Center nacelles and wave pounding in cats.

«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?

 

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.

 

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.

 

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

Or are you just an other fundamentalist?

 

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

 

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?

 

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

 

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.

 

"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 also has some cat design stuff

 

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.

 

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, check them out!
Jeff, where can I see some drawings or pictures of your Egan 12.4?

 

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.

 

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.

 

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.