Light air performance of different multihulls?

[nortonscove]

Originally Posted by Yihang

it's definitely a little cringy watching them ignore all the issues in the 1260 they picked up in other boats, ie just top opening hatches in the cabins. In the other boats they were like 'oh, only top opening hatches means no ventilation in rain.' on the 1260 'two opening hatches, one above the bed, perfect!'. goodluck using that one above the bed in rain or even overnight with dew.

Totally agree. I've found this to be true of all reviews. I own a pickup truck that has been picked apart on numerous sites but then have actually recently had the chance to drive another brand where I noticed issues that were not mentioned on the same site during that vehicles review. Money and the continued access can allow for blinders.

[django37]

Originally Posted by arsenelupiga

Upwind 3 kn VMg you are lookng at 9 kn true. 4 kn vmg need 12 kn. 5 kn vmg, i dont do it as I like my rigging and my wife complains.

I tend to depower at 10 kn boat speed.

8 m/s (or 16 knots) should give you 5 knots VMG upwind

[arsenelupiga]

Originally Posted by django37

8 m/s (or 16 knots) should give you 5 knots VMG upwind

may be, but i have not experienced this situation. If we get this kind of wind tends to be variable and one cant really say what was average wind speed. Also for travel we avoid this kind of forecasts.

In sub 10 kn wind speed environment 90 % of time, we operated last several daysails , wind speeds are remarkably static and one can actually work on particular wind speed wind angle and trim. Excellent for trim practice and physical exercise.

[sailjumanji]

Originally Posted by mglonnro

And in the spirit of my original question: what kind of wind do you need to get the boat moving 4.. 5.. 6.. knots?

For us a light wind is 8 knots, and we will usually do between 5-6 knots upwind.
Below that, we were in the Abacos in 2019 and had a couple of days of really light wind. I remember 5 knots wind, about 3.5 knots boat speed upwind. Below 3 knots boat speed, will usually start the motors to get there.

[clakiep]

There is one situation where a light or even heavy multihull will perform much inferior under sail than a monohull ballasted with a heavy keel : light wind and choppy sees ! The lateral movement of the mast, although of small amplitude, will be fast, making the sails flap so that no steady airflow can establish itself along the sails.

Capt. Claus - ocean tramp of the eighties.

[introverted]

Our boat is a tank in light winds prior to getting a code 0 but I knew that before buying. with main and jib its not worth it in anything less than 7 knots. With our code 0 I can get at least 50% of any wind speed down to 3-4 knots without even trying to trim. Which for me is great. your sails are going to play a big roll in performance. A big light sail is going to make any catamaran more peppy in light air conditions. All things being equal, narrow hull cats will have an advantage.

[wingssail]

Originally Posted by introverted

Our boat is a tank in light winds prior to getting a code 0 but I knew that before buying. with main and jib its not worth it in anything less than 7 knots. With our code 0 I can get at least 50% of any wind speed down to 3-4 knots without even trying to trim. Which for me is great. your sails are going to play a big roll in performance. A big light sail is going to make any catamaran more peppy in light air conditions. All things being equal, narrow hull cats will have an advantage.

So this makes sense to me. With Catamarans generally sailing at lower wind angles than monohulls, a code zero, which is not effective as a purely windward sail, makes sense. You are matching the sail's effective wind angle to the boat's effective sailing angle.

[smj]

Originally Posted by wingssail

So this makes sense to me. With Catamarans generally sailing at lower wind angles than monohulls, a code zero, which is not effective as a purely windward sail, makes sense. You are matching the sail's effective wind angle to the boat's effective sailing angle.

On most purpose built cruising/charter catamarans yes........

[jdazey]

Our threshold for turning on the engines varies on conditions. If we're on a tight schedule and have fuel, we can motor 8+kts. If there isn't a deadline and over long distances, we'll sail as much as possible. Our passage from Ensenada to Sitka was 2600nm+/-. If our speed dropped much below 3.5kts (4-4.5kts wind) I'd give it a half hour. If we're still dropping below 3kts, I'd start a motor. Main and jib only, no code/screecher sail, but I'd sure like one.

[Tupaia]

Originally Posted by wingssail

So this makes sense to me. With Catamarans generally sailing at lower wind angles than monohulls, a code zero, which is not effective as a purely windward sail, makes sense. You are matching the sail's effective wind angle to the boat's effective sailing angle.

We will pull 35 degrees apparent with our Code Zero in flat sea this equates to 8knts boat speed in 6knts TWS or 10knts in 8knts TWS. In light winds the faster the boat goes the more important it is to be able to sail close to the wind. It is possible to be close hauled downwind, the only problem is that might not be in the direction you want to go.

[mikereed100]

Originally Posted by introverted

Our boat is a tank in light winds prior to getting a code 0 but I knew that before buying. with main and jib its not worth it in anything less than 7 knots. With our code 0 I can get at least 50% of any wind speed down to 3-4 knots without even trying to trim. Which for me is great. your sails are going to play a big roll in performance. A big light sail is going to make any catamaran more peppy in light air conditions. All things being equal, narrow hull cats will have an advantage.

Not sure if I agree with this. It is my understanding that, at lower speeds, wetted surface is responsible for most of the drag on a hull. Since a wider hull will have less wetted surface for a given length, it stands to reason that it would have a speed advantage until wave making overtakes wetted surface area as the greatest drag inducer. In other words, a cat with wider hulls will have an advantage at lower speeds in light air, whereas a cat with narrow hulls with have be more easily driven and be faster as the wind fills in.

Of course, I could be making all this stuff up. Hopefully someone with a background in naval architecture will chime in and set us straight.

[jdazey]

Not an NA, but that's my understanding as well.


Cheers,

[KC375]

Originally Posted by mikereed100

Not sure if I agree with this. It is my understanding that, at lower speeds, wetted surface is responsible for most of the drag on a hull. Since a wider hull will have less wetted surface for a given length, it stands to reason that it would have a speed advantage until wave making overtakes wetted surface area as the greatest drag inducer. In other words, a cat with wider hulls will have an advantage at lower speeds in light air, whereas a cat with narrow hulls with have be more easily driven and be faster as the wind fills in.

Of course, I could be making all this stuff up. Hopefully someone with a background in naval architecture will chime in and set us straight.

If you keep the displacement fixed then for a given length I think you will find a semicircular cross section gives the lowest surface area.
This article on duckworks covers it

https://www.duckworksmagazine.com/03...wing/index.htm



https://www.duckworksmagazine.com/03...wing/fig-2.gif

[wingssail]

Originally Posted by mikereed100

Not sure if I agree with this. It is my understanding that, at lower speeds, wetted surface is responsible for most of the drag on a hull. Since a wider hull will have less wetted surface for a given length, it stands to reason that it would have a speed advantage until wave making overtakes wetted surface area as the greatest drag inducer. In other words, a cat with wider hulls will have an advantage at lower speeds in light air, whereas a cat with narrow hulls with have be more easily driven and be faster as the wind fills in.

Of course, I could be making all this stuff up. Hopefully someone with a background in naval architecture will chime in and set us straight.

No, a narrower hull (to a certain point) will have less surface area and therefor less drag. Fine hulls are better in light air.

[mikereed100]

Originally Posted by wingssail

No, a narrower hull (to a certain point) will have less surface area and therefor less drag. Fine hulls are better in light air.

You may be right, but this is counter intuitive for me. We all accept that a semicircular profile provides the least wetted surface as seen in the diagram posted by KC375. It then stands to reason that a semicircular longitudinal profile would present the lowest wetted surface as well. If you combine the 2 you get a hemisphere, which would be the hull shape with the least wetted surface. Not very hydrodynamic, but least wetted surface nonetheless. If you stretch that hemisphere in one direction, keeping the displacement the same, you will get a more efficient hull shape, but the wetted surface area will increase, no?