Bridgedeck Clearance

Gday all

just a few questions about bridgedeck clearance on catarmaran bridgedecks

is there a formulae??? or just the higher the better??

what effect do outboard pods have?? For example a Peter Snell Easy 11.6.....

has anyone seen or heard of a design for a retractable outboard pod that sits flush to the bridgedeck when retracted???

any wisdom greatlty appreciated

yeeeeeeooowww
nick

 

I would consider a motor on each hull.

 

As I see it, their are two effects of poor bridgedeck clearance. The first is simply annoying noise (wave slap) and some potential discomfort in shaking and vibration. The second is more serious, structural degradation due to that same wave slap and shaking. In the 11-12 m overall length you'll want a minimum of 700-800 mm clearance with more being even more desirable.

There are sometimes formulas given with a basis of either beam or lwl but I don't think it's particularly necessary to have it scalable. You need that 800 or mm clearance as a reasonable minimum but going too much more is difficult aesthetically and from a windage standpoint. In larger boats, you will need proportionally less clearance but 1.2 to 1.4 m clearance on a 14-15m boat is good. I suspect that a wider beam for the length may benefit from increased clearance as well. The real authorities will doubtless clarify this.

Outboard pods I have less experience with it but clearance is necessary to avoid inadvertant submersion. A good fairing will minimize damage and resistance to wave impact.

 

The wider the two hulls apart the higher the bridgedeck clearance should be. The reason is the wider apart they are the bigger the swell that can fit in there. I would think that a cube of water at a ton can make some damage, apart from being very uncomfortable. I don't know if there is a formula for calculating the bridgedeck to hulls apart distance, but I know most cat's bridgedeck clearances are too low, one of the reasons they never leave the mooring.

This was discussed before to some length previously. Imo a sea going cat should have a bridgedeck of no less than 800mm and for fresh water probably no less then 500mm. Someone else will probably come along and offer a different view.

The bridgedeck clearance is a compromise between possible deck slap and windage / cabin space if you have a cabin. If I have a choice between a cabin and bridgedeck clearance I'd choose the bridgedeck. Be a bugger if you can never go somewhere with the boat.

 

In reality, there are a couple of factors affecting bridge deck clearance.

First, there is just the clearance itself. This is a factor with extremely steep chop that is more than a LWL apart, or some other similar wave phenomenon. To picture this scenario, picture the hulls staying stationary relative to the waves and the short, steep waves reaching high enough to hit the underside of the bridgedeck. This is the scenario most people think of.

Next, there is another scenario to picture... larger, but more rounded waves that aren't as steep. In addition to the regular bridgedeck clearance scenario, the hulls will lift with these larger, rounder waves. This lifting of the hulls (riding up and over the waves) depends greatly on the reserve buoyancy of the hulls. When the hulls lift up over the waves, the bridgedeck follows and gains height relative to the waves.

This is often the forgotten factor.

So, slamming isn't just about bridgedeck clearance. It's also about total reserve buoyancy. A very low bridgedeck with very wide, fat hulls will slam just as much as a high bridgedeck with very skinny hulls (with no flare) in the case of real world waves.

So, just looking at the bridgeeck height alone does not tell the whole story.

Lastly, in the real world, all cats will slam if you use them incorrectly. The thing to do is this:

If you are slamming, just alter course a bit until you don't slam anymore. It's that easy.

 

thanks everyone for the words of wisdom

i think catbuilder you hit the nail on the head
"If you are slamming, just alter course a bit until you don't slam anymore. It's that easy."

and i guess the bigger the cat the bigger the sea state it will be comfortable in...makes sense

does anyone know if the pods that hang down to house the outboards have any impact on the slamming, or is there smallish surface area and positoning aft mean that they are protected or not a real issue???

 

Well, I don't know about elsewhere but here a lumpy sea is uncomfortable in any direction. Getting over swells is nothing, but I don't see how changing direction will get you comfortable through those lumps. There is only one thing getting you through there and that is bridgedeck clearance.

 

In my experience, lumpy seas aren't what makes a cat slam. It's usually going to windward in rough conditions that does it. Or worse, moorings into rough conditions.

That's why changing course (slightly) stops the slamming.

Lumpy or confused seas just toss the cat around monohull style, but with a quicker motion.

 

In my experience, lumpy seas aren't what makes a cat slam. It's usually going to windward in rough conditions that does it. Or worse, moorings into rough conditions.

That's why changing course (slightly) stops the slamming.

Lumpy or confused seas just toss the cat around monohull style, but with a quicker motion.

 

(motoring into rough conditions) damn phone. Should not post from a phone.

 

The Formula

Quoting from Gregor Tarjans book, "the well-known rule of 1" of bridgedeck clearance per each foot of beam was a safe way to prevent excessive wave slap. As the beam gets wider then the rule would have to be increased to 1.3" and overly square boats to 1.8"." He doesn't elaborate on the change from 1" to 1.3". Also, I have heard that you should maintain a 30" clearance but this isn't really practical on smaller boats. Good luck BOB

 

Link to Renewed Discussions

Please join in with this discussion over on this other subject thread. I'm trying to consolidate some of these bridgedeck slamming discussions into one major thread, and felt that this other one had the more encompassing title.

http://www.boatdesign.net/forums/multihulls/center-nacelles-wave-pounding-cats-19139.html