Unbearable slamming

[Aussiesuede]

OK, I'm a bit reluctant to share this, as I've already gotten plenty of ribbing from friends who have wondered what the hell I was doing. First of all, I'm one who likes to tinker with things. I truly believe that just about any problem can be solved, or at the very least - managed. Put down your cup of coffee so you don't spit it out in laughter with the following "management" of wave slap.

First purchase 2 cheap inflatable rafts @ your local Walmart (or any retailer). Next purchase a 2 inch thick 8'x12' piece of pink or blue foam board and a box of heavy duty trash bags from your local hardware supply. Inflate Both rafts and cut the foam board into rectangles large enough to fill the interior of both rafts. Then insert the cut foam into trash bags and put the foam in the inflated rafts and secure with bungee cord. Now take a piece of rope and the two rafts together bow to bow. Attach two ropes to the front raft @ both bow corners and drop the two rafts between the hulls and release enough line until the rear raft exits slightly between the rear hulls. Now tie 2 more ropes to each bow corner of the trailing raft and then draw in line from both front and rear until both rafts are secure against the bridgedeck.

Ok, after the good belly-ache laugh - grab your cup of coffee and continue on. But for under a hundred bucks, you'd be surprised at just what this little experiment produces. Give it a go the next time the seas are a bit choppy and I suspect you will find your self with a bit of a grin on your face. Doesn't stop slamming, obviously, but manages the slapping "sound" quite effectively as the rafts absorb the initial wave impact, and 'slapping' sound accordingly.


(Amazing what ideas boredom on a sunday afternoon will produce!)

[ldrhawke]

OK, I'm a bit reluctant to share this, as I've already gotten plenty of ribbing from friends who have wondered what the hell I was doing. First of all, I'm one who likes to tinker with things. I truly believe that just about any problem can be solved, or at the very least - managed. Put down your cup of coffee so you don't spit it out in laughter with the following "management" of wave slap.

First purchase 2 cheap inflatable rafts @ your local Walmart (or any retailer). Next purchase a 2 inch thick 8'x12' piece of pink or blue foam board and a box of heavy duty trash bags from your local hardware supply. Inflate Both rafts and cut the foam board into rectangles large enough to fill the interior of both rafts. Then insert the cut foam into trash bags and put the foam in the inflated rafts and secure with bungee cord. Now take a piece of rope and the two rafts together bow to bow. Attach two ropes to the front raft @ both bow corners and drop the two rafts between the hulls and release enough line until the rear raft exits slightly between the rear hulls. Now tie 2 more ropes to each bow corner of the trailing raft and then draw in line from both front and rear until both rafts are secure against the bridgedeck.

Ok, after the good belly-ache laugh - grab your cup of coffee and continue on. But for under a hundred bucks, you'd be surprised at just what this little experiment produces. Give it a go the next time the seas are a bit choppy and I suspect you will find your self with a bit of a grin on your face. Doesn't stop slamming, obviously, but manages the slapping "sound" quite effectively as the rafts absorb the initial wave impact, and 'slapping' sound accordingly.

I can see the frown on your face after the first decent wave rips your gizmo off and you see it floating behind the boat. What sort of raft would not be ripped apart from tons of wave force? It may help wave slap a little if you picked up you cat with a helicopter and dropped it, that is if the sound of the rafts exploding wasn't even noisier. Sorry but I'm only laughing at you not with you...

[Woods Designs]

You are right that slamming can be a major problem and is certainly something that all designers consider when designing a bridgedeck cabinned catamaran. Unfortunately life is a compromise, increase bridgedeck clearance and freeboard, windage and weight will go up while (generally) aesthetics goes down.

My general rule of thumb is that you should be able to get a conventional inflatable dinghy (ie not a RIB) under the bridgedeck, (although I'm not convinced about Aussiesuede's idea!) I would avoid boats that don't have that much clearance if you want to be comfortable offshore.

I'd also avoid boats (like Prouts) with big nacelles as they, in my experience, slam more than boats without nacelles. I'd also avoid sleeping in the bow bunks of any catamaran, the only good sea berths are aft of the mast.

Incidentally I sailed the Capetown to Rio race in a Norseman 43 (similar to the Voyage). Apart from stores for 6 men for a month the boat was empty (no dinghy for example) and it slammed very badly. Loaded for cruising it would be much worse.

My current thinking is to have a gently V'eed bridgdeck floor (as on my Transit 38 design). That way the wave energy can dissipate sideways rather than straight up.

What certainly works extremely well is a big hull knuckle (again as on my Transit and also Eclipse and Sagitta designs) which deflects waves down and helps break the solid water into spray. The fact that you get less spray on deck and have more interior room is a bonus.

It maybe an unsurmountable problem, but as I said at the beginning, bridgedeck slamming is something that concerns all catamaran designers

Richard Woods of Woods Designs

www.sailingcatamarans.com

[Nordic cat]

What do you think of this dinghy position then?


From: Alibi - Catamaran manufacturer - Constructeur de catamarans - Catalibi


Alan

[imagine2frolic]

I can't imagine the dink being so stable there is no wear. If ever it did come loose.....oh my the problems.

[Woods Designs]

How do you get it into/out of that position?? How do you stop it filling with water on the first wave it meets?

Richard Woods of Woods Designs

[josef]

Have a Caribe RIB with 15HP 4 Stroke Johnson which easily drops down from davits and out under the bridgedeck and trampolin on my FP Bahia 46

[imagine2frolic]

josef,

I just looked through several sites, and all show stern davits. Can you post some pics?

[Nordic cat]

Originally Posted by Woods Designs

How do you get it into/out of that position?? How do you stop it filling with water on the first wave it meets?

Richard Woods of Woods Designs

People who think that a heavy boat is more comfortable in a seaway might enjoy this novel method of waterballasting
Under the right conditions no pump is required for filling

No, to be a bit serious, the people behind this design know a thing or two about fast catamarans, so I'm pretty confident that they have these issues sorted.

Lots of thought has gone into this refreshingly different looking design.I remember reading that they had received an order, but can't seem to find any pictures or postings. Gregor Tarjan is the uS rep.for them, and his site talks about a 50-something footer whereas the original Catalibi site only mentions a 47 footer. At 7 tons lightship she should be a fast boat.

Alan

[Adaero]

Originally Posted by Nordic cat

Lots of thought has gone into this refreshingly different looking design.I remember reading that they had received an order, but can't seem to find any pictures or postings. Gregor Tarjan is the uS rep.for them, and his site talks about a 50-something footer whereas the original Catalibi site only mentions a 47 footer. At 7 tons lightship she should be a fast boat.
Alan

I spoke with the designers nearly 2 years ago about the 47, as was then. They told me that the first one was due for launch July last year but to my knowledge they have yet to build one. When you now look at Gregor's site you see the 47 is now 50 plus and still not one in the water although I do remember reading somewhere that one had been ordered. There are loads of high tech designs around that don't always get to market so until we can all see one in the flesh its just another design.

[josef]

Originally Posted by imagine2frolic

josef,

I just looked through several sites, and all show stern davits. Can you post some pics?

I have stern davits.

[rick p]

Originally Posted by Woods Designs

I'd also avoid boats (like Prouts) with big nacelles as they, in my experience, slam more than boats without nacelles.

Would you include the newer Broadblue boats in that?

Rick

[Woods Designs]

I should have said "are noisier" rather than "slam". I haven't sailed a Broadblue, but know the 385 is basically an old Prout 38 tarted up. I have sailed Snowgeese etc.
The newer boats do have more bridgedeck clearance so I guess they are better.

There is a photo of a Prout nacelle on my FAQ's page

Sorry I cannot help you more

Richard Woods of Woods Designs

www.sailingcatamarans.com