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Reverse Bows — Pros & Cons

Question: Why have reverse bows? …and what are the pros and cons?
Sometimes called 'wave-piercing', 'hammerhead' or 'X-bows'.

Reply:
Well, technically, one can initially see several reasons why these bows have become popular of late.

Probably the most valuable one is the claim (and generally accepted fact) that it reduces pitching, which is not only uncomfortable—it also slows the boat. Pitching is aggravated when a full deck bow picks up a large increase in buoyancy as a high wave passes, as this then throws the bow high in the air, until it drops back down again to repeat the cycle. The associated discomfort of pitching is obvious but the boat is also slowed due to both the constant variation of trim (that causes rapid changes in water flow around the hulls), as well as from the disturbance of wind across the rapidly moving mast and sail in a fore and aft direction, being as the rig has no choice but to follow the pitching of the boat.

Another reason is that, theoretically at least, one can achieve the longest waterline length for a given hull size and also save some deck weight in the bow. Even anchoring equipment may have to be moved farther aft as the bow will likely be too narrow.

Reverse Bow

A third is that there will be somewhat less windage and for a large cruising cat this could affect performance as well as when maneuvering in cross winds etc.
While some may also say "it looks fast" I personally believe that despite any initial reaction, we generally grow to like the look of anything that finally proves itself and works or performs well. "Beauty is, as beauty does" was something a well known designer once told me and I believe that, especially after being the past owner of both a VW Beetle and a Citröen! So first, we need to look at all the facts and decide IF a reverse bow works well for our needs, or not!

So with the positives already stated, let's now look at the negative things and see how they balance out for the type of boat we are considering.
First and foremost I believe, are the undeniable fact that reverse bows are wet. VERY wet! …as these bows go more THROUGH the waves and less over them. While a race boat may choose to disregard this 'inconvenience', it could make doing a passage in a steep chop such a miserable experience that you just stay in port—even if we push aside the increased risk of being swept off the bow of the boat! And if you happen to own one of these 'interesting' new catamarans with a forward cockpit "for improved tropical ventilation, mast access and steering visibility", then you can be sure that this cockpit will at times become nigh 'uninhabitable' when combined with 'sleek, wave-piercing bows' and you'd better have some super-large drain holes there as well, or this could even be dangerous.

An experienced Captain recently wrote to me and suggested I also add this to the negatives.

He considers reverse bows are potentially dangerous, as there’s a significant risk that they could scoop up kelp at speed and even trip the boat into a capsize or pitchpole. I consider that a perfectly valid point and even if the boat did not flip, the deceleration could throw a crew off the deck or cause an accident on board”

Large, flared bows have existed and been developed over the years for good reason. We might call these 'a traditional bow' now. Boats that need to be launched through heavy surf, NEED bows with high freeboard or they will be swamped before they even get to sea; and this is the main reason that many traditional boats that need to be launched from beaches directly into the sea, have such high freeboard and flare up forward (see the traditional boats from Micronesia as just one example).

Quite often in fact, dryness might win out over pitch reduction as there's a clear trade-off here. Adding a bulbous bow below the waterline up forward, has also been shown to reduce pitch—but again, do you REALLY want that large 'probe' hanging out there when you might be navigating in waters with lobster traps or even dealing with anchor lines under non-ideal conditions? I don't think so.

A nose-dive

And what about the reverse bow on the new, fast catamarans? Already we are seeing this appearing more and more and it's clearly becoming a fashion and fad. Well, I have personally never been totally comfortable with this long, slender 'upside down' bow. With lots of wind force coming from high-up on ANY sail, let alone a tall wingsail, there is clearly a large forward lever as soon as one bears off wind and the reverse bow has far less buoyancy to oppose that tendency to prevent stuffing the bows under. It only has to go below the shallow depth of the lowered bow, and the very shape of it is now driving the bow deeper from hydrodynamic forces on it and these rapidly increase as the long bow plunges deeper. Just take a look at this YouTube of noted Australian sailor Torvar Mirsky flipping a Formula 40 in China in April 2011 and you will see just how fast the reverse bow is driven down deep, and how much deceleration occurs. And the conditions were clearly not rough or even that windy!
www.youtube.com/watch?v=Xs7jOYGppnw

As I pointed out back in Nov 2012 in an article about the Americas Cup boats, it 'seems inevitable' that these boats will pitchpole if the reverse bow goes under with any speed and momentum. Well, it's now happened to both Oracle and Artemis with quite disastrous results. Without some form of side vanes to oppose that downward plunge, I think we will unfortunately see more such 'accidents' before designers start to withdraw this new fashion from their design files. As I mentioned earlier, unless this is for a race boat where the crew is ready to sacrifice ultimate safety and deck dryness for performance, I do not see this is an advisable trend, particularly for the smaller boats that are already low in freeboard. Even for larger boats, highly protective caddies and coamings will be needed to make the otherwise wet sailing conditions more tolerable.

Now if this reverse bow concept can be combined with enough freeboard, then such boats might indeed benefit from the pitch reduction—but this might only be for large commercial vessels and we already start to see oil-rig supply ships that have their very high accommodation house up forward anyway (to make room for the large oil pipes that they transport on their long aft decks - see new designs from Ulstein). But even these boats typically have large deflectors high above the water level, in order to keep solid water from coming over their decks.

Ques: So is there any other form that can help to reduce pitching, without being so wet and vulnerable to pitchpoling?

AXE Bow

Personally, I am more a fan of the so-called 'AXE' or Canoe bow. Called that way for obvious reasons, versions of this bow have been around so long that it might also be called 'traditional'—being used on some warships since the early 1900s. In this case, the actual freeboard is not reduced so much but the hull forward is slim with almost parallel sides. This permits the passing wave to surge up and down the straight bow sides as the boat cuts through at speed, with a relatively small increase in forward buoyancy, rather than having the wider Vee and broader deck. (I have even used this approach on my small W17 and despite the seemingly boxy form, the relatively narrow, nearly parallel sides allow most waves to pass cleanly along the hull.)

As the boat gets bigger, one can slightly curve back the stem at the deck and really take on the more typical AXE shape, as seen in this nice example by Morrelli and Melvin for their large catamarans. Yes, the bow will be a little wetter than if there were more flare, but with 62 feet and ample freeboard, this will not inconvenience the crew very much and they will still gain the benefit of less pitching—which in turn, will bring less fatigue to the crew when beating up-wind.

Reverse Bow

But this also needs to be considered. There are seemingly a lot of sailors with cruising cats who seldom choose to sail upwind anyway and in such a case, there are far more negative risks when bearing off and sailing downwind with reverse bows than could ever justify their use.

Before closing this, I will mention that it's possible that the use of curved lifting foils—perhaps more forward than is presently seen - might help to offset the present tendency for reverse bows to nosedive—but such a development has still not adequately been tested. Just the use of simple side fins at the bow (as once adorned early catamarans that lacked bow buoyancy), could certainly help check the nosedives if placed at the right angle, and these were tested—way back in the 1950s.

So for everything there is a place. We know what they achieve but we must also recognize the price. Reverse bows are most certainly not for everyone and neither should they be applied to all boat types and sizes. It very much depends on where and how you sail, what sort of boat you are considering and of what size.


Update:

Additional explanation of why the AXE bow might be more forgiving than the Reverse Bow.

With the AXE bow, there is slightly less hydrodynamic downward thrust of the bows compared to the true reverse-bow and this can sometimes make the difference.

Let's look briefly at these rough bow sections, representing a conventional, a reverse and an AXE bow. Simply based on their geometric shape, if we drive these shapes forward into waves, we see different responses.

Bow designs

The conventional bow will tend to lift (see blue arrows). By comparison, the reverse bow will be pushed down, while the AXE bow will stay fairly neutral.

If we now imagine the stem pitched down more, the conventional bow will present resistance against this, and slow the boat, up until the deck goes under—when it will present the large braking surface of the deck—a no-no. The Reverse bow will simply continue to drive down under. (Ed: just like an upside-down sea kayak would, if pushed from the rear at 20k!) Meanwhile the AXE bow will knife through the water without excessive force either up or down, unless the boat turns sideways, when it will start to add a braking component. Keeping these bows moving through the water regardless of pitch angle, is clearly very important in avoiding a pitch pole. One also wants to keep the conventional bow deck above any solid water and the extra buoyancy will help achieve that. (Of course, this all primarily applies to fast, lightweight multihulls, as slower, heavier monohulls are far more likely to broach sideways and heel over, thereby relieving themselves of the driving sail pressure.)

Now watch the two short capsize cuts on this video. Both boats have the AXE bow:
www.youtube.com/watch?NR=1&v=93cSAa2Z758&feature=endscreen

At 2:50 mins, you will see 'Homatro' start to pitch the bow under but the bows do not stop dead as they typically do with a more extreme reverse bow—they still permit enough motion through the water to prevent the full pitchpole, resulting in the stern finally dropping back down, and boat then laying over in a more conventional and manageable way.

At 4:08 on the same video, you will see Oracle-BMW "Ellen MacArthur" pull off 'the same maneuver'.

Of course, on a non-race boat, no one wants to even capsize, so in that case, you really have to start easing off the sail pressure as soon as the lee bow gets close to the water surface. something that takes a lot of concentration as well as immediate action. Reefing early will significantly increase your margin for error.

Also, there's this YouTube of Thomas Coville doing a spectacular nose dip just after starting a single-handed round-the-globe record attempt on board Sodebo, a couple of years back.
www.youtube.com/watch?v=QP-67_sPgGc&playnext=1&list=TLUe6A2INVpUI

But again, the AXE bows permitted just enough forward motion to prevent the boat from tripping past the point of no-return, and she settled back down and sped forward. But definitely, one of those 'OMG' moments!

One thing of current interest is that the reverse bow moves the buoyancy down low and as shown in the sketch above, this can permit an almost flat bottom up forward. If this is combined with lifting foils, then this flat surface can be used to provide early lift to get that bow up, and once up on foils, there is less concern about the reverse bow shape as the bow is now mostly clear of the water. The AC72s of New Zealand, Italy and the USA all use this arrangement to varying degrees. It is just the first Swedish AC72 that chose NOT to use lifting foils—but after their tragic accident, they will surely re-think this one.

Finally, for the record, wave-piercing reverse bows are nothing new. Here are two examples from about 80 years back—we might guess that 'this new idea' was dropped based on the crew finding the result 'just too damn wet'. I'd not be surprised to see the present 'recycled solution' head the same way.

Reverse Bows

Interestingly, during the same time period, the bow of the adjacent submarine used the more neutral AXE bow, in order to dive with more control, and most submarines still do.


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