AAHH…if they have no shape they don’t exist… without displacement they (are) like a tumbleweed… without purpose they have no use… with too much they go too slow… These lines are quoted from a post by waikikin from a forum on boatdesign.net and they’re fitting to this discussion. It’s about trimaran design; the hull shapes, the geometry of the beams and the rig in relation to the hulls, and creative solutions for optimising small trimarans for performance, safety and cruising enjoyment. by Tony Grainger
what makes trimarans fast?
There are two kinds of design features that affect your performance and your sailing enjoyment even if you’re not racing. The active ones and the passive. The active or dynamic ones are features like rotating wing masts, lifting foils, and canting rigs. Most of these active features require skills and crew resources to implement successfully and they can be a handicap if not successfully deployed. They also add to the cost, weight and complexity of the boat. The passive ones are the ones that are built into the platform including the overall geometry of the boat, the hull shapes, the beam clearance, and the amount of buoyancy you have forward of the centre of effort of the rig. It costs the same and takes the same amount of time and effort to build hulls that are compromised in shape as it does to build a great set of hulls on a well configured platform. Hull shapes are critical to trimaran performance, probably more so than any other type of sailing boat. And so the features this article will focus on are the platform, the design of the hulls, and the way the main hull and the float interact as wind strength and sailing angles change.
1. Platform Weight distribution and beam placement For some time multihull design has seen the rig and beams moving further aft on the platform, both for cats and tris and it is common now to see the rig at the 50% mark, even for some cruising boats. There are good reasons. Moving the rig and beams aft, (and also moving the centre of buoyancy aft) means you have longer legs that is; more boat sticking out the front of the centre of pressure in the rig. This is a good safety factor in waves and in higher wind strength. You have more buoyancy forward to resist bow burying and pitch poling. Moving the beams aft, especially the forward beam reduces pitching moment because the beams are now located closer to the centre of gravity. This provides a more comfortable motion and reduces the likelihood of the beams slamming into a wave. And there's a bonus; if you're pushing hard you can get the crew weight right aft and outboard to maximise righting moment just as you would on a high performance cat.
2. Main Hull Design. Broad and flat is fast for planing hulls, but not for multis. Fast ships like destroyers are long and narrow, deep rather than wide for minimum resistance at speed. Relatively broad and shallow sections can help to promote planing in multihulls downwind but the primary reason for efficiency in multihulls is their fine hulls, not planing sections. Fat and shallow to get more floor space in the middle of the boat is a handicap you don’t get a rating benefit for. Monohulls already do the accommodation space thing quite well, and some of them are pretty damn quick too. If we’re going to have a trimaran lets have one that sails impressively. Racing multihulls and beach cats have a high slenderness ratio, or length to beam ratio. In high performance multihulls this ratio is typically above 12:1 and can be more like 16:1 or even higher. The
more weight you have to carry the fatter the hull needs to be so weight is integral to this equation. A lightweight high performance trimaran might have a similar slenderness ratio to a high performance cat, but typically a cruiser/ racer trimaran will have slightly fatter main hull. You can go as low as about 9:1 without any significant increase in drag, and in fact, up to a point, the fatter main hull has an advantage in light air because it has less friction drag as discussed below in Drag Transfer Effect. However at beam to length ratios lower than 1/8 induced drag increases quite markedly and is comparable to that of a displacement monohull.
Hull design and pitch damping. (Main hull and Floats) This is relevant to the shape of the main hull and the float, but also to the interaction between the main hull and float. Excessive pitching or hobby horsing is not only uncomfortable, it severely inhibits performance. Pitching disturbs the flow over the hulls creating more drag in the water, and it disturbs the flow over the sails resulting in less drive. Pitching is resisted by asymmetry in the waterplanes relative to the transverse axis. That is; the water planes
The top float has good pitch resistance, the bottom one not so much, and even less if it has a lot of rocker so it’s full length is only immersed when it’s hard pressed.
forward of the centre of buoyancy are more different in shape when viewd from above. Boats that are pointed at each end in plan view, like a double ender, will pitch much more easily than a boat with a wide stern and a fine bow. Additionally the float shape can make a significant contribution to pitch damping by counteracting any pitch tendancy in the main hull, especially if the rocker is not excessive, the float is at least the same length as the main hull, and the there is generous buoyancy down low.
Can you have good performance and good accommodation space as well? Good design in a cruising boat is about finding creative solutions that successfully integrate performance and seakeeping with the accommodation space and the general cruising ammenity of the boat.
The section shapes at left compare the Airplay 30 (in black) with a popular production trimaran (blue lines) of similar size. A fatter main hull does give you a little more sitting room in the saloon but at price in performance. Raising the cabin height allows you to raise the cabin sole and seats for more width and very little penalty in performance. An effective way to provide internal sleeping is simply to flare the aft sections above the waterline and carry the sheer line high right through to the transom, effectively creating a useful aft cabin space. The transom of the main hull is already in a zone of turbulent air flow and any additional windage here is negligible. The Scoundrel 22 (7m LOA) can sleep up to four or five adults in reasonable comfort,
Two of the layout options for the Rascal 22 trailerable trimaran.
Profile and section drawings for the Rascal 22. Broadly flared aft sections create a roomy cockpit with ample seating, or a cosy aft cabin that can sleep two adults without any impediment to performance.
3. Float design. A trimaran that is powered up and sailing on its float is no different to cat except that there’s an extra hull being carried through the air and a higher righting moment. So there is no reason why a trimaran float shouldn’t closely resemble the hull of a high performance catamaran. The main difference is that a cat at rest has to have its’ centre of buoyancy located at or close to the static centre of gravity of the whole boat. The trimaran can have the centre of buoyancy of the float further forward to allow for the ideal trim in the dynamic sailing condition. This works in the trimaran’s favour and in combination with
overall beam allows the trimaran to be harder pressed in fresh conditions. Float rocker is the most important factor after the overall buoyancy. It determines your hull length when you’re sailing on the float. Flatter rocker with ample buoyancy low down helps to dampen pitching. It also maximises float length for lower drag and a higher displacement to length ratio. A V’d float shape will give a softer ride, but it will heel more than a semicircular bottom shape and probably have less displacement unless the freeboard or beam to length ratio is increased, so float shape is
Five good reasons to have high buoyancy floats Tim Pepperill's Bare Essentials sailing in fresh breeze with the main hull just skimming the surface and the lee float probably not even half way submerged. The beams are well clear of the waves to leeward.
a delicate balance of quite a number of input factors. Float buoyancy and length are critical in 5 ways (i) Firstly, a high buoyancy float acts as a pivot point that enables the main hull to lift out early, significantly reducing drag and providing an immediate speed boost. A trimaran with the main hull just skimming the surface is a very efficient sailing machine and competitive with a high performance cat on a reach. In fresher breeze you are sailing with a higher slenderness ratio precisely when it is most important.
(ii) Secondly the high buoyancy float with the correct amount of rocker enables the boat to sail at less angle of heel, even with the main hull skimming. This reduces the down force and increases forward drive. It also reduces the loads on the boat. When hit by a gust a trimaran with high buoyancy floats will lift the main hull, increase speed and lift higher to weather when sailing to windward. A trimaran with low buoyancy floats leans over, the beams are more vulnerable to the waves, drag increases and the boat is more likely to slow down than pick up speed. (iii) Thirdly with a full length float in the water you have a higher prismatic and significantly higher pitch damping - so the boat sails on a straighter trajectory, not pitching up and down so much as it encounters waves. (iv) Fourthly, with the boat sailing on the float, the beams are higher out of the water and so there is less drag from them encountering waves and less chance of a beam digging into a wave, possibly inducing a pitch pole. (v) Lastly, the longer the hull that is in the water, the higher the displacement to length ratio. The displacement to length ratio is the all important
measure of the speed potential for any boat, not just trimarans. A longer hull for a given weight is faster especially in fresh air where sea conditions and form resistance are determining factors. The longer the hull for a given boat weight, the faster you go and the kinder the ride.
The top float in this comparison sweeps up a lot toward the transom. This is a good safety feature if not overdone, but in this case the float has to immerse quite a lot before it begins to make good use of its’ length, and this creates excessive heeling. The lower float is from the Airplay 30 design.
Float shape and Capsize Resistance Keeping a trimaran upright is a question of balancing the overturning moment (height of the pressure in the rig x the amount of pressure) against the righting moment (buoyancy acting on the float x the effective lateral distance of the that buoyancy from the centre of pressure in the rig).
A Question of Balance
The principle is simple enough but there are some dynamics that come into play that affect your stability and float shape is critical to these dynamics. The heeling of the vessel and the consequent susceptibility to capsize is directly related to float buoyancy. Any reduction in float buoyancy has a cumulative effect on susceptibility to capsize, both from lateral forces due to the pressure in the sails and capsize from dynamic forces such as encountered when sailing at high speed with significant waves. As the float depresses in the water the heeling moment is increased through increased downforce on the sails. At the same time the righting moment is reduced through the effective reduction in beam and the centre of pressure in the rig moving outboard toward the float as the boat heels.
In addition the heeling moment is further increased through the increased profile of hulls, beams and wing nets becoming more exposed to the wind pressure. In a paper written by Jean Sans in 2006 in French (and translated to English by Simon Forbes) Sans calculates the heeling and righting moments for a 36.8m catamaran (using Orange 11 as an example) and a trimaran using an ORMA 60 as the example. In relation to s sideways capsize in open sea conditions Sans demonstrates by calculating the differences in the heeling moment (wind pressure x height) and the moment resisting capsize (buoyancy x beam) that when a trimaran main hull is lifted onto the crest of a wave, the trimaran can be significantly more susceptible to capsize than a catamaran. The paper by Jean Sans is freely available in pdf format at this address: http:// sans.jean.free.fr/ STABILITY_of_MULTIHULLS.pdf John Shuttleworth in his paper “Multihull design considerations for Seaworthiness” states under the topic Stability Curve and Stability in Waves
If the buoyancy of the ama is reduced below 100 % of the weight of the boat, the maximum stability will be reduced not only in proportion to the reduction in buoyancy in the ama, but also by the effect of added apparent displacement from the downward pressure from the sails at high angles of heel. At 20 degrees this would cause a loss of righting moment in the order of 20%. If the ama buoyancy was only 80% in the first place, the total righting moment would be only 60% of an equivalent trimaran of type 3. And later under the same heading: The multihull that fares worst in this situation (relative to a monohull encountering waves on the beam) is the trimaran with low buoyancy amas. When a wave hits the side of the boat, firstly it will roll quicker and much more than a cat, and if the ama immerses to the point where it digs in, thereby stopping sideways movement, all the energy will be transferred into rolling and a capsize is possible. The full article can be found here: http:// www.shuttleworthdesign.com/considerationsfor-seaworthiness.html
One of the illustrations from Jean Sans’ article on trimaran stability
4. Drag transfer effect. Sailing trimarans have some unique characteristics that set them apart from other sailing boats. I call it the drag transfer effect. A sphere encpsulates a given volume with the minimum possible surface area. The more you deviate from the sperical (or demi sphere) form by going deeper, wider or longer for a given displacement, the more wetted area you have. Hulls with a cross section shape that is semicircular in cross section have less drag in light air than sections that are deep and skinny, long and skinny or wide and shallow. Friction drag (which is related to surface area) is the major drag factor in light air. Hull shapes that are smaller in cross section area relative to their length (like a trimaran float shape or a fine catamaran hull) have more wetted area for a given displacement. So they have more drag in the light, but are much better suited to fresh air performance where the displacement to length ratio is the critical determining factor in performance. So what if we could design a boat that had relatively full rounded sections for optimum performance in light air, but could change to a long skinny hull for optimum speed in fast reaching conditions? Well, that’s exactly what we have with the modern high performance trimaran with high buoyancy floats. We see this demonstrated on the race course with trimarans competing against cats of a similar rating.
The graph below shows how the relationship of viscous drag (skin friction) and wave drag (sometimes referred to as form drag or induced drag) varies at different speeds. At low speeds almost all of the drag is viscous. At 7 knots of boat speed the induced drag has become predominant. This is about the speed where a trimaran’s main hull is starting to lift, provided it has good float buoyancy.
At rest a Trimaran floats only on the main hull and so has significantly less surface area than a catamaran of equal length, a distinct advantage in light air where skin resistance (viscous drag) is the primary source of drag. In medium strength breeze a light weight catamaran (or one of similar weight to a trimaran of the same length) will lift the windward hull earlier than a trimaran. So the cat has a distinct advantage in the
medium wind range, especially high speed reaching on flat water. At the upper end of the wind scale the cat reaches a limit of her stability and has to reef. The trimaran flies the main hull and windward float and with more beam and weight can carry full sail longer. The net result is that a trimaran's float needs to somewhat resemble a catamaran hull to optimise the boat for
fresh conditions. Longer is better, reasonably flat rocker will minimise pitching, ample buoyancy forward for bow high trim, and a some fullness in the run aft to provide some lift at speed and even better pitch damping. There is no reason to have the float shorter than the main hull because you get no rating benefit, your performance will suffer, and safety may be compromised as well.
Airplay 30 demonstrates the powerful float shapes that helped propel her to second place (and top trimaran) in the 2015 Australian Multihull championships
Good design is a free lunch for your rating…and a more pleasant ride There are factors we haven’t discussed here that have the potential to further improve performance without incurring a rating penalty, namely the active or dynamic elements such as canting rigs and C foils or other lifting foils. At this time these features are not rated so boats that have them should be winning under OMR - but the boats that have been collecting the trophies, at least on the Australian racing circuit, have been winning without the use of foils or canting rigs. There may come a time when these features come to be easier to deploy and more effective, and so there will be a demand for them to be factored into the rating rules just as rotating masts currently incur a penalty under OMR. Whether these dynamic elements are rated or not - the fact remains that good design of the hulls and platform configuration will never be rated. Whether you’re racing or cruising good design is good for performance, it’s good for safety and your sailing pleasure, and if you are racing it’s a free lunch for your rating.