What is bridge clearance ona cat?

[Lodesman]

henryv once whispered in the wind:
As to Kantor's comments appearing to be mostly anecdotal - the man has sailed more cats than most of us will ever set foot on so I think his opinions are worth considering.

Not disagreeing about Kanter's experience and the worth of his opinions. But I also see the value in the opinions of such other noted catamaran "experts" as the Prouts, Shuttleworth, Woods, White, Wharram, Crowther et al. Understand that there are many divergent views on almost all aspects of cat design, amongst all of these people. Not all of them can necessarily be considered absolute gospel. As for the article you've attributed to Kanter, there are statements like this:

"One example of steps protruding into the tunnel space. There are many other variations on this theme. Even the high performance boats that are designed like this seem to suffer the same fate as the cruising or charter boats. A quick check of the race records will confirm my assumption."

OK - show us the race records; give us some empirical data! Nowhere in the article, does he say "when we shaved off the humps, this catamaran's performance increased by X percent." He provides nothing to back up the statement that nacelles and "planing wedges" reduce performance.

If you look at photo seven in the article, he comments that when this 38-footer was redesigned as a 42-footer there was a dramatic improvement in performance. If I'm not mistaken this is a picture of a Manta - go to the multihull comparison link and compare the Manta 38 to the Manta 42. Gee, he's right; the 42 is a better performer; but, oh wait, there are other factors such as the hull beam. The 42 has narrower hulls - maybe that might play a part, along with the increased LWL, which is the prime factor in theoretical hull speed. Maybe the change in BMAX/LOA did reduce the wake, but since he's comparing apples to oranges there's no way to prove that.

Take a look a photo one - he describes the sea as calm, but there is clearly a 1-2 foot chop that could easily be attributed to the depressed stern he is trying to illustrate.

My personal favourite is this statement: "A fifty foot boat with a thirty-two foot beam may be close to unmanageable." He has just described the ideal length to beam ratio according to John Shuttleworth, but he doesn't say why it would be unmanageable.

I'm not trying to knock Kanter - his book really is very useful; maybe he's just a crappy essayist, but this article is devoid of any hard facts; it is purely anecdotal (I stand firm in that assessment) but it illustrates very well the fact that even the opinions of acknowledged experts should be taken with a grain of salt.

Kevin

[colemj]

Kevin,

I'm not debating what you say or the main point of your post, but I do want to point something out about your Manta example. I went to the multihull comparison database and, while it is a very interesting and somewhat useful site, the data used in the Manta calculations are simply incorrect for many variables. The hull width of the 38, 40 and 42 are not different, but equal. The displacement values are incorrect for all three boats and the sail area is incorrect on the 38 and 40 (all three actually have the same design sail area).

The above leads me to take any comparisons using that database with a grain of salt.

Mark

[Pat Ross]

Multihull Dynamics, Inc, www.multihulldynamics.com , has made every effort to use correct data in its calculations. We collect data from magazines, websites, sales brochures, designers, builders, etc... However we realize that some data may be inaccurate. We request Users, designers, etc... to inform us if they think the data is incorrect. We request that they verify with us where they are getting their data from, so that we may verify it, before posting it on the site. Finally there is a Custom Calculation section on the site, on the entry to the Comparison Page, where it says selection, "Stored Data Base or Custom Calculation", this allows individuals to calculate different data for the same boats, since individuals will load them differently, or carry more or less different sail area, just to mention two possiblilites. If you have feedback for the website you will find several locations to deliver this information, and we greatly appreciate your help. We, like you, love multihulls. Pat Ross

[Talbot]

One of the biggest factors that is missing from multihull specs, is the effect of additional weight. For example, the Prout snowgoose in light weight configuration is a much better performer than the Snowgoose Elite. However, as soon as you add the normal sort of weight required for cruising, the Elite is a much better boat.

[Lodesman]

Mark and Pat,

Maybe the hull widths of the Manta 38 and 42 are the same - the figure I used from the database was hull width at waterline. Perhaps adding four feet in length has reduced the draft and therefore reduced the breadth at waterline of the semi-circular hull form. I can't speak for the other factors, but I think the database is a useful tool even if some factors are not 100% accurate. Most of the comparisons seem to be mathematically-derived and therefore theoretical. Certainly I would like to see even more comparative data - moulded hull depth, windage, height of sail centre of effort above waterline, total immersed surface area (wetted surface), payload, etc. and if possible, empirically measured data as top speed (sail and motor). Cheers.

Kevin

[Pat Ross]

Kevin, Those are excellent ideas. Unfortunately getting the data from the designers to calculate all of that is very difficult. For example getting the information to calculate the Primatic Coefficient is a real challenge. I have tried. So we take what is available and offer people the opportunity to vary displacement and sail area to see how that will affect perfomance and stability.
Pat Ross

[Slrman]

What I am seeing from all of these posts is that there are many variables in what the "experts" say (they each of their own preferences and ax to grind) and that it is very difficult to compare apples to apples.

A boat that is designed for performance may be a slug when loaded for cruising, and a cruising boat may be a rocket when stripped for racing and safety considerations are tossed out the hatch.

What we must do is decide where the personal priorities lie, examine the available data, (want performance, look at racing boats and their design) and focus on how you will really be using your boat.

I have seen people with multis that were absolutely dismal in sailing qualities that were ecstatic with their boats because of the increased safety, convenience, and living conditions aboard. I've seen others with great speed complain about no storage, constant maintenance, and the work required to keep it moving at maximum.

"So you pays your money and makes your choices." Just be sure you know what your choice really is. Imagining sailing to far horizons is fine but if you're really going to be doing coastal cruising with no more than week or two aboard, be honest with yourself.

[fhrussell]

Back to the original question about wing clearance... In general, if there isn't 24 inches between the water and the that wing, the boat will pound, it will be unsettling, and will suffer in performance. Unfortunatley, most (probably every) new designs do not have this clearance, mainly because the market prefers to have standing headroom and berths positioned in the hulls at waist height. So to keep the lines of the boat low and attractive they build the wing at 12 inches off the water and the berths and steps and other appendages into the hulls at sometimes less than 9 inches!

[Jeannius]

fhrussell...

Not all modern cats have poor bridgedeck clearance. Here is an example of a good cat... My Privilege 435...

[fhrussell]

Thanks Mike! I know there are some good ones out there, but they are becoming fewer and far between.... seems to be the trend. Again, thanks for showing an example of good design. Any others? It would be a good referance list for other visitors to this topic thread.
Kurt Hughes always has decent wingbridge clearance on his boats, but of course, his boats are not production boats for the most part.

[CaptainK]

Hey fhrussell.

I noticed on your profile, that your a boatbuilder. Which boatbuilding company do you work for?

[fhrussell]

Hi Capt.,
I work with a small custom yacht builder, Sportsmen Yachts/ Big Sandbar Multihulls. We only do custom work, but will be introducing a production boat this year, a 28' sportfish, Sportsmen 28. It's similar in philosophy as the Albin 26.
Presently, we are building a 40' Sportfish, a 35' Downeast style cruiser, a 48' Kurt Hughes charter cat, a 25' overnighter/ daysailor catamaran of our design, and a Nelson Zimmer 28' mahogany runabout.

[CaptainK]

Wow!!

So how long have you been doing this line of work? And exactly where is this company based at. What town or city?

It sounds like you really love your job. Sometimes I wish I could work for a company like yours. I did work for Sea Ray once back in 1999. I worked in their upholstery department. But they had the stupidest rules there.

They wouldn't let me bring my own tools. And we had issued tool boxes with padlocks. Like if they're worried someone would steal my tools? And I developed problems with my hands, due to straining them. (Due to not having the right tools!!) SO after a couple of weeks. I told them to go !uck themselves, in a polite way.

Wouldn't mind working for another company. I have worked as a mechanic in the past. Wouldn't mind trying my hand at that in a boat yard!! Big $$$

[fhrussell]

Hey Capt,

We are in Manorville, NY. Also have a small shop in Wading River, NY. We are known as Sportsmen Yachts/ Big Sandbar Multihulls. Two companies working under one roof basically. Sportsmen was originally OffSoundings, then changed names about 10 yrs ago.

I've been building boats off and on for 20 years, but have gone full time for the past 3 years. My partner has been fulltime for 30 years. We are a small shop and have only 2 other employees. We do all phases of the building. Occasionnaly we send out for SS work or upholstery.

I know there are positions around for a good marine mechanic. You should look around. It seems to be one of the popular job offerings in the marine industry. But I guess you'd want to find a position with a smaller shop, to avoid the silliness of those bigger shops....

OK..back to wing clearances!!

[Joli]

Our experience racing against multi's has been that while they were faster on certain points of sail they were not fast around the course. They suffer greatly when going up and down. I would guess that many are not in racing trim and since our waves here are very short and square the bridge clearance issue become exagerated. When sailing the Great Lakes, pounding is a problems for everyone.

Bryan