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Old 26-09-2009, 19:50   #16
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"K Value"?

I'm planning to buy a medium Cat (read saving to buy). I could buy a Mono right now but I'm waiting and the reason I'm waiting is that as a cruiser I'll be lucky to sail 5% of the time I'm on board. The rest of the time I'm living in it. What good is K Value calcs to me?

Down wind the cats are faster that seems to be a given. Pointing into the wind is a different story but (as someone else has mentioned on this forum recently) who wants to do that.

How does the "K Value" compute when your in port? Now that caculation I would like to see.
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Old 26-09-2009, 23:58   #17
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Originally Posted by barnakiel View Post
I would love to see the day when boats stop being seen as vessels traveling through the rarefied space of relative factors and start (once more) being taken for what they are - sailing vessels. And this 'sailing' thing is probably the first factor we should put into any (serious or only half so) equation.

So - whatever the god of moderation suggested, PLUS "divided by the body of water that is to be sailed in the craft".

Deal?

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It was meant as a little humor. So much of deciding on a boat is qualitative decisions and personal preferences.
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Old 27-09-2009, 01:17   #18
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Originally Posted by Live to Dive View Post
as a cruiser I'll be lucky to sail 5% of the time I'm on board. .


These always gt back to same old, same old. This thread was about speed

Anyway

For a floating Gin palace in the Caribbean Cats are wonderful and if the 5% sailing is from St Martin to St Barts I'll agree with you.

If the 5% sailing time is between Ushuaia and the Antarctic then you won't find any cats. Probably because that 5% is now a significant percentage!

So the 5% is flexible.

Some people we met point blank refused to do 2 days on a beam reach in cats and would only go where its aft the beam. Why? Because some cats can have a horrendous motion at sea. Maybe they do, maybe not, but cat people expect there to be no healing so they think this means no motion. Well as anyone who has sailed their cat in choppy quartering ocean seas can tell you Cats certainly can have motion in all directions! At least we mono-hull people are over most of our motion sickness!

Anyway, as so many people have said its all a personal choice. If I had a million dollars I would have more choices. But it would probably be a new 1/2 million dollar Beneteau and half milliopn dollars to spend getting drunk because 95% of my time I'm not in command

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Old 27-09-2009, 01:51   #19
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............half milliopn dollars to spend getting drunk because 95% of my time I'm not in command
Thus spoke a true "son of the sea"
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Old 27-09-2009, 15:29   #20
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The normal way to assess vessel performance is to calculate it's 'hull speed factor' - that is what multiplier of square root of waterline it sails.

Doing that to this dataset the result is that both multi's and monos acheived exactly the same - averaged 1.27 times sq rt of waterline. To do that calcualtion I made some some average assumptions about waterline length because monos tend to have longer overhangs than multis.

So, I would have said the conclusion from this data is the two designs sailed pretty much the same speed (liven their length), within the data quality we have.
Getting back to the topic, I'm not sure how you came to your conclusion here. There's a 36'cat which was nearly as fast as a 60'cat, and faster than more than half the cats, all of which are bigger. (And faster than every mono under around 44 feet.....just how long do you think the overhangs are on these boats?)

From the very small sample here, it would seem that cats aren't as dependant on waterline length as monohulls for their speed. A 46, 45 and 40 foot boat were faster than the 60, and a 35 footer was almost as fast.

With the mono's the trend is pretty clear - bigger is generally faster.
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Old 27-09-2009, 17:08   #21
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Displacement hull speed, simply put, is dictated by the waterline length. The longer it is the faster she'll go. When the vessel reaches 'hull speed' it will not go any faster unless horsepower is applied 'exponentially' to climb the bow wave.
The formula for working out theoretical hull speed is: = boat speed in knots.
e.g. The square root of a waterline length of 25 feet = 5 x 1.34 = 6.7 knots.
How much horsepower do I need for my displacement boat? Rule of thumb is 1 hp per 500lb total weight. or 1kW per 304kg. This gives adequate performance in most wind and tide conditions.

I found this at BOAT PERFORMANCE. Now if my thinking is correct, it won't matter wether you're talking about a leaner or multi hull it's all about the energy required to climb the bow wave. This is the Thearetical maximum only in that the energy required to go faster becomes exponential and therefor un-economical, it doesn't mean you can't do it. Keep in mind too that this would be in ideal conditions ie no head wind, no current etc.

Now one could argue that 2 hulls, more mass etc means more energy to climb the bow wave but this doesn't really concern us as upto that point the energy doesn't climb exponentially and so we can use the same coefficient of 1.34. Both hulls would reach this point simultaneously.

I think a better guide for actual hull speed would be the wetted area x sail area (sail configuration would also need to be considered) This should in theory give you a guide for the speed of the boat in different conditions. Really it comes down to simple thrust vs resistance. Obviously the cat has a greater wetted area but because of stability can also have an increased sail area. BTW just look at the motor HP supplied with cats vs monos obviously cats have more resistance in water.

This is all fine but I stick to my earlier comment. When cruising you're going from one place to another and then staying in that place for significant time. Cruising for 1 year how much time are you actually sailing? What we really need is a mono with fold out living areas and berths and such like the campervans which are on the market. Now that I'd like to see. Lol.
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Old 27-09-2009, 17:26   #22
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Please remember that length plays a vital part in resistance calculations: At the same speed/length ratio boats of similar form will have roughly equal resistance per ton. That is to say a 25 footer at 5 knots; a 49 footer at 7 knots; and an 81 footer at 9 knots. All of these examples will would require the same horsepower per ton - though speed has nearly doubled. Clearly then a Solar boat with limited power should be long and thin to gain the maximum performance. We should also consider that the shape giving the least wetted area is the arc of a circle, but that such a shape is not practical except in multihulls. That is another reason why Solar Navigator is at the moment a catamaran design.

And this at MEASURING HULL DRAG AND CALCULATING HORSEPOWER REQUIRED FOR LOW DRAG HULL DESIGNS - SOLAR NAVIGATOR WORLD ELECTRIC NAVIGATION CHALLENGE. This is very interesting and explains a lot regarding the percieved speed advantage of cats.
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Old 27-09-2009, 19:16   #23
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Originally Posted by Live to Dive View Post
Displacement hull speed, simply put, is dictated by the waterline length. The longer it is the faster she'll go. When the vessel reaches 'hull speed' it will not go any faster unless horsepower is applied 'exponentially' to climb the bow wave.
The formula for working out theoretical hull speed is: = boat speed in knots.
e.g. The square root of a waterline length of 25 feet = 5 x 1.34 = 6.7 knots.
How much horsepower do I need for my displacement boat? Rule of thumb is 1 hp per 500lb total weight. or 1kW per 304kg. This gives adequate performance in most wind and tide conditions.

I found this at BOAT PERFORMANCE. Now if my thinking is correct, it won't matter wether you're talking about a leaner or multi hull it's all about the energy required to climb the bow wave. This is the Thearetical maximum only in that the energy required to go faster becomes exponential and therefor un-economical, it doesn't mean you can't do it.
This is only correct up to a point. A study done some years ago (I think by Edmond Bruce) found that with increasing LWL:BWL ratios, bow wave generation reduces to the extent that there is virtually no "hump" at ratios over around 10:1.

Which is why boats such as rowing sculls, hobie cats, Nacras etc, can exceed theoretical hull speeds by large amounts without needing enough power to make them plane.

For instance:
A boat with a theoretical hull speed of around 15 knots sailing at in excess of 40 knots without planing.
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Old 27-09-2009, 19:30   #24
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Thanks for the boat porn 44'cruisingcat,(I lust after Banque Populaire big time) however I have to wonder,is a tri sailing on one ama "planing" to a degree,as in its exposing(if thats the right word) less wetted surface than if it was sailing along with the main hull submerged??
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Old 27-09-2009, 19:44   #25
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Some useful info here: Multihull Dynamics, Inc. - News Article


"Hull Beam and Hull Length to Beam Ratio
A very significant factor in boat performance is the ratio of waterline length
(Lwl) to hull beam (Bh) (not overall beam), Lwl/Bh vs. Length, plotted in Fig. 1
below. In Bruce and Morris work Design for Fast Sailing, it was shown that a
good performing cruising catamaran or trimaran (main hull) should have a ratio
of at least 8. In displacement hulls, the bow wave includes a trailing trough
that a boat stern tends to set down into, limiting the speed of the boat. This
is called hull speed. Slender hulls, those with Lwl/Bh ratios greater than 8, do
not create a large enough bow wave for this to be an appreciable effect[3]."
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Old 27-09-2009, 19:56   #26
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Originally Posted by creakyboy View Post
Thanks for the boat porn 44'cruisingcat,(I lust after Banque Populaire big time) however I have to wonder,is a tri sailing on one ama "planing" to a degree,as in its exposing(if thats the right word) less wetted surface than if it was sailing along with the main hull submerged??
It's about the underwater volume rather than the area.

Think of a 1 ton speed boat -at rest it displaces 1 cubic metre of water. There is 1 cubic metre of boat underwater.

And motoring at or below hull speed it still displaces 1 cubic metre of water.

But when planing there is much less than 1 cubic metre of the boat under water.

When a big tri like Banque lifts it's main hull, the ama becomes much more deeply immersed than it is at rest. It is still displacing a volume of water which weighs what the boat weighs.
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Old 27-09-2009, 21:10   #27
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Cheers for the clarification 44'cruisingcat

(still wondering/dreaming bout a cruising conversion for Banque...wonder if the old one is up for sale for a reasonable price hehehe)
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Old 30-09-2009, 15:11   #28
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Vector average for the different boats

It's difficult to see the average boats speeds and what it entails. It looks like the monohulls above a certain length increase in proportional speed above around 55 ft. I think that would make sense as your talking about multimullion dollar boats competing against each other at that point. Also, in the catamarans above say 50 ft it's difficult to get a sense of speed as there's only really one data point. But from the 40-50 ft range of catamarans, which is by and large the vast majority of ocean crossing catamarans, it you look at the center of the cloud it would seem the catamarans are around 20-25% faster. It also looks like there are two real groups of catamarans those that are really around 38% faster and those that are maybe only around 12% faster. Again, this really does translate into two design philosophies of catamarans. Many of those cats are charter boats and are heavier and slower. For charters being faster can be a detriment (who wants their charter boat raced?) and also being heavy with little sail means its impossible really to flip. Some though are owner aimed boats and keep their performance advantage. And there's a real cat dog in those statistics too for the catamarans. It's a 42 ft long boat whose SA/D is worse than any other catamaran out there. I'll let you figure that out.
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Old 30-09-2009, 17:05   #29
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MarkJ doth protest too much

some people's concept of voyage is how fast do I get to the next port. Some people would consider this to be foolish. You get there when you get there!

A multihull speed is derived from hull design and weight. A heavy loaded speed design is potentially slower than a reasonably loaded cruising design.

A heavy loaded cruising design is probably faster than a mono, unless the journey is entirely to windward.

A gentleman does not sail to winward.
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Old 30-09-2009, 17:55   #30
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In to heavy weather all boats are the same & its the people which will give in sooner not the boats!! I have just returned from a week away & found it was 6kts boat speed with 30kts on the nose & 14kts with shelter from the land.(pasted a few monos down to less than 4kts & not looking any more comfortable) The level of comfort is closely supervised by the other captain. Fuel burn 2X7.5hp 1gms 7+kts @ >3ltrs/hr 5.5Kts @ 1800RPM >2ltrs/hr 36X21' approx 5T. Lift the dagger boards & rudders .4mtr water lets you get very close to the beach & at night whilst sitting very flat just look at other boats mast lights. How do they sleep? The big thing is marina fees for me.
Not bad for a boat launched 1985.
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