We raced Wildling in the Outremer Cup regatta last weekend, and while we were careful about our sails and rig loads and didn’t break anything, we did push the boat a bit, and there was one point when we decided we should furl the Code-D because it was getting overpowered, and right when we were furling, the gennaker on the boat next to us literally exploded in a wind gust, creating a vertical tear from head to tack!
It was a scary reminder of the need to pay careful attention to rig loads on a catamaran, and it got me thinking, how much is too much, and how do you know when you’re pushing too hard?
When I learned to sail on monohulls, we were taught the two golden rules for managing an overpowered boat.
Rule #1: When the boat heels too much, depower. Usually first by letting the traveller down and then by reefing. Since Rule #1 varies based on experience and sea state, i.e. 20 degrees of heel feels OK to me, but is too much for Robin, we have golden Rule #2.
Rule #2: It’s time to take a reef when you first think about it. Or in our case, it’s time to take a reef when Robin tells me to stop leaning us over so far, damnit! Gusty conditions on a monohull are not a big deal either, just reef to give a comfortable heel most of the time, and let the boat heel further every now and then to spill the power from the gusts.
So how do we apply these two golden rules to a catamaran? It turns out, not very easily. It becomes less of a feel thing, (at least until you get to know your boat) and more of a numbers game. Catamarans don’t heel, so they can’t spill the power of the wind, all the power has to be absorbed by the rig. In gusty conditions, this becomes dangerous, as rig loads can become unsafe very quickly, so the golden rule for catamaran reefing is to reef for the gusts. But since the gusts are occasional, how do we know when and how much to reef, and what happens if we carry too much sail? We’re not going to capsize in 25 knots of wind, so no big deal right? Since stories of rig failure, broken masts and exploding blocks seem more common on catamarans than monohulls, I think it might be a bigger deal than I realized.
Our catamaran came with a reefing plan from Outremer. The plan tells us at what wind speeds we need to reef, or change headsails to avoid overloading the rig or capsizing. Here’s the reefing plan for Wildling:
We were above this plan at times during the Outremer Cup, and nothing broke, we even flew our Code-D in 20 knots, so we have proven that Wildling can handle more load than this plan dictates, so is the plan just a conservative suggestion from the manufacturer, designed to make sure no boat ever gets damaged, or is it a prime directive to be broken only at great risk? After our racing experience, the answer wasn’t as clear to me, so I did some calculations to try and find out.
Before I go into the calculations, I want to give a plug to the Attainable Adventure Cruising website. John, the author, has published a large amount of excellent information on offshore cruising and I have learned a lot from him. I was reading his article on rigging a “proper” jibe preventer (because after our autopilot went crazy last year and jibed us without warning, I need to rig a system to protect the boom when it happens again). In the article he talks about mainsheet loading at different wind velocities and how to calculate the forces the preventer needs to be able to withstand. I started calculating the loads on our mainsheet using the Harken formula in John’s article, and the results were pretty interesting, so I ended up building a spreadsheet to determine Widling’s mainsheet loads at different wind velocities and different reefing points.
Here are my calculation results:
What’s impressive to see in these results is how much the rig load increases from a gust. For example, with a full main at 20 knots, a gust of just 10 knots higher, more than doubles the rig load! My calculations are probably not that accurate, so the actual numbers are not very useful, but what is interesting is to see the relative changes in load as wind increases, and as we add or remove reefs.
I wondered how much load we were putting on our boat when we exceeded the reefing plan. The hardest we have ever pushed Wildling was during our delivery test sail, when we had full main and jib in 30 knots on a beam reach in flat water. Our boat speed was 19.7 knots! Sailing at these conditions gives a calculated load of about 3,750kg, which is about 10% higher than the maximum load in the reefing plan at 3 reefs and 50 knots. So I’m going to use this as the maximum safe limit for our boat.
If we plot the loads on a chart, and define a safe zone below the Outremer reefing plan, and a danger zone above our maximum load, and then put a racing zone in between, it looks like this:
This chart helps to put everything into context. The Outremer reefing plan might look conservative in steady conditions, but it becomes much more realistic when you take into account the loads from gusts. For example, with no reefs at 25 knots steady, we’re under the danger zone, but we now have only a 5 knot gust margin. That might be OK in a race when you’re watching things very carefully, but for cruising, it’s just not enough. Taking Outremer’s advice and reefing at 20 knots, gives us the margin we need to handle changing conditions.
So what are the conclusions from all this? For me there are two important conclusions:
- BEWARE OF GUSTS! – The rig loads they generate are extreme and they happen very quickly. Reef early when sailing in gusty or building conditions. Reef deeper if sailing around squalls.
- Follow the plan! – This exercise has given me greater respect for the Outremer 5X reefing plan. It is well designed and has a good balance between performance and realistic gust tolerance
We know from experience that we have a strong boat that can go beyond the reefing plan, and that’s great because it gives us a safety margin, but it’s important not to confuse occasional race conditions with long distance offshore cruising conditions, and the need to sail conservatively. The nice thing about a performance cruising catamaran, is that you don’t have to push it in order to sail fast and be safe.