Real cruising multihull speeds

[44'cruisingcat]

Originally Posted by BigCat

Well. of course, the problem with sailing fast in a catamaran is the danger of capsizing.

Depends on what you call fast. I've sailed at 16 knots plus, which I'd call pretty fast, (for a 38 foot boat) and there was no danger whatsoever of capsizing.

IMHO the real problem with sailing that fast AT SEA is comfort.

As Richard Woods stated earlier, you slow down to make life more comfortable.

If you happen to be in an area with smooth water, you can comfortably and safely sail fast.

[Woods Designs]

Originally Posted by BigCat

Well. of course, the problem with sailing fast in a catamaran is the danger of capsizing. You don't want to end up upside down with your wife and kids.

A capsize will probably spoil your day as well. But in fact there isn't a real correlation between speed and capsizing. After all many slow multihulls have capsized.

The sea is a dangerous place and I like to sail as fast as possible between two safe anchorages. But I want to be comfortable, relaxed and rested as I do so.

That is the main reason why people cruise slowly. It isn't because they fear capsize.

Richard Woods of Woods Designs

Woods Designs Sailing Catamarans

[BigCat]

Originally Posted by Woods Designs

A capsize will probably spoil your day as well. But in fact there isn't a real correlation between speed and capsizing. After all many slow multihulls have capsized.

The sea is a dangerous place and I like to sail as fast as possible between two safe anchorages. But I want to be comfortable, relaxed and rested as I do so.

That is the main reason why people cruise slowly. It isn't because they fear capsize.

Richard Woods of Woods Designs

Woods Designs Sailing Catamarans

And yet, one hears time and again that it is only racing catamarans that capsize. I notice that the cruising catamarans that have capsized are usually smaller than 40', often under 35', and of course, stability increases drastically as boats get longer, especially if there is a proportional increase in beam.

[imagine2frolic]

Pretty ship, but to my disappointment I was thinking of the CSK Aikane from the 50's....

[BigCat]

If you put big engines in a cat, you can motor really fast, which isn't true of mono sailboats, because a cat can usefully absorb much more power than a displacement mono. The issue of pushing hard with wind is the danger of capsizing or pitchpoling, which doesn't exist when motoring. You get this benefit without in any way compromising the sailing qualities of your cat, except for the added weight of larger engines and engine gear. Since you can motor under one engine in a cat, this doesn't mean that you must always use a lot of fuel, either.

Paradoxically, if you want to safely sail a cat fast, you can give it a low aspect ratio rig. This is usually though of as slower, as your lift / drag ratio isn't as good hard on the wind, but you can keep more sail up and go faster, until the seas themselves become a danger.

[Troutbridge]

I have a Broadblue 385. I left the Channel Isles (UK) in June and am currently in the Spanish Rias. That is about the sum total of my cat experience so far. I've found that I sail very conservatively (not much cat experience). The boat is heavy to start with, I'm talking real weight here, not the brochure weight which is not with full water/fuel or any stores. I estimate that the boat weighs about 8.5/9000 kg right now, half fuel tanks, full water and pretty much fully provisioned.
I need 10kts of wind to get the boat moving, well.... to make any meaningful progress and I've found that running under the screecher only in 20kts true wind from 160deg gives me about 6-7.5kts, depending on sea state.
Beating to windward (the other day, as it happens) in sheltered water, gusting up to 25kts true, I could get to within 30deg of the apparent wind and I was getting 6+ kts most of the time.
For the Atlantic crossing in Jan I'm passage planning on 150nm per day and all things being equal (are they ever?) I expect to beat that figure most days. I'd fully agree about comfort. I used to have a 39ft mono (Freedom 39, the LWL was 5ft less than the BB) and I have to say that life onboard is much more comfortable now. I reckon that the max comfortable off-wind cruising speed will prove to be 7/7.5 kts. I've been told by BB & others who have many miles on a 385 that the max safe speed is 15kts, at which point it all gets a bit like walking on egg shells. I suspect that real-life cruising will produce much the same sort of averages as my old boat, just more comfortable!
On the point of wind speeds, to produce an average figure you need a wide range of speeds. In Jan, the average Trade wind strength is around 20kts, on my last crossing we saw speeds in excess of 30 kts for a couple of days.
Commenting on Richard's figures, I have friends who own a Banshee. They are in NZ at the moment, having also sailed from the Channel Isles. It's a fast boat, but with sitting-only headroom in the saloon I didn't find it overly comfortable for cruising when I visited on a couple of occasions. I reckon it was designed with fast coastal cruising in mind, Richard will correct me here if I'm wrong. I'm sure high average daily runs are more easily achieved in cats BUT, for me at least, the aim is to arrive having had a comfortable trip AND not having broken anything en-route. I'm retired on a pension so I want to avoid repair bills like the plague!

[GordMay]

Originally Posted by Troutbridge

... I'm sure high average daily runs are more easily achieved in cats BUT, for me at least, the aim is to arrive having had a comfortable trip AND not having broken anything en-route. I'm retired on a pension so I want to avoid repair bills like the plague!

Sounds to me, that your sailing (like most of mine) is an avocation & lifestyle.
Since no one's paying us (cruisers) to do this, it ought to be enjoyable.

[Troutbridge]

Got it in one Gord. I don't know how long I'll be cruising, but the boat is my home, I own no bricks & mortar anywhere in the world, virtually everything I own is on board. That kind of makes you a cautious sailor, but in any case you can get in enough trouble being cautious without going out and asking for it
I can't remember the full quote or who said it, but it goes along the lines of "the sea is essentially unforgiving...."
I have a feeling it may have been one of the early NASA astronauts

[GordMay]

Originally Posted by Troutbridge

...I can't remember the full quote or who said it, but it goes along the lines of "the sea is essentially unforgiving...."
I have a feeling it may have been one of the early NASA astronauts

The seas are neither brutal, unforgiving, nor cruel – they just “are” (what they are, & will be), and their they’re merely unknowing of, and indifferent to we mere mortals.

[BigCat]

The Broadblue 385 is one of those boats where the builders give you a displacement without telling you what its basis is-loaded, empty, or half-tanks. Looking at it, one can see that there is no trampoline, and that the bridge deck takes up a very high percentage of the boat's overall length-well above average. Clearly, the design goal was a maximum of accommodations for its length, and other considerations were secondary.

Using the figures given by the builders, it has a DL ratio of 156, a SA/D ratio of 18.75 with the gennoa, and the waterline square root is 5.97. Looking at drawings of the interior, I guesstimate the hull waterline beam at 5', which would make the hull L/B about 7.2. So, one wouldn't expect a lot of speed from this design. If, as I suspect, the displacement is unloaded, one would have to correct the DL and SA/D figures, and speed expectations would be even lower. As the saying goes, 'built for comfort, not for speed.'

(Further research tells me that the displacement given is the 'light' displacement. Add a hundred gallons of fuel and the same of water, at least two people and their gear, and you have added a good 3000+ pounds. That lowers the SA/D to 16.8, and the D/L increases to 186. This combined with beamy hulls will give those who understand these figures the expectation that the Broadblue's performance would indeed be similar to an equivalent monohull.)

[Therapy]

Originally Posted by BigCat

. This combined with beamy hulls will give those who understand these figures the expectation that the Broadblue's performance would indeed be similar to an equivalent monohull.)

Of equivalent length or accommodations?

[BigCat]

Originally Posted by Therapy

Of equivalent length or accommodations?

Length of the designed waterline divided by greatest beam at the designed waterline. To understand this statistic, read Southwinds - January 1999 . Skinnier hull beam = faster boat, all else being equal, at least up to 16 beams per DWL or so-but less accommodations in the hull are possible with skinnier hulls.

[Troutbridge]

Originally Posted by BigCat

The Broadblue 385 is one of those boats where the builders give you a displacement without telling you what its basis is-loaded, empty, or half-tanks. Looking at it, one can see that there is no trampoline, and that the bridge deck takes up a very high percentage of the boat's overall length-well above average. Clearly, the design goal was a maximum of accommodations for its length, and other considerations were secondary.

Using the figures given by the builders, it has a DL ratio of 156, a SA/D ratio of 18.75 with the gennoa, and the waterline square root is 5.97. Looking at drawings of the interior, I guesstimate the hull waterline beam at 5', which would make the hull L/B about 7.2. So, one wouldn't expect a lot of speed from this design. If, as I suspect, the displacement is unloaded, one would have to correct the DL and SA/D figures, and speed expectations would be even lower. As the saying goes, 'built for comfort, not for speed.'

(Further research tells me that the displacement given is the 'light' displacement. Add a hundred gallons of fuel and the same of water, at least two people and their gear, and you have added a good 3000+ pounds. That lowers the SA/D to 16.8, and the D/L increases to 186. This combined with beamy hulls will give those who understand these figures the expectation that the Broadblue's performance would indeed be similar to an equivalent monohull.)

Not quite. Obviously I talked to the builders, their figures are basically with half tanks, so +50 (imperial) gallons of fuel and the same for water.
The design criteria was for 'Blue water cruising' for a couple & occasional guests. The ability to carry a decent load featured high in the priorities but work was done on the underwater profile to 'speed the boat up' a bit. People with more experience on the boat than me get better average speeds, but then as I said I sail very conservatively. The hull form is a compromise between speed and load carrying, they weren't quite so concerned about maximising living space as the boat was not designed with the charter market in mind.

[Therapy]

HUH?
..........

[BigCat]

Originally Posted by Therapy

HUH? ..........

Huh, what? It seems pretty clear to me. Skinny hulls are faster than fat hulls. Push your hand through the water, finger tips forward--easy. Pushing your hand through the water sideways--harder.