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Old 08-01-2015, 10:57   #46
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Re: Cat Bridge Deck Clearances?

Seaslug, I am with those who disagree with you concerning the effect of adding transom extensions - the additional LWL will decrease hobbyhorsing and thereby reduce slapping and slamming. Think of two teeter-totters with the fulcrum at the same height. Persons at the end of a 6 foot teeter totter will move up and down much further than ones on a 12 foot teeter totter, even though the forces exerted are higher due to the lever principle. Only if the fulcrum is raised proportionally will the ultimate heights reached remain the same.

Since hulls typically have some rocker, the extensions aft also operate so as to make use of the upward slope aft in the bottom of the hull: extend this upward slope over a further distance and you raise the height of the transoms so that they are out of the water. This improves hydrodynamic efficency.

This is to say nothing, of course, of the fact that theoretical hull speed is directly proportional to LWL: increase one and you increase the other. In the end result, properly designed/constructed transom extensions can reduce hobbyhorsing, slamming/slapping and increase speed.

I also disagree with your formula for assessing bridgedeck clearance based solely upon LWL. Indeed, while such formulae are sometimes cited, the width of the tunnel is far more relevant to slamming than the LWL. Indeed, if once considers two boats where all else is equal (BWL, hull shape, tunnel width, Cg, Cb, hull shape, loading, SA and Ce for the sailplan, etc.) then although both boats have the same bridgedeck clearance, the one with the longer waterline will tend to hobbyhorse less and therefore pound/slap less than the boat with the shorter LWL. Yes, on its own, increased LWL reduces slamming/slapping.

Tunnel width is the critical factor and a figure sometimes cited (including one by an earlier poster in this thread) of 1" per foot of BWL is a good starting point in catamarans that have BWL/LWL ratios of about 50%. Some suggest, hoever, that as beam increases much beyond that ratio ( further increasing relative tunnel width), the figure may actually need to be closer to 1.3" per foot. Why?

In head seas, a cat with an 8 foot tunnel width will permit 50% less water into the tunnel than a boat with a 12' tunnel width. The impact will therefore be 50% higher. Furthermore, as there is a slick, or shadowing effect from the windward hull into the tunnel, that effect will also be greater where the tunnel is narrower. In terms of seas off the beam, a narrower cat operates much like a narrow jeep/offroad vehicle by being better able to straddle irregularites in a road.

Of course, a narrower tunnel can increase slapping to the extent that the bow waves can converge more readily (and closer to the front of the bridgedeck). To best optimize a relatively narrow tunnel, it is critical that the bridgedeck either start further aft, or that it have much higher bridgedeck clearance towards the front of the bridgedeck. It is also helpful to have small, upward sloping knuckles towards the bows in order to divert the spray downwards.

Factors such as bridgedeck shape are also critically important in determining the impact of seas in the tunnel, regardless of the width. A bridgedeck with a relatively blunt leading edge to the bridgedeck (as often occurs with transverse berths over the leading edge of the bridgedeck) is far more susceptible to slamming than one with a gradual curve aft (and preferably, also a gradual curve back up at the end of the bridgeck).

Once water comes into contact with the bridgedeck, it operates not much differently than the hull of a boat in the water. A flat bottomed scow with a blunt bow will pound much more than a hull with some rocker and a graceful entry. For this reason boats such as the Manta, with relatively low bridgedeck clearance, nevertheless are known for being quite effective at avoiding slamming.

Anyway Alias, sorry but bridgedeck clearance is not just a matter of one simple equation that can be applied to all boats!

Brad
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Old 08-01-2015, 17:25   #47
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Re: Cat Bridge Deck Clearances?

**So Guys.... looking at the underside of this Corsair 3600 Cat I had shown some interest in, would I be correct to assume that the unanimous consensus is to run away??
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Old 08-01-2015, 17:43   #48
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Re: Cat Bridge Deck Clearances?

You could always try to get your advice from unbiased experts rather than the internet. EG engage a good multihull surveyor and talk to him about the various designs. Talk to some designers, whilst they may be biased you can at least get a good understanding of their design parameters etc. Internet forums are - with all due respect to my keyboard warrior friends - the least reliable place to get information.

Like many things in life the answer is rarely simple. I have sailed a Corsair 3600, indeed a couple of them, yes it would be better without the corridor (trench), but they are not a bad boat at all, Chamberlain is a good designer and they are pretty good value usually.

If you are in the market then money spent with a good MULTIHULL surveyor who can advise as well as check out boats, is money well spent.
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Old 08-01-2015, 18:01   #49
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Re: Cat Bridge Deck Clearances?

Quote:
Originally Posted by Factor View Post
You could always try to get your advice from unbiased experts rather than the internet. EG engage a good multihull surveyor and talk to him about the various designs. Talk to some designers, whilst they may be biased you can at least get a good understanding of their design parameters etc. Internet forums are - with all due respect to my keyboard warrior friends - the least reliable place to get information.

Like many things in life the answer is rarely simple. I have sailed a Corsair 3600, indeed a couple of them, yes it would be better without the corridor (trench), but they are not a bad boat at all, Chamberlain is a good designer and they are pretty good value usually.

If you are in the market then money spent with a good MULTIHULL surveyor who can advise as well as check out boats, is money well spent.

That sounds like a playing it safe 'pearls of wisdom' coming from a boat broker perchance?

Cutting to the chase...................

Would you be happy to spend extended periods of time on a Corsair 3600?

Would you sail one to NZ or Noumea with your family on-board?

Would you buy one for your own usage?
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Old 08-01-2015, 18:13   #50
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Re: Cat Bridge Deck Clearances?

Quote:
Originally Posted by ALIAS View Post
That sounds like a playing it safe 'pearls of wisdom' coming from a boat broker perchance?

Cutting to the chase...................

Would you be happy to spend extended periods of time on a Corsair 3600?

Would you sail one to NZ or Noumea with your family on-board?

Would you buy one for your own usage?

Does this "boat broker" have the listing on the Corsair your looking at? I believe he has nothing to gain, but knowing his experience with multihulls I think you would have a lot to gain listening to his opinions.
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Old 08-01-2015, 18:30   #51
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Re: Cat Bridge Deck Clearances?

[QUOTE=smj;1719591]Does this "boat broker" have the listing on the Corsair your looking at? I believe he has nothing to gain, but knowing his experience with multihulls I think you would have a lot to gain listening to his opinions.[/QUOTE


Well smj............ don't even know whether 'FACTOR' is a boat broker.I'm a new kid on the block.

Was just asking the questions if you will take note as his response sounded like to ME, the type of response I had gotten/would expect from a salesman.

Accordingly lets wait and see what sort of answers he gives me to my questions.

But thank you for your input even though it sounds a bit like a buddy giving support to a buddy even though I only asked some pertinent questions it appears to me that some folks around here are kinda touchy with questions.

Are called for?
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Old 08-01-2015, 18:40   #52
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Re: Cat Bridge Deck Clearances?

[QUOTE=ALIAS;1719599]
Quote:
Originally Posted by smj View Post
Does this "boat broker" have the listing on the Corsair your looking at? I believe he has nothing to gain, but knowing his experience with multihulls I think you would have a lot to gain listening to his opinions.[/QUOTE


Well smj............ don't even know whether 'FACTOR' is a boat broker.I'm a new kid on the block.

Was just asking the questions if you will take note as his response sounded like to ME, the type of response I had gotten/would expect from a salesman.

Accordingly lets wait and see what sort of answers he gives me to my questions.

But thank you for your input even though it sounds a bit like a buddy giving support to a buddy even though I only asked some pertinent questions it appears to me that some folks around here are kinda touchy with questions.

Are called for?

Factor is a Seawind dealer but im sure he has sailed and reviewed more multihulls than anyone on this forum. I for one would be interested in the Corsair cat if it were in the US.
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Old 08-01-2015, 18:48   #53
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Re: Cat Bridge Deck Clearances?

Quote:
Originally Posted by ALIAS View Post
Would you be happy to spend extended periods of time on a Corsair 3600?

Would you sail one to NZ or Noumea with your family on-board?

Would you buy one for your own usage?
If you want opinions,

No, No, and No. Too small for me.

2 Hulls Dave
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Old 08-01-2015, 18:56   #54
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Re: Cat Bridge Deck Clearances?

Quote:
Originally Posted by 2Hulls View Post
If you want opinions,



No, No, and No. Too small for me.



2 Hulls Dave

Your right Dave, but if an owner of a Gunboat 55 was asked the same questions regarding your Catana 47 his answers would probably reflect the answers you just gave. It's all relative.
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Old 08-01-2015, 18:59   #55
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Re: Cat Bridge Deck Clearances?

Quote:
Originally Posted by smj View Post
Does this "boat broker" have the listing on the Corsair your looking at? I believe he has nothing to gain, but knowing his experience with multihulls I think you would have a lot to gain listening to his opinions.
[QUOTE=smj;1719605]
Quote:
Originally Posted by ALIAS View Post


Factor is a Seawind dealer but im sure he has sailed and reviewed more multihulls than anyone on this forum. I for one would be interested in the Corsair cat if it were in the US.
Thanks for bringing me up to speed on that one smj, appears my hunch was correct. Btw wasn't questioning Factor's experience, just how his response came across.

As for the Corsair, hey its just a hop and skip of a sail to the US of A and think of the over the yard arm tales you could post.

Btw would you feel safe,contented and confident sailing the Corsair across 1500k open oceans??

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Old 08-01-2015, 19:02   #56
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Re: Cat Bridge Deck Clearances?

Quote:
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Your right Dave, but if an owner of a Gunboat 55 was asked the same questions regarding your Catana 47 his answers would probably reflect the answers you just gave. It's all relative.
Well of course, my friend. Alias asked "Would you..." It's clearly a personal choice. He's smart enough to consider all the answers.

Besides, it's a 471. I don't particularly care for the 47s. Too slow.

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Old 08-01-2015, 19:04   #57
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Re: Cat Bridge Deck Clearances?

Quote:
Originally Posted by 2Hulls View Post
If you want opinions,

No, No, and No. Too small for me.

2 Hulls Dave
Yep, wanting opinions............

And why specifically No to the first two questions?
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Old 08-01-2015, 19:04   #58
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Re: Cat Bridge Deck Clearances?

Quote:
Originally Posted by 2Hulls View Post
Well of course, my friend. Alias asked "Would you..." It's clearly a personal choice. He's smart enough to consider all the answers.



Besides, it's a 471. I don't particularly care for the 47s. Too slow.



2 Hulls Dave

Ah, and I'm a fan of the older 48'ers! But then an early 90's Catana 39 or mid to late 80's 40'er would be fine by me.:-)
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Old 08-01-2015, 19:21   #59
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Re: Cat Bridge Deck Clearances?

If you say so. It's your choice!

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Old 08-01-2015, 19:33   #60
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Re: Cat Bridge Deck Clearances?

FWIW, Previous thread including discussion on the subject is here:

Ocean-Going Catamaran Size and Bridge Deck Clearance

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