Three Ways to Break the Boom on your Amel Super Maramu.

Here are three good ways to damage, bend or break the boom on your Amel. In the very unlikely event that this is your objective, it will be even better if you  do more than one!

  • Option #1:  Move the mainsheet attachment forward on the boom.
  • Option #2: Sail downwind with the traveler centered on its track.
  • Option #3: Replace the fixed length vang with an adjustable block and tackle.

I am going to discuss these in order, and then go deeper down into the weeds with the calculations and details about the whys and wherefores.

Do Not Move the Mainsheet.

On the Super Maramu people sometimes get frustrated with not being able to open the window in front of the helmsman while sailing because it interferes with the mainsheet. Or they modify the dodger and its new configuration interferes with the factory arrangement of the mainsheet. To “solve” these problems, the mainsheet is moved forward along the boom to the point that was designed to attach the downwind preventer. This increases the loads and bending moment on the boom, and moves them to  a place not designed to handle them.  Do not do this. Find another solution.

Use the Traveler.

When sailing downwind with the mainsail always move the traveler to the limit of its track on the leeward side. This limits the amount of mainsheet that is out, and therefore limits the swing and speed of the boom in the case of an accidental jibe. The faster the boom moves during an accidental jibe, the more damage can be done when the mainsheet comes tight to stop it. Keeping the mainsheet as short as possible limits this damage.

You can make this mistake even worse if you ease the boom out so far it is touching–or is very close to touching–the shrouds.  Now, in the event of an accidental jibe the boom accelerates all the way across the boat, reaching much higher speeds, and it can hit the shrouds on the new leeward side as all the lines and fittings stretch to absorb the huge forces needed to stop the boom and sail.  In this case, breaking the boom might actually be the BEST case.  The worst case is the impact of the boom causes the aft lower shroud to fail, resulting in the mast  buckling and going over the side. If you think this is not possible, I have seen it happen on another type of sailboat.

On Harmonie if we are sailing so far off the wind in wind strengths that an accidental jibe is a worry, we almost never use the mainsail.  If we are not using the twin headsail rig, we go with just a single headsail.  We lose a little bit of speed, but it’s a whole lot more relaxing.

Do Not Change the Vang.

Do not replace the fixed length line that acts as a vang on the mainsail with an adjustable vang you use to hold the boom down when sailing off the wind.  This is usually done by former racers who are used to using this type of vang and  don’t appreciate the genius of the Amel system, and the forces involved.  

The fixed length vang that Amel installed does two things.  In the event of an accidental jibe, it prevents the boom from rising high enough to impact the mainmast backstay.  This would obviously be bad for both the boom and the backstay.  Failure of either would ruin your day.  The fixed length vang is also a critical part of the bulletproof Amel furling system.  With no input at all on the part of the crew, it holds the boom at the exact height needed to properly furl and unfurl the sail.  You can just release the mainsheet, and the rest is “automatic.”  In neither of these cases is this line and its end fittings subjected to significant loads.

If you DO use a multiple purchase adjustable vang here, you are potentially putting HUGE loads on the boom and mast that the designers did not, and could not, anticipate.  In addition to pulling the boom DOWN against the forces of the sail (which that attachment point was NOT designed to do!), the force geometry jams the boom HARD forward into the gooseneck fitting at the mast.  This loading was totally unexpected by the designers of this system. Adding an aftermarket vang to any boom is almost always a mistake.  The loads are very large, and even booms designed with those loads in mind break at the vang attachment.

The thing is, this is totally unnecessary for efficient sailing of the boat.  The Amel is blessed with a REALLY long traveler that gives you really exceptional control of mainsail shape over a huge range of wind angles from close hauled, all the way past a beam reach, and down to a broad reach.. As long as the mainsheet attachment point is over the traveler, the mainsheet does all the vanging the sail needs.  If you are sailing SO far off the wind that the boom is further out than that, use the Amel supplied vang that attaches to the rail. It loads the boom in ways that the engineers who built it expected, and accounted for.  

This is NOT a J-24 or a J-105.  Do not try to treat it like one!!!

The Details and the Math

These are the factory installed locations for the boom rigging on Super Maramu #160. There might have been small changes with earlier or later hull numbers, but nothing significant. The position of the outhaul car changes with sail trim of course, but this is a pretty typical location.

Let’s do some simple math to calculate how the loads change as we move locations. I’ll make some simplifying assumptions, but I think they do not change the conclusions at all.

The basic equation we need here is the force exerted by the mainsheet downward on the boom to counter the force of the sail pulling up.

F_{sheet}=\frac{F_{sail} \times L_{outhaul}}{L_{sheet}}\\

If we assume the sail pulls upward with a force of 100 kg, then the correct position of the mainsheet gives us the following:

F_{sheet}=\frac{100 \times 324}{280}=115.7kg\\

Now, let’s move the block forward to where the preventer should be…

F_{sheet}=\frac{100 \times 324}{234}=138.5kg\\

An increase in loading on the boom of about 20%. Not a huge number, but why would you want to do that?

Now let’s attach an adjustable vang and use it to pull down the boom.

F_{sheet}=\frac{100 \times 324}{106}=305.7kg\\

Does that get your attention? Loads almost 3 times higher than the mainsheet was designed for, and in a part of the boom not expected by Amel to carry any rigging loads at all!

And there is more!

If that’s not good enough, there is more cleverness that is easily missed. If you look at the attachment point of the sheet to the boom when the boom is centered, it angles aft. Why? Amel rarely did things without good reason, and the reason for this is genius.

The traveler follows the curve of the deck—downward to each side. If the sheet attached to the boom directly over the traveler when the boom was centered, as you moved the traveler to the side, the sheet would come tighter and tighter as you eased the traveler to leeward, requiring you to ease the sheet, or risk breaking something.

With the sheet attached further aft on the boom, as the traveler is eased, the attachment comes forward, keeping the distance from the boom the the traveler constant. A very clever use of geometry!

But! But! But!

I can hear the questions now…

“Amel’s design uses the preventer to vang down the mainsail at the forward attachment point. Why is that OK?”

The preventer attached to the rail is designed to be used when the boat is far off the wind. Under these sailing conditions there is less upward force from the sail than when you are sheeted in on a close hauled course.

“I have been doing this for years, and it’s OK.”

Yep. It it will continue to be OK–right up until it isn’t.

“Surely Amel built a bit of safety factor into the design.”

Of course they did. Loads on sailboats are notoriously hard to predict. You always add a “fudge factor” for unknown situations. Are you comfortable using up some of that safety factor for things that are not needed?

“Is the removable block and tackle that Amel supplied a vang or a preventer, or what?”

While Amel calls it a “preventer” its primary job is as a vang, to hold the boom down and control sail twist when you are outside the range of sail angles where the traveler allows the mainsheet to do this job. It will also act as a preventer in the case of a jibe, but it is not a terribly good one. The geometery is not optimal for this job, and if you DO backwind the sail here, it would be very difficult to release this and get the boat back on her feet again. In very strong winds, I would worry about this system breaking in the case of a serious backwinding of the main. A broken preventer is certainly not the worst thing that can happen, but it’s not ideal either.

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