The Offshore Voyaging Reference Site

In-Mast, In-Boom, or Slab Reefing—Convenience and Reliability

Part 1

The tragedy on the CNB 66 Escape, in which Volker-Karl Frank and Annamarie Auer-Frank were fatally injured while attempting to shorten sail in a gale with in-boom roller reefing, got me motivated to take a deep dive into the benefits and drawbacks of each of the three primary options for mainsail handling.

Before writing, I:

  • Read through many comments from owners of both automated systems and slab reefing.
  • Interviewed my friend Hans who has owned and done challenging ocean passages with all three systems.
  • Went out with Hans on his Far 56 Pilothouse Cutter for a demo of his in-boom system and produced a video.
  • Had a very interesting email exchange with John Kretschmer, (John-K) who has slab reefing on his beloved Quetzal, but has done a trans-Pacific (California to Hawaii) and five East Coast to Caribbean voyages with in-boom systems, and even made some illustrative videos for Forespar/Leisurefurl in the past.
  • Over the years, talked to many other owners of all three systems.

I also have personal experience with both automated systems, albeit limited, and, of course, some 50 years of using slab reefing offshore while racing and cruising on all kinds of different boats, ranging in size from a Sea Sprite 23 up to our previous boat, a McCurdy and Rhodes 56.

Rank By Criteria

Analyzing the advantages and disadvantages of each system turns out to be surprisingly complicated, so to simplify things let’s use the following criteria to rank each system on a scale from one to five, with five being the best in that criterion:

  • Convenient to use
  • Low physical demands
  • Low skill required to use
  • High reliability
  • Easy to repair
  • Fault tolerant
  • Good sailing performance
  • Reasonable cost
  • Low risk and safe to use

Personal Needs and Wants

Of course, the problem with any scoring system like this is that we all have different needs and wants. For example, I might put safety first, another person cost, and a third person performance—all are valid ways to look at this, as long as we are realistic about the downsides of each option.

So I have also created a spreadsheet where each of us can weight the above criteria, much as I did with anchor selection.

To make this weighting easier, I have included my thoughts on factors that affect weighting at the end of each criterion.

I also assumed that each system is best-in-class designed, engineered and installed, since it’s stupid to reject a system and buy a different one just because the incumbent was a poor example of its type.

The whole idea here is for each of us to come up with the system that’s best for us, rather than turning this into the bullshit of “mine is best” so typical on forums and YouTube.

Let’s do it:

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More Articles From Online Book: Sail Handling and Rigging Made Easy:

  1. Six Reasons To Leave The Cockpit Often
  2. Don’t Forget About The Sails
  3. Your Mainsail Is Your Friend
  4. Hoisting the Mainsail Made Easy—Simplicity in Action
  5. Reefs: How Many and How Deep
  6. Reefing Made Easy
  7. Reefing From The Cockpit 2.0—Thinking Things Through
  8. Reefing Questions and Answers
  9. A Dangerous Myth about Reefing
  10. Mainsail Handling Made Easy with Lazyjacks
  11. Topping Lift Tips and a Hack
  12. 12 Reasons The Cutter Is A Great Offshore Voyaging Rig
  13. Cutter Rig—Should You Buy or Convert?
  14. Cutter Rig—Optimizing and/or Converting
  15. Cruising Rigs—Sloop, Cutter, or Solent?
  16. Sailboat Deck Layouts
  17. The Case For Roller-Furling Headsails
  18. UV Protection For Roller Furling Sails
  19. In-Mast, In-Boom, or Slab Reefing—Convenience and Reliability
  20. In-Mast, In-Boom, or Slab Reefing —Performance, Cost and Safety
  21. The Case For Hank On Headsails
  22. Making Life Easier—Roller Reefing/Furling
  23. Making Life Easier—Storm Jib
  24. Gennaker Furlers Come Of Age
  25. Swept-Back Spreaders—We Just Don’t Get It!
  26. Q&A: Staysail Stay: Roller Furling And Fixed Vs Hanks And Removable
  27. Rigid Vangs
  28. Rigging a Proper Preventer, Part 1
  29. Rigging a Proper Preventer—Part 2
  30. Amidships “Preventers”—A Bad Idea That Can Kill
  31. Keeping The Boom Under Control—Boom Brakes
  32. Downwind Sailing, Tips and Tricks
  33. Downwind Sailing—Poling Out The Jib
  34. Setting and Striking a Spinnaker Made Easy and Safe
  35. Ten Tips To Fix Weather Helm
  36. Running Rigging Recommendations—Part 1
  37. Running Rigging Recommendations—Part 2
  38. Two Dangerous Rigging Mistakes
  39. Rig Tuning, Part 1—Preparation
  40. Rig Tuning, Part 2—Understanding Rake and Bend
  41. Rig Tuning, Part 3—6 Steps to a Great Tune
  42. Rig Tuning, Part 4—Mast Blocking, Stay Tension, and Spreaders
  43. Rig Tuning, Part 5—Sailing Tune
  44. 12 Great Rigging Hacks
  45. 9 Tips To Make Unstepping a Sailboat Mast Easier
  46. Cruising Sailboat Spar Inspection
  47. Cruising Sailboat Standing Rigging Inspection
  48. Cruising Sailboat Running Rigging Inspection
  49. Cruising Sailboat Rig Wiring and Lighting Inspection
  50. Cruising Sailboat Roller Furler and Track Inspection
  51. Download Cruising Sailboat Rig Checklist
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P D Squire

Is there much variety between the three systems’ ability to enable reefing downwind, or is that determined more by other factors like spreader angle?

Arne Henriksen

Adding a storm trysail on a boat with in-boom furling, could move the Fault Tolerance score from 1 to 2, or even 3 if the trysail has it’s own track and halyard.

That would be the same for in-mast furling.

On my 30 year old Selden in-mast furling setup the mast extrusion has an additional luff groove for a storm trysail or spare main.
In an emergency even a foresail with rope luff could be hoisted, if I can trust the docs.. 🙂

Douwe Gorter

Have you ever tried to hoist a foresail into a grove in anything above 15 or 20 knots of wind, offshore? Well I did and I promise you that only works when you have two more experienced and strong crew members available.

George L

If you compare likes with likes then I don’t think that the physical demands of slab reefing with a powered winch for the halyard and separate leech and luff reefing lines is any more physically demanding than the alternatives.

But I am biased – to me roller systems are a brilliant solution to a non-existing problem.

Edward D Simper

We have sailed Roundabout II about 25,000 offshore miles with Selden mast furling. I go up the mast regularly to grease the bearings and have just replaced the Lewmar deck line organizers with Selden. I regularly reef and furl off the wind without using electric winches. Provided attention is paid to reducing friction a moderately fit man or woman should not need powered winches. My wife will use the winch to furl in stronger winds, I am able to simply able to pull the lines in.
Our boat is a Moody 40 which is small by comparison to many we see out cruising. We bought that size because we do not want nor trust electric winches. We have seen some horrific malfunctions leading the loss of fingers and a forearm in one case.
We consider that in mast furling is safe and convenient requiring only the it be maintained properly.

Edward D Simper

We also have a trysail on a separate track for redundancy. So far have never need to use it
Ted – Roundabout II

Michael Lambert

What about those behind-the-mast furlers? Ugly, and probably smacks the mast at anchor, but it seems to me that if you want ease of use it’d be hard to beat.

Stein Varjord

Hi Michael,
If I wanted to have some alternative to slab reefing, that would be the only option I could live with. It also has the benefit that it gives a bit less load on the mast tube, as it only gives compression, no mid section side loads. I don’t really think it’s any uglier than a roller furling head sail.

That said, I’m also not in love with roller furling head sails. The simplest, cheapest and most reliable there is, is hank on sails. That’s off topic here, so I won’t elaborate.

Douwe Gorter

You’re missing the support a reefed (slab) mainsail gives to the mast section.

Stein Varjord

Hi Douwe,
I would also never choose anything but slab reefing, so we’re on the same page. What I was aiming at here was only that a behind the mast furler is just as reliable as a furling headsail, which is also not perfect, while an in mast furler is dramatically more vulnerable.

It’s true that a reefed main that is attached along the mast can give support to the mast tube. However, support is mainly needed to counter the uneven loads from that very sail, like when it flaps, etc. The problematic loads from a deeply reefed main on the mast are dramatically bigger than the support the same sail gives. One of the reasons a cutter rig is good for ocean sailing, is that the cutter stay helps with these loads. A behind the mast furler gives no such load on the mast.

Either way, my choice is slab reefing. No doubt.

Stein Varjord

Hi John,

I have a hunch 🙂 that, after these articles, your preference will still be slab reefing. I’m probably more prejudiced than you. I try to keep an open mind about these alternative “comfortable” systems, but so far I totally fail to see their benefits. In my mind, using the mast or boom as sail cover is just silly.

That’s the only benefit I can see they have. I’m convinced that I can both hoist and lower the main, as well as reef, on average quicker with a good slab system than any of the other systems. Taking in a reef on our 40 foot cat (equivalent loads to at least a 50 foot mono) takes less than a minute upwind and less than two minutes downwind in a blow, when I do it alone. We have no fancy systems. All manual and simple. I have strong arms, but even when that diminishes (62 y.o. now), simple adaptions and a bit more time will be plenty remedy to keep me sailing.

I can see that in the right conditions, your ratings here are not wrong, even though you probably intentionally give the mentioned systems a bit of a positive skew. That way, the coming inevitable conclusions can’t be criticised for unfairness. I applaud that.

My view, so far, remains:
– I will never, no matter what circumstances, consider in mast or in boom furling, as they give me zero benefits and very important disadvantages.
– The reason these systems get gradually more prevalent is that the sellers promote them, because it gives them greatly increased profitability. They make far more from the extras than from the basic boat sale. This creates a false market for “upgrades” that in my mind are close to scams.

My dislike for these systems often finds little resonance with other sailors, as people tend to defend what they have with strong emotions. Still, I’m always right, so I don’t worry. 😀

George L

fair assessment

(except for you being always right 😉 , of course)

Raj Laud

People do tend to defend what they have or what they are used to. Being used to slab reefing, I viewed in-mast as a negative when we evaluated our current boat, which otherwise fit our needs well. Now that we have it and are used to it I see a lot to like. John’s article hits the tradeoffs well.

It seems silly to say there are no benefits. It is faster to set, reef, and take in. We sail more on short passages or in marginal conditions, because it is so easy, we are not hung up on whether it was “worth it” to set the main. It seems more reasonable to weigh the benefits against the downsides, and the cost/hassle of making a change.

If our boat had slab reefing, I would happily use that system and not change it. I would probably choose slab reefing over in-mast if I was starting from scratch, but I’m not so certain now after using the system as I would have been without that experience. And I have become convinced that used and maintained properly, in-mast is a reliable and seaworthy system. It seems to me that the most important thing is to know and understand your system well and get experience with it in a variety of conditions.

Rod Jones

I terrsting that nobody discusses the issue of inmast furling jamming versus inboom when you can still lower the mainsail and flake over the boom. This weekend have just used a Leisurefurl inboom here in Greece versus my Mainfurl boom and both have advantages, although the Mainfurl is better suited to a self tacking jib. IMHO.

Douwe Gorter

i often am not in complete agreement with John but this time I completely agree. Having owned a Swan 46 (slab) reefing, rather complicated since the 15winches were spread all over the deck, mast and boom. A Hallberg Rassy 53 with hydraulic in mast furling, never had so many jams in the mainsail until we replaced the entire mainsail with a new one that when handled with care never failed to unfurl or furl. The only one on board allowed to reef was myself by the way which underlines your observation. The next owner waisted the only two year old (15K) Hydranet mainsail within two weeks of ownership. When we build our Dijkstra 56 I specified slab reefing, Two lines per reef all led back to the cockpit (electric) winches, Never an issue, also the new owner never had issues with this system. On the Gallant 53 (1968) I have slab reefing at the mast, although a bit wet sometimes never problems. We are now sailing a new to us Bestevaer 53 with single line slabreefing managed from the cockpit. I had hesitations with single line reefing due to earlier experiences on a delivery of a Bestewind 50. Initially there was way to much resistance in the single line system but after replacing the reefing lines (and clutches) with slicker and thinner lines (Gleistein dyneema) it works remarkably well. My wife and just crossed the Bay of Biskay in winds going from 15 to 40plus, we had to put in reef 1,2 and 3. We never had to go into the wind to take the pressure out of the main and effortlessly reefed the main. It’s really important to mark the reefing lines and the halyard to be able to do this right first time, especially at night time.The downside is the very long tail ends of the reefing lines but the Bestevaer has a huge amount of space for that in the cockpit.
you didn’t take single reefing into account but when well set up it might make things easier for a small crew.

John Crossen

After sailing Asia, the Medd, and now the (eastern) Caribb over the past 22 years, with just 2 of us on our Taswell 43 monohull, our vote is solidly in the in-mast camp. We did have an issue with our in-mast furler….turned out the upper swivel had 3 sets of bearings in it, and 2 of the 3 sets froze. It occurred, and was repaired, when we were in Athens, Greece, in 2012…and we have had “0” issues with it since. The ease of sail handling, the lack of pulling the M/S up, especially since we’re cruisers, not racers, the (slightly) smaller M/S-no roach-and the lack of battens has minimal impact for us. Our previous boat, a TaChou 33, had slab reefing-what a PITA! And yes, we have a separate parallel track for a Trysail if we should ever need it-we haven’t yet-we just reef in the M/S as we need to.

Charles Butler

And can you reef off the wind?

Ian McMahon

I would like to share our experience from 15 years sailing a Sabre 452 (45’) with slab reefing. The mainsail is a heavy-duty Dacron cruising sail made by North Sails for the 2000 season, on an Anatal mast track, fully-battened with Shaefer’s Battslide System batten receptacles, and with a Dutchman sail flaking system with three control lines. Obviously, one cannot put a fully-battened mainsail on an in-mast or in-boom furling system. The Dutchman sail flaking system has exceeded all our expectations, possibly because we have been blessed to have had it installed perfectly, and we adjust the control lines as needed. I believe a 45’ boat may be ideal for this system.
In an emergency one can dump the mainsail in an instant by simply releasing the halyard (no matter what the point of sail), and then center the main sheet. The mainsail flakes neatly on the boom.
Slab reefing is similarly convenient: ease the halyard, go forward to attach the ring to the horn, tension the halyard and take in the reefing line. To hoist, I jump the halyard at the mast and my wife takes in the slack in the clutch (companionway). I wax the Anatal mast track twice a season, and the benefit is amazing. As John stated, use-it-or-lose-it. Personally, I enjoy the exercise benefit from jumping the halyard and even climbing the mast (ATN mast climber) a few times a season to inspect the rig.
I rate this setup: Convenient to use 4, Low physical demands 2 (I like exercise), Low skill required to use 5, High reliability 5, Easy to repair 5, Fault tolerant 5, Good sailing performance 5, Reasonable cost 3 (more expensive mainsail design and hardware – Anatal track and Shaefer’s Battslides), Low risk and safe to use 5.
In summary, the ideal solution for any sailboat depends on so many personal considerations, and I am unqualified to comment on in-mast and in-boom furling systems. Rather, I simply wish to share our experience and suggest additional considerations in case someone finds this beneficial. 

Dick Stevenson

Hi all,
I am curious, with regard to in-mast roller furling of the mainsail, how all that weight aloft affects the boat at anchor. There is the sail, the gear, and, I would think, a weightier extrusion to handle the large hole to accommodate the sail and remain strong.
I would also think, but have not heard mentioned, that designers would have to design a taller mast (more sail area) to achieve the same horsepower to make up for the loss of roach and the efficiency that battens provide.
Thanks for your thoughts, Dick Stevenson, s/v Alchemy 

Matt Marsh

The in-mast system is heavier in total, and has a higher centre of gravity when furled. Thus, the boat’s overall centre of mass is higher and its roll and pitch moments of inertia are larger.
All else being equal, then, an anchored boat with an in-mast furler will have a slower roll period and a larger roll amplitude than you would experience on the same boat with an in-boom furler or a slab reefing system.
And yes, if a yacht designer is considering a hollow-roach sail, then they must either accept some loss of power or increase the P dimension (and therefore the mast height) in comparison to what would be required with a straight-roach or positive-roach sail. (I assume John will address this further in part 2.)


Hopefully not front-running part 2, but might it be better in at least some cases to mitigate the loss of main power by increasing the E (foot) dimension vs P?


Hi John,

I see why a longer foot would require a new boom, but not yet why a new mast would be needed… I expect I’ll understand after reading part 2! Thanks for digging into the details.


Dick Stevenson

Hi Matt,
Thanks for your thoughts. So, if I read you correctly, it sounds like there would be a difference at anchor, but not that the difference would appreciably affect comfort.
I would be interested (in the next article perhaps) in the percentage increase in mast height to match horsepower. I met someone a good while back who opted for conventional main design over in-mast because he knew he would be on and around the ICW where mast height is limited to its 65-foot bridges. If I remember accurately he calculated 10-12% increased height would be needed taking into account both sail size differences and the decrease in efficiency of no battens etc. that affects shape and horsepower generated.
My best, Dick Stevenson, s/v Alchemy
PS. Speaking of comfort, can those with roller furling in-mast speak to how easy it is to keep their rig quiet at anchor (or in marinas). This is a what some would call a pet peeve of mine: I hate noises at night.

Raj Laud

Hi Dick,

At anchor, in-mast doesn’t seem any different on the noise front than slab. Just need to make sure the halyards can’t bang against the mast. However, I’ve heard that the mast can “whistle” if docked with the wind on the beam; the gap in the mast acting like a flute or the top of an empty bottle.

We actually haven’t experienced this, perhaps since we are mostly anchored or moored. We do have a flute stopper you can run up the gap to fill it and stop the whistling, but haven’t used it, so can’t speak to its effectiveness. The theory seems sound.


Wilson Fitt

Hi Raj, Dick

When we were spending time in Antigua quite a few years ago, big yachts at the marina with in mast furling were emitting unearthly howls at various pitches as the trade wind played across the slots in their masts. Very spooky. I recall someone saying that closing the slot with a fabric cover was fairly simple but it certainly was not standard practice in those days!


Dick Stevenson

Hi Raj and Wilson,
Thanks for the feedback. I was thinking more of the internal equipment making noise and hard to quiet, but I suspect the sail wrapped in there dampens the sound a lot.
As for the “howling”, I would not have guessed at that, but I suspect that may explain a spooky sound I heard at zero-dark-thirty one night in a marina and wondered what it was without the slightest inclination to explore and find out.
My best, Dick

William Murdoch

During hurricane Irene in 2011, the fabric tape that closed the mast slot of a boat at the dock off my stern at Northwest Creek Marina in New Bern, NC popped out of its slot during the storm and streamed from the masthead like a long pennant. It entangled with the mast of the boat in its portside slip, and as the storm continued the tangle grew drawing the two masts together at their tops. Many of the expensive bits up there were damaged.

Dick Stevenson

Hi John, Interesting: a trade-off clearly. I look forward to part 2.

Scott Arenz

Hi Dick,

I recall from my days living and working in a marina that the only time in-mast furling systems would be intolerably noisy is when the mainsail was unbent. It’s then that the extruded aluminum luff rod has room to move about in the empty interior space, impacting the sides of the mast. What racket it makes!

Fortunately one could probably fix this either by increasing the luff rod tension, or by looping some kind of padding around it and hoisting it halfway (after attaching a downhaul).

With the sail bent, however, the mast is generally no more noisy (or less neighborly) than any other boat.


Dick Stevenson

Hi Scott,
Thanks for the fill. Makes sense.
I doubt whether luff rod tension alone would stop the noise if the sail was unbent without it being triangulated somehow, hard to do inside the mast. Tensioning halyards rarely works in my experience: just changes the harmonics/period/ etc., but not the overall noise.
My best, Dick

Scott Arenz

Hi Dick,

Good point! The luff rod would still be like a huge guitar string in the mast, maybe just clacking faster. And, the excessive tension probably wouldn’t be good for the furler’s bearings and other wear parts.


Raj Laud

We had the mainsail down for a while last fall while on a mooring. An icicle hitch around the foil brought back to the outhaul block on the boom restrained the foil and kept it from banging around. Agree that luff rod tension alone wouldn’t do it. And yes, with the sail bent on, nothing bangs around at anchor inside the mast.

Anyway, I agree with Dick that banging halyards and other noises at night are annoying. Sailors who keep a slab rig quiet will be able to keep an in-mast rig quiet, I think; it’s those who can’t be troubled who will have noise regardless of setup.

Dick Stevenson

Hi Raj
Good information, makes sense. And agree, noise reflects the willingness of the skipper to have a quiet boat and be a good neighbor.
My best, Dick