Technical » Bridgedeck Clearance for Catamarans Revised 1-'10

Release Date: 1/19/2010

Bridgedeck Clearance for Catamarans 2007 Evaluation (See re-evaluation below)

 

 

The following standards for Bridgedeck Clearance (BdCl) have been discovered through the literature and direct contacts with the sources:


 

Source

Standard

Jeff Schionning

Cruising Cats 24-30 inches (61-76 cm)

Performance Cats 27-35 inches (69-89 cm)

Racing cats 30-40 inches (76-102 cm)

Westlawn Institute of Marine Technology

The bridgedeck should be at least 30 inches above the waterline, or 80% of waterline beam overall, or 66% of WL length, whichever is higher. The higher the better. On small cats less than 30 feet, use the 80% of WL beam OA.

Tony Grainger

6-7% of waterline length

Ian Farrier (F-41)

31 inches (79 cm)

The Multihull Source

2.5-4 feet (30-48 inches or 76-122 cm)

Sail Magazine

Greater than 2 feet (61 cm)

Sailnet.com

30 inches (76 cm)

 


There is a wide disparity in these values, reflecting a wide difference of opinion within the design community. The objective in establishing bridgedeck clearance is avoidance of pounding of waves against the underside of the bridgedeck, a phenomenon that can be both nerve-wracking and damaging to the boat structure.

Schionning indicates that the clearance should vary depending on use. The Westlawn standard suggests that beam is the driving element rather than length, and a narrower boat can get by with less clearance than a wider boat of the same waterline length.


For the purposes of evaluating this specification element on the Multihull Dynamics, Inc. website, the MINIMUM clearance is defined as follows: Bridgedeck Clearance should be at least 6.25% of Lwl for boats up to 40 feet Lwl and 30 inches for boats greater than 40 feet Lwl.


Boat Waterline Length

MINIMUM Bridgedeck Clearance

Less than 40 feet

6.25% of waterline length*

Greater than 40 feet

30 inches


*It should be noted that 6.25% of 40 feet is 30 inches.


NOTE: Bridgedeck Clearance vs. Minimum Clearance is shown in the boat data on this site. This is shown as plus (+) or minus (-) results, i.e. the number of inches or centimeters the Bridgedeck Clearance varies from the MINIMUM Clearance for catamarans of that waterline length. Plus (+) means more clearance than the minimum, minus (-) means less.


In cases where bridgedeck clearance varies in height above the waterline on a boat (i.e. forward clearance is higher than aft or vice versa), the lowest height is used for reference to bridgedeck clearance for a boat in the database.


2010 Re-evaluation


In the three years since the original search for a standard for bridgedeck clearance for catamaran sailboats, two significant things have happened: 1) bridgedeck clearance data for many more boats has been obtained and 2) the number cruising catamaran designs in the 50-75 foot range has increased dramatically. As a result, it is possible to establish a trendline using the capabilities of Excel. This represents the apparent practice by current designers in this regard. It can be noted in the graph below that as waterline length is increased, bridgedeck clearance is being increased linearly.


A graph of the 246 boats in the database for which we have this data with the current trendline and minimum recommended clearances is below. It is interesting that the trendline crosses through 30 inches at 40 feet Lwl.



It can be noted that many of the boats in the database are below the minimum line. You can read about the wave pounding results in books and articles about sailing these boats


.

There is more involved, however, in wave pounding under the boat deck than just bridgedeck clearance. Three others items come to mind: 1) pitching characteristics of the boat, 2) distance of the bridgedeck from the bow (more is good) and 3) the fullness of the bows and hulls (slimmer is good). We do not have data or analysis for these factors, but the things to look for that minimize pitching are significantly different shapes of the forward half of the hulls from the aft half, and concentration of the weight of the boat and its payload in the center, not in the ends and aloft.


Builders of catamarans with the bridgedeck extending well forward between the hulls argue that they do not have slamming problems as long as the bridgedeck clearance is adequate, but it would be interesting to have some boat-to-boat comparisons.


 

Whether the waves that pound under a bridgedeck are 1) those the boat is sailing into or 2) those the boat forms while moving through the water is also not well defined. Some of the argument for a given bridgedeck clearance is based on bow waves interacting between the hulls and against the deck. To the extent that this is the case, slender bows and slender hulls that minimize bow wave formation will improve the wave pounding characteristics of the boat.