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Old 15-05-2013, 00:53   #181
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Re: What are the Slowest Cats?

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Originally Posted by downunder View Post
Lar keels displace water and can also increase load carrying vs daggerboards to some extent although you example you have negated increased bouyancy by adding a water load.
Why is adding water negating the buoyancy? I assume the water is tankage? Would the centerboard version be carrying no water? If they both have the same tankage the one with the lar keel tanks will still sit higher regardless to the location of the water tanks. The keels are just a convenient place to put the tanks.

Lar keels actually have a slight negative impact on the righting curve as they act as anti keels (ballast wise). I guess if you manage to make them heavier than water (pretty impossible in practical terms) then they would benefit the righting curve. Also LAR keels increase pitching motion. To what extent I am not sure.
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Old 15-05-2013, 01:09   #182
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Yes, the wave-making ability of hulls goes down as they get narrower. Which reduces the energy required to overcome hull speed. But the wetted area drag climbs rapidly. Nothing is for free.

Same goes for the number of hulls. There are some advantages to increasing the number of hulls. But nothing is for free. More than one hull causes irrational sensitivity, lack of humor and insecurity in the owners. Apparently.
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Old 15-05-2013, 01:09   #183
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Re: What are the Slowest Cats?

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Originally Posted by Cheechako View Post
Well, there are traditionally three kinds of motion on the water; displacement, semi displacement (or semi planing), and Planing. By definition, if part of the hull leaves the water from the normal "sitting in the water" position, it is called planing. Thus a Sea Ray at 30mph with 1/2 to 2/3 of the hull out of the water is planing. That is... instead of being submerged to it's normal position, it's riding to some extent up on the water. If the hull is still somewhat supported on the water by displacement, then it may be semi-planing. If it is only supported on the water by it's displacement, then it's displacement. If someone put a yanmar 30HP diesel in a SeaRay, then it would be displacement boat, not because of hull shape, but because of the fact that at any speed it can attain, it is supported on the water purely by it's displacement.
Maybe I should have said "Semi-planing" in my original post. Evidently that is what a cat does.... really not sure...is a power cat at 40 mph driven by screws planing or semi planing...? I'm not even sure when a boat is up on foils if it fits any of the tradional 3 definitions....

Ignoring foils, which are out there on their own, you can have planing cats - usually trailerable power cats, and displacement cats, which are still capable of going well beyond (multiples of) theoretical hull speed, while still operating at design waterline.

Here's am example: 80 foot boat, hull speed 12 knots, operating at 30 knots with the hulls at waterline.



It's not planing!

What you notice is that even though it's a big boat going fast, there is very little in the way of bow wave generated. That's due to the high LWL:BWL ratio.
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Old 15-05-2013, 01:37   #184
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Re: What are the Slowest Cats?

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What is a good accepted sailing speed on average?

6 knots 8 knots or 10 knots??
G'day mate!

Hope you & family are all in good health and spirits?

We do all our trip planning using 5 knots. So we always arrive early, but we're OK with that....and where there's bars to cross we try to aim for the mid-flood tide thinking we're more likely to average 6, rather than 5. Our long delivery voyage averaged 6.5kn and that was with very little motoring (1500litres in ~14000nm, most of that burned around land/marinas/etc) i.e. there were quick days and slow days. So it seems to us that 6 knots would seem a reasonable average for an L440. That said, our last little scoot from Coffs to Yamba covered the 56nm in 7 hours with 15-20kn breeze and the current supposedly against us...and our top speed (3 times now) was >21kn, basically surfing...mmm... As you know, the little puppies can swim!
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Old 15-05-2013, 01:39   #185
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Re: What are the Slowest Cats?

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A catamaran doesn't have foils - I think it's absurd they allow those in a "catamaran" race. It should be called a hydrofoil race. Any they confuse the public with regard to real catamarans. You know those things flip over almost every time they run them.
If you follow that argument to its logical conclusions it is also "absurd" that they allows cats to race at all. Its confuses the public in regard to real boats which are monohulls.

My point being that I do not agree with your original statement.
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Old 15-05-2013, 01:49   #186
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Re: What are the Slowest Cats?

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Originally Posted by 44'cruisingcat View Post

What you notice is that even though it's a big boat going fast, there is very little in the way of bow wave generated. That's due to the high LWL:BWL ratio.
That looks like a very nice boat and is going well in those swells but the way it is pitching it makes you wonder how wet it would get in more telling conditions!
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Old 15-05-2013, 01:51   #187
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Re: What are the Slowest Cats?

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Originally Posted by dennisail View Post
Why is adding water negating the buoyancy? I assume the water is tankage? Would the centerboard version be carrying no water? If they both have the same tankage the one with the lar keel tanks will still sit higher regardless to the location of the water tanks. The keels are just a convenient place to put the tanks.

Lar keels actually have a slight negative impact on the righting curve as they act as anti keels (ballast wise). I guess if you manage to make them heavier than water (pretty impossible in practical terms) then they would benefit the righting curve. Also LAR keels increase pitching motion. To what extent I am not sure.
Yes will sit higher with same tankage. No problem putting water there but I was assuming additional water there would negate the boyancy of keel. If it allowed smaller tankage onboard then the keels would give additional boyancy than can offset load.

Just one of the reasons (boyancy) apart from cost/room that the production charter builders use LAR keels
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Old 15-05-2013, 02:05   #188
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Re: What are the Slowest Cats?

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G'day mate!

Hope you & family are all in good health and spirits?

We do all our trip planning using 5 knots. So we always arrive early, but we're OK with that....and where there's bars to cross we try to aim for the mid-flood tide thinking we're more likely to average 6, rather than 5. Our long delivery voyage averaged 6.5kn and that was with very little motoring (1500litres in ~14000nm, most of that burned around land/marinas/etc) i.e. there were quick days and slow days. So it seems to us that 6 knots would seem a reasonable average for an L440. That said, our last little scoot from Coffs to Yamba covered the 56nm in 7 hours with 15-20kn breeze and the current supposedly against us...and our top speed (3 times now) was >21kn, basically surfing...mmm... As you know, the little puppies can swim!
Our last trip to Yamba:



From Southport. We had to slow down for the second half of the trip, we were going to arrive at near low tide! Ended up putting the mainsail away altogether around Cape Byron.
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Old 15-05-2013, 02:07   #189
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Re: What are the Slowest Cats?

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Originally Posted by Sos View Post
That looks like a very nice boat and is going well in those swells but the way it is pitching it makes you wonder how wet it would get in more telling conditions!
There's always the option of slowing down!
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Old 15-05-2013, 02:11   #190
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Re: What are the Slowest Cats?

Impressive times. I averaged close to 8K from Toau in the Tuamotos to Tahiti. I was going for broke as I would have arrived after dark. There is no way my average speed is anywhere near that. Prob under 5.5K (ocean passages) although I have done the occasional 190nm day. Never quite got to 200.

I just checked and my average from Sandiago to the Marquesas was 5.3k when using the distance the crow flies. We done it in 22 days. Had a few days bobbing in no wind etc. That is considered a fast for that crossing.

The only real way to compare boats fairly is on long crossings like this, preferably at the same time. As boats will be fully loaded and sailed in all conditions.

Remember, coastal cruising averages will be faster than ocean passages since they will be shorter with good forecasts and therefore its less likely you will have "slow" weather and wont be bobbing in no wind or battling headwinds.
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Old 15-05-2013, 08:14   #191
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Re: What are the Slowest Cats?

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If you follow that argument to its logical conclusions it is also "absurd" that they allows cats to race at all. Its confuses the public in regard to real boats which are monohulls.

My point being that I do not agree with your original statement.
My point is that hydrofoil races should not be called catamaran races.

I disagree with your statement, and do not see any logical relationship to my original comment.
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Old 15-05-2013, 13:15   #192
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Re: What are the Slowest Cats?

Were you talking about the Americas cup? Which was traditionally a monohull event. When multis first appeared there was an outcry similar to the one you had regarding the hydrofoils.
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Old 15-05-2013, 16:11   #193
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Re: What are the Slowest Cats?

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Were you talking about the Americas cup? Which was traditionally a monohull event. When multis first appeared there was an outcry similar to the one you had regarding the hydrofoils.
It seems these boats are known as "AC72" boats. The name "catamaran" seems to be used as a historical reference to the basic style.

If I mention the word "catamaran" in random company, I'll often get the response "Isn't that the kind of boat that flipped over and killed someone last month"?
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Old 15-05-2013, 16:11   #194
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Re: What are the Slowest Cats?

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Originally Posted by dennisail View Post
Were you talking about the Americas cup? Which was traditionally a monohull event. When multis first appeared there was an outcry similar to the one you had regarding the hydrofoils.
So True

I fact traditionally an event where the challenger had to sail on their own bottom across the Atlantic prior to racing.
Quote:
If I mention the word "catamaran" in random company, I'll often get the response "Isn't that the kind of boat that flipped over and killed someone last month"?
Yep, thats it exactly the same. No difference at all between a full on F1 car and a Totyota Corolla.

BUT more importantly how about we await the enquiry before suggesting that the boat flipped, there is many versions of what happened, as in broke then flipped or vice-vcersa and variations on that theme.
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Old 15-05-2013, 17:40   #195
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Re: What are the Slowest Cats?

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UT more importantly how about we await the enquiry before suggesting that the boat flipped, there is many versions of what happened, as in broke then flipped or vice-vcersa and variations on that theme.
Well, my point is that whatever happened, it's not likely to happen on your Lagoon, and they are really not the same kind of boat at all even though they are both called "catamarans".

If they're going to race hydrofoils, I wish they'd call them that!
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