Choosing a Cat - Bridge deck clearance

[Yes maybe]

Hi,

Researching for a cruising cat for.

Leopards (lots to choose from) - Bridge deck clearance, seems lower than most?
Is there a greater problem with slamming on these models?

Boris.

[BigBeakie]

Here ya go, a bit of educational reading on bridgedeck clearance from Multihull Dynamics which is a very good resource to learn about specific designs.

Multihull Dynamics, Inc. - News Article

A rule of thumb is below 40' Lwl minimum clearance should be 6.5% of Lwl, and above 40' Lwl it should be minimum 30".

Despite the minimum clearance factor, also consider any surfaces that may extend below the bridgedeck that present a forward facing surface. They may slam even if the underside of the bridgedeck doesn't.

happy hunting

[belizesailor]

Also, more to the dynamics than just simple vertical clearance. Take a look at Mantas for example. Bridge deck clearance moving aft is not very high...not an issue.

[maxingout]

I can testify that when you sail around the world and accumulate lots of heavy stuff on your catamaran, your bridge deck clearance gets significantly less. That is one of the reasons I like sailing in the trade winds downwind. A lower bridge deck clearance is not so much of a problem running downwind on our cat.

I would love to have a higher bridge deck clearance, but taking stuff off the catamaran involves people giving up some of their stuff. Not easy.

I would love to have a higher bridge deck clearance by getting a bigger cat, but then there are all sorts of new challenges sailing and taking care of a larger yacht, not to mention the much bigger expense.

I would stay away from low bridge deck cats, because in the real world of sailing, there is a high probability the clearance will get lower once the cruise gets underway.

[BigBeakie]

Originally Posted by belizesailor

Also, more to the dynamics than just simple vertical clearance. Take a look at Mantas for example. Bridge deck clearance moving aft is not very high...not an issue.

If you watch the wave train and convergence of the bow waves under the boat, you'll see that actually you need more clearance at the transom if anything.

[valhalla360]

It's complicated.


It's more to do with the beam than the length but also how far the bridge deck extends forward, what direction you are taking waves from, how fat are the hulls, do you have a nacelle.


It is true that bridge deck forward is the bigger issue for slamming. In smooth water the wake isn't typically an issue if they meet towards the stern and by the time waves get to the stern, they already slammed further forward or the boat has had time to rise with the wave.


Then you have the non-sailing aspects. We recently switched from a Gemini (lower bridge deck) to a Catalac (higher bridge deck). Interior living, the steps on the Catalac are much steeper to accommodate the higher bridge deck. This makes it far less convenient carrying stuff down into the hulls (ie: groceries).


Also, docking the lower deck makes it easier to reach low floating docks from the boat. As you get into larger boats, many of the high bridge deck models leave you 6-7' off the water and the dock may be 12-18" off the water.


So while you need a boat that is reasonable for sailing, remember you will probably spend 95% of your time living on the boat at anchor or in a marina.

[Barra]

Originally Posted by valhalla360

It's complicated.


So while you need a boat that is reasonable for sailing, remember you will probably spend 95% of your time living on the boat at anchor or in a marina.

I love that the time spent at anchor as a % keeps getting bigger and bigger. The rule of thumb used to to 70 / 20 / 10 ie 70% at anchor 20% motoring and 10% sailing on average. A more performance oriented boat used to play around with the 20/ 10% trade off by being able to make decent speeds in lighter winds.

Now its 95% at anchor or in marinas which leaves max 2% of time actually sailing hehehe just buy a trawler i guess....

[belizesailor]

Originally Posted by BigBeakie

If you watch the wave train and convergence of the bow waves under the boat, you'll see that actually you need more clearance at the transom if anything.

Yes, but waves from convergence are small. A bit of a slap at most. Thats why so many designs, including the Leopards in question and most other South African cats, can get away with low bridge deck clearance aft.

Ive sailed a lot of different Leapords and its not a significant issue. They have good forward reserve bouyancy, high bridge deck clearance forward...which both help prevent pounding in the bigger primary waves. By the time the waves get further aft and converge, its usually just a slap.

The Leopard bridge deck tunnel is relatively flat though and, in the right conditions, I think that can contribute to pounding. By contrast, look at the tunnel of the Manata and other Larouche designs...I think the concave shape helps to dissapate wave impact forces better than many other shapes that have been tried.

[valhalla360]

Originally Posted by Barra

I love that the time spent at anchor as a % keeps getting bigger and bigger. The rule of thumb used to to 70 / 20 / 10 ie 70% at anchor 20% motoring and 10% sailing on average. A more performance oriented boat used to play around with the 20/ 10% trade off by being able to make decent speeds in lighter winds.

Now its 95% at anchor or in marinas which leaves max 2% of time actually sailing hehehe just buy a trawler i guess....

When was this mythical time when 30% of time was spent under way? While I'm sure you might find the occasional cruiser who reaches that level, that's roughly 110 -24hr days under way per year. Possible but real hard core traveling.

If you are talking about off shore cruising, your motoring time is much higher than typical both now and historical (exemption for the rare open ocean motor cruiser).

If you are talking about coastal cruising, it basically implies you travel daily for 8hrs with no days off...pretty much ever (and it's probably closer to 90% of time under way uses the motor).

That said, the point remains the same, the vast majority of a cruisers time is not spent under way slamming the bridge deck into waves. Our Gemini has a comparatively low bridge deck but even under way, it was only an issue on certain points of sail and easily corrected by bearing off 20 degrees and in large swells a non-issue, so don't make it the be-all end-all unless you plan to do a lot of bashing head on into short steep waves.

The point being consider it but also consider the trade offs. We never understood the galley up vs galley down on the Gemini as the 5'0" misses was right there even when down. With the Catalac, it's not a huge issue but definitely much more separated from the main cabin area.

[spiv]

Hi Boris,

you've open a can of worms......
Everybody has an opinion, normally biased towards the type of boat they own.

On cats, lighter is always better, higher bridgedeck is always better.
I once owned a 40' cat with just 60cm (23.5") and the furniture cracked and you could not leave anything on the table when beating. And you can't always take a 20 degree detour if it takes toward the coast.
In my Salina 48 with 85cm (33.5") clearance it was much better, BUT, there have been many times during my circumnavigation when I wished I had more, especially when I wanted to sleep off duty......

So, compromise if you want/need, but you will have to live with the consequences, especially if you intend to cruise as comparing with just taking a little holiday every now and then.

[Emerald Sea]

There are no hard and fast rules on what the bridge clearance should be however evidence suggests that 0.7m (2.3ft) would be a minimum. More is better of course. Having a nacelle under the deck will also soften the slamming effects. Slamming, whether under the bridge deck or against the hulls (inner and outer) will always be given of catamarans.

[akprb]

I wrote a bit on this at the bottom of a post entitled "Catamaran Storm Tactics" (www.sailingohana.com).

There are many more who have looked at the design and physics of Cats, I'm not one of those. I'm more of a go cruising guy with what you got. An informal survey of other cat owners confirmed that they all slam to one degree or another in various conditions. The only one who really was disturbed by this was the owner of a Venezia 42 who commented his table would "dance" while underway.

Are there any cat owners out there who find their boat unsailable due to bridge deck slamming?

I also agree with the poster who commented that cruising the bridgedeck naturally gets closer to the water over time :-)

[iliohale]

I found this article pretty helpful when considering the trade-offs on bridge deck clearance.

14 Things to Remember When Buying a Catamaran - Sail Magazine

[caradow]

Well this has always been a sore spot for me to discuss.
For I own probably one of the lowest bridge deck clearance cats made.
This is my 6th year in my owners version Voyage 440. I have sailed this boat from Maine to Venezuela and many places in between and throughout the Caribbean on many occasions.
There has been many times that I have witnessed the dinning table raise up an inch or two when hitting a wave. The noise is sometimes scary but I live with it. I have learned that changing your angle to the waves help a lot but it never completely go's away. It is sometimes unnerving.
Yet when we turn the corner the boat takes off and the ride is smooth and exhilarating. Very well built boat and fun to steer. Does not feel like you are driving a house like some of the other cats I have sailed. Much room for storage and living spaces.
Also I have never been as uncomfortable as I have on monohulls beating to weather and I did that for 30 years before buying a cat.
I guess everything in life is a trade off unless you have money, money, money

[rallyman1122]

Originally Posted by iliohale

I found this article pretty helpful when considering the trade-offs on bridge deck clearance.

14 Things to Remember When Buying a Catamaran - Sail Magazine

And for all those that hate the the condomaran, the Leopards, the Fountaine Pajots and the Lagoons, look at the last point on that list. You my love the hard core go fast daggerboard cat but your significant other just may want something more sedate and confidence inspiring.