MULTIHULL WEIGHT AND LENGTH COMPARED

We'd all like our next set of wheels to be a cool sports car with room to pick up the kids from school, do a bit of shopping, and maybe make the occasional trip to the hardware store. But we understand we're going to have to compromise somewhere. Same with boats. First across the line will probably not be the most comfortable cruising boat unless it's big and/or expensive. 

But how do you decide how fast you can go without sacrificing too much room, or without having to go bigger and more expensive? The purpose of the information in these plots of boat weight against length is to give an indication of what's achievable in the real world for fairly conventional construction methods and materials, and a relatively modest budget.

A lot of the boats on the plot are well known world wide including the Seacart 30, Corsair 36, the Multi 50 Arkema, G32 Foiling cat, the Gunboat G4, an ORMA 60 and a typical Gunboat 60 (weight estimated) which is off the page in the top right hand corner of the first plot.

This is the big picture graph that plots the weight against length trend line for catamarans and trimarans up to 60' LOA and also includes a MOD70 which is off the scale to the right. The blue line is the trend line for catamarans which are represented by blue squares. The grey line is the trimarans. Some images to follow will zoom in on various parts of the graph so you can read boat names.

It's interesting that the plot lines cross over at about the 40' mark with cats becoming relatively heavier as size increases. We shouldn't try to read too much into this in terms of the weight of the bare structure. It's a natural consequence of cats being preferred for long term cruising in the larger size range while trimarans are the weapon of choice for ocean racing. They employ exotic materials, refined engineering and meticulous manufacture to achieve low weight. 

The plot above focuses in on the 35' to 45' size range. The three trimarans Arkema, Carbon3 and 3 Itch are all sitting well below the trend line. Three Itch is the only one of these three for which which we have an OMR weight. Jessica Rabbit is sitting above the line but we used a weight figure provided by Martin Fischer which is an estimate of sailing weight including crew and equipment. Jessica Rabbit would sit closer to the black line under an OMR measurement.

Arkema's number is the quoted "displacement". If that is the actual weight it's a very light boat for its size but if we were to draw a trend line for Gunboat G4, Arkema, the ORMA 60 and MOD 70 all of these four boats would be close to that line. They are also all quite high tech construction.

The six Stealth catamarans (shown by the red squares) which are built by Asia Catamarans in Thailand provide an excellent benchmark for a high performance lightweight cruising catamaran that doesn't use exotic materials or construction methods. Most of the these boats have carbon rigs and synthetic rigging and they employ carbon in high load areas but are otherwise relatively conventional in construction using foam core with epoxy/glass skins. The three Corsair 36 trimarans are firmly above the line even though two of them were built in carbon.

This plot in the 22' to 34' size range is mostly made up of trailerables. Trimarans on the black trend line and grey triangles, cats in blue squares. We didn't have enough cats under 30' in the data base to reliably carry the catamaran trend line into this plot.

Leaving aside the fitout and inventory that goes on the boat (we discuss that in another article) there are three critical elements to a light boat; engineering, materials and build methodology.

Engineering is critical for understanding the loads on the structure, having a good knowledge of the materials properties and applying the materials accordingly. Connecting loads with material properties means that a close working relationship between the builder, the designer the materials people and the composites engineers is absolutely fundamental. Where this relationship doesn't exist the materials specification has to be more conservative. That's one good reason why light boats take more time, more expertise and more effort, and consequently are more expensive.

Material selection is critical for making sure things are as strong and as stiff as they need to be. Carbon won't make for lighter hulls but it will reduce the weight and provide added stiffness in load critical structures like beams, rigs and foils. PVC foam is the ideal core unless you go to honeycomb cores - but that puts you in a whole new price bracket and ups the build skills requirements.

Build Methodology involves good laminating practice; getting the right fibre ratios and resin cure times and achieving good secondary bonding. If the engineer or designer is not sure of the level of skill on the shop floor then once again the specification needs to be conservative.

All of the boats that we see below the line on these plots have been built with due attention to these elements and the further they are below the line the more they will have focused on these elements in the respective design and build stages.