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Old 23-08-2008, 20:37   #1
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Importance of a Cat's center clearance

Probably not the right term, but have noticed a great deal of variation in the center clearance of various large crusing Cat's and was wondering what the ramifications of high or low clearance might be. I saw the video of a sleeping Dave on Exit Only while the sheets were getting bumped around by the waves slapping the bottom... my take away was the design needed more clearance, but then get the latest edition of MultiHauls World and on the back cover there's a great shot of a new Lagoon that is almost a trimaran with a center "haul" just touching the water line... so was just wondering if one should be concerned at all about the center clearance

All thoughts greatly appreciated...

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Old 23-08-2008, 20:39   #2
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center "haul"
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Opps... supposed to read "Hull"...

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Old 23-08-2008, 21:20   #3
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I know next to nothing but I remember hearing that it has alot to do with center stability and beating the boat up if it slaps too much. how valid that is I have no idea at present. Just reiterating what I heard.

PS can anyone tell me how to start a new thread? sorry but I am new to this.
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Old 24-08-2008, 00:55   #4
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You are talking about bridgedeck clearance. However, the height alone is not the whole story. It is also a factor of width and design . some designs are shaped to take the wave crest and deflect it through 90 degrees and can thus reduce the bridgedeck clearance. some, like the Prouts have a central nacelle which does the same thing.

Heavy slamming onto the bridgedeck is not good, but there will always be some course/wave that will cause a slam. It is very good tactics to change the course and reduce that slamming, but sometimes this is not possible - it is then that having a more solidly built cat like the privilege is a good thing.
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Old 24-08-2008, 05:27   #5
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Slamming under bridgedeck is common when:
- bridgedeck clearance is low
- L/B ratio off hull is high and
- displacement of catamaran is high
- inner hullsides are vertical
- bridgedeck is long
- beam between hull centers is too narrow (interference of waves)

Under way, slamming is more common when:
- angle of waves is unfavourable
- swells are across waves
- current is against wind and wave direction
- sailing at high speed
- loadings are at the ends

These were my first toughts of bridgedeck slamming...

Terho
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Old 24-08-2008, 05:43   #6
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Slamming is annoying and it can happen to any catamaran.

It has a lot more to do with your angle of attack w/ respect to the waves than anything else.

I have a high clearance on this boat, but we can still slam if I screw up and take steep waves wrong. Mine will slap back by the cockpit (aft) when I do this.

It's mostly a matter of learning how to handle your boat... if you handle her properly, you will be able to avoid slamming completely. In Dave's case, he was on a passage and probably would have lost days of time made good if he were to have changed course to accommodate the slamming.

These are the downside of cats - slamming and the way the bows fly around motoring into seas. Other than that, everything else is great!
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Old 24-08-2008, 06:48   #7
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http://www.sailcopress.com/elusive_c...aran_perfo.htm theres some info for you its a good read
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Old 24-08-2008, 08:59   #8
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SCTPC, Thanks for that article, it was very helpful in understanding this issue better... not sure what Lagoon is going for with their 420 "trimaran" design, but perhaps they are looking to slice the water into two channels...

Thanks also to Ragner, Talbot and Capt Sully.. really appreciate your thoughts on this....

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Old 24-08-2008, 11:53   #9
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You referred to a video of the sleeping skipper's feet bouncing in the bunk. That was not bridgedeck slap; as explained in subsequent discussion, the photographer was kicking him to wake him up!

However, the phenomenon is real, and varies from merely annoying to threatening. As boat speed increases, bow waves (which are always present, but not very significant at lower speeds) converge and their combined energy creates a central standing wake much higher than either one alone. Add the height and energy of an external wave, such as a wake or another local peak, and a slam follows, sort of like driving into a barracade of water.
Theories abound for dealing with this, and the continuing differences of opinion indicate that one clear answer has not emerged. Higher bridge deck clearances mean that only those waves that are higher will hit, and that the boyancy of the bows will lift the boat over a lot of lesser occurances. Narrower beams reduce the likelyhood of straddling a random peak without being lifted above it. Wave-spreading center hulls will spread the offending peak and its mass providing you hit the offender directly, and will impart lateral thrust if you do not, making dishes slide more than bounce. Big long heavy dangling pendulums (lets call them 'ballasted keels' for the sake of discussion) dampen the motion but those kind of boats seem to lean a lot.

All of these solutions come at a cost of one kind or another. Additional wetted surface area equals drag. Large frontal area has to be pushed through all that water, and if your thrust and momentum aren't up to it, you slow down or even stop.

Its hard to fit a lot of accomodation into the most water-friendly shape, so performance is traded for liveability. The only vessel that is impervious to wave interaction is a submarine, but these have not caught on with the cruising set.
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Old 24-08-2008, 13:36   #10
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6%

You typically want for the bridgedeck clearance to be about 6% of the length of the boat for the typical cruising boat, lightship, and the bridgedeck should be no more than 2/3 of the boat's overall length. If the bridgedeck is longer, you need more bridgedeck clearance.
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Old 24-08-2008, 15:51   #11
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Quote:
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You typically want for the bridgedeck clearance to be about 6% of the length of the boat for the typical cruising boat, lightship, and the bridgedeck should be no more than 2/3 of the boat's overall length. If the bridgedeck is longer, you need more bridgedeck clearance.
And if you go to high then you get High Windage and that can be just as bigger problem.
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Old 24-08-2008, 18:19   #12
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Slamming is really annoying

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And if you go to high then you get High Windage and that can be just as bigger problem.
Maybe a little slower, or a little more leeway on the wind, but that is much, much less annoying than bridgedeck slaming.
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Old 24-08-2008, 21:27   #13
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how high?

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You typically want for the bridgedeck clearance to be about 6% of the length of the boat for the typical cruising boat, lightship, and the bridgedeck should be no more than 2/3 of the boat's overall length. If the bridgedeck is longer, you need more bridgedeck clearance.

To be a meaningful number you really have to compare the wing-deck clearance to the width of the tunnel (inside of port hull to inside of starboard hull at approx centre of loa). This is the bit that's straddling the waves beam on in the worst case scenario.
Crowther and others then use around 18 to 20% of the tunnel width as the required wing deck clearance, from memmory.

So a cat with a 6 meter beam will have a tunnel width of around 3.5 meters and a wing deck clearance of around 630 to 700 mm.

If the bridge deck is long you will require more bridge-deck clearance at the ends. Maybe a fore and aft curved under-wing.
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Old 24-08-2008, 21:36   #14
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The tunnel width rule works, too.

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To be a meaningful number you really have to compare the wing-deck clearance to the width of the tunnel (inside of port hull to inside of starboard hull at approx centre of loa). This is the bit that's straddling the waves beam on in the worst case scenario.
Crowther and others then use around 18 to 20% of the tunnel width as the required wing deck clearance, from memmory.

So a cat with a 6 meter beam will have a tunnel width of around 3.5 meters and a wing deck clearance of around 630 to 700 mm.

If the bridge deck is long you will require more bridge-deck clearance at the ends. Maybe a fore and aft curved under-wing.
Yes, this typically works out about the same as the 6% guideline, given that most catamarans have similar proportions.
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Old 10-11-2008, 19:40   #15
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You typically want for the bridgedeck clearance to be about 6% of the length of the boat for the typical cruising boat, lightship, and the bridgedeck should be no more than 2/3 of the boat's overall length. If the bridgedeck is longer, you need more bridgedeck clearance.
Let me make sure I understand this. You are designing a 36' cat, so the bridge deck should be about 6% of 36' above the water line? So in the case of our 36' Cat, the bottom of the bridgedeck should start 2.16' above the water line.

Also, on your second point, you say 2/3 of the boats overall length. So given 36', the bridgedeck should be no longer than 24'. Correct?
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