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Old 25-04-2008, 10:40   #76
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I used to do it quite frequently on my Hobie 18 on the San Francisco Bay. Of course I did not have a cabin with vertical forward windows and a flying bridge like a Lagoon, but I did it. Besides, once in the tropics, how often do you need to beat to weather for days on end? Wouldn't your engines come into play if you could not make any headway against a lee shore?
Pointing high is fine until 25 to 30 knots above that the higher waves and windage start playing a significant role, it is possible but not pleasant
I prefer reaching any time of the day, however having said that
when sailing towards Miami with 30 knots on the nose and reasonable wave action we did ok probably the same as a monohull of 50 ft.

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Old 25-04-2008, 10:56   #77
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sailboat performance

Here is an interesting article about 200 mile days on cruising boats. Mostly about monos, but still relevant i think.

http://bethandevans.com/pdf/200mile.pdf


regards

Alan
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Old 25-04-2008, 11:03   #78
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Here is an interesting article about 200 mile days on cruising boats. Mostly about monos, but still relevant i think.

http://bethandevans.com/pdf/200mile.pdf


regards

Alan
Mostly the article is right however it looks at races only but not at courses chosen for the wind and current tracks , for instance when sailing from Cape Town to Barbados with the wind during the day at 120 to 140 degrees at 15 to 18 knots and during the night same course but wind between 18 and 24 knots the crossing speeds go up a factor 1.3 or well over 200 miles per day. In a straight line we averaged 8.95 however with pointing taken into account the speed over ground was 10.95 faster is possible but than one really has to work hard and I sail for pleasure.

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Old 25-04-2008, 11:39   #79
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Originally Posted by David M View Post
I used to do it quite frequently on my Hobie 18 on the San Francisco Bay. Of course I did not have a cabin with vertical forward windows and a flying bridge like a Lagoon, but I did it. Besides, once in the tropics, how often do you need to beat to weather for days on end?

Wouldn't your engines come into play if you could not make any headway against a lee shore?
I've read a number of articles about total potential loss of windward ability due to drag. To try to paraphrase: the resistance of the superstructure is higher then the lift of appropriately reefed sails (IE windward ability goes away). Some designers pay attention to the design of the deck to avoid this situation, some don't.
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Old 25-04-2008, 12:03   #80
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Originally Posted by Joli View Post
I've read a number of articles about total potential loss of windward ability due to drag. To try to paraphrase: the resistance of the superstructure is higher then the lift of appropriately reefed sails (IE windward ability goes away). Some designers pay attention to the design of the deck to avoid this situation, some don't.
I feel all designers pay atttention to this situation with exception for the yachts and cats that are strictly designed for chartering the better the windage the higher one can point cats with very poor wind dynamics are pretty good downwind sailors but can hardly point at all, look at the Lagoon 42 and 44 for instance I have not seen their polars but am sure downwind is ok upwind over 60 degrees almost not possible.

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Old 26-04-2008, 01:48   #81
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The more I read, the more I am confused...
Everybody seems to agree that the ratio SA/LD is the main factor (if not the only one) when predicting how the cat will perform.For the sake of consistancy, if we take 3 FP's listed, this ratio is in the order of (from good to worse) Mahe, Lavezzi and Orona.
If you check the polars which are on the official website of FP, actually .it is just the contrary.. According FP, Orona is faster than Lavezzi and Lavezzi faster than Mahe...(At 120 degree and at 20 knots true, they are respectively; 11,5 kn, 11,1 and 10)

I have no idea whether they are actual numbers or just hypothetical ones.. But since this is the same manufacturer, we may assume they are all consistant beetween them.
If so, according FP, the bigger the boat, the faster she is, regardless the SA/LD ratio.
I also assume that hull sections, bridge deck clearance, etc are pretty similar.
So how this can be true ???

Pls help me out. ...

Yeloya
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Old 26-04-2008, 02:42   #82
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Originally Posted by yeloya View Post
The more I read, the more I am confused...
Everybody seems to agree that the ratio SA/LD is the main factor (if not the only one) when predicting how the cat will perform.For the sake of consistancy, if we take 3 FP's listed, this ratio is in the order of (from good to worse) Mahe, Lavezzi and Orona.
If you check the polars which are on the official website of FP, actually .it is just the contrary.. According FP, Orona is faster than Lavezzi and Lavezzi faster than Mahe...(At 120 degree and at 20 knots true, they are respectively; 11,5 kn, 11,1 and 10)

I have no idea whether they are actual numbers or just hypothetical ones.. But since this is the same manufacturer, we may assume they are all consistant beetween them.
If so, according FP, the bigger the boat, the faster she is, regardless the SA/LD ratio.

I also assume that hull sections, bridge deck clearance, etc are pretty similar.
So how this can be true ???

Pls help me out. ...

Yeloya
Hello Yeloya

the answer is simple , if the SA/LD factors are the same the water line lenghts are not , the longer the waterline lenght the lower the resistance and therefore a higher speed will result form larger cats with the same SA/LD ratio

The old but still used Texel rating takes this into account but to keep it all simple the SA/LD ratio works for yachts in the same lenght.
For instance a FastCat 455 has a waterline lenght to width ratio of around 1:13 while a charter cat will have a ratio of 1:9 or 1:10 , an all out racing cat will be closer to 1:16 or 17 creating a better ( lower ) hydrodynamic resistance. Other factors also have an effect like normal or folding propellers, on a cat this can make a 5 to 10 % difference in speed, even better is a retractable motor like an outboard to even remove the drive leg from the water. daggerboards also can be of help with lifting one or both out of the water creating a lower drag. , Besides have drag in the water there is the drag thru the air that also has its effect however in minimizing this drag the comfort of yachts is also minimized and thta is fine for an outright racer but a disaster for a cruiser or performance cruising cat.
Lots of tweaking can be done under the waterline to minimize drag, the biggest factor in having a slow of faster yacht ( Multihull )
The first item is offcourse weight , the lower the weight of any given yacht the lower the drag, then shape has its effect, a lighter yacht will have a better lenght to width ratio .
Other items that haved their effect are underwater openings like all the water in and outlets, we always try to minimize these and all outlets are placed above the water line and inlets are combined to one on each hull.
Then recessing grounding plates , log and depth sounders and cooling plates has its effect.
Shapes of rudders , keels ands daggerboards also play a role and so do items like a skeg in front of a rudder or saildrive, creating the optimal shape has its effect.
We have recently changed the bow intake to a finer one losing 3 % of the drag ceated.( and this is why we did it originally making less spray at higher speeds )Another item we are succesfull with in creating a higher speed are ballast tanks , our fresh water and diesel tank fluid can be moved from the leeward hull to the windward hull by pump and this can help considerably in longer voyages. The difference is only 0.2 or 0.3 of a knot but after a 30 day voyage it can save you a day in sailing time. you do have water and diesel on board so using this to your advantage helps.
By using all these possible items to reduce drag a cruising cat can almost perform like a racing cat. Many of you are not interested in a faster sailing boat but it can help in safety or in my case my wife, given a 3 week vacation likes to sail but arriving in a harbor after shorter voyages helps a lot.
Last summer we sailed from Brest to La Coruna in 26 hours 331 NM and the same trip back in 29 hours crossing the Bay of Biscaye.
In other yachts I have owned , among them a Halberg Rassy and a Moody 44 this trip would have taken a minimum of 48 hours in the same wind conditions and I doubt very much she would have come along.

Greetings and have a good weekend
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Old 26-04-2008, 10:54   #83
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Last summer we sailed from Brest to La Coruna in 26 hours 331 NM and the same trip back in 29 hours crossing the Bay of Biscaye.
331 / 26 = 12.73 knots average speed hard to believe..Fastcats parameter is close to Catanas, so I dont believe it is faster..I see your speed on the forum only, and wanna see also in the ARC results..why dont you join ARC? good luck
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Old 26-04-2008, 12:42   #84
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this is my new table to compare the cats

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Old 26-04-2008, 13:46   #85
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Thats great Mamo...you want to add the two Gunboats, Catana 57 and the Fastcat 57 to it?
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Old 26-04-2008, 13:47   #86
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I've read a number of articles about total potential loss of windward ability due to drag. To try to paraphrase: the resistance of the superstructure is higher then the lift of appropriately reefed sails (IE windward ability goes away). Some designers pay attention to the design of the deck to avoid this situation, some don't.
Its called "windage". It makes perfect sense, the more there is the less your efficiency at going to weather.

wind·age (wndj)n.1. a. The effect of wind on the course of a projectile.
b. The point or degree at which the wind gauge or sight of a rifle or gun must be set to compensate for the effect of the wind.
c. The difference in a given firearm between the diameter of the projectile fired and the diameter of the bore of the firearm.

2. The disturbance of air caused by the passage of a fast-moving object, such as a railway train.
3. Nautical The part of the surface of a ship exposed to the wind.

windage - definition of windage by the Free Online Dictionary, Thesaurus and Encyclopedia.
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Old 26-04-2008, 14:07   #87
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Good work Mamo.

May I suggest doing the SA/D when adding a load that one would expect a cruising boat to have.

My suggestions:
Fuel 250 kgs
Water 250 kgs
Options added to the basic boat, like extra anchors and rodes, biminis, electronics, tender, outboard, extra sails, watermaker, tools and spareparts, charts, books, extra batteries, cooking utensils etc.
Say 500 kgs.
Provisions for 4 crew for 30 days @ 3 kg/person/day = 360 kgs
4 crew including luggage = 440 kg

Total say 1800 kgs. This is in my opinion a conservative figure for a 45-50 ft cruiser.

This should give a better idea. Maybe leave out the boats under 40 ft as they will probably not be as heavily loaded.

Ideally, the SA should only be the main + jib, as some quote a 130% foresail as standard.

There is a formula for "performance index" that gives an estimate of what percentage of wind speed the boat can achieve in optimal conditions ( a reach) but of course things like pointing ability and windage are not considered.

Regards

Alan
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Old 26-04-2008, 14:17   #88
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Are polars generated by computer or are they based on actual real world sailing? I would think if they are computer generated then they are worthless.
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Old 26-04-2008, 14:23   #89
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this is my new table to compare the cats


Thank you so much for all the work.
I am out of your categories.
I would like to see the under 40ft bunch.
Care to work even more.......for us handicapped?
Particularly the Gemini 105Mc, Mahe set.
And the DLR column would show if it was made a little wider.
Thanks again.
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Old 26-04-2008, 14:39   #90
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Windage is not too difficult to calculate, and there are some guidelines regarding the wind resistance coefficients to be used. There are 2 basic versions, one for calculating wind resistance when sailing, and another when anchoring, as the boat is expected to be facing directly into the wind.

The basic formula is: Resistance= ½ x air density x V2 x Area x Cw

Air density = 1.3 kg/m3
V2 = wind velocity squared in m/s
Area in square meters
Cw = wind resistance coefficient.

For sailing conditions you can use:

Hull windage Cw = 0.5
Mast windage Cw = 1.0
Rigging windage Cw = 1.2

Resistance is given i Newtons


I did some rough calculations for wind resistance based on a wind speed across the deck of 20 m/s. 27 knots wind and 13 knots boat speed.

The wind resistance was much more than the water based resistance!!!

I found a figure of 4.5 N/m for 12 mm stainless wire @ 20 m/s!

For a 20 meter mast you easily get more than 100 meters of rigging. This gives 450 Newtons - probably comparable to having 3 or 4 dinghies in tow.

So Yes, windage is a major factor in performance, both in speed and pointing ability.

I just launched my boat yesterday, and did a test without the rig - not very scientific, but I get 6.5 knots with the rig and 7.2 without under similar conditions of less than 2 m/s wind, and same revs on the engine. And the rig only has 3 stays and a bit on the mast. Total of about 75 meters of rigging, no sails mounted.

This is one of the reasons I'm going for an unstayed rig on the next boat

Regards

Alan
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