center of the daggerboard, and you'll be right in the middle of the required balance. It's as simple as that. If for some reason you get it wrong and the boat just does not balance right, then something has to change a few inches one way or another. But this guideline has worked over time based on the experience of those that sail the most.

Eric

 

Can anyone recommend some books or online articles that cover all aspects of catamaran balance? For example CG, LCB and all the other factors that play into balance.

 

To get a boat "balanced" you need everything in equilibrium. The boat will bear off, luff, make leeway, heel etc until that state is reached.

A monohull will have a big lead because it heels. In general the more heel the bigger the lead. So 30 years ago I thought that multihulls needed no lead. But slowly I increased it to 2% and then 3-4%. That was because I found that as the boat heeled and trimmed forward as speed increased the CLR moved forward

A trimaran is different from a catamaran because you have an asymmetric drag from the lee hull further forward. So many trimarans have daggerboards further forward than you might expect

But really it "all depends". Hull shape makes a big difference, so too does the AR of the sailplan, the type of cloth used, the predicted boat speed, the ratio between rudder and board size, whether you include the rudders in the CLR or not.

So I don't think you can generalise too far, while different designers calculate the lead in different ways.

Model tests don't always give the right full size answers. The 12m Mariner being a good example. Even Australia 2 rarely performed as the tank suggested it should. RC trimarans often have to fit rudder Tfoils to stop the bows burying and the boat pitchpoling. You don't need those on full size boats.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

I think Richard hit upon the correct answers in his reply.

What I didn't see in most all of the replies was a little more full explanation of why we utilized such large 'leads' originally. When the boat (monohull) heels over its center of sail effort moves over a considerable distance from the centerline drag of the keel, there is a considerable 'moment of two forces' (fwd drive of the sails vs backward drag of the keel) created that tends to what to turn the vessel up into the wind. To counter that 'turning moment we needed to move the sails CE forward a certain amount (lead) so that a two other forces could create a counter moment,...( the heeling force of the wind vs the leeway reducing forces of the keel), trying to turn the vessel off the wind

In level sailing multihulls (non-flying ones) we don't displace these sail centers and keel (or board) centers to such a great degree, so much less 'lead' in needed.

However sometimes the multihullers forget this little fact when operating their 'boards' in the outer hulls of their vessels.

i began to look more seriously at this when I was drawing up the idea of my Dyna-Rig cat Lets see now ,where is the CE on that Dynarig? ...and perhaps I can set it right over the CLR of that single centerboard arrangement??

...and that central board arrangement might just make tacking this vessel easier (going to be a challenge to get that Dynarir to tack on a catamaran

 

An older discussion, but I have a related question:

What is the sailplan you calculate the CE for? The maximum upwind situation? If you reef the main or hoist a big foresail the CE shifts forward by up to 9 %. So is lead also a way to get a balance for different sail combinations?

 

They essentially said, keep the mainsail luff above the leading edge of the keel - with reference to fin keel sloop monohulls. Sailing catamarans don't seem to be all that picky, but 2 - 3% lead would be common.

 

Yacht designers use the working rig, that is, fore triangle and mainsail in sloops. CE changes little with foresails that overlap mainsails.

 

It makes no difference whether the foresails overlap or not. When a boat heels the CE of the sails moves to leeward creating weather helm. The lead is to prevent too much weather helm.

 

The shorter your mast (s), the flatter your sails, and the less your boat heels, the less weather helm your monohull will develop weather helm from this effect. In a catamaran, of course, this effect will be almost non-existent. Also, asymmetry in heeled waterlines greatly increases this effect in monohulls.