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Old 14-07-2008, 12:48   #1
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Real cruising multihull speeds

I have been away sailing on my catamaran so haven't read this forum for some time, my apologies. I'll try not to do it again (BG)

So I am making a new post rather than replying to what is now a very old one.

Following is part of what I wrote in an article "Why sail a catamaran" published recently in Practical Boat Owner magazine (the largest UK boating magazine).

"....Most catamarans are advertised as being "fast" for many people are easily seduced by the thought of more speed. However, when cruising, speed must always be related to comfort. Just because a catamaran sails upright to windward and doesn't roll downwind, it doesn't mean you should always sail one fast.
On a monohull you tend to sail as fast as possible all the time. On a catamaran you only sail as fast as you want to go.
I often compare boats with cars: 6 knots is 60mph, 8 knots is 80; 20 knots is 200. So in reality not many cruising multihulls genuinely do 20 knots in flat water.

A monohull is like an old car; you can keep your foot on the gas all the time. In a new car it's often hard to judge how fast you are going, it's so comfortable. In practice, few people buy the fastest car; rather most want a car that is comfortable to drive and handles predictably. Even so, you quickly learn not to drive fast in traffic, in the dark or in bad weather.

It's the same with a catamaran, where the real trick to successful catamaran sailing is to know when to slow down. Thus I prefer to cruise offshore at a 6 knot average rather than 9. That's because even 9 knots can be uncomfortably fast when passagemaking, mainly because you are living on board, not just out for a day sail.

I've found that peak speeds are about double the average speed. To average 9 knots you'll often do 18 and later you'll swear the log never read below 12.

Sheer speed is not the main reason we go sailing, for if we really wanted to get somewhere in a hurry we'd use a powerboat. Instead, what is important is to have a boat that is fun to sail.

Having a responsive boat and one that will do what you want when you want it are the real differences between boats that sail well and those that don't......"


You can see the whole article by reading the link found on my Latest News page, or by buying a back issue from PBO

In 60,000 miles of ocean sailing I have twice done 650 miles in three days, once 1000 miles in 5. All when racing full crewed and hand steering. I doubt if anyone really averages 200 miles a day if living on board (wife knitting? baby crying?? you sleeping for some of the time???)

So the real questions to ask are not "How fast do you sail" but:

"How comfortably do you sail when averaging 175 miles a day?" and "How much wind do you need to do so?"

Good sailing, at any speed
Richard Woods of Woods Designs

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Old 14-07-2008, 14:47   #2
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So far our longest sail has been 500 miles, and a 1/2 dozen 450 mile trips. With 20+ knots of wind on the quarter we can easily maintain 175 miles a day. A double reef in the main while using all, or half the headsail. Depends on how much wind.

We have managed 360 miles in 36 hours starting out with 30 knots on the quarter. The last half ending up with 10 knots of wind as close hauled as we could get. We began in the midteens consistantly, and at 6 knots against a current in the end. The whole trip in comfort.While maintaining the midteens I had a cup of coffee on the roof sitting against a winch.

Sailing down the wave was the mid & low teens. Photographing the dolphins we were at about 10 knots ( pics)

The P.O. claims an avg of 180 miles under spinnaker alone from the Canaries to the Caribbean. I myself am waiting for the chance to sail the trades!

Then again June the 6th it took us 17 hours to cover 45 miles. Miami to Bimini in 20 to 30 knot winds from the east. It was a little uncomfortable, but very little pounding occurred. We still managed to prepare food, and drink along the way.
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Old 14-07-2008, 17:03   #3
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Quote:
Originally Posted by Woods Designs View Post
I have been away sailing on my catamaran so haven't read this forum for some time, my apologies. I'll try not to do it again (BG)
Thanks Mr Woods, it great to see you posting here.
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Old 15-07-2008, 11:31   #4
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I could typically average 40% of windspeed up to around 20 knots of wind, in cruising mode. Lightship I can get up to around 10 to 12 in 20 knots of wind on my FP Tobago 35, cruising in the relaively protected waters of the Baltic, but with short choppy waves. This was with the standard rig.

I have now got a rotating wing mast(33 cm chord) and a large flat top main that is 1.5 meters taller. During the last week of cruising we have been able to average around 7.5 knots in 12 to 16 knot winds, mostly forward of the beam.

Had a lovely sail this morning between some islands where we averaged around 8.5 knot in 12-14 knots TWS. We are fully loaded with 4 adults, all their stuff plus dinghy, outboard etc, so I'm really pleased with the new rig and sails

So adding around 45% to the mainsail area, rotating mast, new main and self-tacking jib have improved performance from 40-50% of windspeed to 60-65% at 15-20 knots of wind. The improvement is even more at low wind speeds of say 5-6 knots, now I can actually sail!


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Old 15-07-2008, 11:45   #5
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Originally Posted by Nordic cat View Post
I could typically average 40% of windspeed up to around 20 knots of wind, in cruising mode. Lightship I can get up to around 10 to 12 in 20 knots of wind on my FP Tobago 35, cruising in the relaively protected waters of the Baltic, but with short choppy waves. This was with the standard rig.

I have now got a rotating wing mast(33 cm chord) and a large flat top main that is 1.5 meters taller. During the last week of cruising we have been able to average around 7.5 knots in 12 to 16 knot winds, mostly forward of the beam.

Had a lovely sail this morning between some islands where we averaged around 8.5 knot in 12-14 knots TWS. We are fully loaded with 4 adults, all their stuff plus dinghy, outboard etc, so I'm really pleased with the new rig and sails

So adding around 45% to the mainsail area, rotating mast, new main and self-tacking jib have improved performance from 40-50% of windspeed to 60-65% at 15-20 knots of wind. The improvement is even more at low wind speeds of say 5-6 knots, now I can actually sail!


Regards

Alan
Hi Alan, Sounds like your mods have paid off, well done. Just as a matter of interest, how have you configured your self tacking jib, when if I remember the Tobagos forward hatch butts up very close to the mast, have you had to alter the hatch etc?
Also by having a (I assume) smaller jib, have you had this designed more as a foil so as to direct the wind more efficiently onto a larger main?
I also assume that you would not see any greater speed by having a larger genoa with this rig set up?
Looking forward to your replies.
Cheers
Ian
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Old 15-07-2008, 11:56   #6
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I'll start by saying that we live aboard and try to keep our tanks topped off or at least above half full. However, that's a bit unfair as we aren't carrying lots of spares, single use tools etc etc as we're not going too far from the N American continent these days.

I usually sail somewhere in the 50-65% of true wind range in winds under 20 kts. The percentage is higher in the lower wind ranges below 8-10 knots. This is to weather. I seldom have used light sails. The screecher and pole are stowed to limit marina fees these days. No spin.

So how fast are we? I can easily beat to weather at 8.5-8.8 kts in 18 knots true. I can point with any cruising sail boat and 'go where she's looking' without abnormal leeway. However, I usually sail at 38-42 degrees apparent going to weather. Likely you do to unless you're a racer type with a racer type boat and headsails. I have not genoa as the jib is self tacking.

Overall, the speed advantage is minimal over most equivalent monohulls but perhaps a bit greater off the wind. The real advantage is that my wife works comfortably at her computer or in the galley most of the time when a monohull would be heeled well over. We sail in our shirtsleeves and slippers when the rest are bundled up in their foulies/survival jackets etc.

At anchor, no rolling and lot's of space. No contest.

Sure enjoying our summer cruise to the Broughton Island group in BC this summer. Comfy, warm, and living large even while sailing with a single reef in winds gusting to 30 kts.
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Old 15-07-2008, 13:39   #7
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Factoring in the DWL, we can adjust speed expectations

If the 175 mile days were on a catamaran with a waterline length (DWL) of 36', that would have a square root of 6, and the speed relative to the DWL would be 1.29 x. The equivalent in a boat with a 25' waterline would be about 155 miles, for a 49' waterline, 217 for a 49' DWL, and the equivalent in a 64' waterline woud be 248 miles a day. So, the DWL should be considered when discussing speed.

Weight is of course, another issue. The Tobago is said to weigh 4 tons, so with a DWL of about 34', would have a DL ratio of 100. Hull beam is unknown to me, but I doubt if it is slim. So, if NordicCat can get the equivalent to 225 miles a day in 15 knots of wind, or 1.6 times the square root of the DWL (assuming a DWL of 34',) then a boat with a lower DL ratio and a waterline of 64' could get 12.86 knots x 24 =308 miles on the wind, if its rig is equally good. Of course, we can't count on 15 knots of wind, but trade winds are about that usually.
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Old 15-07-2008, 14:14   #8
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Maine Cat 41 considered

The Main Cat 41 has a dwl of 40.5, and a rather svelte hull beam length ratio of 10.5. It is somewhat heavy loaded, at a DL ratio of 129. The square root of its waterline is 6.36. At 8.65 knots, it is going at a 1.36 times the DWL square root. Of course, 18 knots is a lot of wind. Times 24 hours, that would be a 208 NM day.
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Old 15-07-2008, 17:54   #9
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Of course, 18 knots is a lot of wind. .
A lot of wind? Hell, I don't even hit my max hull speed unless it's blowing in the low 20's. Where exactly do you sail that 18 knots is a lot of wind??
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Old 15-07-2008, 18:13   #10
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18 knot is a higher than average wind

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A lot of wind? Hell, I don't even hit my max hull speed unless it's blowing in the low 20's. Where exactly do you sail that 18 knots is a lot of wind??
I don't mean it's a gale, I mean it is above average, and so you can't assume that your boat will average over a long time what it averages in 18 knots of wind. We are considering the question of average speeds on this thread-

Here for example is Lahaina, Hawaii, a fairly windy place, in mph:
LAHAINA-KAPALUA AP AWOS |PHJH|1996-2006| 14.4 15.1 14.9 16.7 15.7 16.9 17.1 16.8 15.8 15.1 14.2 14.6 | 15.6

The different wind speeds are monthly averages, 15.6 mph is 13.5 knots, which is the annual average for Lahaina. (I sailed across the Pacific, and it goes from zip at the equator to a pretty stiff blow sometimes in the trades, not counting gales, but the average isn't all that high.)

Here's an interesting link: http://www.gipsymoth.org/weatherpics...therimage1.gif
It shows the wind speeds encountered in a fall / winter crossing in the NE trades in the Atlantic. Even in November and December, it doesn't look like it averaged 18 knots.
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Old 15-07-2008, 18:52   #11
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Real time wind & weather report NW Hawaii Buoy is a link showing the wind speed at bouys offshore in Hawaii, as well as onshore locations. Even offshore buoys many miles from the nearest land in Hawaii, which is notoriously windy, average only 13 knots yearly. (Click on a buoy, and then click on 'wind statistic.)
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Old 16-07-2008, 07:37   #12
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Quote:
Originally Posted by BigCat View Post
If the 175 mile days were on a catamaran with a waterline length (DWL) of 36', that would have a square root of 6, and the speed relative to the DWL would be 1.29 x. The equivalent in a boat with a 25' waterline would be about 155 miles, for a 49' waterline, 217 for a 49' DWL, and the equivalent in a 64' waterline woud be 248 miles a day. So, the DWL should be considered when discussing speed.

Weight is of course, another issue. The Tobago is said to weigh 4 tons, so with a DWL of about 34', would have a DL ratio of 100. Hull beam is unknown to me, but I doubt if it is slim. So, if NordicCat can get the equivalent to 225 miles a day in 15 knots of wind, or 1.6 times the square root of the DWL (assuming a DWL of 34',) then a boat with a lower DL ratio and a waterline of 64' could get 12.86 knots x 24 =308 miles on the wind, if its rig is equally good. Of course, we can't count on 15 knots of wind, but trade winds are about that usually.

Just a note, the Tobago never weighed 4 tons except on the sales brochure (4.2 tons) Empty is around 5 , so in cruising mode with full tanks and 4-5 adults, dinghy, ouboards etc. we must be close to 5.8 - 6 tons.

Hull beam is around 100 cm. Waterline length is around 34 ft loaded.

Just a reminder, this is a "souped" version with nearly 50% more sail area in the mainsail, good laminate sails with 8 battens in the main and a rotating wingmast. Spectra rigging. Bowsprit etc...

regards

Alan
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Old 16-07-2008, 09:58   #13
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Alan's Tobago 35 considered

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Originally Posted by Nordic cat View Post
Just a note, the Tobago never weighed 4 tons except on the sales brochure (4.2 tons) Empty is around 5 , so in cruising mode with full tanks and 4-5 adults, dinghy, ouboards etc. we must be close to 5.8 - 6 tons.

Hull beam is around 100 cm. Waterline length is around 34 ft loaded.

Just a reminder, this is a "souped" version with nearly 50% more sail area in the mainsail, good laminate sails with 8 battens in the main and a rotating wingmast. Spectra rigging. Bowsprit etc...

regards

Alan
Hi, Alan, So, loaded a DL of about 148, on a 34' DWL. If it's hull beam is 100 cm, though, it has a rather slender 10.36 hull length / beam ratio.

I noted that you had a good rig, and what I have read about rotating rigs makes them sound very good indeed. Is your mast fairly airfoil shaped?

If you must have standing rigging, I like the idea of Spectra-less metal to fatigue. I have seen many a ss shroud and stay with broken strands where it enters the swedging.

I haven't looked at SA/D yet in this thread, but that is also of interest. The original mainsail would have been 430 sq. ft. with a total area of 731 sq. ft., so 925 sq. ft. now, and your SA Disp loaded would have gone from 21.9 to 27.7.
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Old 16-07-2008, 10:22   #14
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Maine Cat Sail area Displacement ratio

If on its marks, the Maine Cat 41 would have a SA / Disp of 23.6 with its 'windward' sail area, with a displacement of a bit above 17,000 pounds.
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Old 16-07-2008, 10:41   #15
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Our cruising cat is a MVP/VLP designed Aikane 56. With a 52' waterline and a l/b ratio of almost 13. Fully loaded she weighs around 23,000lbs. We have had this boat for about 4 years now and have put over 10,000 sea miles on her.

While she has proven capable of speeds on many downwind runs in the high teens to mid twenties, We have found that she can reliably be counted upon to consistantly turn out 180 to 220 mile days. However, the faster she travels, the less comfortable she becomes. Powered up in 20k of wind she will go upwind at 11k, but it is so much more comfortable at 9k. In any event, average speed is certainly a function of waterline and weight. Dut driving that function is comfort for a family.
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