What is bridge clearance ona cat?

[vilanomark]

I've seen several remarks about "bridge" clearance on a cat? I imagine it is different than the mast clearance of asailboat. IS it the height of the cabin from the waterline? I would imagine a lower bridge clearance would be less of a hindrance in the wind.
Am I in the rigt neighborhood, or way out in left field?

[GordMay]

Good question!

The Bridge Deck Clearance on a Catamaran is the distance between the waterline and the underside of the middle deck, joining the two ammas. A higher bridge deck clearance reduces pounding in waves.

A Bridge Clearance (all boats) can also refer to it’s “Air Draft”, which is the height of the mast above waterline. This is the height a bridge must have, in order for the boat to clear under it.

From: http://www.graingerdesigns.com.au/considerations.htm
”... Many cruising cats have low bridge deck clearance to provide maximum possible headroom in the bridge deck saloon, without creating an excessively high structure with poor appearance and high windage. In some conditions this is fine, but there are occasions, especially beating to windward in steep seas, when the slamming under the bridge deck will become a limiting factor on how hard the boat can be driven, and could severely impede performance in some conditions. Slamming under the bridge deck is also affected by the shape of the hulls and their separation, and also by the length of the bridge deck, and could be expected to be worse if the hulls are excessively wide or the width between the hull centrelines is unusually narrow, thereby causing the bow waves to converge under the bridge deck. Because of the complex factors involved in determining bridge clearance no specific rule can be provided to cover all designs, however, in regarding to sea keeping, the more clearance the better ...”

HTH,
Gord

[vilanomark]

Thanks, that helps alot.

[Lodesman]

Some cats, such as Prouts, have an additional hull-shaped bulge or appendage in the centre of the bridgedeck to "cleave" rather than slam into waves. There seems to be some controversy as to whether or not this actually works, but it would seem to make sense. If you look at wave-piercing fast cats, they generally have this sort of design.

Kevin

[GordMay]

Suspended Nacels, such as the Prout sport, may cleave the water, as Lodesman suggests - or they may merely lower the effective bridge deck clearance, creating more pounding.
Possibly, they do both. I suspect the appendage might reduce the wave height at which some pounding/slamming occurs, but reduce the impact of that slamming.
I'll be interested to read experienced comments.

My fantasy Cat' has the bridgedeck set back from the bow, with trampolines foreward, and a fairly flush nacel (not deeply suspended) providing fwd. access and a fair chain lead. These are purely theoretical musings, with NO experience-base.

[henryv]

Charles Kantor comments on suspended nacelles in his book Cruising in Catamarans. My recollection is that his experiences and comments were not terribly positive.

Even in relatively flat water the bow waves meet in the middle of the boat and produce a single wave that will slap the bottom of the bridge unless there is adequate clearance. Putting a nacelle there may reduce the noise but there will be a price paid in reduced performance.

[Lodesman]

You're right that Kanter was anti-nacelle. Of course it seems that Prouts are considered one of the more "ocean-worthy" designs, and the follow-ons (Privilege and Broadblue) seem to have kept the nacelles. For every feature that one experts touts, there's another expert that is completely opposed to it. The nacelle is also supposed to provide reserve buoyancy, to keep the bows from burying. Since cat hulls are mostly very fine forward, this is probably a good thing. Frankly the "reduced drag" that would be caused by the nacelle is irrelevant if the seas are such that the bows are plowing into the waves.

Kevin

[Talbot]

Prout nacelle is also designed to provide massive additional buoyancy up forward when running into a wave. The disadvantage of them is that when overloaded the nacelle is actually underwater, thus adding to the drag in what is not a very fast boat. The broadblue nacelle is very different.

[GordMay]

Let me reiterate, that I’m not a Cat’ expert.

It seems to me, that the Prout & Broadblue catamarans have very different nacelles.
The Prout is located farther forward, and is more deeply suspended than the Broadblue.
I’d expect these differences to have significant implications for slamming, with the Broadblue more closely approximating my preference.
See the comparisons at: http://cruisersforum.com/photopost//...php?photo=1820

I’ve never seen a significant 'empty stowage’ space, aboard a cruising boat. Give me volume, and I’ll give you weight ... and even "light-weight" ground tackle is heavy.

FWIW,
Gord'

[henryv]

If you want a good outline of Charles Kantor's take on catamaran bridge issues see article at: http://www.sailcopress.com/elusive_c...aran_perfo.htm

[Lodesman]

Thanks for the good read henryv. It certainly gives a lot of stuff to consider, but it doesn't give a lot of fact to back up the claims. It seems mostly anecdotal, and he even seems to contradict himself by saying that some of the designs' performance were improved by lengthening the hulls. No doubt length/bmax ratio is important in performance, but it doesn't support the "performance-robbing protrusion" theory. I have a hard time believing that hull-flare makes for poor performance when it seems to be such a common trait in many so-called performance cats - Shuttleworth and Crowther designs included. As I said, what's anathema to one designer is an absolutely necessity to another.

Kevin

[vilanomark]

HenryV...thanks for the link...After reading the article it was surprising to me the guy is a fan of multihulls. I almost felt depressed after reading about the performance of some of the hulls. However that info will be useful to me when looking at hulls, but hopefully it wont keep me from getting a cat.

[Lodesman]

Here're some links to various builders/designers/etc. that cover a lot of design aspects and philosophies. The more you read, the more contradictions you'll find, but don't let that put you off. I figure in the end you decide on those things that you: must have, want to have, will grudgingly accept, can't have, or are ambivalent about - then find the boat that results in the fewest compromises. Don't get frustrated by the search.

http://www.john-shuttleworth.com/default.html
http://www.multihulldesigns.com/
http://www.runningtideyachts.com/multihull/
http://www.adagiomarine.com/
http://www.sailingcatamarans.com/
http://www.chriswhitedesigns.com/concept63/index.php

Enjoy.

Kevin

[laser]

If you are interested in comparing the theoretical performance of different catamarans based on their technical specs I found this site to be very interesting

http://www.multihullcomparisons.com/default.asp

Just go to the "compare multihulls" link near the upper right.

[henryv]

As with most things there are many compromises made when designing catamarans. Good performance is much easier to achieve if you accept minimal accomodations however that is not going to satisfy the demands of most buyers.

In spite of the compromises I think that catamaran designs have evolved considerably over recent years and performance has improved and become more consistent.

As to Kantor's comments appearing to be mostly anecdotal - the man has sailed more cats than most of us will ever set foot on so I think his opinions are worth considering.