Bridge Deck Clearance . . . How Much Have You Got ?

[webejammin]

We get an occasional slap but no real slamming, mostly from wake board boats plowing a big wake.

[Barra]

Originally Posted by quartersplash

I have found that it has more to do with the sea state. When we had large washing machine seas from every direction at the same time, in the Carrib, we had major pounding of the bridge deck. When we had seas of any size from any direction that had a pattern, there was no pounding.

I totally agree with this. A well designed cat with adequate clearance for that design shouldnt slam in regular sea states but take that same cat and put it in a confused sea state and I cant sea how any cat wouldnt slam. ie when a few differnent direction swells heap up on themsleves under the bridgedeck an extra 100 or 200mm wont make any difference. Its then that a nacelle can help.

I dont really understand how guys can say their cat "never slams" (unless they have been lucky enough to have avoided confused seas)

[colemj]

Originally Posted by Mark Johnson

IF you want a serious "cruiser", and not just a marina bound condomaran, then you will need to be able to handle considerable amounts of going hard to windward, often in a gale. Survival storms will eventually be served up as well! I've been through more than a dozen hurricanes for example...

MOST, (but not all) FRP production cats are/were built to a design that does NOT fill these requirements, and they were NOT intended for this service. You need to pick from the <10% that were, NOT a belly dragger! ALL sales propaganda and claims from biased owners, that fly in the face of good naval architecture, should be ignored.

A true "cruising cat" has really good wing clearance for one, (enough to drive a high bow RIB through the wing tunnel). It needs to be wide as well, for stability.
(I know of Prouts for example, that I surveyed and found to be rather flimsy in construction. Due to their low wing clearance design flaws, they were driven back to the boatyard for months of repairs, after the first attempt to drive them into 35 knots of wind, and 8' seas)! The Gemini is similar... They and their type, are NOT the boats for this! Getting lucky only works for a while...

A good SEA BOAT would have JUST enough head room in the main bridge deck cabin, that you don't hit your head, and a cabin that is small, leaving GOOD visibility forward, (UNDER a high clewed jib), AND good walking space on its side decks.

It is thus designed to have relatively low windage, and applying these parameters, light but STRONG construction techniques & materials, AND low down locations for tools, tankage, engines, and all heavy items. This keeps the COG low. Remember... BRIDGE DECK CABINS SHOULD BE STREAMLINED AND RELATIVELY SMALL!

This stable, low COG, low windage, good visibility, minimally pounding platform, is combined with light weight, so that a small but easily reefed rig will drive her, EVEN hard to windward.

With skill, experience, and good judgment, she could easily take you around the world... with safety, speed, and relative comfort.

Many of the best "designs" are older ones, for the custom "one off" builder. (Back in the day when, IF you wanted a multihull, you built it)! It is how I got all three of mine...

These "better designs", were better because they were not designed to sell boats, they were designed to sell plans to build boats. The priorities of the experienced multihull home builder, were generally "wanting the more serious cruising type of vessel", for real world cruising. We didn't expect as much of an "apartment at sea".

IF you look carefully, there are still some great, still seaworthy examples of these craft out there. A few production cats qualify as well. Of all the characteristics that one must look for, they are ALL important, in just the right measure, but good wing clearance is #1. Pounding is not just noise. If it is bad enough, for long enough, it is the sound of your boat coming apart!

Good luck in your search!

How about a little perspective?

We have been seriously cruising full time for over 4 years with no marina time and have never had to drive hard to windward in gales for days on end. And we regularly meet people cruising even longer than us on catamarans that do not meet your strict and pedantic requirements who, like us, have not had any problems with their boats or ability to get to new cruising grounds.

In fact, the vast majority of the catamarans cruising and sailing all around the world do not meet your ascetic requirements. And somehow everyone is happy.

If you have been through more than a dozen hurricanes, you are in serious need of some weather forecasting education, or at least taught how to use your radio or tv to get the news. Really? All cruisers need to have boats that can sail through hurricanes or other survival storms because they will be encountering dozens of them?

If there is one common thread among all of the cruising books out there, it is how surprised people were with how few bad weather experiences they had. Sure, be prepared with storm sail systems and bad weather plans, but it is very easy to avoid major systems and, if you are caught in some bad weather, to slow down, forereach, use a drogue, heave to and other tactics rather than drive full on hard into the teeth of a storm.

Violent storms and long, heavy windward work is what everyone fears when starting out cruising, but few actually encounter.

Mark

[Tomahawk]

Originally Posted by Mark Johnson

IF you want a serious "cruiser", and not just a marina bound condomaran, ...

Mark

Whilst I agree that many cats are really floating condo's... It would be helpful if you could give us the names of some Cats and Tris that you consider meet your, quite demanding, specification..

[Southern Star]

I think it is important to bear in mind that virtually all things in the design of a yacht, including bridgedeck clearance, involve compromises rather than a win/win situation. For example, all things being equal, increased bridgedeck clearance:

1. Raises the centre of gravity (Cg) of the vessel, thereby reducing transverse stability. Simply put, the higher the bridgedeck, the higher the bridgedeck accomodation and the higher the rig, placing weight further aloft.
2. Raises the Ce (center of effort) of the sailplan, again reducing transverse stability. If the coachroof is higher (required in order to maintain acceptable headroom in the accomodation above a higher bridgedeck), then the sailplan must be correspondingly higher.
3. Increases windage (again, a higher side profile results from the higher bridgdeck accomodation). This is something that is clearly detrimental when attempting to make ground to windward in heavy conditions. It also causes the boat to wander more under anchor, increases the forces on the ground tackle and makes the boat more difficult to dock in cross winds.
4. Increases freeboard which in turn makes it more difficult to get on and off the boat at dock (and boats with particularly high freeboard can be potential ankle-breakers when docking without assistance from ashore).

Prouts have been cited by at least one as boats that have very low bridgedeck cleearance, but which nevertheless have great reputations as safe sea boats. The reasons for that include the aforementioned benefits from a lower bridgedeck (i.e., lower, Ce for the sailplan, lower Cg, less windage, less freeboard), but also the following:
1. The Prout rig is well suited to heavy-weather sailing as it has a dedicated staysail/storm jib and tends to orient the sailplan on a more fore/aft basis, thereby further reducing the Ce of the sailplan and increasing transverse stability.
2. The relatively heavy displacement of Prouts also tends to improve transverse stability.
3. The relatively narrow beam of the Prout means that the required bridgedeck clearance of the boat is lower than on a wider boat:
a.) The volume of water that can make contact with the bridgedeck is directly proportional to the width of the tunnel ie., a 12 foot wide tunnel will permit a 50% greater volume of water to contact the bridgedeck than an 8 foot wide tunnel.
b.) When taking seas diagonally off a bow, the windward bow actually blocks off a portion of the wave from entering the tunnel (think of a 'shadow', or 'slick' to leeward). The closer the bows are together, the greater the area of the tunnel that is shadowed.
c.) Just as a narrower jeep or off-road vehicle can do a better job of straddling boulders or irregularities in the ground, so too can a cat with a narrower tunnel do a better job of straddling irregularites in the surface of the sea.

Of course, this is to say nothing of the importance of the shape of the bridgdeck. Yachts with large forward doubles over the bridgedeck often have a blunt leading edge that can virtually bring the boat to a stop when they contact seas; on the other hand, the gently curving under-bridgedeck shape of the Manta, for example, permits much less violent impact from seas and therefore a lower bridgedeck.

Brad

[REsCat]

"ALL" Cats slam, bang, pound, slap, bone crunch(thankfully few!) in certain sea state conditions, any multihull owner (20 ft>) that says their boat doesnt... hasn't sailed in those sea state conditions or has some singular definition of the above results.
High, Big, Run a high bow rib underneath, bridgedeck clearance is helpfull... and most certainly helps to reduce the above but certainly doesn't eliminate it in those sea states.
Sailing hard (or close to hard) to weather (20 + knots true) for extended periods of time with building sea states in any cruising boat generally talked about in this forum sucks... we all no that

How about a little perspective?

We have been seriously cruising full time for over 4 years with no marina time and have never had to drive hard to windward in gales for days on end. And we regularly meet people cruising even longer than us on catamarans that do not meet your strict and pedantic requirements who, like us, have not had any problems with their boats or ability to get to new cruising grounds.

In fact, the vast majority of the catamarans cruising and sailing all around the world do not meet your ascetic requirements. And somehow everyone is happy.

If you have been through more than a dozen hurricanes, you are in serious need of some weather forecasting education, or at least taught how to use your radio or tv to get the news. Really? All cruisers need to have boats that can sail through hurricanes or other survival storms because they will be encountering dozens of them?

If there is one common thread among all of the cruising books out there, it is how surprised people were with how few bad weather experiences they had. Sure, be prepared with storm sail systems and bad weather plans, but it is very easy to avoid major systems and, if you are caught in some bad weather, to slow down, forereach, use a drogue, heave to and other tactics rather than drive full on hard into the teeth of a storm.

Violent storms and long, heavy windward work is what everyone fears when starting out cruising, but few actually encounter.

Mark

Totally agree with this rebuttal....

my cat falls into many of the must have list of features that Mark Johnson espouses but to have 6+ foot salon height and high bridge deck
clearance which my cat has comes at a cost!, it has very tall hulls which produce big windage for docking,more to scrape, polish ,fix etc.... you cannot get away from this...
( look at any, new, current, high bridgedeck clearance, 6 foot salon height design multihull)...and I do not want 5+ - foot salon interior height for a liveaboard... that would be a total head(DEAL) breaker!


Bob