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Old 13-01-2015, 03:46   #31
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Re: Why I like Catamarans.

That is the trouble with the internet. Trolls can throw a hand grenade and step back. They would never have the intestinal fortitude to do it face to face. The web allows weak willed spineless people to run amuck.
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Old 13-01-2015, 04:57   #32
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Re: Why I like Catamarans.

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Personally, I have never found the need to denigrate ANY vessel designed for a water based purpose. From coracles to ocean going cruise liners. Its all about the water and being on it.
Not even those d*mned sea-lice aka PWC?
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Old 13-01-2015, 05:33   #33
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Re: Why I like Catamarans.

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Not even those d*mned sea-lice aka PWC?
(There may be exceptions)
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Old 13-01-2015, 05:45   #34
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pirate Re: Why I like Catamarans.

Would love to see this 'Real Man' Sloopguy circumnavigate a Tiki 21...
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Old 13-01-2015, 06:47   #35
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Re: Why I like Catamarans.

my cat is huge compared to most of the mono hulls i looked at prior to deciding on a cat .. i am having to sell it and it is listed here on this forum .. it is a solaris 42 and is moored in the bay of panama .. i have over 70,000 in it right now but would accept any reasonable offer .. it has an 18 foot beam which i found very few mono hulls that wide ,, i looked at a lot of cats like the catalac that were not as wide as some mono hulls i saw .. even the prouts and geminis i saw were not near as wide ,,
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Old 13-01-2015, 09:30   #36
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Re: Why I like Catamarans.

I would get real life confirmation on the 8kt speed if that is important to you.

The proposed boat is far different than the day sailor with super slim hulls posted later in the discussion. That is essentially half a step from a beach cat and those will easily push into double digits. Fun for an afternoon on the water but not so great on the accomodations.

Our gemini had a 40hp when we first bought it and flat out before we loaded her for cruising was just shy of 8kts...and most of that was noise and wake. We would never travel at that speed short of an emergency. Sailing in the right conditions is a different matter. We have occasionally touched double digits but I wouldn't count on it to get from point A to point B where you can't control the angle of the wind.

This is a significantly shorter boat with presumably a lower hull speed (maybe the hulls are much thinner and accomodations much less but it doesn't look like it from the picture)
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Old 13-01-2015, 09:41   #37
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Re: Why I like Catamarans.

My understanding is that cat's don't conform to the conventional hull speed limits. Rather the ratio of the lwl to the beam of a single hull was the determining factor. A 10 to 1 ratio seems to be some sort of dividing point for faster cats, assuming the boat is not over loaded or under canvassed.

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Old 13-01-2015, 09:47   #38
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Re: Why I like Catamarans.

There is a Hirondelle 23 footer in the marina here.
It is glass over ply. Uses an 8 horse, similar in design to the Diamond 24.
I have struggled to keep up with it in the Westerly heading out of the marina.. must be pushing 6 or 7 knots on 2/3rd throttle. Fast off the blocks too.


A moderately thin entry for an older design and has extended scoops on the back.

I see no reason that it wont cruise at 6 knots and a couple more when wide open.

In a flat sea state of course.
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Old 13-01-2015, 11:43   #39
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Re: Why I like Catamarans.

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My understanding is that cat's don't conform to the conventional hull speed limits. Rather the ratio of the lwl to the beam of a single hull was the determining factor. A 10 to 1 ratio seems to be some sort of dividing point for faster cats, assuming the boat is not over loaded or under canvassed.

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Hull speed still applies.

The confusion is the 1.3 at the begining of the formula is not actually a constant. It is a function of the L to B Ratio. The confusion comes from the fact that ratio changes so little for traditional monhulls that it's not worth accounting for.

This is why I mentioned at the end that it may have unusually narrow hulls but of course those come with the downside of less load carrying capability. Great for a lightly loaded day sailor but not as great for cruising.

I wouldn't be at all suprised if lightly loaded it can motor at 6-6.5kts quite comfortably but the HP requirements go up very quickly as the speed increases over even a small range, so it's almost like hitting a wall when you hit hull speed.

Again, I'm not saying it can't hit 8kts but with just the one picture, it doesn't look like a design that would easily hit those kinds of speeds.
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Old 13-01-2015, 13:20   #40
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Re: Why I like Catamarans.

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Hull speed still applies.

The confusion is the 1.3 at the begining of the formula is not actually a constant. It is a function of the L to B Ratio. The confusion comes from the fact that ratio changes so little for traditional monhulls that it's not worth accounting for.

This is why I mentioned at the end that it may have unusually narrow hulls but of course those come with the downside of less load carrying capability. Great for a lightly loaded day sailor but not as great for cruising.

I wouldn't be at all suprised if lightly loaded it can motor at 6-6.5kts quite comfortably but the HP requirements go up very quickly as the speed increases over even a small range, so it's almost like hitting a wall when you hit hull speed.

Again, I'm not saying it can't hit 8kts but with just the one picture, it doesn't look like a design that would easily hit those kinds of speeds.
Not really. Tank testing done by Edmund Bruce found that at LWL:BWL ratios above about 8:1, bow wave propagation reduced to the point that "hull speed" limitations ceased to apply.


Multihull Dynamics, Inc. - News Article
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Old 13-01-2015, 15:43   #41
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Re: Why I like Catamarans.

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Not really. Tank testing done by Edmund Bruce found that at LWL:BWL ratios above about 8:1, bow wave propagation reduced to the point that "hull speed" limitations ceased to apply.


Multihull Dynamics, Inc. - News Article
Actually, it does not say that.

It says "for practical purposes".

If I recall the math properly, it's a function of the tangent of the wake angle and while it still exists, hull speed increases so rapidly that surface friction quickly becomes the dominant drag force.

Waves propogate as a function of length. Longer waves propogate faster. Hull speed is closely related but the boat travels at an angle to the wake(ie:wave). A short fat hull will typically send a bow wave off at something around a 45 degree angle. As the L to B ratio gets higher, the bow wave angles further and further back.
- If you hold the boat in a single trough going directly with the wave, it will travel at the speed of the wave (doesn't happen with wakes)
- If you run at a 45 degree angle, the speed of the boat is traveling around 140% of the wave speed.
- By the time you get to say a 70 degree wake angle, the boat is running around 225% of wave speed and surface friction is the limiting factor. Many people confuse this with a lack of hull speed.
- This also assume a calm smooth surface and a simple symetrical hull with no interference from the wakes meeting. Add in surface waves or asymentrical hulls and other complications and the math gets messy but the underlying principal still holds true.
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Old 13-01-2015, 16:40   #42
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Re: Why I like Catamarans.

Well, experience shows that multis frequently blast right through "hull speeds" with nary a twitch. With slenderness ratios above a threshold where wave making is no longer the limit, it seems friction becomes the limiting factor. And friction can be overcome simply by more propulsive force without the worry to overcome gravity as well by climbing a wave. Indeed, when gravity becomes a contributor, e.g. when surfing, it seems the sky's the limit. Actually friction is the limit and stable multi speeds well in excess of 20 kts are not unusual. Less friction and the faster they'll go. Here, other variables and forces disrupt monos regarding directional stability and broaching becomes the major threat.

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Old 13-01-2015, 17:35   #43
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Re: Why I like Catamarans.

It is also a bit more complicated than the ratio.

For example, I know that the speed potential of my Stiletto was radically different when stripped for racing vs. cruising. The beam ratio was not much different, and the transoms did not drag, but the lighter boat could climb out of the hole (semi-plane) far more readily than the loaded boat. Much has to do with the hull shape, and the Stiletto has fairly flat aft sections. On the other hand, my PDQ is far less weight sensitive (and slower), the result of more rounded sections with more rocker and beam.

My gut says the PO's boat may hit those numbers light ship (stripped of everything but paint) but that as cruised he will be more subject to the hull speed limit, motoring ~ 6 knots and sailing a bit faster.



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Old 13-01-2015, 17:37   #44
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Re: Why I like Catamarans.

A westerly Centaur is quite a sedate vessel.
This will be faster
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Old 13-01-2015, 18:00   #45
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Re: Why I like Catamarans.

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It is also a bit more complicated than the ratio.

For example, I know that the speed potential of my Stiletto was radically different when stripped for racing vs. cruising.
When unloaded it had less immersion, and thus, less skin friction. And remembering F=ma, it had better acceleration.

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