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How to assess your capsize risk and bring it down to near zero
Ques: When should I reef a trimaran (like the W17?) and what other precautions might I take to avoid a capsize ? RB, Antalya, Turkey ... June 2022
Ans: Well, the ‘dumb’ answer is, ‘reef before you capsize!”, but to figure out when that is likely, a little figuring can surely help;)
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There is really a long list of possible answers that depend on a long list of cases, but we have to start somewhere. The sea and wind can throw you such an infinite variety of possibilities but let’s look at 3 the most common scenarios as if you can control these, you will cut your risk of capsize by about 90%.!
#1: Typically, a near-stationary boat with sails up will be easiest to capsize sideways, by just being blasted with excess side wind. This is not uncommon and can occur just after tacking when the boat is moving too slow for the rudder to be effective ... giving you almost no ability to luff up. To see what your particular boat can potentially resist, it’s worth comparing the capsizing moment (force x lever arm), to the static righting moment (weight x lever arm to the buoyancy center) when at maximum effectiveness.
First, the capsizing arm. In this case it will closely relate to the wind force on the sail area you have up, multiplied by the height from the center of lateral resistance* to the center of pressure on the sails you have set.
*While there are arguments to using the vertical center of buoyancy instead of the center of lateral resistance, I prefer using the latter as it factors in the effect of a drop keel into capsizing .. increasing as it does, the capsize lever when its fully down. Of course, if the drop keel is weighted, then its otherwise negative effect may be compensated by a small increase of the righting moment but not unless the boat takes on significant heel.
For the righting moment, the more weight the better but its location is critical. (A lighter boat will typically capsize more easily). On a heeled multihull, the center of buoyancy will quickly move to the leeward hull or ama and for that reason, these boats are VERY stable - up until the buoyancy stops moving out, which is when the windward hull lifts above the surface. This maximum stability point can be as low as 10 degrees heel on a wide catamaran but will be more like 30-35 degrees on a trimaran, as it will take that angle to lift the center main hull out.
What happens after that will depend on the amount of buoyancy in the ama. If it’s less than the total boat weight (with crew), the ama will be pushed under and the tri will rotate with its main hull just lightly in the water, only displacing the difference between the total weight and the buoyancy offered by the ama. So if the total weight were say 800lbs and the ama buoyancy was 700lbs, the center hull would float high on its bilge, needing to donate only 100lbs of buoyancy (about 1.5 cuft of intact volume) for a state of ‘pre-capsize equilibrium’.
Whether it would stay there would depend on 1) how high the buoyancy is in the ama and 2) how low the weight is in the main hull. If the buoyancy was low in the ama and the weight high, the trimaran would turn upside down, unless this capsizing couple (leverage) could be offset with mast-top buoyancy. In practice, that’s often proven quite hard to achieve as the load on that buoyancy pod can be higher than its structural strength & attachment, or even that of the mast, though something auto-inflatable can in some cases be made effective.
But let’s look at how we might avoid this in the first place, and a look at sail area and wind loads would be a start.
To show an example, I have taken the wind pressure at different speeds and applied that to different W17 sail area combinations to see when the normally available righting moment is overpowered. I say ‘normally available’ as in this little study, I have not considered crew on a trapeze or even on the windward ama, so for short periods, one could survive somewhat higher gusts IF one is prepared to move crew members outboard, to (or even beyond) the windward ama. This would normally be done only in sport or race mode, but it’s worth noting.
Obviously, any crew sitting to leeward, would have the reverse effect and as for any small boat, the crew should be ready to ‘quickly move to the high side’.
So here is a chart to think about. While this one is specifically for the W17, it would be wise to create one for your own trimaran, as many of the commercial models are not as stable as this one.
So one should plan on reducing sail BEFORE the wind gets up to the level indicated, as this shows the approximate static capsize point with a given side wind.
It’s important to realize that once a boat is heeled beyond its point of maximum stability, things can change fast. While the boat will still want to right itself, the 'rate' of this applied stability will get less and less with each degree of heel and eventually disappear to zero at some very high angle. This point of no-return will be particularly hard to judge in waves as the boat will change its heel angle relative to the horizontal as wave-forces of their own act on the immersed hulls of the relatively lightweight trimaran … especially if there are surface white crests that represent ‘moving’ water. (See this note on Max Stability of Multihulls)
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Maximum Stability for Multihulls For a catamaran, this will be when the windward just lifts off the water. At barely 10o on a very wide cat but up to perhaps 15o on a narrow one. Trimarans may range from having their maximum at 25o for a wide tri with large volume amas supporting over 100% total weight, to even 40o for a tri with less ama volume & less beam. A cat will typically have significantly more initial stability up to say 20o heel but then have significantly less beyond that compared to most other boat types, often becoming zero at around 70o of heel, compared to 80o for a trimaran, or even 90o on a tri whose amas are set high relative to the main hull. Typically, between 35o and 65o heel, a trimaran will have about 40% more righting moment (RM) than a catamaran, though this RM does drop with increasing heel at about the same rate for both types. |
For this Scenario #1, having the ability to quickly relieve wind pressure on sails will generally annul the situation. There are a couple of approaches to do this.
If you are near stationary, releasing the sheets, or at least the mainsheet will be essential. While this ‘sounds’ straight forward, it may not be. A very strong gust can put so much load on the mainsheet of a stable trimaran that the mainsheet practically fuses to the cleat teeth, so it’s essential that the cleat be set up correctly. Many boats I’ve sailed were set up to permit the mainsheet to be easily cleated but this is wrong .... dead wrong. This cleat must be set up to be easy to get OUT of, and that typically means it will be hard to get IN. On my current boat, the camcleat is on the lower triple block and it can be swiveled up and down. I’ve set mine in the highest position, so I actually have to stand up and even stretch up to engage the cleat. But as I am always engaging this cleat in moderate conditions, it’s easy to do. Then, should I have a gust with the potential to capsize me, I only have to pull the mainsheet straight towards me to pull the rope downwards out of the cleat. So with a quick ‘tug & release’, the mainsheet runs out every time. With the maximum load* on my boat never exceeding 65lbs and generally MUCH less, that’s always doable. But if your max line load goes much over 75lbs, you will need to add more blocks to your mainsheet and also consider another cleat type (like the Spinlock rocker that releases easier). But on a larger boat say over 7m (22ft), there’s another important issue that often factors into a capsize and that is the skippers loss of balance in a highly heeled cockpit and the resulting difficulty to apply the release force quickly. As this leads to a relatively high proportions of the capsizes, I’d certainly consider an EMR (Emergency Mainsheet Release), so I will add more on this later (see NOTE at the end of this article).
*If you want to roughly calculate what your mainsheet load might be, here is a simple formula that works acceptably for me. Let’s define ‘Mainsail Area’ (sqft) as MA and the No. of Parts on your mainsheet as MP. Then your Max Mainsheet Load (MML) will approximate: (MA/MP) x 2. (There IS a little science to this as the 2 represents 2lb/sqft wind load ….the normal maximum point at which you would reef and reduce area). For my W17 this is (167/5) x 2 … or 67 lbs. I have not seen this level as yet, but I am aware it’s possible.
On the 25ft Magic Hempel my initial MML was 300/6 x 2 = 100 lbs and once that proved impossible to uncleat as the wind gust was enough to totally lock-up the camcleat. With an ama driven nearly 2ft underwater, it was the closest I came to capsizing her (2003) and had me changing both the no. of tackle parts AND the cleat after that. (It also triggered my first thoughts to develop an EMR).
#2 is related to the above, as it’s also a conventional capsize risk to leeward. But this time, you are moving along, dealing with high gusty winds .. caught in a situation of two much wind for the sail you have up. Ideally this should never happen of course, as by now you know you ‘must always reef in time’. (see Hoving-To [point #4 below] for that option)..
But you will occasionally run into situations when perhaps it’s either too dangerous to stop (due to a shoreline or whatever) to take a reef .. or you see from a passing black cloud that these conditions will only last a short time. You are clearly over-canvassed but need to safely survive for say 10-15 mins.
In such a case you can generally find ‘a sweet spot’ in the direction of a close reach. First, ease out the traveller to the limit and then concentrate on sailing the boat at a constant but moderate speed while retainng as constant a heel angle as you can. With the wind gusting perhaps to 50% more than the average and the direction changing 20-30 degrees, you will have your hands full, but put your mind to it as it’s not for a long period. The way you will control both heel and speed, is with the tiller. You will sail a very wiggly course as you will turn upwind when speed and heel increase, but downwind when you are headed and slowing down. About 5-6kts is a good target speed, offering excellent rudder control without excessive high speed from bearing off that can put you into the next situation of Case #3. Obviously, if the conditions have really changed for the longer term, then reefing or better still, substituting a modern storm mainsail, will be far safer and more relaxing. This is particularly effective with a rotating wingmast. More on this here.
#3 The other equally common way of capsizing a multihull is what we might term, the ‘diagonal pitchpole’. This is when one ama bow is pushed under by a complex combination of 5 variables, namely wind, heel, pitch, speed and waves. As all these things generally factor into a pitchpole, we need to focus on the ones we can control. A diagonal pitchpole typically starts when bearing away, heading on a broad-reach or downwind.
First, we can do little ourselves about either wind and waves, so we are left with heel, pitch and speed to control, so while we are comfortable ashore, let’s analyze each.
Heel can be reduced in 4 ways.
1) Adding weight to the high side. Although effective for small boats this is not always easy as you may need to stay in the cockpit. Also, while weight might be required outboard for a short burst, the need may not be there all the time, so it will depend on how athletic the crew can be. So this is not a very reliable way, especially for those no longer very athletic.
2) Head upwind more to ease pressure on the sails. This will lead to the Solution #2 above, but may not be an available option if the shore is to windward. Even when it’s possible, this must not be executed too quickly or the centrifugal forces can momentarily increase the heel.
3) Head downwind more to have the pressure pushing more downwind, when boat speed will reduce the driving wind. This can work if the waves are low enough to ride over. But can be dangerous if the bows dig into a large wave, preventing the bow from lifting. If the bows are deeply digging in, cutting speed is essential and that means either sailing as in condition #2 above, or reducing your mainsail area significantly.
4) Reduce the sail area by reefing or changing sails. If the conditions look like they are there to stay for a while or even worsen, then this is the wisest option but you will need to first hove-to. Here is how, and it’s worth practicing when conditions are moderate.
Bring the boat into a closehauled position and then tack towards a safe direction (considering any obstacles) but leave the jib sheeted as for the previous tack. Once you have tacked, slacken the mainsheet completely, push the tiller down and partly slacken the weather jib sheet until the forward half of the jib is laying on the centerline of the boat. This is the ‘hove-to’ position and with a small adjustment to the rudder and jib sheet, the boat will lay there about 50-55 degrees to the wind and only advance at about 1-2 kts. See here for a short 'how-to' VIDEO
The mainsail will be slack and closely inline with a rotating wingmast, allowing you to readily lower the sail to take in a reef (another advantage of a rotating mast). If you lower the sail completely to change to a stormsail, the boat will likely turn more downwind so you will need to ease the weather jib sheet, and lightly pull in the leeward one. You now have the safe option of sailing downwind with only the jib, or raising a stormsail to sail back upwind. Control is now back to you and you are now at very low danger of a capsize.
Forward PITCH & TRIM can be reduced in a few ways,
1) Moving weight aft
2) Using a foresail rather than a mainsail while going downwind, as the vector off the sail will lift more upwards. Spinnakers do this also.
3) Pitching can be reduced with design changes and changing weight location.
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After sail area, the next most important way to reduce a Pitchpole is SPEED Speed can be reduced by either easing off the sail, changing the attack angle to the wind or by reducing sail area. Of course, in extreme conditions, one can further cut speed by towing something to add resistance over the stern, to act as a drogue. In this case, dragging this from a bridle is much preferable as this can then be trimmed to pull from either side or from the center, to give the best directional effect. As far as drogue diameter size, something about L^2/230 will typically start to be effective. (ft. units) … or L^2/70 in meters.
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When sailing in a strong wind with perhaps more sail than is really needed, the best prevention against a capsize is to keep ones eye glued on the bow of the leeward ama. (If a sail often blocks your view, mark where a window is needed and get that added at the first opportunity). At the ama bow, keep a MINIMUM of 1% of your boat length** as average minimum freeboard. (Suggest to use even 2% for safe cruising).
**This is based on having the ideal ama length of at least 90% of the boat length. So if its under 90%, take 1% of the mean of the boat length and ama length. ie: 1% of (Boat L + Ama L) / 2.
This means that while a few short wave tops can pass above that line, the solid average waterline at the bow must not. If it starts to, immediately ease the sails a little or adjust your angle of attack to the wind if navigation space permits. Having a clear view of the leeward bow is ESSENTIAL for this safety and for larger boats sailing at night time, it’s definitely an added safety feature to have a flood or spot light mounted on the cabin or hatch top that can be rotated in different locations. (This can be achieved with a simple rotating mount (plywood is fine) with a sprung lever that holds a peg in each location). These suggested positions are Port quarter, Port beam, Port bow, Mainhull bow, Starb’d Bow, Starb’d Beam, Starb’d Quarter.
In such a case, adding 3 reflective stripes on the inside of each ama bow to show up in the dark would be great, with one at the deck, another at the 1% of L point and the lowest down at 2% L. (The middle one could be a colored reflector to contrast with the others). One of several reasons I am not a fan of reverse, wave-piercing bows is that we lose this critical 
reference point to judge how much reserve bow buoyancy we have and once a fast boat pushes that reversed bow under water, a capsize is now FAR more likely. Just look at this 53 footer flying along with the critical leeward ama mostly hidden by spray. It should be no surprise to anyone that she capsized soon after, even though waves were not a factor.
Seeing the ama just above the water level is assurance that we have a small, but ‘critical to survival’ buoyancy reserve .. and is akin to seeing a decent shoulder on a road when we drive fast. Without such a definition point, it’s far too easy to exceed the safety point. In fact if I were sailing a boat with reverse bows, I would definitely want some indicator installed to represent 1/100th L below the deck line, or I would make my guidance safety line much lower down for common-sense security.
Note: re earlier pre 1990 designs. One case where a lower bow line will be needed is when considering an early designed ama with high sheer .. such as we see for the Hobie 14/16 beachcats but also for those impressive ‘big birds’ designed by Dick Newick. Virtually all designs since 2000 have more moderate or near straight sheers for their amas, and my above recommended indicators apply fine for these. For the Hobie cats, the high sheer is still sometimes of value, as it’s often needed for boats being launched off beaches with high surf conditions. But for boats without this need, high sheer is no longer recommended. Although the high bows can be of added comfort in short steep waves, it’s now generally accepted that this buoyancy only becomes fully effective when the boat pitches a lot, so effectively allowing it to do so, plus the high bow freeboard adds much windage when less is needed ... not to mention the shorter waterline length such amas offer. So while there was a certain elegance to the banana-shaped ama hulls, they are no longer considered the most efficient for either performance or resistance to pitching. It’s worth noting that not all designers of the 1980’s followed Newick and one major exception was the Australian Lock Crowther whose aeronautical design background had him well ahead of his time with light weight, straighter lines and greater beam, such that his designs still perform remarkably well today. (It’s no coincidence that my first trimaran was indeed a Crowther design .., they just made sense to me at the time ... ie 1980's)
So to correct for the high sheer of banana hulls, I would suggest to first mark down the bow to remove at least 1/2 the actual sheer and then mark the L/100 down from there. Otherwise, you will be assuming there is acting buoyancy ahead of the forward aka (beam) that there is not.
I think it’s time that more multihull sailors realized that despite their great initial stability, their often large sail area is there for light wind conditions (to offset their generally higher wetted surface that particularly adds drag at low speeds) and to therefore be more ready and accepting to reduce sail down to more sane proportions as the wind increases. Unless you are sailing a heavy roomaran, the slim hulls of multihulls do not need a large sail area in strong winds to drive them and it’s just being foolhardy and irresponsible to drive such a boat beyond its limit every chance you can. Take this 60ft cat being sailed by a seasoned circumnavigator (Henk de Velde). The boat is triple reefed yet still clearly sailing fast and looking very comfortable in her wild environment. We should learn from such wisdom. The low center of effort of this reduced rig is great for running downwind like this, but a high-aspect ratio storm mainsail behind a rotating wingmast, will show surprising benefit going upwind.
So here is a short summary wrap-up.
* Consider carrying a storm mainsail and use it in winds above 20kts.
* Watch the leeward ama bow and use 1/100th of the boat length as a minimum freeboard at the bow and add a window in any sail that is blocking your view of that leeward ama bow
* Add a reflective tape there at 3 levels, (about 1/70th of Length “long”). For a pure cruising boat, I would even recommend using 1/50th of the boat length as a guide line for minimum ama bow freeboard to maintain more reserve there.
* Set you mainsheet cleat to release with a simple straight pull
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NOTE re the use of an EMR …. an Emergency Mainsheet Release. Although these have been experimented with and used on large expensive boats, their price and complexity would not generally bring them into the conversation for more modest craft that most of us might sail. But the sheer numbers of multihulls now in circulation changes things and there is little doubt that one of the aspects most talked about in the monohull vs. multihull debate, is the risk of capsize. So if we can find a way to reduce this with a solution that is both affordable and acceptably reliable, it’s certainly well worth considering. Having learnt about hard-to-avoid capsizes firsthand from both clients and friends, I joined forces with another sailor and engineer in 2021 to create SeaSafetyTech LLC to research a possible solution, so here is a link to an Introduction to the W-B EMR. (PDF). If this potential anti-capsize solution interests you, until the product is officially launched (likely in 2024). please contact me via the Questionnaire with your questions and potential needs. (2023 update: a production prototype is now installed on a 27ft racing trimaran in Europe and undergoing on-water tests).
Copyright : Mike Waters: June 2022
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