Light air performance of different multihulls?

[wingssail]

Originally Posted by barnakiel

I have noticed most any wind will get you moving in flat water.

This all changes in open water (ocean swell etc.)

From my observation nearly all cats can keep wind in the sails longer than most monos.

Only flat racing monos and some very big monos will sail fine in light winds ;-(

b.

Mostly, my experience indicates exactly the opposite. In ocean swells, where the movement is gentle, a monohull can keep sails filled. The rise and fall, gentle rolling, or slow pitching, of a monohull in an ocean swell does not disturb the flow of air over the sails. The fact that you can heel a monohull in light air to help the sails hold a good shape, improves the flow and enhances light wind performance. All racers know or sense these things.

Flat monos, racing or otherwise, suffer in light air due to the increased wetted surface. For an extreme example which illustrates this, look at the 100' Commanche vs the 100' Wild Oats IV in Sydney Hobart. Commanche is ultra wide while Wild Oats is narrow. In heavier winds Commanche's for stability enables her to take off, but in the light stuff Wild Oats is faster.

I observe the same effects in the racing I do. The fastest boats in our fleet are wide and flat compared to most of the others. In light air they are sticky and can't really get moving. As soon as the wind gets up they are gone.

In choppier waves the disadvantage of a multi hull is that the hull is supported by two points in the water. If the boat is beam to the waves or pointing into the waves the two points are lifted independently and this causes the jerky motion often referred to regarding multihulls. That jerky motion is not good for air flow over the sails or for light air sailing performance.

In my opinion.

[Tupaia]

Originally Posted by barnakiel

5, 10 or 15knots all sound fine.

But are your boats sailed to nowhere (I mean imagine a typical weekend sail on a bay).

Then there is long distance passage making. And here you will not only care about how fast you are sailing to nowhere but also at what angle.

Cats sail different angles than mono. Many destinations are either downwind ones or upwind ones.

So the point is how fast you can go towards the target.

So, if you are into further trips, think not just how fast the boat sails in light winds but also what angles you can sail - fast, and comfortably.

b.

This very relevant as I stated in an earlier post

"It is possible to be close hauled downwind, the only problem is that might not be in the direction you want to go".

This why it is so important to have small sheeting angles. The faster the boat relative to the wind speed the more the apparent wind will be bought forward.

Ice boats capable of speeds in excess of 50 knots are sheeted in on all points of sail. They are extremely lightweight and don't suffer leeway. Their ability to "point" is restricted because they are unable to sheet IN any further.

A cat with relatively large sheeting angles, average power to weight ratios and mini-keels, will tend towards poor light wind performance and mediocre all round performance.

For light wind performance a cat requires, low weight, high power, effective hydrodynamics (dagger boards) and very close sheeting angles.

[barnakiel]

Yes. It is so when you think about the cans where much of the distance covered is upwind work.


Now think about a typical downwind work. Say a sail from Canaries to St Lucia or from Galapagos to French Polynesia.


A cat will deploy a spinnaker and sail effortlessly outstanding miles - because the spinnaker will be fully set and 100% stable flown of the bows. Additionally he main will be square headed and so with plenty of SA high - where it should be.


So now suddenly a boat that was thought of as a pig is suddenly flying.


I too was all about sheeting angles and weight when I raced (dinghies, mostly) but the optics changes whne you start going along the trade wind route. Now my boats stellar performance on paper is pretty useless.


So to say everything has many facets. We often think in round the cans performance terms while we are bound to sail mostly down wind passages in our cruising life.


b.

[barnakiel]

Originally Posted by wingssail

Mostly, my experience indicates exactly the opposite. In ocean swells, where the movement is gentle, a monohull can keep sails filled. The rise and fall, gentle rolling, or slow pitching, of a monohull in an ocean swell does not disturb the flow of air over the sails. The fact that you can heel a monohull in light air to help the sails hold a good shape, improves the flow and enhances light wind performance. All racers know or sense these things.

Flat monos, racing or otherwise, suffer in light air due to the increased wetted surface. For an extreme example which illustrates this, look at the 100' Commanche vs the 100' Wild Oats IV in Sydney Hobart. Commanche is ultra wide while Wild Oats is narrow. In heavier winds Commanche's for stability enables her to take off, but in the light stuff Wild Oats is faster.

I observe the same effects in the racing I do. The fastest boats in our fleet are wide and flat compared to most of the others. In light air they are sticky and can't really get moving. As soon as the wind gets up they are gone.

In choppier waves the disadvantage of a multi hull is that the hull is supported by two points in the water. If the boat is beam to the waves or pointing into the waves the two points are lifted independently and this causes the jerky motion often referred to regarding multihulls. That jerky motion is not good for air flow over the sails or for light air sailing performance.

In my opinion.

Yes. 100' boat examples all right.



Ours is only 26' and will not keep wind in anything below 5knots on a passage. as a rule swell is 5-7 feet and most of the time from two different directions. At times anything below 10 knots is a waste of time as we are forced by swell to take angles that lead us to Nova Scotia while our destination is Lisboa ...


I am with you on big boats. Watching J racing teaches us that in a mono it makes so much sense to be BIG (or else flat - like a Mini Class boat ...)


b.

[mikereed100]

Originally Posted by fxykty

Your intuition seems to point to a coracle as being optimum, which is silly. Shape (wave making) drag is a much higher factor at speeds above about 5 knots, but still matters even at lower speeds.

With a fat hull you must have less depth to the hull and will have flattened semi-circular hull sections. With a skinny hull the hull can retain optimally semi-circular hull sections. Skinny hull will generally have less wetted surface for a given displacement, unless it gets so skinny that it has vertical sides.

A coracle, if shaped as a hemisphere, would indeed have the least wetted surface, but, as I mentioned, would not be very efficient. My point is that, given the same displacement, a hull with a length to beam ration of, for example, 8:1 would have an advantage at lower speeds than a cat with a L/B ration of say 14:1. Once speed increases and wave making overtakes wetted surface as the greatest drag inducer, the narrower hull has the advantage, allowing it to reach higher speeds that the fatter hull.

Here are some wetted surface numbers lifted from a thread on Boatdesign.net that illustrates my point:

#1__13 ft long, l/b 15.7:1, cp 0.62, wetted area 4957 in/sq, hull speed 4.8 knots
#2__9.2 ft long, l/b 9.1:1, cp 0.62, wetted area 4409 in/sq, hull speed 3.5 knots
#3__6.7 ft long, l/b 5.7:1, cp 0.62, wetted area 3654 in/sq, hull speed 3.0 knots


These numbers are for a kayak, but the principle remains the same. As length to beam ration increases, wetted surface increases as well. If you look at #1 and #2 above you will see that the hull with l/b of 9.1:1 has about 9% less wetted surface than hull #1 with a l/b of 15.7:1. At low speeds this should translate into a slight speed advantage.

This is relevant to this thread in that, if you value light air ability over top speed, one might want to look at cats with fatter hulls. For a cruising boat this provides better load carrying capacity as well.
Your point that a wider hull must have flat sections which increase wetted surface is well taken, but is that necessarily true? Is it not possible that even a relatively wide cat hull could be drawn with semicircular hull sections?

[wingssail]

Originally Posted by barnakiel

Yes. It is so when you think about the cans where much of the distance covered is upwind work.

Now think about a typical downwind work. Say a sail from Canaries to St Lucia or from Galapagos to French Polynesia.

A cat will deploy a spinnaker and sail effortlessly outstanding miles - because the spinnaker will be fully set and 100% stable flown of the bows. Additionally he main will be square headed and so with plenty of SA high - where it should be.

So now suddenly a boat that was thought of as a pig is suddenly flying.

I too was all about sheeting angles and weight when I raced (dinghies, mostly) but the optics changes whne you start going along the trade wind route. Now my boats stellar performance on paper is pretty useless.

So to say everything has many facets. We often think in round the cans performance terms while we are bound to sail mostly down wind passages in our cruising life.

b.

Definitely a cat, even a cruising cat, will likely sail faster on a broad reach or downwind on passages than a monohull. Disregarding the fact that many ocean passages wind up having significant portions of upwind work, another factor comes into play: Danger of capsize. This danger is tangible. Many accounts I've read confirm that skippers are aware of that danger and rarely push their boats with large downwind sails while cruising. In squally conditions or even just overnight, skippers reduce sail to ensure that unexpected squalls or stronger winds don't overpower their boats. Some often drop mainsails to be safe.

The result of this is that the cats don't normally have faster crossing times. Of course a high performance cat crewed by keen sailors will be faster on an offwind passage.

Normal cruising boats? The passage times will often not be shorter.

Looking at the 2019 ARC results we can see that the normal cats (not the top 10 high performance boats pushed by aggressive crews and not the bottom 10) made the crossing in 20-22 days. Looking at the overall results for cruising monohulls, (excluding the top 20 and bottom 20) the crossing times were 18-25 days. https://www.worldcruising.com/arc/ar...x?eventid=117#

The cats I am referring to were virtually all over 45 ft. The monohulls included boats down to 36 feet.

So this does not support the theory that cats are faster crossing oceans.

[barnakiel]

Originally Posted by wingssail

Definitely a cat, even a cruising cat, will likely sail faster on a broad reach or downwind on passages than a monohull. Disregarding the fact that many ocean passages wind up having significant portions of upwind work, another factor comes into play: Danger of capsize. This danger is tangible. Many accounts I've read confirm that skippers are aware of that danger and rarely push their boats with large downwind sails while cruising. In squally conditions or even just overnight, skippers reduce sail to ensure that unexpected squalls or stronger winds don't overpower their boats. Some often drop mainsails to be safe.

The result of this is that the cats don't normally have faster crossing times. Of course a high performance cat crewed by keen sailors will be faster on an offwind passage.

Normal cruising boats? The passage times will often not be shorter.

Looking at the 2019 ARC results we can see that the normal cats (not the top 10 high performance boats pushed by aggressive crews and not the bottom 10) made the crossing in 20-22 days. Looking at the overall results for cruising monohulls, (excluding the top 20 and bottom 20) the crossing times were 18-25 days. https://www.worldcruising.com/arc/ar...x?eventid=117#

The cats I am referring to were virtually all over 45 ft. The monohulls included boats down to 36 feet.

So this does not support the theory that cats are faster crossing oceans.

+1!


My clients in a Lagoon 450 sailed Las Palmas to Barbados in 13 and 17 days respectively. Off course, they had that amazing weather router on their side ... ;-)


My observation is that a smartly rigged cat is the ultimate light-wind, down-wind cruising weapon. They keep the wind much longer and they have much better choice of downwind sail-able angles. They will go SLOW ddw in light winds, just like anybody else, but they will be able to shape their course to the target better than a mono. Thus their VMG downwind is, in cruising terms, stellar.


Not to bash monos. We have a mono and I love monos. I find them more technical, more dependent on skillful trim and on driver's ability to drive in light airs.


I also love sailing in light airs. Although given the tool in hand, our happy episodes are limited mostly to the return trip once we near well up with the Azores. Elsewhere, swell is the killjoy.



Cheers,
b.

[barnakiel]

Originally Posted by mikereed100

(...)


Is it not possible that even a relatively wide cat hull could be drawn with semicircular hull sections?

+1!


Same question here.


b.

[Training Wheels]

Originally Posted by mikereed100

My point is that, given the same displacement, a hull with a length to beam ration of, for example, 8:1 would have an advantage at lower speeds than a cat with a L/B ration of say 14:1. Once speed increases and wave making overtakes wetted surface as the greatest drag inducer, the narrower hull has the advantage, allowing it to reach higher speeds that the fatter hull. ?

I would think that would be true only if displacement and SA/D are similar.

[wingssail]

Originally Posted by barnakiel

Yes. 100' boat examples all right.

Ours is only 26' and will not keep wind in anything below 5knots on a passage. as a rule swell is 5-7 feet and most of the time from two different directions. At times anything below 10 knots is a waste of time as we are forced by swell to take angles that lead us to Nova Scotia while our destination is Lisboa ...

I am with you on big boats. Watching J racing teaches us that in a mono it makes so much sense to be BIG (or else flat - like a Mini Class boat ...)

b.

Barnakiel, I only used big boats because the photos were easily found and the comparisons were visually evident. I think that, unless your boat is quite different then mine, even smaller boats like yours, can sail well in ocean swells, mono or multi. In rough weather things become different, and upwind non-racing multihull boats are at a disadvantage. Offwind, multihull boats are faster in most conditions, especially stronger winds, but skippers often reduce sail for safety, so actual crossings of mono and multi hull cruising boats are similar.

[wingssail]

Originally Posted by mikereed100

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?

Do wider hulls have less surface area?

No

Look at the drawings, taken from KC375 (post #43) and duckworks (https://www.duckworksmagazine.com/03...wing/fig-2.gif)

Yes these are not scientific measurements, but you can clearly see that wide hulls have more surface area than normal hull shapes.

But the multi hull example deserves more thought. Based on the numbers from post 43, for the same weight, the surface area will be greater. But we know that narrow hulls, such as Outremer, are faster. The difference is in two factors, weight carried, and wave making. It seems to me that the narrow hulls of a multi make less waves and a lighter multihull immerses less of it's hull in the water, reducing wetted surface.

[barnakiel]

But now. This is a simplification.


A wide boat can do without balast. A narrow one cannot. Hence the wide one will only weigh (ha ha, another simplification) some 50% of the heavy one.


Now draw those hulls again, now measure their wet area.


You used same weight, but narrow and wide weight will NOT be the same weight. Ballast normally is range 50% of the weight, and ballast is OPTIONAL in a wide boat.


Ballast is also optional in a cat ... ;-)


There is also one more factor. Assuming wide hulls are sailed flat. They are not. Look at Comanche again THEN watch Ken Read talking about Comanche becoming a mono-maran when heeled. A canting keel is such a nice device ... it works both ways!



b

[Mr B]

My Gemini weighs 4.5 Tons,
Its a cork, It sits on top of the water, not in it,
It needs 3 knots to move on the 150 Genoa,
It sails usually 2 knots under wind speed,
I like to sail between 5 and 10 knots,

I spent 7 days sailing (motoring) in Bass Strait, I had one day with wind,
9 Knots of speed, 3 metre waves,
The rest of the time it was dead calm and no wind,
The Genoa hung on the boat like a wet rag, The rest of the time,

We have either no wind or screaming winds,

[jdmuys]

Originally Posted by wingssail

Looking at the 2019 ARC results we can see that the normal cats (not the top 10 high performance boats pushed by aggressive crews and not the bottom 10) made the crossing in 20-22 days. Looking at the overall results for cruising monohulls, (excluding the top 20 and bottom 20) the crossing times were 18-25 days. https://www.worldcruising.com/arc/ar...x?eventid=117#

The cats I am referring to were virtually all over 45 ft. The monohulls included boats down to 36 feet.

So this does not support the theory that cats are faster crossing oceans.

Of course: you are cherry picking. You exclude the fast cats just to support your hypothesis.

I beg to differ. The first cat, a Marsaudon TS5, was sailed by a competent, but not out of the ordinary crew composed of two relatively old couples. The difference more than anything else, is the boat. If you select a condomaran such as the lagoons, then of course they will not perform. If you choose a light catamaran designed for good performance, such as a Marsaudon, then you get a completely different picture.

I chartered a Marsaudon TS42 two years ago, and I can tell you that its performance was remarquable with commonly 14 knots on a close reach (with 20 knots of wind), this despite several handicaps:

- mini keel version, not daggerboard
- charter boat with all the consequences, including worn out Dacron sails
- my own incompetence (I never raced, therefore I suck at trimming sails, and additionally, that was my first time on a cat)

My point is, choose a light cat designed for speed, you’ll get speed. Choose a floating apartment designed to stay put in comfort, you’ll stay put. Comfortably.

[arsenelupiga]

Originally Posted by jdmuys

Of course: you are cherry picking. You exclude the fast cats just to support your hypothesis.

I beg to differ. The first cat, a Marsaudon TS5, was sailed by a competent, but not out of the ordinary crew composed of two relatively old couples. The difference more than anything else, is the boat. If you select a condomaran such as the lagoons, then of course they will not perform. If you choose a light catamaran designed for good performance, such as a Marsaudon, then you get a completely different picture.

I chartered a Marsaudon TS42 two years ago, and I can tell you that its performance was remarquable with commonly 14 knots on a close reach (with 20 knots of wind), this despite several handicaps:

- mini keel version, not daggerboard
- charter boat with all the consequences, including worn out Dacron sails
- my own incompetence (I never raced, therefore I suck at trimming sails, and additionally, that was my first time on a cat)

My point is, choose a light cat designed for speed, you’ll get speed. Choose a floating apartment designed to stay put in comfort, you’ll stay put. Comfortably.

obviously you have not heard of CF member Django with lagoon 38 that won both ARC races and left behind many racy boats that you promote, couple years ago.

Unfortunately for 'cat performance pushers', cruising cats like lagoon provide way more value for money and more comfortable drive. You can have appartement on water or/and fast passagemaker if you put effort into it.

Now feel free to start crying.