We talked to Pat and Cal of Multihull Dynamics which is an excellent site to compare sailing characteristics of cruising catamarans. We walk through what their website can do by doing a comparison of the Catana 42 and Lagoon 440. They discuss why they started the site, various features of the site, touch on overall lessons they have learned from over a decade of analysis, and more.
They are launching a new site within the next couple months, so please stay tuned and look for their enhanced site which will have more functionality, better look and feel, and an even larger database of cruising catamarans to compare to each other. You can go to their website and get in touch with Pat and Cal with questions, suggestions, and custom analysis.
Welcome Pat and Cal! Can you give me a little bit more information on Multihull Dynamics?
Hi. I’m Pat Ross. I live on the Gulf Coast of Florida. I was an Army aviator and combat pilot. I’m one of the original people who designed the website. It was a task that Cal Markwood, who’s an engineering analyst for Multihull Dynamics, who will speak shortly, and myself got together some 13 or 14 years ago and through a series of processes we have this site now, which we’re also getting ready to upgrade.
Multihull Dynamics was designed to help the average person who’s interested in multi-hull sailboats to evaluate the differences in performance and stability from one boat to another, to include an individual boat’s ability to handle rough weather, be a good cruising boat, or a good racing boat.
It happened as a result of the fact that the formulas for evaluating multi-hull sailboats are more complicated than the formulas for evaluating mono-hull sailboats, and takes much more time, and is a whole different process. While there were several websites available for mono-hull sailboats, there were no sites available, and still to this day, some 13 years later, we’re still the only site doing this for multi-hull sailboats. Although there are other sites, of course, that talk at length about multi-hull sailboats. We’re the only one that does this type of thing with them.
Wow! Exciting, very exciting. Cal, would you like to introduce yourself?
I’m Cal Markwood, I’m from Denver Colorado. I’m a retired Air Force vet, 25 years as a fighter pilot, aerospace engineer, and program manager. Then I worked for Lockheed-Martin as an aerospace engineer. I started doing multi-hull analysis way back, probably the mid-1970s, when I found little information on how to compare boats. So I developed some formulas out of some basic engineering approaches, some dimensional analysis. Then over the years improved on those from information I got from other analysts and so forth, to what we have now.
When Pat contacted me, I had about 90 boats in the database and a couple of published articles on the analysis approach. Then we started digging into everything we could find and built that database up. Added trimarans to it after a while, far less than the catamarans. The catamarans date back to the 1950s, actually when they were just starting to to really get popular.
I concentrate on cruising boats. Although I have one or two true racing catamarans in the database, mostly it’s cruising boats. That goes from cruising racers to just plain cruisers. For the two boats that we were talking about, the Catana 42 tends to have race qualities to it, as well as being a pretty phenomenal cruising boat. The Lagoon 440, like most Lagoons, are just cruising boats, with little racing potential unless you’re racing against another Lagoon 440, then it’s sailor and sailor.
We have 151 trimarans on the website and 937 catamarans at this time now. We may have one or two more than that, that’s our best number right now. I think when we upgrade the site, we’ll have a good number.
Take us through the website and what you can tell us about the Lagoon 440. For example, how likely is this boat to capsize? What is the bridgedeck clearance?
One of the things that’s important about the website is that comparisons are relative one to the other. These formulas that I use are not fourth decimal place precise, they’re best estimates.
The Lagoon 440 is a fairly heavy boat and it’s fairly lightly powered, the sail area is not great, the stability speed is pretty high. That means the speed at which it would start to lift the upwind hull is 25 knots.
Whereas the Catana 42, which has more sail and far less weight, is at 18 knots. So you can see the difference comparing the base speed (BSpd) and the stability indicators (SSpd and KSI).
Doing these two boats surfaced a couple of problems that I have with data. One of them is that most of the boats are advertised as light displacement. That’s a boat with no people, no gear, no provisions for cruising on it, and the manufacturers are reluctant to give me a true cruising displacement weight. So I have to estimate cruising displacement from either a light boat number, or some of them will list a light and a max, so I go two-thirds of the way between those two numbers, but those numbers are close estimates.
The second missing item is bridge deck clearance and very few designers list that. If I can find a good three-dimensional drawing of the boat, I can estimate what the bridge deck clearance is. But I do without it in a lot of them, and I didn’t have that for either of these two boats.
Catana tends to have quite a bit of bridge deck clearance and Lagoon tends to have relatively small because, like I said, it’s a cruising boat.
So can you tell us a little bit about the difference in what those comparisons are on the bridge deck clearance?
Catana tends to be about 30 inches, which is what we have established as a good standard. We got our standard from information from a number of designers.
What makes that a good standard?
The issue is that waves tend to form between the hulls. When the waves get high enough to where they begin to impact the hull, it can cause a rough ride and it can cause damage if it’s severe enough. Over 30 inches for the kind of boats we’re talking about. We’re talking about 30 feet to 200 that seems to be enough to do the job.
Related to that is the comparison of the spacing between the hulls at the water line, and if that gets too small, then the tendency to make these waves between the two hulls gets larger so they play together.
Another problem that shows up is that I don’t analyze the difference between a boat that has dagger boards, and a boat that has fixed keels on the hulls. I have no capability of doing that fine grain analysis. You’ll find the dagger boards tend to be on the race boats, and the fixed keels tend to be on the cruising boats generally.
When we were talking about the stability speed earlier, it is called Shuttleworth’s formula. The reason I’m bringing this up is because you asked about capsize. Shuttleworth’s formula, which gives the number at which point the windward hull would begin to lift, not capsize. If you look at the stability speed for the Catana 42, Shuttleworth suggests that this windward hull would begin to lift at 18.37 knots. What Shuttleworth incorporated in that formula was a 40% gust factor. What we tell people is, we think a good time to start reefing is that 18.37 is the number you should be reefing at, because Shuttleworth is saying if you’re carrying windward sail and you have a a beam wind of 18.37 that the windward hull would begin to lift.
Shuttleworth's formula = 9.48((0.5*Bcl*Disp)/(SA*Hce))^0.5 Inputs in US units and result is statute miles per hour of wind speed.
So logically, we suggest that you would start reefing, in this case, probably around 17.5, 18 at the latest. But you would start planning on reefing as the wind picks up, assuming you have that kind of bearing of beam. I say that’s important because it’s not a hard fast number because he’s put a safety factor in there, and it’s a number that you should, if you are not really familiar with your boat, start reefing before it gets there.
The Kelsall stability index, which is the KSI ,which we do have for the Catana is 30.89 knots. Both of these formulas have to do with boat tipping over. Shuttleworth says, if the wind is from the starboard side, the starboard hull would begin to lift at 18.37 knots. Kelsall, on the other hand, has a Kelsall stability index, which says that at 30.89 knots, or right at 31, the boat will turn over. So the Kellsall formula is it’s going over at 30 knots, and Shuttleworth’s formula is that the windward hull will begin to lift at 18.
How about the Lagoon 440?
The Lagoon is 25.1 for Shuttleworth. The Lagoon can handle more wind because that windward hull is not gonna, according to Shuttleworth, begin to lift until 25. Where the Catana will start lifting, according to Shuttleworth at 18.37.
That definitely helps a little bit on the stability factor in regards to the Catana 42 and the Lagoon 440. Would you like to talk a little bit about the Bruce numbers on these boat designs?
It’s a sail area divided by displacement, so the more sail area you have for a given displacement, the higher the number will be. The heavier the boat is the lower the number will be. Sail area to displacement is a number that that we put on the site and don’t pay too much attention to the Bruce number. In the earlier years that was considered the number to determine how powerful a multi-hull would be, how fast it would be, that kind of thing. However, it is missing the consideration of the length of the boat. The base speed, covered below, includes the waterline length of the boat and is a better approach to judging speed performance.
Let’s move on to expectations of upwind performance on both the multi-hulls, the Lagoon 440 and the Catana 42. What can you speak on regarding the numbers on those?
The base speed is the number that I used to to judge speed performance. Base speed was the number that a gentleman over in England, who is an analyst of boats, derived from a from a series of races that were done around Great Britain by a lot of different boats, different types not just catamarans, but conventional boats as well.
Remember, we talked about the Bruce number and about sail area divided by displacement. Sail area divided by displacement and Bruce number are kind of like power-to-weight ratio in a car. But boats have another factor that affects their speed a lot, and that’s the waterline length of the hulls.
So the base-speed formula includes waterline length and sail area in the numerator and displacement in the denominator. The exponents on those factors in the equation are what was determined for the base-speed formula. The values for the two boats considered above are not the top speed, but should be viewed as an average over 24-hours of racing.
The Catana’s base speed is 10.8 knots and the Lagoon 440 is 9.79 knots.
When you buy a boat, whether it’s a racer or not, weigh the boat. There’s some great designers who go back that say you even weigh the mattresses. In other words, you take everything off the boat and you weigh the things as you bring it on.
What are the biggest trends that you see in catamaran design and your biggest takeaways from researching the catamaran data?
Weight is the biggest problem that multi-hulls have in regards to being successful. That’s why if you look at the history of how cargo has been shipped, it’s been shipped on monohulls, not because they’re faster, because they can carry more weight, and therefore by design they’re more efficient. Multihulls cannot carry the same amount of weight on the same water line length. So you can’t take a 40-foot monohull and move to a 40-foot catamaran, and expect to do well. You’d have to go to something, and this is off top of my head now, but if I had a 40-foot monohull, and I was trying to get something that could handle the weight that I’m carrying on a 45-foot or a 40-foot monohull, I’m probably going to look in the 50-52-foot range of a multi-hull to be able to carry that weight.
What Cal was talking about earlier in regards to manufacturers not wanting to give out that number, I mean simply, it’s a marketing thing. So we encourage people who are going to purchase a multi-hull to have it weighed during the survey to find out what she actually weighs.
Then on our website is a page called the custom evaluation page, and it allows you to plug all these numbers, all the specifications for any boat, and run our formulas on any boat you want for free. Then you could run those accurate numbers based on your custom weight of their boat.
I think the other thing to understand too, about these formulas, is that when we talk about the stability speed being 18.37 on the Catana, and the stability speed of 25.1 on the Lagoon, that these are relative to each other. That’s what Cal was saying earlier, they may not be the exact numbers, but the relationship to each other is is correct.
We try and get people who use our data, and you were saying, where have we failed. We can’t seem to get people who own the boats to tell us how the formulas fit for them after they take them out. He would like to know, hey did you really have to reef at 18, or did you drift at 22? Did you reef at 12? Does she actually do that in the base speed?
You’re looking for owner stats of how the boats perform compared to your formulas?
Yes but regardless of whether we get those, the relationship between the equations from boat to boat, we believe are real, based on those specifications.
So tell us about the new updated version of Multihull Dynamics.
Currently the display of the site is a half page, so remember that. It’s only half the page of whatever screen you’re using. It’ll be a full page website to begin with, so it’ll be easier to read.
The boats will be depicted more with pictures, as well as names. We hope to have pictures of every boat, and we believe selecting those you want to compare will actually be easier.
The one part of the website that I think is really cool, is Cal’s Evals. That’s where Cal does evaluations of specific boats, whether we pick it or somebody asks us to do one. We’ve done evaluations where we’ve had four or five boats together, where we’ve charted all that, and graphed all that for people who were looking at four or five different boats. I try to pick the boats that are about the same length so that I can get that kind of a comparison. Some of them are 18 or 20 boats in the set, depending on what the database contains.
In the new website, you’ll actually be able to choose perhaps up to 20 boats to compare at a time. I’m not sure if we’ll have that many options. Right now, you can compare two.
If you go to the multi-hull comparison page, it will take you to a choice of where you can choose to compare catamarans and trimarans. Below that is a little-used device that we are going to expand, called the advanced search option. In the advanced search options, once you’ve done the readings, and you’ve kind of begun to understand what you’re looking for in numbers. Because in time, you will be like, I want something that does more than 9 knots, I want something that can handle at least 15 knots, before I have to reef. Once you begin to get these things that you know you want in the advanced search option, you can actually post those specifications you want, and it will bring up the boats that match that.
It’s been there all along but it’s just kind of tucked away, and unless you really dig into it, it doesn’t get to you.
So between that, Cal and I both think the custom page where you can put in the data for any boat you like. For example, all of our boats are calculated on windward sail area, which is the smaller sail area. We do that so that it’s an apples to apples comparison. A lot of boats have large jibs, large Genoas, and so forth. So we just use the four triangle, just not the biggest sail they have. But the fore triangle and the main, and again that that gives us a a fair comparison of one boat against the other. Because anybody can put on more sail. Anybody can add a chute or make their boat faster. We’re trying to get them where they’re equal, so a person may want to know: okay, this is great that my boat is faster than that boat when we’re on windward sail dimensions, but what’s it like when I have my genoa out and I’ve got my spinnaker out? You can put all that on see how it will affect the speed of the boat, how it will affect the stability of the boat, whether it will increase or decrease its stability. Logically, it will decrease it, but my point is you’re not stuck with the numbers we give you. You can modify that and expand your own understanding of the boat you’re interested in.
That sounds very exciting for all those curious about different multi-hull boat designs.
If I could get one thing back, and I think I would feel the same way if we could get the real displacement weights, we could get feedback from those who use the site about how it fits for the boat. That would be a big thing to us.
Did you hear that, all those multi-hull owners out there? Let us know how all these owners can contact you with all their specifications.
Just email us. Just say: I’ve got a Catana 42 and this is what my experience was at these winds, at this direction, et cetera.
So what are your emails, how can people contact you directly?
Right on the website, we have contact button, and up will come a box. You just tell us what it is you want. We ask people if they want boats for us to add, let us know they do that.
The website is MultihullDynamics.com, and you can reach out to Cal and Pat directly through that website. Thank you so much for all the fantastic research that you’ve done.
Cal has done the lion’s share of that. I’ve had the fun part of putting kind of putting the articles together on the site. We have a really good technical page and we have an interesting waypoints page where we have designers and other information like that, that’s really helpful.
|Formula||Units||Catana 42||Lagoon 440|
|Kelsall Stab Index||knots||30.93||42.19|