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Cool New Gimmicks...?   A Cautionary Note

Copyright 2000 -2006 Michael Kasten
 

Looking around among the many various "trawler yacht" designs currently being offered, one will observe that many of these designs are genuinely innovative, while others may perhaps be overly quirky. That said, I am not intending to pick on any one design or designer in particular.  Instead, it is the intent of this brief article to substantiate the following cautionary statement:   
 

It is not always the best idea to latch onto off-beat concepts "just because they are different."
 

Builders and designers of "trawler yachts" have recently experienced quite a lot of activity.  As a result, they are all competing for the attention of a relatively small client base who have the means to purchase a new power yacht.  Among the various heavily advertised  "trawler yachts" we can see examples of the over-hyping of several offbeat concepts, many of which are, in my view, highly irresponsible.

In order to reveal the most troubling of these, we need only look at all the noise currently being made in the media about double bottom construction; hydraulic primary propulsion; Z-drives; propeller nozzles; maintenance strakes; bow bulbs; and engines located far aft for the sake of the accommodation spaces.

The following is a brief explanation of each of these "features" and what we find to be troubling among them.

 

Engine Room Location

It is well known among sailors and among competent designers that weights located in the ends of a vessel will virtually always create trouble in terms of pitching moment. In other words, the vessel will be slow to react to the sea. With a large weight such as an engine located far aft, the vessel will be readily pooped.  That same vessel will also tend to bury her bow due to the mass of trimming ballast which will have to be located far forward in order to correct for the location of the engine.  The same applies if the engine or a large tank is located too far forward; there will have to be ballast located at the other end of the ship to compensate.

We cannot responsibly encourage the placement of large masses such as engines, tanks, batteries, etc. right in the ends of a vessel, regardless of the possible benefits to the interior accommodation....

 

Bow Bulbs

Several well known trawler designers seem to favor bow bulbs on nearly everything that moves.  In my view, placing bow bulbs on relatively small vessels cannot be encouraged for any vessel under around 65 feet or so, due to potentially dangerous handling characteristics in following seas.  This is particularly so for short and fat boats which will already exhibit excessive yaw in following seas.  What is favorable in the towing tank is not necessarily a benefit in the ocean, where wave-size-to-boat-size quickly becomes an overriding factor.

Bow bulbs may often pandered on the basis of pitch reduction.  The benefits in this regard are measurable.  It is important to realize though that the benefit of using a bow bulb simply to attenuate pitch on a well designed hull form is controversial at best.  For example, we have observed that on smaller craft a bow bulb can increase the likelihood of slamming in short steep seas due to its relatively flat bottom shape.  For more information on the possible hazards of bow bulbs on small craft please see my article on "Running vs. Heading Up."

 

Propeller Nozzles

Propeller nozzles at least are not dangerous...!  Nozzles are known to provide very substantial advantages to slow speed vessels doing towing or trawling.

On free moving vessels there is not the same degree of benefit, even at the relatively slower passage making speeds.  Even if a nozzle were able to provide a small percent advantage at a certain free running speed, but requires a 300% increase in the cost of the system, it is at best a questionable "feature."  Still, a nozzle can at times be justified on a free running vessel where there may be a requirement for extreme shoal draft, or where there may be the danger of propeller tip damage due to debris, ice, or shoals.

 

Hydraulic Drive

Hydraulic propulsion is most often promoted in order to enable placement of the main propulsion engine in a location other than amidships, in other words to make use of an unfavorable location in terms of trim and pitching moment.  Although ordinarily highly reliable, any hydraulic propulsion system must be engineered correctly or it will be very unsatisfactory.  Even if engineered and installed correctly, the use of hydraulics for primary propulsion is expensive to install, highly complex, ordinarily very noisy, and incredibly inefficient when compared to a simple direct shaft drive. This can hardly be viewed as a "feature."

On the other hand, one might very rationally consider a modest hydraulic drive system for a get-home installation, primarily to save weight and to provide flexibility of machinery location.  A PTO on the generator might for example be used as the motive force.   In this case, the hydraulic propulsion equipment will still be somewhat expensive, noisy, and inefficient, but it will not be asked to operate continuously, nor at full vessel speed.

 

Z-Drive

A Z-Drive may be justifiable in certain extreme cases, for example aboard a working tug that needs all-around maneuverability.  A Z-Drive as primary propulsion on a trawler yacht however falls into the category of being both extremely expensive and unnecessarily complex.   For use with the main propulsion engine aboard a yacht there is the same caveat regarding weights located toward the ends of a vessel...  it cannot be recommended due to the likelihood of adverse pitching behavior.

Still, on a yacht one might effectively employ a Z-Drive or Sail Drive with a "get-home" engine, perhaps even using hydraulic power - whether from the generator or the main engine.

 

Twin Keels

Twin Keels are defined as being in the form of a pair of bilge fins, usually foil shaped, located more or less amidships.  By contrast, Bilge Keels are usually defined as being relatively long and shoal, ordinarily made of flat plate.  Both forms, if implemented correctly, can be very effective at reducing roll, so can be quite justified on that basis.  If they are not sized and located correctly, there can be rather severe handling anomalies.

For more information about the design and use of Twin Keels on passagemakers, please check out the article on Roll Attenuation at this site.

Similarly, we have observed much hype put forth in favor of using a tri-keel arrangement; in favor of unconventional LCB locations; and in favor of propeller nozzles for free moving vessels.  The "tri-keel" keel arrangement in this case refers to the vessel having a deep fore-body and twin "skeg keels" aft.  This concept was originally developed by the US Coast Guard primarily for use on fast patrol vessels intended for shoal water, where the propellers needed good protection and the vessel must take the ground upright.  These were not created as "trawlers" but as high speed chase boats, and the shapes were optimized for planing performance rather than being intended for displacement speeds.

Among these "features" the use of twin props with twin keels aft is an arrangement that can be very favorable, however not for the "efficiency" reasons espoused.  This arrangement will mainly be used for the sake of engine and drive line redundancy, and for achieving shoal draft while maintaining good maneuverability.

 

Double Bottom

Among all the "features" mentioned, the double bottom may have a certain amount of merit, most particularly on larger vessels having a very adequate bilge capacity.  One can certainly consider this option, provided that the double bottom does not result in overly wide or overly long tanks that may seriously compromise the stability of the boat due to the free surface effect of the liquids within.  In other words, the bottom tanks must be subdivided in order to reduce the stability-degrading effects of the free surface of the tank contents.

A double bottom can be a good thing as long as it does not compromise the vessel's handling characteristics or limit one's access to the hull.

 

Quoted Stability Range

To make a vessel "survivable" in a 360 degree rollover is the ultimate goal for any true blue water boat.  For any vessel, survivability requires water tight integrity of all openings such as windows and hatches, etc.  Despite this fairly obvious requirement it is quite common for trawler yachts to have a relatively large windows for the sake of achieving a good view.  True survivability can hardly be claimed for boats with big picture windows and lightly built superstructures...!

On the other hand, a 180 degree range of positive stability for power vessels that do have strong and rugged superstructures and with small and robustly constructed window and door openings will not ordinarily be an excessive claim.   If structure is approached conservatively, if the openings are designed with an adequate support structure, and if the glazing is of the right materials and adequate thickness, this degree of survivability amounts to only a slight penalty in terms of weight and cost.

For additional reading on the calculations required in order to support various stability claims, and on what information one should expect to have readily available with any new design, please review our web article, Essential Design Data.

 

Compromise...

Inevitably, all boats involve compromise!

Many of the above 'gimmicks' are pandered in order to get the attention of potential buyers, or worse, in order to justify a compromise that has been made necessary by another compromise...!

Such gimmicks can in fact become a real obstacle to good design.  Many who are fairly new to boating will be attracted to these offbeat "features."  There is the tendency for the novice to become fixated on some unusual feature or other which he may have recently read about, often being unwilling to abandon it until much effort (and consequent engineering expense) has been expended in pursuit of that feature, and if the notion is bizarre enough it may ultimately need to be corrected on the finished boat.

These various gimmicks should be seen for what they are: simply marketing tricks put upon an inexperienced and unsuspecting public. For every claimed advantage, one must relentlessly ask what the possible disadvantage may be.

 

Innovation

This article does not mean to imply that we should blindly follow the "status quo."   Far from it!   Innovation is very much to be encouraged.  It is the only way we can progress. 

The salient point is that yes, we should of course look at all favorable options when creating a new vessel design, but we should always approach the new and unusual with a healthy skepticism. These are not mere idle warnings.  I have seen the unfortunate results many times... 

Please take the above as it is intended: A simple red flag in terms of encouraging good "boat sense" when considering the various 'features' being pandered for a new vessel.

Having exercised good judgment regarding what makes a proper boat for voyaging, one can enjoy many miles of safe and efficient boating...!
 

Michael Kasten   Metal Boat Quarterly #21 - Winter 2000 Editorial - Updated 2003 & 2006 BACK TO ARTICLES...