Surprising results looking at Catamaran Performance Numbers

[Tupaia]

Originally Posted by smj

Your ability to easily tack probably comes from momentum due to weight, not anything to do with aerodynamics.

Agreed, it is much harder to tack a light fast cat than a heavy slow cat because the acceleration and deceleration is so much greater.

[arsenelupiga]

Originally Posted by smj

Your ability to easily tack probably comes from momentum due to weight, not anything to do with aerodynamics.

acknowledged one advantage of heavier cats. I thought you are "light and narrow is always better" camp ?

[arsenelupiga]

Originally Posted by Tupaia

Agreed, it is much harder to tack a light fast cat than a heavy slow cat because the acceleration and deceleration is so much greater.

yeah, fast cats have larger tack angle, so maybe that is why is harder ? We do routinely sub 90 deg tacks.

[Tupaia]

Originally Posted by arsenelupiga

yeah, fast cats have larger tack angle, so maybe that is why is harder ? We do routinely sub 90 deg tacks.

Nothing to do with tack angles. It just requires a bit more skill and experience when sailing a light cat but it is worth it to get where you are going so much quicker.

[smj]

Originally Posted by arsenelupiga

acknowledged one advantage of heavier cats. I thought you are "light and narrow is always better" camp ?

For sailing performance light and narrow is always better, something I enjoy.

[arsenelupiga]

Originally Posted by Tupaia

Nothing to do with tack angles. It just requires a bit more skill and experience when sailing a light cat but it is worth it to get where you are going so much quicker.

we travelled with light dagger cat, 38 ft, direct upwind in bumpy conditions and it was bit faster, say 10 % faster VMG. However I noticed quite jerky movement of that boat compared to ours. Wondered how comfortable that is? My wife said she enjoyed our trip so i guess we were moving considerably less.

[fxykty]

Originally Posted by arsenelupiga

we travelled with light dagger cat, 38 ft, direct upwind in bumpy conditions and it was bit faster, say 10 % faster VMG. However I noticed quite jerky movement of that boat compared to ours. Wondered how comfortable that is? My wife said she enjoyed our trip so i guess we were moving considerably less.

Not to be pedantic, but movement of a boat looks much worse from outside than it does when you’re on that boat. As an example, when racing in a one tonner regatta a while ago, going upwind in near gale conditions close to a sistership, we looked over at the other boat and thought “wow, they’re really getting thrown around, glad we’re not on board her”. Meanwhile, we had two crew debilitated by sea sickness throwing up into the cockpit and a full main completely down traveller and flattened (no reefs) to try to limit heel to 30*. We of course were moving around just as bad.

The 38 foot cat you were looking at likely weighs half as much as your cat, so of course it will be moving around more. Comfort is in the eye of beholder - did the light cat’s crew complain?

[fxykty]

Originally Posted by arsenelupiga

if ease of tacking is any indication of aerodynamics above water, then you are wrong. Virtually never had tacking problem, even in sub 5 kn winds.

Well, at 5 knots of wind there isn’t much effect of the aerodynamics. But at 20+ knots? In commensurate seas? I think you will find (perhaps have found) that the tacking of a less hydro and aero dynamic boat will be more difficult.

That might be one of the reasons one doesn’t often see less sea kindly boats sailing upwind?

[fxykty]

Originally Posted by Tupaia

Agreed, it is much harder to tack a light fast cat than a heavy slow cat because the acceleration and deceleration is so much greater.

In very light wind and flat sea conditions possibly.

Increase the wind and seas - the hydro and aero dynamics start to matter a whole bunch more than displacement. Too much drag and a tack won’t be possible without engine assistance.

[44'cruisingcat]

Originally Posted by arsenelupiga

if ease of tacking is any indication of aerodynamics above water, then you are wrong. Virtually never had tacking problem, even in sub 5 kn winds.

Neither have I.

In fact one of the first sails on my boat was in 2 - 4 knot winds. Never missed a tack, and we did plenty.

The thing is, while a lighter boat might lose more speed during a tack, it would have been going much faster to start with.

[44'cruisingcat]

Originally Posted by fxykty

Not to be pedantic, but movement of a boat looks much worse from outside than it does when you’re on that boat.

Very true. Or more accurately, it looks worse than it feels.

Fat hulls with lots of rocker definitely appear to pitch far more than slender hulls with little rocker. Which is exactly what you'd expect.

[arsenelupiga]

Originally Posted by 44'cruisingcat

Very true. Or more accurately, it looks worse than it feels.

Fat hulls with lots of rocker definitely appear to pitch far more than slender hulls with little rocker. Which is exactly what you'd expect.

i duno about your theory, my wife likes going upwind so it is quite comfortable. Sure looked rocky that boat 30 deg forward, 30 deg back. Maybe he was pushing too much. In saying that I do have some rules for upwind. Basically keep hulls glued to sea. This improves comfort and boat structure says thanks. If I cant do that increase angle and if my VMG falls enough then motors until tack. Sometimes one gets waves at angle from wind and one tack is useless, however the other is usually close to ideal.

[rexripley]

Originally Posted by john manning

My comment on your goal is why are you thinking fast. If you extend your argument to its maximum, just fly to the destination and charter.

Is your goal sailing or are you just travelling somewhere.

Every boat sails in good winds.
A fast boat sails in light winds.
A heavy boat motors in light winds.

If your goal is sailing, you want a "fast" boat. I don't care about top speed, I'm not interested in racing. In high winds we will keep the boat below a max speed target with reefing.

All airplanes exceed that max speed target.

[john manning]

Originally Posted by rexripley

Every boat sails in good winds.
A fast boat sails in light winds.
A heavy boat motors in light winds.

If your goal is sailing, you want a "fast" boat. I don't care about top speed, I'm not interested in racing. In high winds we will keep the boat below a max speed target with reefing.

All airplanes exceed that max speed target.

That's a very good response to my comment. I own two heavy yachts and while they both sail in light winds its a bit like watching paint dry.