Catamaran: Protruding shelves in the tunnel and resistance

Hi,

There is a discussion in article http://www.liveantares.com/pdf/BridgedeckClearance.pdf
about wingdeck clearance and protruding “shelves” into the tunnel for bigger and larger accommodation rooms in the hulls.
Such protruding shelves create slamming and resistance in case of higher waves. Slamming perhaps is tolerable for short cruising periods but resistance is not.
Maybe there is a compromise between spacious hull accommodation and shelves with a big distance between hulls, e.g. 65% of LOA and high clearance, e.g. 7% of LOA.
How to estimate the increase of resistence caused by the shelves in function of wave height? Any rule of thumb?
See attached pictures of Cat tunnels with shelves.
Thanks and Regards
APP

 

I tend not to use any protrusions in tunnel, they work as brake on seaway. For cats designed for marinas/brochures only this is acceptable

 

great post

Good work, App. Only problem with the article was they didn't identify the boats. I did notice the telltale Lagoon v protrusion just inches off the water and I think there was a Leapord in there. 2 questions, when was the article written and anyhelp on ID on those boats. BOB

 

Reading that paper, I would disagree about submerged transoms. Submerged transom becomes justified at FnL>0.3.

 

To estimate the (significant) increase in resistance, use some software designed for the purpose.

What you would do is envision the boat's waterline creeping up higher and higher, as it does when you encounter waves and the inertia of the boat keeps it still while the wave rises up.

This creates a raised waterline.

This new, momentary waterline will often include the protrusions you are talking about in the tunnel, which changes the beam of the hulls, wetted surface area, etc...

From those changes in the new underwater profile, you could calculate the change in underwater drag.

Suffice to say, it's pretty bad.

Good sailing boats don't have these protrusions.

Also, there is no compromise as you are referring to, IMO. A proper sailing catamaran uses the bridgedeck area to open up the hulls. ie: The berth is placed up on the bridgedeck, so you climb up onto the bridgedeck from the hull to get into bed. This creates plenty of space and a perfect hull shape.

 

Yes. That is what I though. Using e.g. Freeship you can raise, as you said, the DWL waterline, so you get an increased "BWL". Probably this occurs however for let us say a 30% of the time as the wave going through the tunnel does not affect the entire length of the Hull. The question is that if you raise the DWL in the software, the program recalculates a higher displacement which perhaps is not the case. This has to be checked. Further if you do a coastal cruising you may encounter higher waves less often. Now for longer Cats e.g. above 18 m imagine an overall clearance of at least 1.2-1.3 m height and the "shelves" height of 0.7-0.8 m from the waterline plus a distance between hulls of something as 65% of LOA, then maybe we have a good ... compromise!? The third picture of the ones posted above shows a rather good compromise. Berths placed up on the bridgedeck is good but you lose space from the bridgedeck, unless you create a nice Flybridge above that, further to climb on bed you need a raised Hull floor and of course steps inside the hull and higher hull headroom. I noted that protruded shelves are used more in sailboats as they have a small engine room. In Power Cats berths inside the hulls are mostly longitudinal as probably the designer has to put the engine under a “raised” longitudinal bed. I think so.

Unfortunately Cat builders do not give out drawings with the sections of the Cats, so if you see in the layouts beds in the transverse way (and not on top of the Bridgedeck) you should become suspicious about the correct clearance. Maybe you should look for a video of the Cat cruising and try to look underneath!

Best Regards
APP

 

Whilst overall I think the article is good, one point it "picks" on is the protrusions in to the bridgedeck clearance. However in all but more extreme weather (in which case you are likely to be sailing differently anyway) most of the small protrusions sit "in" behind the bow wave where there is often a depression in the wave height.

 

Slamming is tolerable?

"Slamming perhaps is tolerable for short cruising periods but resistance is not." Actually, I'd say the opposite. At any rate, I avoid 'shelves.' See http://bigcatcatamarans.com to see how I do it.

I just read the article you linked to. I don't think his rules of thumb are universal - bigger boats have less bridgedeck than his rules of thumb, typically. A 65' catamaran with a 4.5' bridgedeck clearance is unheard of, I think. His beam overall based rule of thumb is even more extreme. For my compilation of catamaran rules of thumb see:

http://www.multihulls4us.com/forums/showthread.php?t=1117

 

Lagoon protrusion

Hi,

Attached is a picture of Lagoon 560 protrusion. I think such a "soft" protrusion does not create problems and is a convenient one, at least we have not heard any negative comments so far.

Regards
APP

 

What is that protrusion for?

It appears to be part of the engine room??

I see vents on it and also it is parallel to the hatch in the teak deck I assume goes to the engine room.

Strange.

 

With my limited experience with Cat boat design, I found in aluminum Cat construction, connection between bridgedeck and hull require, for lack of a better term, a fillet for strength. If you look at the tow tank wave interaction for a sailing Cat if the bridge deck is high enough, and the fillet not too large, there no real danger of slamming even at max heel. But, the benefits to strength are enormous.

Mark

 

Mark,
That chamfer is relevant to most builds as it does allow bulkheads to spread the load and distribute pressures better.

Look at mine, http://www.boatdesign.net/forums/boat-building/my-little-piece-peace-25962-101.html#post468378 use the links and open in new tabs, then just click between tabs... The "ribs" fore and aft are convenient for carrying hydraulic and electrical cables etc...

Bridge-deck clearance is the issue... beam is about 6.3m, LOA is 11.99m and bridge-deck clearance is about 750mm... It is NOT regarded as a "true-ocean-going" yacht as it is intended for the short steep inshore seas around the islands on the periphery of the Coral Sea, with maybe the occasional quick 'short-cut' when there is a good weather window...

 

Very good observation. I am not familiar with strength matters and I always had a question in aluminum Cats regarding the relation between the distance of the centers of the hulls (beam between hull centers: BCB) and how to say the “thickness” of the bridgedeck (Height of deck walking floor from waterline minus the Clearance).
For example in a long Cat with BCB 8 meters (26.2 Ft) what is estimated to be the Bridgedeck thickness for a good Cat strength? Considering always the “fillet” you mentioned. Is it something between 0.4 – 0.6 m (1.3 – 1.9 Ft)? Considering e.g. a Bridgedeck length = 45% of LOA). Is the aluminum Cat bridgedeck less thick than the composite one? Probably is a stupid question for many here, but nevertheless I would need a hint.

Best Regards
APP

 

In the later stages of my design the bridge deck was not flat (underneath surface). With the hull fillets and a center mini V (viewed from the bow looking aft), to dissipate wave strikes in heavy seas. The bridge deck formed a sort of small letter m shape. It was this curved symmetric shape which also aided greatly to strength. The center v acted somewhat like a single truss. To accommodate the small wave cutting V I raised the bridge deck slightly.

So in designing for strength, its all about shape, and looking at the overall design for common techniques like monocoque structures for light weight.

My direction for aluminum design is such that the bridge deck thickness is really apart of the main cabin structure, all my Cats had a cabin. The cabin shape in relation to connection to the hulls formed the main load bearing structure. Really just a sophisticated truss tying the hulls together.

For charter sailing aluminum Cats of between 65 to 74 feet (see Sunreef for examples), there are a lot of proportional tradeoffs to overall cat design which many on this site have talked about at length. Hull Beam to LOA and hull centerline distance, for example.

I consider myself a hull amateur compared to some of the well known Cat designers on this site.

One thing I will say is the final hull design has to do as much with the customer requirements as does its "pure" theoretical hull performance.

For example, a requirement for large diesel engines to make time with no wind or make port ahead of a storm. Can be more important to an operator, basically meeting their destinations on-time and protecting the passengers and crew (safety), than high speed performance under sail with an ultra-light structure with very small sail drives.

However, large engines are heavy and need to be near the center of the hulls which drives much of the design, so we add a little more sail area, but under sail it will never out pace an ultra light vessel.

This is why it is so hard to compare one design against another. It really comes down to owner requirements.

For structures, if you have an improvement for more strength at reduced weight, it helps to study the basic shape for a single frame or section and run a quick simulation (FEA). Keep paring down the design and then consider how it is built, and rerun given as welded strength.

Given the time I would like to explore the direction of more transverse frames of thinner plate with cutouts for weight savings, rather than current scantling ISO frame spacing and thickness.

Mark

 

Masalai,

Very nice example of the complex fillet. I actually had a similar large fillet design that looked identical to your design. I was able to reduce with a mini m shape to the bridge deck. The bridgedeck height fully loaded is 33 inches to the bottom of the mini V. However, in relation to the photograph from another member, of a similar mini m shape, my arch was much more pronounced, like a bridge arch. Harder to construct (complex curves), but stronger.

Thanks for the links, very interesting stuff. Perhaps you could help answer APP questions about proportions.

Mark