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  #71  
Old 02-10-2013
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Re: Lets have a chat about multis.

Fair enough Paulo. You seem to have done a lot of research and your right you have to buy a boat that suits you.
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Old 02-11-2013
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Re: Lets have a chat about multis.

Quote:
Originally Posted by smj View Post
I have found two sites on the Internet that state the class a certification of the Gemini. ---
Would you mind sharing those sites?
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  #73  
Old 02-11-2013
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Re: Lets have a chat about multis.

Class A CE certification was an option for the 105MC, it required modifications while it was being built (not extensive, but not add on at a later date).

For example, some of the options were additional bilge pumps, high toe rails, a higher sill from cockpit to inside, bigger drainage holes in the cockpit. It may or may not have included actually sealing the float tanks.

My Gemini (a 2007) was not CE class A certified; I didn't opt for it.

I don't know if Hunter (who now builds it) has continued to provide that option or not.
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Old 02-11-2013
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Re: Lets have a chat about multis.

The RCD is continually being upgraded regarding safer and less pollutant boats. This year a new version come out with mandatory new rules about engine and noise pollution, the need of all cruising boats to have holding tanks and for all cruising multihulls to have escape hatches accessible when the boat is inverted :

"All habitable multihull craft that is susceptible of inversion when used in their design category shall be provided with viable means of escape in the event of inversion".

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  #75  
Old 02-12-2013
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Re: Lets have a chat about multis.

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Would you mind sharing those sites?
Here's one of them.
A Gemini 105 Page
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Old 02-12-2013
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Re: Lets have a chat about multis.

Questions.
How much deck clearance is really necessary?
What is a good beam to length ratio?
Is a traveler really necessary on a cat since they don't point well?
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Re: Lets have a chat about multis.

Melissa,
For a cruising cat:

36 inches is a rule of thumb at the bow, it's the underbody that matters (and what slams disconcertingly). I don't have a number there, but more than the Gemini's 15 inches is a good number.

depends, classical is 9:1 - but for weight carrying capacity less is best. Depends more on where it's 'fat', bows that are too sharp require care as they will plunge and pitch pole.

Yes, traveler isn't just for pointing, it's for trim/balance and heeling also. Very important.
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Re: Lets have a chat about multis.

Deck clearance depends on the size and design of the boat. More is usually better but if you have full standing headroom on the bridgedeck cabin on a 32' cat plus 3' of bridgedeck clearance you would have really tall windage. This would raise your center of gravity and center of effort and probably make the boat unstable. If you take the same boat but make it open bridgedeck, no bridgedeck cabin, then your CG and CE would remain lower. The Gemini for example has a low bridgedeck because it has a narrow beam and needs to keep the CG and CE low to maintain stability.
Some cats go as low as a 7to1 ratio and other 16to1. If you have the length to be able to handle all the weight then the 16 to 1 would be a much quicker boat. If the cat is smaller and needs to support a good sized cruising load they would lean towards a fatter hull.
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Re: Lets have a chat about multis.

Melissa -
If you're asking about overall beam vs length (as opposed to hull ratios) most designers of offshore cruising cats seem to like overall beam to be at least 50% of overall length in boats up to around 50' length. Over that length the ratio gets a bit narrower.
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Old 02-15-2013
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Re: Lets have a chat about multis.

mdi posted a link with interesting information about multihulls, including stability on another thread. I think it is useful to have it here also. The link:

Multihull Dynamics, Inc. - Account Login

and my comments:

Quote:
Originally Posted by PCP View Post
Thanks, interesting stuff.

But in what regards stability the ratios they use are a bit limited. Both take into account the sail area but in extreme situations one does sail in bare poles or with very little sail area so in fact those ratios don't tell about the absolute stability of a boat but about the stability for the max sail area the cat can carry (you can always reef).

Multihull Dynamics, Inc. - News Article

Looking at the formulas they attribute the same importance to three factors: Length, beam and weight. That seems a bit arbitrary to me.

I remember some years ago of some huge racing multihulls (60ft) that were capsized on a huge storm, carrying no sail, just for the windage on the mast. If I remember rightly 70 to 80K winds.

A good way of having an idea of an absolute static stability measure on a multihull would be to know what the intensity of wind would be necessary to capsize the cat only with the wingage over the mast and cabin. I don't understand why that is not used. It is not difficult to computer predict that and surely If I sailed a cat I would know more about that than about a relative stability regarding Stability/sail area. I can always reef or go to bare poles.

..
Interesting that the stability ratio used by the designer of Tom cats multihulls is not the same:

"The stability formula gives a result which is the wind speed (in Knots) required to just lift the windward hull (the point at which, in a cruising boat, not a beach cat, you scream "Let go the sheets!", or better still "Let's put a reef in her").

Stability = 13.7 sqrt(Wt * 1/2Bm/SA * HCE)"


Welcome to TomCat Boats

As you can see they are different:



Multihull Dynamics, Inc. - News Article

Anyway on this times of CAD design, with powerful programs I don't understand the need of approximated ratios when it would be easy for the designer to calculate for each boat (taking into consideration all boat's dimensions) this value and most of all the force of wind needed to capsize the boat on bare poles. This could be a king of AVS point for multihulls, not related to heeling but with wind force. If those values were mandatory we could have a pretty good idea of the stability of each cat.

Anyway, as we cam all see, Beam, length and weight are all determinant factors in a multihull stability and that's why I had made some personal reserves to the use of small light cats for offshore work.

...
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Last edited by PCP; 02-15-2013 at 07:44 AM.
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