What differences do wider or narrower tunnels make?

The Nabi's would need a wide beam to deal with stability with the high capacity crane mounted on the starboard side of the boat.

 

Yes - that is something that the aquaculture place I worked at overlooked when they designed/ordered a 12 metre version of a Nabi catamaran - the consequence was that even with the crane stowed it had a decent list to the starboard stern quarter where the crane was mounted just due to the weight of the crane. It also had other ridiculous problems such as the port fuel tank had a crack in it near the top so there was a post-it note on the dash saying not to fill the tank to more than 3/4 - except the fuel gauge for the port tank didn't work... and you couldn't judge how much fuel to put in based on how much you put in the starboard tank because the generator ran off the starboard tank - the whole place ran like that.

 

Does he really claim that a deck of the same height with more separation has less slamming. It is simple geometry to see it is the opposite. Look at a car and a commercial truck. The truck has more clearance or it would get stuck going over a curb or railroad track.

 

I hate to reply for Richard, but he really did not say either of these things unless taken out of context.

Here are some of his words, where he did say wider is better, but out of context, doesn’t mean 20 feet wide apart hulls without consideration of adding clearance.

But he does suggest the same deck clearance and wider is better. He basically suggests the area under the bdeck is a factor, but also applies the dinghy passing under minimum. Not sure my hull makes the dinghy test, btw.

And the general rule is 5-6% of LOA, not beam. My boat got extended a bit and so I am waiting to really test the issue someday and a bit nervous we are low.

Anyhow, I’d want 6% of LOA if I were RSD and also to follow the idea to get a veed bdeck bottom.

I’ll add a picture of what I ended up doing later today or tomorrow.

I would not mind hearing from an expert on the issue either. My opinion is the foredeck needs to allow water to pass, and the cockpit needs to be higher as well and the bottom veed. In my case, I also veed the area that could strike a wave.

Just to add, the increase of the bdeck clearance has offsets. The boat gets more wind and/or head clearance can be reduced, vcg increases as well, etc.

Slamming waves and open bridgedeck crossbeams. https://www.boatdesign.net/threads/slamming-waves-and-open-bridgedeck-crossbeams.22833/page-2

 

Just to make sure - the 6% is for water to bdeck in the tunnel?

 

Yes. So if you 12M requirement is LOA

72cm

Or

28 inches

Keep in mind, that is 6%, a bit less is probably okay. Woods, despite some issues I have with him, also makes wise recommendations about increasing the aft cockpit clearance to water and having a wave breaking structure underneath. This is a bit of a problem as smaller cats and a double bottom are in a bit of a conflict afa weight goes.

If you have a forward pilothouse, I’d assume a bit more clearance is best.

But the NAs know best.

 

That is measured at midships....as the minimum wet-deck clearance.
But at the bow you need 10%, to avoid slamming.

In this MGN you can find guidance. This came on the back of additional reserach that we did in the early to mid 00's.

 

Thanks so much for sharing that detailed paper. For semantics, I will measure the boat after the mods.

As some of you know, I built a Skoota 32 demountable design, but ran into some issues with the bdeck getting too heavy and being set too far forward among others. The boat was 8.5” below design lines forward. This meant my bdeck clearance was going to be really bad.

I got some expert help and I can report the end results for the bow area and midships and aft, just for semantics, not guidance.

 

Cheers - will take a look at that!

 

He does claim that.

I think the idea is the hulls displace water and half of it ends up under sthe bridgedeck in the form of waves. If the bridgedeck is wider that volume of water is distributed over a larger area and less likely to contact the bridgedeck. Or something like that.

I'd say it's plausible, not saying it's true.

Sailing Catamarans - Bridgedeck Slamming https://www.sailingcatamarans.com/index.php/faqs/19-sailing-and-performance-questions/108-bridgedeck-slamming

 

Reducing the amount of water being sent through the tunnel is one advantage I mentally can foresee with asymmetrical hulls on a cat.

 

Would this be a good way to solve the problem?

 

That would not be a catamaran. Trimarans are a completely different design, and there are several threads about them and their behavior.

 

Oh I realize that it would then be a trimaran, was just wondering if in simple terms (without going into deep calculations) whether adding a larger demihull in the middle would be something that a naval architect would consider/might choose as a way to solve that problem on cats with very wide tunnels?

Of course this then raises an even more curly question of why are they called catamarans instead of bimarans?