Originally Posted by yeloya
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. ...
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
, 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