Center nacelles and wave pounding in cats.

[kerosene]

would something like this help - the boyancy would grow at least somewhat gradually and all the pressure wouldn't hit "at once"

http://kerosene.smugmug.com/photos/140771122-L.jpg

 

[doug kay]

Well it would certainly increase the fore and aft strength of the bridge deck but I can't see it doing anything to stop pounding since it would reduce it at the peaks but increase it in the troughs. Do you have special software to produce those beautiful pictures.

 

[brian eiland]

Nacelles and CB mounting possibilities

I touch on some of this topic in these discussions:

http://www.boatdesign.net/forums/showpost.php?p=36348&postcount=36

http://www.boatdesign.net/forums/showthread.php?p=36348#post36348

..under ASYMMETRIC CENTERBOARDS
http://www.boatdesign.net/forums/showthread.php?t=13389&page=2

DynaRig MotorSailer
http://www.runningtideyachts.com/dynarig/

Basically you need adequate clearance and surfaces that don't present a 90 degree impact surface to the water being thrown at it.

 

[Pericles]

A centre nacelle in the style of INCAT ferries, Hull Number 062 would permit a deep bury for an freestanding carbon mast suitable for the Omer Soft Wing Sail.

http://www.incat.com.au/news/delivery_archive.cgi

http://www.omerwingsail.com/

Pericles

 

[brian eiland]

ReOpen this Subject

I am looking to reopen this subject, and examine the largest number of existing designs out there that have made an attempt to provide a modified surface area in the wingdeck area to combat the wave slamming that can occur on many catamarans.

I searched the forum for "bridgedeck slamming" and came up with primarily 3 subject threads with that wording in their titles, but this one's title seems to embrace concept most closely.

Here is another subject thread with some good postings on the subject:
http://www.boatdesign.net/forums/multihulls/slamming-waves-open-bridgedeck-crossbeams-22833.html
....and a more recent one here:
http://www.boatdesign.net/forums/multihulls/bridgedeck-clearance-37666.html


I recall bring this subject up back in 2003, not long after joining the forum:
http://www.boatdesign.net/forums/boat-design/big-cat-alt-cbs-sail-rigs-2225.html
First, imagine a flat plate, on edge, mounted down the centerline on the underside of the bridge deck. This flat plate will act as a rib to strengthen the fore-to-aft rigidity of the vessel, a somewhat weaker characteristic in a catamaran structure vs. a keeled monohull. If a tow bundle (rope, etc) of carbon fiber (kevlar, PBO, etc) was laid along the bottom edge of this flat plate, the rigidity could be even greater (sort of akin to a bottom truss structure, or a flange of an 'I' beam).
The front of this nacelle/plate could be configured to act as a wave splitter to actually attack, up front, the formation of those peaky waves under the tramp areas that eventually slap at our bridge deck underside. We kind of slice those waves down a bit. A lightweight fairing might also be added to this 'flat plate nacelle' so it appears outwardly much more esthetically pleasing, as well as more curvature to shed those peaky waves.

My 'nacelle plate' actually extended out to the front of the bridgedeck quite a bit to "attack" the water formation that might slap at the wing.

Hopefully we will get a number of participants supplying photos of good and poor examples, that can be analyzed as to why or why-not they worked.

 

[Wavewacker]

So, as I'm trying to follow this, yes, I'm a beginner, The Micromega 5, sailed by the french twins across the Atlantic had a center pod or control cockpit that sat (I believe) slightly higher than the hulls and was as pointed as a Formula One car. Is that type of arrangement a solution?

Another small cat, the ECO6, is rather strange looking but seems to be well received as a shall coastal cruiser that has a flat cabin that dives down to the leading edge of the boat incorporating both hulls. I wondered how waves might slam on the cabin, or would these boats be lighter and go over waves?

And, how badly would a dory style hull slam, if it were say 42" high at the bow, at about 20' and say 800 pounds (hope that makes since) being the same asymetrical design?

 

[brian eiland]

So, as I'm trying to follow this, yes, I'm a beginner, The Micromega 5, sailed by the french twins across the Atlantic had a center pod or control cockpit that sat (I believe) slightly higher than the hulls and was as pointed as a Formula One car. Is that type of arrangement a solution?
Could you provide a link to that 'vessel'? I can't find it

Another small cat, the ECO6, is rather strange looking but seems to be well received as a shall coastal cruiser that has a flat cabin that dives down to the leading edge of the boat incorporating both hulls. I wondered how waves might slam on the cabin, or would these boats be lighter and go over waves?
Could you provide a link to that 'vessel'? I can't find it

And, how badly would a dory style hull slam, if it were say 42" high at the bow, at about 20' and say 800 pounds (hope that makes since) being the same asymetrical design?

Could you provide links to those vessels...can't seem to find them. posting photos also really helps. If you need help with this just ask...by email preferable

 

[brian eiland]

A centre nacelle in the style of INCAT ferries, Hull Number 062 would permit a deep bury for an freestanding carbon mast suitable for the Omer Soft Wing Sail.

http://www.incat.com.au/news/delivery_archive.cgi
Pericles

Hey Pericles, could you provide an update on the link?...doesn't seem to work now

 

[brian eiland]

would something like this help - the boyancy would grow at least somewhat gradually and all the pressure wouldn't hit "at once"

http://kerosene.smugmug.com/photos/140771122-L.jpg

Could you provide an update on that link? ...it doesn't seem to work now.

 

[brian eiland]

Lagoon 43 'sneezing'

...Much of this slamming is self inflicted. Imagine two hulls close together pitching into a wave. The water they displace has to go somewhere, and it piles up just as the bridgedeck sails over it. Clearly a wider hull spacing will turn a narrow high peaked mountain of displaced water into a low flat molehill. A wide knuckle and flared hull will also help reduce the size of the induced wave.
Richard Woods

I believe this 'self inficted' phenomenon is what is occuring on the Lagoon 43 powercat and its reported 'sneezing' problem. The bows of that vessel are fairly full in shape, and fairly close together. The forward motion of the vessel is forcing a lot of water into a more restrictive channel down between the hulls. Add to this a bit of choppy sea, and/or a downward pitching bow, and that extra water gets 'sneezed' back out front of the vessel to be blown back over the bow as a spray.

And that very minium the V-shape nacelle they provided does little or nothing to really help....mostly cosmetic if you ask me.

I wrote about this sneezing problem somewhere else on some forums, but can't seem to recall where

In all fairness I do not recall reports about slamming on these vessels,...only sneezing. Would be great to hear first hand from some owners. I tried several times to get a ride on one of these vessel in some nasty conditions, but never got the chance.

 

[Frosty]

The owner of the Catana 45 power cat behind me right now as I type can confirm this.

A sneeze out of the front can do nothing in my opinion for forward momentum.

It handy for stowing the anchor chain.

 

[Dryfeet]

So did/does the Gougeon 32 have a sneezing problem or noticeable slamming issue? It is afterall, both narrow AND low bridgedeck clearance. Or does the fineness of the hulls minimize this issue (versus something fat like the Lagoon43)? While width can help reduce wave train interaction from the hulls, it seems to me that the need to 'straddle' more waves might result in increased slamming. Bridgedeck clearances for offshore cats often are given in terms of vertical clearance but surely slamming would be greater for a 3' Brdk clearance on a 20' beam versus a 26' beam with the sam clearance?

 

[brian eiland]

Sneezing vs Slamming

The owner of the Catana 45 power cat behind me right now as I type can confirm this.

A sneeze out of the front can do nothing in my opinion for forward momentum.

I agree Frosty, it would do nothing for momentum, but I'll bet you could live with this much better than slamming.

Do you think you could elicit some personal stories from that other owner??

 

[brian eiland]

So did/does the Gougeon 32 have a sneezing problem or noticeable slamming issue? It is afterall, both narrow AND low bridgedeck clearance. Or does the fineness of the hulls minimize this issue (versus something fat like the Lagoon43)?

Fineness of the hulls can be both good and bad. Fine can reduce the squeezing between effect. But fine can pitch down more by the bow before gaining sufficient bouyancy

While width can help reduce wave train interaction from the hulls, it seems to me that the need to 'straddle' more waves might result in increased slamming. Bridgedeck clearances for offshore cats often are given in terms of vertical clearance but surely slamming would be greater for a 3' Brdk clearance on a 20' beam versus a 26' beam with the sam clearance?

In most cases these vessels are not staddling multiple waves.

 

[Alik]

Minimal vertical clearance t for powercats I use for our designs:

For displacement cats:
- t=4.5...6% of length
- t=0.5*h1/3, where h1/3 is anticipated significant wave height

For planing cats:
- t=2...3% of length

These and other recommendations are published in some my papers on catamaran design.