Take a Bow: Wave Piercing Bows

Subject: Is there a counter point to Wave Piercing Bows?

Hi Ted

As the debunker of many a design fad I thought you might like to comment on the appropriateness, or otherwise of “wave piercing bow” designs to the cruising application. As their application to the racing scene seems fairly well established at this juncture, and some cruising cat companies are starting to trumpet them as state-of-the-art in cruising cat design.

Cheers Russ

TC

Russ,

Take a bow.  (I couldn’t resist)

How wet do you want to be?

The merits of all bow treatments especially unconventional ones (bulbs, reversals, etc), are dependent on a particular water line approximation.  Lines of intersection with the water’s surface above or below the theoretical range change the parameters and the benefits/detriments equation. Also, size does matter, e.g.; common wave heights relative to your boat, the proportional wave generation physics, and your tolerance for pails of water thrown rhythmically at your face.

wake

The wake starts at the bow

The bow curves, especially the cutwater profile, are so significant to our aesthetic appraisal of a vessel that some compromise is nearly always made to create the prettiest boat possible within the bounds of necessity.  However, if necessity is perceived as a fashion or a bandwagon marketing ploy, the compromise may be inappropriate for the vessel’s use.

clipper

Thus we get ‘clipper’ bows persisting on vessels with neither functional bowsprits nor even sails

clipper 2

Clipper bows may be handy for practical work, like salvaging smaller vessels with ‘wave piercing’ bows.

Bow design has to practically consider the speeds expected for a particular vessel and its range of motion in all sea states. Regardless of the vessel, (subs excepted); ultimately the bow should have an increasing buoyancy reaction as it is immersed if it is expected to lift the vessel above the waves (volume below the immersion line is already immersed and effectively neutral). Bow shapes that increase in volume quickly will decelerate the bow immersion abruptly with accompanying large G forces (impact and high resistance). Sections that increase volume quite gradually (e.g. destroyer) allow the bow to plunge a long way before recovery, resulting in low decelerations, slower motions, less resistance, but deeper submergence (wet decks). We may tend to think in terms of the vertical when thinking of the bow volumes interacting with waves but the consideration is actually 3D; the horizontal sections aft of the cutwater are equally as significant as the vertical cross sections.  An extreme example of this is the typical barge.

cargo ship

Handsomely rakish profile is side view but simplistic treatment in plan view; cargo capacity trumps resistance and everything else apparently.

The prevailing sea conditions and duties may dictate a variety of expedients for various vessels.  Fishing boats often display the results of evolution (survival of the fittest in the physical and economic sense).

Northumberland Straights boat

This Northumberland Straights boat illustrates the local preference for a fine entry with dramatically increasing vertical volume close to the sheer (flare); makes it very tough to immerse the foredeck.

Our intuition of what is ‘right’ or appealing for boats in general may have strange influences.  The classic battleship bow profile (above the waterline) is an example, with the vertical or aft raked stem being identified with speed and power, whereas in fact they have mostly to do with the ultimate weapon, the invisible ram below the waterline.

cutwater

The cutwater is interrupted by the torpedo tube and the requisite filigree, so you suspect that keeping water off the deck was not a high priority

Regardless of how fine the entry is, it is common the get immersion recovery volume by overhanging the bow longitudinally. The resultant forward rake makes you look like you intend to go fast (cool).

Queen Mary

Bow overhang has the disadvantage of decreasing the length of the hull waterline in relation to the overall vessel length, which in the case of a displacement speed range vessel will impair its performance against vessels of like ‘size’. It also adds suspended structural weight forward.  Sailing cats have generally had very mild forward rake angles for many years to maximize the waterline length and offer some added immersed volume in the bow for fore/aft stability.  This has put a damper on the ‘clipper’ approach, (damn, the cats already suffer enough in the classic aesthetics department).

When something touted as ‘new’ comes along with a strong marketing profile, we may unthinkingly accept it (in name at least) as appropriate for general use.  The relatively recent advent of the ‘wave piercing’ catamaran ferry is a case in point.

If a ship has enough altitude and acreage to accommodate some volume sacrifice behind a sharpish cutwater, and other significant parameters will tolerate it, more extreme contours/contortions may be accomplished. This is the case with the true ‘wave piercing’ fast catamaran ferry boats pioneered in Australia  which are substantially larger than the average sailing cruising cat and have their buoyant recovery volume well above the operational wave impact zone and a semi-submerged snoot to lead the way below the cutwater.

wave piercing

True modern ‘wave piercing’ bows

Although properly engineered for very a specific speed and wave characteristic envelope, adventurous applications pop up using questionable rationale but lots of enthusiasm.  Go pierce them waves!

cat

Apparently amphibious too

cat

no comment

Polynesian

Eons old Polynesian wave piercer design (small waves anyway)

The beam length ratios of our catamaran hulls may tempt us to imagine them comparable to ship hull forms, yet the operating parameters and sensitivities are dramatically different. For example, constantly immersed bulbous bows on ships that operate at a rigorously consistent cruise speed have demonstrated benefits, but they are not translatable to the small scale vessel with a different wave making physics scale and a highly variable mix of speeds and sea states to negotiate.

bulbs

A nice pair of bulbs

This same inappropriateness of scale makes the true ‘wave piercing’ bow, as identified in the naval architecture sense, inapplicable to our cruising catamarans, so the industry is left with just a cool name to work with (something like the term ‘hybrid’).

The temptation in cruising catamaran marketing may be to adopt racing cat elements that may be within scale but not within the vessel operating parameters.

deck

No attempt here to keep a deck dry

This appears to be the case with a bastardization of the term ‘wave piercing’, used to excuse some cruising cat bow design liberties. The current low volume racing cat bow that is designed to run at speed intermittently submerged, thus ‘piercing’ waves, while its windward partner flies high and dry is not practically applicable to the cruising cat. You can see the action demonstrated in videos of fast racing cats and tri’s operating in even moderate sea conditions, it’s not ma and pa cruising. The leeward bow in fact must be extremely mild in its reaction to the seas or it would tear off.  As nobody is expected to inhabit the territory, its low volume and miniscule deck area are of no consequence. The possibility, if not the probability of capsize, requires the constant vigilance of the crews;

boat style

So, the cruising boat stylists present us with a name and some compromised wet geometry for the bow

crumb

…and a peculiarly comforting sense of deja vue…(apologies to R. Crumb)

A less readily apparent factor in bow design is its effect on a boat’s handling and hydrodynamics.  A knife-like deep forefoot acts as a sort of bow rudder, sometimes with a mind of its own in a following sea, and as it always directed straight ahead, it impairs a boats ability to turn.  ‘Rocker’ in a boats bottom profile indicates a propensity to turn more expediently, straighter keel lines will likely be steadier on course.  There are many variables and factors to consider in this regard, especially with sailing vessels, but the bow profile and ‘fineness’ really ought to compliment the overall hydrodynamics and handling.  ‘Steering’ bow sections that are immersed intermittently with the waves (see above) may have a very peculiar effect on the boats motion and handling in a seaway. Fashion and cosmetics should be sublimated to the hydrodynamics in a serious cruising vessel.

Large scale bow section parameters have to be reconciled with a desirable cutwater, which is an exercise in compromise especially tricky in catamarans such as we deal with; smaller vessels encounter comparatively larger waves. A very fine cutwater will have little value if it can’t be reconciled with the large hull volume necessary right behind it to avoid the ultimate danger (pitchpole/capsize) to a sailing cat that may result from the submergence of a leeward bow when running in bad sea conditions, (at which time you then hope there is substantially increasing volume in the plunging leeward bow).

Cruising cats never fly hulls intentionally and adopting racing boat hull or bow sections will be inappropriate and dangerous in some conditions.  Even in port or at anchor (where most boats spend a lot of their time) the ability to readily access the bow of the boat from an adequate foredeck may make your day.  I guess you could straddle the bow of the boat above and scootch your way out to the end to fend off, clear a fouled rode, examine the damage, etc.  But would you want to?  Is it worth it?

Actual complications of hull shapes have consequences in regard to performance, structure, weight, construction, repairability, costs, etc., but trumpeting a fashionable facsimile may have marketing value, even if it is only demonstrably beneficial to non-analogous vessel applications and is represented by a token effort. There are always consequences though they may not be life threatening. However, even a small amount of salt spray being thrown around needlessly can be very annoying, causing the cruising owner to second guess his fashion preferences if not his ultimate confidence in the vessel.

The ‘enjoyability’ of your cruising cat may be comprised of serviceability, safety, pride in aesthetics, pride in performance, ease of operation, comfort, etc.  Different vessel designs weight these values according to the imagined preferences of a speculative owner, you maybe.

Fortunately jargon is often adopted with little concession to an actual design definition, thus a ‘wavepiercing bow’ may in fact be nothing more than putting lipstick on a pig, (harmless if the pig co-operates).

(BTW – All bows are inherently ‘wave piercing’ if you can find a steep enough wave so we may all consider ourselves fashionable without paying the price).

cruise ship

Cruise ship inadvertently piercing a wave

Regards,

TC

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6 thoughts on “Take a Bow: Wave Piercing Bows

  1. interesting post, our cat has a bow that is far from the norm, take a look through our site, there will be more bow photos on soon as I update the refit process. look forward to reading through your blog as I get the time! thanks

  2. Take a bow for another excellent and entertaining article. I notice from the video clip that wave piercing bows work brilliantly er… when there are no waves. Be interesting to see how these vessels fair in say, a 50ft Southern ocean swell, like we get in my neck of the woods.

    Chris

  3. Speaking of the Alpha 42, it has 1.5″ 316 stainless rudder shafts. Kurt Hughes and Derek Kelsall pronounced the diameter very inadequate, which explains why they bent. Contrary to what one might suppose, ordinary mild steel is quite a bit stronger than 316 stainless.

    • Tim,

      I regret having stated my own perception regarding the rudder stock size based solely on its appearance in the published photo; “the boat appears to have greatly undersized rudder stocks”. In fairness I should have inserted some qualifiers as in the absence of data concerning the nature of the rudders, their areas, and the expected vessel speed, I was propounding an opinion off the cuff. Presumably the lads below did their homework, (I did too, but a dog ate it, honest).

      Any calculations applied to the rudder stock take into account the niceties of the materials used. A mild steel shaft would be subject to a corrosion degradation factor based on an acceptable depth of pitting. The expansion factor for rust makes bearings considerably more problematic and short-lived. Even on steel boats, you will seldom discover a non-stainless stock though I have dealt with them on aged craft.

      My comments regarding limiting the rudder swing and the importance of the attached steering gear still stand. There are lots of theoretically undersized stocks out there that never have to withstand the ultimate test of being broadside on and consequently never come up as an issue. I would still suggest that the steering system be viewed as whole and if it at all looks consistent with the potential loads, perhaps the weakest link never need be discovered the hard way.

      Regards,
      Ted

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