Center nacelles and wave pounding in cats.

[xarax]

«The idea is that the force of the wave collisions will be dampened by the cushioning effect of the V section (of the nacelle)". ( Chris White, The Cruising Multihull)
On the other hand, the wave collisions are more frequent on a catamaran bridgedeck with a center nacelle, aren’t t they?

[Frosty]

Bridge deck pounding is a terrible experience. If your not used to it it feels like the boat will be smashed apart. Its the one thing I don't like about cats. It may stop me buying another.

Bridge deck height is all important. I dont think the "nacelle" ( is that what they call the middle bit) will help.

[jehardiman]

For an open ocean vessel, you cannot prevent bridge deck pounding. You must accommidate it. There is a very fine decision between designing for impact with less clearence and a V-section vice increasing clearence. The trade-off is buried in response (waterplane and mass interaction), speed, and structural strength-to-weight considerations. These arguments are well covered in the SWATH reasearch done in the late 1960's/early 70's.

[terhohalme]

What is such an unpleasent catamaran? Sure they all can't be that bad.

Or are you just an other fundamentalist?

[Richard Woods]

You are right to be concerned about bridgedeck slamming on catamarans. Having said that, it is usually a comfort problem rather than a structural one. Fortunately it is, or should be, a thing of the past.

Early catamarans had low freeboard, partly because of the heavy materials which were all that were available at the time, and also because low freeboard boats always look better. The latter is still true today of course, but people have now got used to seeing high boxy catamarans.

As Chris White points out, in an attempt to cushion the slamming, many builders, and Prout Catamarans in particular, developed the central nacelle. Prouts took this nacelle to it’s extreme and their last designs were more like three hulled catamarans with the nacelle in the water at rest.

I have never believed this approach to be a good one. I always say that for offshore sailing one should be able take a conventional inflatable dinghy under the bridgedeck. I sailed the S Atlantic from Capetown to Rio in a Norseman 43. Its bridgedeck was very low (but it looked a very nice boat due to its low freeboard) and it was very uncomfortable living on board. A few years earlier I had sailed from the UK to the Canaries in another low bridgedeck catamaran (again not one of my designs). The slamming was so bad that we were unable to use the saloon table as plates would jump off as we hit every wave.

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.

The best solution is to start the bridgedeck well back and have it low only where needed. That’s why most offshore catamarans have nets or trampolines forward. The boats to avoid if you plan any offshore sailing are those with bridgedecks taken right to the bows. Furthermore, the water that goes in at the bow also has to come out at the stern, for as the bows pitch out of a wave the sterns will pitch in. So bridgedecks should also be high near the stern. Have a look at the stern of a Prout catamaran, you’ll see there is very little space for the bows waves to get out. No wonder they are so noisy to sail – and the waves trying to force their way out must slow the boat down.

I do fit a nacelle on some of my designs, notably on the Gypsy and Romany. I accept the compromises as I wanted standing headroom in a small boat. But I designed these nacelles as footwells so they are as small as possible, (they are only 600mm/2ft wide). I sailed my own Gypsy thousands of miles and didn’t find slamming to be a problem. Mind you, I also had a Veed bottom to the nacelle. Had it been flat then I am sure the slamming would have been noticeable.


I hope this helps

You can see more of Gypsy and Romany on my website

www.sailingcatamarans.com

Richard Woods

[xarax]

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.

Thank you Richard,
If the sides of the hulls are less curved inward, or even if they are not curved inward at all, (like the hulls of some power catamarans), would this reduce this self inflicted slamming?

[Richard Woods]

I am doing exactly that on my new powercat designs, but people haven’t found much benefit from using asymmetric hulls on sailing catamarans. No doubt because of the speed variations, not to mention heeling and leeway considerations.

Best wishes
Richard Woods
www.sailingcatamarans.com

[Spiv]

Hi everyone,
I am new to this forum, but not new to catamarans having owned a few power (7.5m x twin 225HP, 9.5m x twin 275HP and 12m x 320HP jet) and a few sailing ones (Windrush, Nacra 16sq and 18sq, Hobie17 and Schionning 12.5m) .
I always tell people cats should be compulsory!

I had the 12m jet cat designed and built during the America's cup days in Fremantle, she had asymmetric hulls (top speed 30kn). There were 4 ton of alloy in the factory before fitting out. She did not perform well and needed a lot of power to get up and go. she also used to slam a lot into the "Fremantle doctor" (25 to 28kn sea breeze).
The 7.3m was a Shark Cat (top speed 32kn), beautiful boat, she did well once on the plane, but one need to be careful into the wind: never head on or she would slam hard.
The 9.5m (36kn) had a smallish wavebreaker in the middle of the bridgedeck, all the way to the transom; now, this boat had a softer ride than both the others, but suffered by too much weight and low bridgedeck clearance. Once on the plane you could feel the wake of the two hulls rise onto the underdeck and meet in the center where the wavebreaker would send it back down increasing lift, but with the low clearance she would start hitting waves a little too early.
That is it for the high speed stuff, now I want to give you my thought end experiences on my passion: sailing.

Most of you would know the little cats, they behave differently from the bigger cabin boats and even though they do not have a bridgedeck, you would find the tramp slamming into waves, sometimes slowing you to a halt.
On sailing cats you soon realize what is the critical factor: Bridge deck clearance.
Charles Kanter in his great book "Cruising in Catamarans" explains the various factors affecting slamming: fine bows fat sterns, finesse coefficient, over 'square' design etc; but he gives a general rule of thumb: bridgedeck clearance should be at least 1 inch for every foot of hull centerlines distance.
So for a 'square' 42' cat the beam should be 21', the centerlines distance would be approximately 19' and the clearance should be at least 19".
I found that to be not enough.
I took my 12.5m Schionning 3 time on a 1600NM voyage from Fremantle to Broome along the treacherous West coast of Australia. We always had a bashing on the way South into the predominant Southwest sea and wind and we also could not leave plates on the table. Several times we could not sleep for the noise and vibration.
And my bridgedeck clearance? 620mm (24"). A full 5" more than Charles recommended as minimum for comfort.
I believe that a lightweight modern cat should be able to slice through 1m (39") waves and climb over bigger ones without hitting them.

Please keep this discussion going as I am most interested in hearing about other experiences. :)

[yipster]

"a lightweight modern cat should be able to slice through 1m (39") waves and climb over bigger ones without hitting them"
sounds reason and do-able to me, good to hear experience sailing bigger cats toni grainger (http://www.graingerdesigns.com.au/construction-resources.php?id_rca=1) also has some cat design stuff

[waikikin]

Stefano, nice post, sounds like your really keen for cats, I had a 10 metre Beach Marine cat that had 530mm underwing clearance, the bridgedeck started well back(at mast beam) from the bow & only slammed 2-3 time in plenty of sailing over 7 years although plenty of spray kind of thrummed onto it, also had a centre nacele but this was more an outboard fairing, also had spray chine to hulls & this could be noisy(& keep me awake), my next cat(in build) is an Egan 12.40 & the designed clearance is 860mm, she is to have a double champher panel & step to the under wing section but definatly not having spray chines again, hopefully this clearance will be adequate but it'll be a while before I test it:) All the best from Jeff.

[Spiv]

Yipster, I agree with Toni Grainger's comments. In the last months I have been reading all I can find about cat design, and some designers still have it wrong.
Some really good ideas from Windspeed Yachts (http://www.windspeedyachts.com.au/sailing_multihulls.htm), check them out!
Jeff, where can I see some drawings or pictures of your Egan 12.4?
:)

[waikikin]

Stefano, if you pm me I can post(mail) the GA & profile, theres nothing online 'cos I aint to IT savvy. All the best from Jeff

[Spiv]

Stefano, if you pm me I can post(mail) the GA & profile, theres nothing online 'cos I aint to IT savvy. All the best from Jeff
Jeff, how can I contact you, there is nothing visible on your profile?
Please email me your ph and I will give you my address.:)

[waikikin]

Stefano, if you click on my blue waikikin there will be an item to send a private message, once sent it comes up on page head for me to click & see. Regards from Jeff.

[doug kay]

when I lived in Wivenhoe Essex UK Sailcraft and Prout were the multihull masters, they were the ones breaking the rules and being very successful but we've moved on since those days. A 2x1 was the norm but now I have a Ground Effect which I built with a 9 metre OA and 6 metre beam. It survived Wilma in Ft. Lauderdale 3 years ago but got hit by a freak twister,it now needs extensive repairs. I'm thinking of building a centre board into the bridge deck which i can adjust fore and aft, comments please.

[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]

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]

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]

...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 :confused:

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]

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.

[brian eiland]

Minimal vertical clearance t for powercats I use for our designs:....
Does that take into account where the bridgedeck begins along the length of the vessel?

[Alik]

Does that take into account where the bridgedeck begins along the length of the vessel?

This is minimal clearance at midships section. Front 25% of wet deck should be either raised or cut out.

Moreover, one should look at structural considerations. Say, GL HSC rules apply much higher slamming loads on bridgedeck structure if local vertical clearance is below 0.05*L.

[brian eiland]

Here is a more favorable shape I found in my files...even while it extends almost to the bow.

I forgot what this belongs to :confused:

[mariedukat]

Hi Brian, keeping in mind that DOMINO is a power cat, not a sailing cat, our wing deck is highly anti-slamming. The Hight above water is one component, the knuckle at the base of each hull allows for added lift, the double-barreled round wing deck, widening aft, allows for energy dispersion. In 12,000 miles and all sorts of seas, we never slammed. Check the design on Malcolm Tennant website or on our blog, http://dominocatamaran.blogspot.com

http://dominocatamaran.blogspot.com

[Pericles]

In 2007, I mentioned the centre nacelle of an Incat ferry as a source of design inspiration for a sailing catamaran, with the added bonus of being deep enough to mount the Omer wing sail rig designed by Ilan Gonen. http://www.omerwingsail.com/

In addition, an diesel generator and a single dagger board could be mounted inside the nacelle.

Looking at another favourite of mine, although there seems to be adequate clearance, I wonder if the Gunboat 66 slams at 30 mph or if anyone cares? :p :p


http://www.gunboat.com/66-gallery.php#

[mariedukat]

Yes, Pericles, the Gunboat is an ultimate design. Here is DOMINO (white) compared to PELICANO (blue). Their profiles are slightly different, Pelicano being much wider and longer.

[Spiv]

...and Spiv please bring along your diagrams and personal knowledge


Hi Brian,
good idea to try and consolidate this great knowledge we all share into one thread.
I'll copy and paste what I can find first and as time allows I will write some new.

This from a 2008 post, I still consider this true:

Charles Kanter in his great book "Cruising Catamaran Communique`" explains the various factors affecting slamming: fine bows fat sterns, finesse coefficient, over 'square' design etc; but he gives a general rule of thumb: bridgedeck clearance should be at least 1 inch for every foot of hull centerlines distance.

Therefore, for a 'square' 42' cat the beam max should be 21', the centerlines distance would be approximately 19' and the clearance should be at least 19". That equates to 8%.

I found that to be not enough, my last 12.5m (41') cat had centerlines beam of 6.5m (21') and a bridgedeck clearance of 620mm (24"). She would slam badly, tossing things off the table, we even had damage to the saloon seats..
So, my view (shared by a local cat builder on his 47th cat) is that the same size cat should have a minimum 900mm (35') or 14%.
I was looking on Kurt Hughes web site and most of his 13 to 15m cats have 920mm clearance.

If your cat ends being 35', I would guess a BMax of 18' and CL beam of about 15'; using 'my' min 14% you would need 2.1'.

That is bridgedeck clearance, forebeam should be higher as it is going to hit waves much earlier and before the boat starts to rise."

[Spiv]

This also from a 2008 post:
,
I had several power cats of different size (23 to 40') and driven others on test runs to try and make up my mind on what to buy. All cats are not the same.
I know this is off the tread subject, but I hope Bob doesn't mind...

What cats have in common is that they (in general) go faster and are more comfortable than monos of same length and power as they (should) ride on a cushion of foam. All my cats did more than 25kn, the Manta 33' did 36kn and it was by far the best.

The idea of a wavebreaker has been used by different design, but the manta carried all the way to the stern and the half bridgedecks were round.
Also, it had asymmetric chines so it would climb on the plane smoothly and react to waves smoothly.

If you are considering doing some mods to your cat, add the wavebreaker, keep it off the waterline and carry it all the way aft. I am sure it will make a big difference, especially at speeds over 20kn.

http://www.boatdesign.net/forums/attachments/multihulls/23418d1216175523-slamming-waves-open-bridgedeck-crossbeams-mantasketch.jpg

http://www.boatdesign.net/forums/attachments/multihulls/23419d1216175523-slamming-waves-open-bridgedeck-crossbeams-mantasketch2.jpg

[Alik]

I don't see much advantage of having such developed and long wavebreaker specially on planing cats. Yes, they do soften the slamming, but if the wavebreaker itself is lower than normal surface of tunnel then it does not work.

We have done some tunnel photos/video (http://albertnazarov.blog.ru/114050851.html) on number of cats. Off course we were unable to test in extreme slamming conditions otherwise we would loose the camera.

[Spiv]

My first post refers to sailing cats, my last to power cats doing 15 to 30+kn.

I thought for a long time about a wave breaker for sailing cats, but I am not sure if it would work at slower speeds.

On sailing cats, I think it is safer to stick with high bridgedeck clearance and round chines.
As Richard Woods suggests, the addition of a 'knuckle' above waterline, is all I would recommend.

This section is of a lightweight 17m cat Richard Wood designed for me.

[Alik]

On sailing cats, I think it is safer to stick with high bridgedeck clearance and round chines.

Fully concur.

On powercats, we use 'wave breaker' shape at bow mostly as placement of anchor and natural stiffener. We do not use wave breaker at full length of wet deck as it actually reduces either vertical clearance or headroom.


As Richard Woods suggests, the addition of a 'knuckle' above waterline, is all I would recommend.
This section is of a lightweight 17m cat Richard Wood designed for me.


Smooth knuckle works though sharp one would create unpleasant noise (due to wave sloshing) especially at rest.

[Boston]

dont any of these designs have pressure relief, seems to me that one reason slamming is so sever is that the upward thrust of the water is completely resisted by the intransigent nature of the deck. If there were a space between the hulls and the bridge pod that allowed "breathing" ( pressure from the upwardly moving fluid to escape the hull structure ) then the intensity of the slamming action might be relieved somewhat. Say a typical hull configuration suspended between the hulls of a cat with some form of rigid mesh between, wouldn't elevate slamming completely but might make it bearable in less than ideal conditions ;-)

[Frosty]

That would make a bit of a mess of my lounge carpet Bos.

[Boston]

well they could roll up in ugly conditions, ;-) Thing is why does there have to be a large solid flat surface rather than a nice streamlined one with space for the deflected water to bleed off between the hulls? Could be something rigid but porous that might hold that carpet just fine in pleasant conditions. Maybe even with a canopy for some shade. Whole things stows away for rough conditions.

[brian eiland]

Hi Brian, keeping in mind that DOMINO is a power cat, not a sailing cat, our wing deck is highly anti-slamming. The Hight above water is one component, the knuckle at the base of each hull allows for added lift, the double-barreled round wing deck, widening aft, allows for energy dispersion. In 12,000 miles and all sorts of seas, we never slammed.
Marie brought up 2 interesting points, power vs sail vessels, and the 'double arch shape' that Tennant experimented with quite a bit.


The height, shape and span, both transverse and longitudinal, of the wing deck, will vary from design to design, and from designer to designer. When looking at the wingdeck structures of power catamarans it is necessary to recognize that they may be operating in conditions that will never affect a sailing catamaran.

Most sailing catamarans will never be going any closer to the wind and waves, under sail, than 40-45 degrees. This to some extent can reduce the slamming effect of the waves. But keep in mind that a sailing rig itself can impart a downward force to the bows on many points of sail.

The power cat on the other hand may very well be 'punching' straight into a seaway and this should be kept in mind. Likely this is the more difficult case to design for.

In the case for a motor-sailer which is a great interest of mine, and particularly a vessel that can perform reasonably well in both modes, at sea, the good bridgedeck designs of the power cat are most likely the ones to be incorporated on the motorsailer

[brian eiland]

Hi Brian, keeping in mind that DOMINO is a power cat, not a sailing cat, our wing deck is highly anti-slamming. The Hight above water is one component, the knuckle at the base of each hull allows for added lift, the double-barreled round wing deck, widening aft, allows for energy dispersion. In 12,000 miles and all sorts of seas, we never slammed.
Marie brought up 2 interesting points, power vs sail vessels, and the 'double arch shape' that Tennant experiemented with quite a bit.


The height, shape and span, both transverse and longitudinal, of the wing deck, will vary from design to design, and from designer to designer. When looking at the wingdeck structures of power catamarans it is necessary to recognize that they may be operating in conditions that will never affect a sailing catamaran.

Most sailing catamarans will never be going any closer to the wind and waves, under sail, than 35-45 degrees. This to some extent can reduce the slamming effect of the waves. But keep in mind that a sailing rig itself can impart a downward force to the bows on many points of sail.

The power cat on the other hand may very well be 'punching' straight into a seaway, and this should be kept in mind. Likely this is the more difficult case to design for.

In the case for a motor-sailer which is a great interest of mine, and particularly a vessel that can perform reasonably well in both modes, and at sea, the good bridgedeck designs of the power cat are most likely the ones to be incorporated on the motorsailer

[Spiv]

..........In the case for a motor-sailer which is a great interest of mine, and particularly a vessel that can perform reasonably well in both modes, and at sea, the good bridgedeck designs of the power cat are most likely the ones to be incorporated on the motorsailer

Brian,
Speed is a major consideration.
Alik also agrees that hard knuckles would not be much good to a sailing cat and also create a lot of undesired sloshing at anchor.

Also, the times that I cursed the most the designer of my old 42' sailing cat with 600mm bridgedeck was when motoring through steep 1 to 1.5m waves.
Once I had a 5h demolition ride at no more than 4kn into a 25kn wind, any faster and I would have had structural damage.

If you look again at my drawing of the Mantacat at speed, the design of the 'double arch' relies on the speed to create turbulence/ foam on which the powercat rides. If you slow down, then you start pounding, till you really slow to a few knots and start climbing the waves rather than cutting through.
Never been on a sailing cat with 'double arch', but my guess is that it would not work the same. Other benefits might apply: extra stiffness, some pounding reduction etc.

IMO for a sailing cat you must make it long, light and with high bridgedeck, this way you climb over big waves and slide through little ones without bashing.
Anything diverging from that will be a compromise.

[masalai]

Look at mine, on the trip up to South Percy beaut... BUT

Know the performance traits and travel to their advantage for comfort (fastest travel option...
a) - Small messy bay chop (westerlies on the East coast) where fetch is 1 to 3 miles, travel close inshore, enjoy the view and pleasant ride....
b) - When the south Easter brings a good swell go offshore to ride the surf and get a nice push (my best was 19.5 knots), or against the wind/sea play the tidal currents or (coming back wait for calmer weather and stay close inshore)...
c) - If you cannot avoid the small 1 to 1.4M slop tack off a bit so that the wave first touches one bow and directly in the same position the stern of the other hull, - coming or going, the same applies and adjust your speed to best comfort in ride...

I find that the 45degree chamfers leading to the bridgedeck lift the boat nicely and, with the high clearance no slamming... The latest variation has the chamfer extending up to the deck at the bows for nicer lift and less chines to tape and fair ... and slightly smaller trampoline area... The hull section in the water is symmetrical, has a strengthened bottom so can be beached... I have mini-keels to protect the sail-drive legs and still allow grounding in shallows with no hassles draft is about 1.3M

[boatn00b]

So...

Symmetric hulls will pile up/accelerate water in the tunnel

Asymmetrics leave the flow/water level in the tunnel basically as it is?

So banana shaped hulls (idiotic in any other sense obviously) would suck the water level down (save cross flow across hull bottoms)? Configure bows like drooping wing leading edges maybe (not serious really...)? Turn bows inward when slamming is more of a problem than efficiency...

[masalai]

Errrr, Not Quite...

Older (original) Hobie cats, for example, had lots of "banana" so they could turn quickly in the surf, lots of fun to sail at the time...

Slamming, (where the waves beat against the underside of the bridge-deck), is reduced by the 45 degree chamfer on the inside - above normal water-line / wetted-areas to add buoyancy as the wave-form passes ... AND ... by increased clearance under the bridge-deck... 700 to 850mm clearance on my boat ... AS WELL AS ... extending bow and stern beyond the bridge-deck to increase buoyancy and reduce weight at the ends...

Some boats have a lower bridge-deck clearance and rely on 'spray' and air-pressure mix to reduce slamming, but the consequence can be a very stiff ride (like a "pile-driver", if you are so medically afflicted :D )

High speed (150mph) boats like "maritimo"... http://www.boatdesign.net/gallery/showphoto.php/photo/15580/ppuser/22903 have straight inside faces and turn at very high G forces...

I have seen others that seem to have a fuller hull form on the inside (what advantage that is I do not know)...

The shape of the wetted area determines the "efficient cruise speed" as the wave generation is the most significant factor in limiting maximum speed and efficiency when in "displacement" mode... Again I am NOT an NA or Engineer and am not using strict Engineering terms... but you should get the general idea...

Wetted area, (rounded as opposed to hard chines has minimal adverse impact for everyday cruising), and may come into play when seeking that last 1% performance - but I reckon that crew capabilities far outweigh that... In multihulls, the greatest performance bugbear is WEIGHT...

[boatn00b]

I was thinking convex/concave/straight (parallel) tunnel sides (at the waterline) as seen from above/below. Purely theoretically, of course...

[masalai]

No real advantage at "displacement" speeds, - A big pain in the arse to ensure PERFECT parallel alignment, both horizontally and vertically and no twist of the hulls in general, - and the connection to the bridge-deck will be less than optimum being at a right angle, exposing a very high stress factor at the join...

With the 45 degree chamfer panel there are the advantages of :-
- facilitating the placement of access steps down to the hulls,
- in the placement of the bulkheads there will be better load spreading,
- the 45 degree panel can also be designed to spread loads and stresses,
- when the bulkheads are positioned, strengthen the join with a couple of runs of glass tape on the inside of the hull... and several other considerations to spread the loads and diagonally stiffen the hulls...

[Pericles]

Two hull forms of interest. Hickman was the subject of a number of threads here and Ken Hankinson designed a high speed catamaran.

http://www.google.co.uk/search?q=hickman+sle+sled&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-GB:official&client=firefox-a#hl=en&client=firefox-a&hs=rfg&rls=org.mozilla:en-GB:official&sa=X&ei=m2U-Tr7DO4HChAfOhqT8BQ&ved=0CDIQvwUoAQ&q=hickman+sled+sled&spell=1&bav=on.2,or.r_gc.r_pw.&fp=28f38f828a2aa9f0&biw=1024&bih=578

http://www.boatdesigns.com/Slot-Machine/products/761/

http://www.boatdesigns.com/46-Slot-Machine-tunnel-slot-high-speed-express/products/874/

[yipster]

I find that the 45degree chamfers leading to the bridgedeck lift the boat nicely and, with the high clearance no slamming... The latest variation has the chamfer extending up to the deck at the bows for nicer lift and less chines to tape and fair ... and slightly smaller trampoline area... The hull section in the water is symmetrical, has a strengthened bottom so can be beached... I have mini-keels to protect the sail-drive legs and still allow grounding in shallows with no hassles draft is about 1.3M

Slamming, (where the waves beat against the underside of the bridge-deck), is reduced by the 45 degree chamfer on the inside - above normal water-line / wetted-areas to add buoyancy as the wave-form passes ... AND ... by increased clearance under the bridge-deck... 700 to 850mm clearance on my boat ... AS WELL AS ... extending bow and stern beyond the bridge-deck to increase buoyancy and reduce weight at the ends...

With the 45 degree chamfer panel there are the advantages of :-
- facilitating the placement of access steps down to the hulls,
- in the placement of the bulkheads there will be better load spreading,
- the 45 degree panel can also be designed to spread loads and stresses,
- when the bulkheads are positioned, strengthen the join with a couple of runs of glass tape on the inside of the hull... and several other considerations to spread the loads and diagonally stiffen the hulls...
been looking at your boat mas, nice and about done eh? that hitchhiker wil come along as well some day
keep aiming for those tropical coconut islands! do like that chamfer and no slaming eh?
spoke with a guy that sailed cats acros the atlantic and like to hear you sailors talk

[masalai]

Hi Yipster,
Long time no see,
Boats usually seem to me to be 'tailored' to meet 'local' conditions, and I find that in Australian cat designs, which seem to have greater bridge deck clearance and a bit more beam overall (similar to South African designs, as the Southern Oceans are a hard task-master, and will severely punish most designs that venture too long in that region...

[Boston]

been following this one for a while and I might have mentioned it before but how about a design that allows for a lot of venting between a central cabin and the sponsons. Kinda like this one

http://psipunk.com/wp-content/uploads/2009/10/solar-powered-velocity-7-catamaran-02.jpg

the underbelly of the central cabin area is still pretty flat in this rendering ( not my rendering or design by the way ) but if it were more v shaped it might handle pounding pretty well.

just a thought
cheers
B

[Mr Efficiency]

I have had power cats without the wavebreaker/nacelle and I always thought tunnel slap was worst when quartering than head-on to waves.

[rapscallion]

So, how much slamming does the G32 experience? Or a Jarcat? Perhaps bridgedeck slamming is the explanation for jan's latest catamaran design "project x" as it was called before it was named strings.

[rapscallion]

Here is an interesting catamaran design...

http://www.wellscoates.org/inventions.htm

[Pericles]

On another page, in 1941, Wells Coates sketched a yawl, to be rigged wirh what he called "Wingsails".

http://www.wellscoates.org/laura.htm

His catamaran had a fully battened mainsail set on a very curved mast. Could that have been his Wingsail? He certainly was ahead of his time.

[Doug Lord]

Here is an interesting catamaran design...

http://www.wellscoates.org/inventions.htm
===========
Pretty cool stuff ,Raps-thanks!

[brian eiland]

Here is an interesting catamaran design...
http://www.wellscoates.org/inventions.htm
Perhaps you might have placed that posting over on a mast, or sail, or wingmast related discussion?...probably better response?
...maybe here?
http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-30.html

http://www.boatdesign.net/forums/sailboats/wing-drive-6698.html

http://www.boatdesign.net/forums/multihulls/homebuilt-wing-mast-30519.html

I just don't see where it relates to wingdeck slamming?

[Doug Lord]

Perhaps you might have placed that posting over on a mast, or sail, or wingmast related discussion?...probably better response?
I just don't see where it relates to wingdeck slamming?

-----
Brian, the raised center nacelle on Coates cat seems relevant to the discussion to me.......Maybe you were thinking of post #60?

[Boston]

what if the center nacelle is able to move virticaly, so when its smooth you can ride on the outside sponsons and when its rough you can dip the center nacelle to be in contact with the water. The drag issues are not the primary concern in a high seas scenario and by dipping the center nacelle your eliminating a lot of pounding and lowering the CG. All you'd need is some hydraulics and the connecting arms could be the only real moving part.

just an idear
cheers
B

[rapscallion]

-----
Brian, the raised center nacelle on Coates cat seems relevant to the discussion to me.......Maybe you were thinking of post #60?


Righto Doug!

The boat reminds me of strings, the new gougeon catamaran.

[geodude]

Hi all.

I'm new to the forum but bare with me.

It seems from what I read that a "square" 40ft cat should ideally be greater than 0.9m clearance, and the bridgedeck should be a short as possible.

But that would mean a total superstructure height to LOA ratio of around 1:4 if you want to have full standing headroom in the bridge. That is a seriously boxy boat, and what about the effect of all that windage in heavy weather scenarios.

Surely the bridgedeck need not be that high across its entire length. Couldn't it step down to 0.7m or even less at the saloon/cockpit bulkhead while retaining a minimum of 0.9-1m clearance elsewhere. Afterall, the most vulnerable areas for slamming are those nearest the ends of the craft. By the time a wave gets to the middle of the bridgedeck wont the boat have risen somewhat to the wave? That would get the superstructure height down to a more reasonable 2.5m on a 12-13m LOA while retaining a high bridgedeck over 90% of the bridgedecks surface. Also wouldn't breaking up the flat surface, or even giving the bridgedeck positive curvature fore and aft, strengthen the deck whilst reducing the area that slams at any particular time. Particularly the latter seems like a sensible way of of reducing slap as no matter how the wave hits the deck, its curvature would deminish the force of the blow.