Upwind performance of condomarans

[Just Another Sa]

Originally Posted by 44'cruisingcat

Extra buoyancy at the centre is like adding more rocker, or reducing the prismatic coefficient. Boats with higher prismatic pitch less.

Boats with higher prismatic typically also have higher waterplane area coefficient, resulting less pitching when damping dictates. Not so with adding buoyancy at the center as minikeels, as those have no effect on waterplanearea if hull sides are vertical at waterline and/or weight is added as well.

Originally Posted by 44'cruisingcat

Another way to look at it - adding buoyancy to the centre is effectively the same as adding weight to the ends.

Not so.
Adding weight to the ends increases moment of inertia, while adding buoyancy to the center does not. You would also need to add weight to your mast top to have the same effect.

[arsenelupiga]

Originally Posted by django37

The big overlap part of the headsail dont do much good and slows tacking. Important though to have some overlap to reestablish the flow on the lee side óf the mainsail, that our thick nonrevolving mast has spoiled. The selftacking jibs on the new Lagoons make them slow.
I want some weaterhelm, rudder is some 2 dgr. of centerline, it helps going to windward
Mainsail: as the foot and luff is unchanged but with more area, most will say, that my main has lower aspect ratio.

But, yes: in light winds, say under 5 knots, we are very slow. When the wind gets above 20 knots we are faster to windward than all monohulls of the same length. As the wind increases above 20 knots, we do not try to sail above hullspeed, but point higher. As the lee hull is pressed down, it feels like it gets better bite in the water and leeway gets less - about 5 dgr. in 30 knots of wind we point higher and COG over ground is higher than the monohulls, they heel over and slide to lee. In 37 knots true wind we lift the weater hull with full sails - and reef - oh, we reef ealier when cruising

some good info there, thanks.

yeah, that is my experience as well. But I chicken out at around 28 kn app, actively observing rig and structure for any sign of stress. All appears in order, no shaking no cracking etc. Where would one look for stress points in this kind of sailing ?

[catsketcher]

Originally Posted by Just Another Sa

Boats with higher prismatic typically also have higher waterplane area coefficient, resulting less pitching when damping dictates. Not so with adding buoyancy at the center as minikeels, as those have no effect on waterplanearea if hull sides are vertical at waterline and/or weight is added as well.


Not so.
Adding weight to the ends increases moment of inertia, while adding buoyancy to the center does not. You would also need to add weight to your mast top to have the same effect.

If we take a minikeel and make it absurdly wide and deep then the whole cat could float on the keel and the hulls would have a very low pitch resistance. In the 80s Randy Thomas put a large NACA minikeel on his Cross trimaran and he stated that the boat lifted out of the water slightly and was more "rocky". The added volume had reduced the immersion of the hull and reduced pitch resistance.

Plotting a curve of areas that includes minikeels would be the better way to understand distributed volume. The keels will provide buoyancy and therefore will produce a peak in the volume in the centre. The hull designer can take note of this by increasing volume in the ends during the design process but you will have a lump of buoyancy, doing its thing, in the middle of the boat.

A quick calculation of a 4.0m by 200mm wide keel 400mm deep gives a volume of about 200litres. On my lightish cat that is about 10% of the hull volume so it would be really significant (if I had a big minikeel) but less for a heavier cat. Not too much flotation in the middle of a condocat but it will provide some buoyancy on the middle and influence distributed volume calculations. Some mono designers don't include appendages in calcs but considering cat keels are positively buoyant it seems silly not to in multis.

cheers

Phil

[44'cruisingcat]

Sailing Catamarans - Keels or Daggerboards, the pros and cons


"Unfortunately, using LAR keels results in a slower boat and more pitching. That is because to optimize speed you need a hull with buoyancy in the ends, not in the middle (technically you need a high Prismatic Coefficient - Cp). Clearly adding a LAR keel adds buoyancy in exactly the wrong place. Furthermore the midships buoyancy makes the boat pitch more as the hull is more "diamond-like". "

[arsenelupiga]

Originally Posted by 44'cruisingcat

Sailing Catamarans - Keels or Daggerboards, the pros and cons


"Unfortunately, using LAR keels results in a slower boat and more pitching. That is because to optimize speed you need a hull with buoyancy in the ends, not in the middle (technically you need a high Prismatic Coefficient - Cp). Clearly adding a LAR keel adds buoyancy in exactly the wrong place. Furthermore the midships buoyancy makes the boat pitch more as the hull is more "diamond-like". "

but if one stores stuff with the weight of bouoyancy above keels, near bottom of hull, this would more or less cancel this effect ?

[PaulinOz]

Originally Posted by 44'cruisingcat

Sailing Catamarans - Keels or Daggerboards, the pros and cons"

Very interesting read but his summation at the end says it all.

In short
If you don't ever expect to race then keels are OK
If you think crosscut Dacron sails are good enough then keels are OK
If you trail your boat regularly fit dagger boards
If you have wheel steering and inboard engines and expect to dry out often fit keels
If you want the best performance then fit dagger boards, but be prepared to use them.


I guess the third point is not that relevant to Cruising Catamarans.

[44'cruisingcat]

True. But the point was about the buoyancy/pitching.

I've been saying all along that there are minikeel boats that sail well. And daggerboards alone don't make a performance cat.

[Just Another Sa]

Originally Posted by 44'cruisingcat

Sailing Catamarans - Keels or Daggerboards, the pros and cons

"Unfortunately, using LAR keels results in a slower boat and more pitching. That is because to optimize speed you need a hull with buoyancy in the ends, not in the middle (technically you need a high Prismatic Coefficient - Cp). Clearly adding a LAR keel adds buoyancy in exactly the wrong place.

Lower prismatic coefficient leads to less wavemaking drag above hullspeed, thus faster boat. No surprises in that.

Originally Posted by 44'cruisingcat

Furthermore the midships buoyancy makes the boat pitch more as the hull is more "diamond-like". "

An empty claim with nothing at all backing it up.

[Just Another Sa]

Originally Posted by arsenelupiga

but if one stores stuff with the weight of bouoyancy above keels, near bottom of hull, this would more or less cancel this effect ?

No, the lower prismatic increases drag, and weight distribution or amount of weight has nothing to do with that.
There is no effect on pitching. It is caused by 2 separate causes:
wave induced and aerodynamically produced. The former is most of the time far more important. How much an incoming wave lift bows depends on added displacement, and therefore waterplane area times height of that extra immersion (which is not a constant). And on leverage of the force produced, damping and moment of inertia about the port/starboard axis. There is no connection whatsoever with either static value or distribution of initial buoyancy. It's an unsupported myth to be busted. Only the rate of change of buoyancy as a function of pitch angle matters.

[Just Another Sa]

Originally Posted by Just Another Sa

Only the rate of change of buoyancy as a function of pitch angle matters.

https://en.wikipedia.org/wiki/Pendulum_(mathematics)
To oppose the claim one would have to prove wrong all the associated mathematics, physics and engineering, in addition to the naval architecture.
Only the rate of change of moment counts, any constant component of moment or force is irrelevant, such as buoyancy of a minikeel remaining fully immersed during pitching motion.

[44'cruisingcat]

Originally Posted by Just Another Sa

Lower prismatic coefficient leads to less wavemaking drag above hullspeed, thus faster boat. No surprises in that.


An empty claim with nothing at all backing it up.

So you reckon you know more about multihull design than Richard Woods?

[catsketcher]

Originally Posted by Just Another Sa

Only the rate of change of buoyancy as a function of pitch angle matters.

This will be pretty closely related to the prismatic under the water. Fuller ends above the water reflect a higher prismatic underwater.

Sometimes a lower prismatic is faster, in lighter winds a low prismatic has lower drag. In fact the case for a higher prismatic was demonstrated very well in the early 80s. Two 42ft Crowther cats were racing together in Sydney during the 1982 AMOC. Both well sailed and well campaigned. The Spindrift 42 had a low prismatic shape and Bagatelle - the newer 42 - had a much higher prismatic.

Inside Sydney harbour the newer boat, Bags, was not faster but when they headed outside Pennant, the Spindrift, was left for dead. She sat and hobbyhorsed whilst Bags pitched on a wave, dampened quickly and sailed away. Even though she had a higher drag hull - a higher prismatic in light winds is slower in flat water. It was a telling moment in cat design and designers no longer draw fine ends on cats. Crowther wrote about it because it was such a clear example of pitch resistance and hull design. (Bags also had bulbs which he liked as well).

So if we design a cat to take a load and make the ends finer because of the volume of the keel, it will have a lower prismatic. But yeah, I get it, the rate of change of buoyancy will not be changed if we load it up another 200kg.


The Uni of Southampton did tank tests of model cat hulls. Their conclusion was that "a low cP was beneficial at low Froude numbers (lower speeds)" and that at high speeds "there was little difference"

https://eprints.soton.ac.uk/46460/1/086.pdf

Pitch resistance and nice stern steps are pretty important, more important than smooth water tank tests.

cheers

Phil

[arsenelupiga]

i tell you what:

get all 40 feet sailing cats ever built, racing super racing, cruising, whatever, load with 3T of stuff. And let them go racing.

I bet Lagoon will win

[PaulinOz]

Quote "load with 3T of stuff."

Only going light ship then not very fair.

[arsenelupiga]

Originally Posted by PaulinOz

Quote "load with 3T of stuff."

Only going light ship then not very fair.

Ok, lets do 8 T then. Here is what I will add so wont have reason to go to shops that often. And see who is the fastest !!!