What “good” bridgedeck clearance is and why is it important? – Richard Woods Interview # 4

Editor’s Note: These videos are meant to be introductions to basic catamaran design considerations. For more in depth discussions please contact Richard Woods via his website or comment below on this article. We welcome questions and suggestions.

In this interview, we talk to Richard Woods about one of the most important characteristics of cruising catamaran design – bridgedeck clearance. It is a feature that is often too low especially on small catamarans. Richard explains that while bridgedeck clearance seems like a simple yet elusive measurements, it is in fact a very complicated design characteristic that catamaran architects have to thoroughly think out.

For more information or to purchase build plans from Richard Woods, please go to his website at www.SailingCatamarans.com.

Please see Episode 1 for an introduction of Richard, Episode 2 to learn about galley locations, and Episode 3 for a primer on why cruising catamaran bows are an important purchase design decision.

Tell us a little bit about what bridge deck clearance is?

The bridge deck is essentially the part of the catamaran that you stand on. It is the space between the hulls. It’s not just the saloon but also the cockpit and the forward anchor lockers. It’s one of the most important design features that you have because the waves can come up and hit underneath. Structurally a bridge deck slamming into waves is not as bad as it sounds, but it sounds really really scary when you have too low a bridge their clearance.

That’s why it’s always important to check on the clearance that you’ve got if you’re buying a catamaran.

I’ve been on boats where you haven’t been able to put food on the saloon table while we’ve been sailing because it just gets thrown off. It’s just so horrible especially when you’re sailing to windward or in a confused sea.

What kind of distance are we talking about – feet inches meters?

What I tend to tell people to do is to make sure that they can get a conventional inflatable dinghy underneath their boat. A high bridge clearance would be three feet. But just looking at the boat and saying, “Can I get an inflatable dinghy underneath?” which is essentially about 18 inches.

Obviously the bigger the boat the easier it is to have a good clearance because people don’t get any bigger when the boat gets bigger, so you don’t need more than standing headroom in the saloon.

On a small boat that’s going to be really challenging and so anything under 30 feet it’s essentially impossible to have standing headroom and good bridge deck clearance.

I guess you’d end up with a really tall boat.

So you talked about slamming when you don’t have enough bridge deck clearance, but is that the main thing that occurs? Do you run into any problems with steering the boat? Is slamming just a physical discomfort?

You want the bridge deck to be as short as possible, and that’s because there’s three factors.

One can be from the waves just sailing along in flat water. Two hulls make waves, and they cross underneath the bridge deck. You can get a lot of interference just in flat water with the waves hitting underneath. If you have a low bridge deck clearance and the hulls close together, you can have the water hitting the bottom with no waves. The further apart you have the hulls, it becomes sort of a a mole hill of interference rather than the mountain that you’re sailing over. So that’s one factor that contributes to the slamming whatever sort of boat you have.

Then the second factor is that as you start sailing into waves the boat’s going to pitch up and down and so the bars go down and up. That’s why you want the bridge deck clearance as far back and as high as possible. The bridge to clear should start as far back as possible, so it doesn’t bash into the waves.

Then there is a third factor with that. If you can easily imagine that if you had the bridge that starting a long way back, but it had a vertical front. Every time a big wave came, it would slam into that front, so you want to have a gentle curve in front which is essentially usually under the anchor lockers. It’s got the benefit that the anchor lockers drain easily because the water will run to the back.

So you want (1) a high bridgedeck, (2) that clearance you want it to start as far back as possible, and (3) you want a gentle slope.

When we’re talking the gentle slope you mean the underside of the boat?

When the bows pinch up, the stern goes down. The same applies to the stern and the cockpit. You really want to have the cockpit higher than the middle of the boat. The boat pitches around the middle right, so the lowest point can be in the middle, and you want it as high as possible.

The stern is lower, but obviously at the stern you’re not going to have waves slamming into the back unless it’s really bad.

So those are the priorities. A high bridge deck underneath, a sloping front that starts as far back as possible, and a higher cockpit then is a good thing.

Then you’ve got the bows as well because the hulls are making waves, and so a lot of people including myself make a quite an exaggerated s-shaped knuckle. The waves that hit come up and break away instead of going up and hitting the bridge deck .

On the inboard side you often see boats with either quite a chambered inner hull side or the knuckle arm to help deflect the spread and waves down.

Then there’s a another factor which is that most bridge decks are flat underneath across the boat, but it’s actually much better to have it slightly fade.

(Editor’s note: Richard models with his hand during this portion the effects of the wave. Please refer to video).

One of the times when it’s very obvious is if you have an outboard in a cell with a flat base. An outboard in the cell is very close to the water. Even if the bridge deck is high, you can hear that slamming badly.

When somebody’s looking for bridge deck clearance if the boat is in the water, what kind of clues are you looking for?

The anti-fouling line is going to be a big factor, but on a lot of boats like for example the Fountaine Pajots being one in particular the anti-fouling line is deliberately taken quite a bit above the real water line.

There’s good reasons for that. One is that it saves all the dirty scum along the water because the anti-fouling paint goes higher.

The other is that your eye is taken to the top of the antibody line so effectively reduces the freeboard. That’s why people do it.

So that makes it a bit misleading as to where the real clearance is. So another good one is that basically no one designs a catamaran with transoms immersed. If you look from the transom and look forward, you should probably get an idea of how much clearance there is, and you should always take a tape measure with you. Then you can just very quickly see what it is. A tape measure is always handy when you’re looking at things.

I know that one of the things that will bring down your bridge deck clearance is having too much gear on board. So when you’re looking at a new boat that’s emptied out of gear, are there any rules of thumb to know how far down that’s going to sink as you as you load it up?

That’s another thing. You’ve always seen pictures of boats. They’re saying we just launched it and it’s floating on its marks. It’s brilliant. Well no that’s awful because they haven’t got half a ton of water and fuel on board. They haven’t got a ton of people on board.

The boats always get heavier and heavier and heavier and over the lifetime they’ll go down hugely. Inches.

The other problem with that of course is when the boat’s heavier it doesn’t respond to the waves so quickly. So you get more bridge deck slamming the heavier you are which is another factor that people say. You want to keep the boat light, so it goes fast.

But in fact you want to keep the boat light, so that it’s more lively and bounces over the waves rather than plows through them. So it’s just more comfortable all around.

That was a big thing that I found from going ocean sailing that when I decided that I was going to go and not come back. I took everything with me, and then every time I flew home back to England, I would take stuff off the boat and fly home with it because I realized that I didn’t want all that stuff.

Definitely. It’s easy to think you need all the all the comforts of a home aboard where you really need to divest stuff.

Thank you for bridge deck information. It sounds like there’s more to look for than just sort of how many inches above the water. There’s all the design shape and everything. That’s really great information. Thank you Richard!