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Cool New Gimmicks...?

A Cautionary Note...

Copyright 1998 - 2011 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 boating 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 more accommodation space.

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

Engine Room Location

It is well known among experienced sailors and competent boat designers that weights located in the ends of a vessel will virtually always create trouble in terms of pitching moment. In other words, where weights are concentrated, the vessel will be slow to react to the sea. With a significant weight such as an engine located far aft, the stern will not so easily rise to the sea and the vessel will be readily pooped. Given balanced lines, that same vessel will also tend to bury her bow due to the mass of trimming ballast that will have to be located far forward in order to balance the weight 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 floats.

In terms of the reduction of wave-making resistance, there is much to be said for bow bulbs. However it must be realized that in order to be effective a bow bulb must be designed for a specific speed through the water, at which speed it will serve to partially cancel the induced resistance due to wave making. Vary the speed and the bulb will no longer be optimum. For a tanker or container ship, where speed is held constant, there can be a big efficiency gain.

For small craft, say around 75 feet or so, speed through the water is rarely constant. We must keep in mind that what might be shown to be favorable in a flat-water towing tank will not necessarily be any benefit in the open ocean where wave-size-to-boat-size quickly becomes an overriding factor.

It is also my view that in spite of any potential reductions in resistance, placing bow bulbs on relatively small vessels cannot be encouraged due to potentially dangerous handling characteristics in following seas. This is especially so for short and fat boats that will already experience excessive yaw in following seas.

Bow bulbs may also pandered on the basis of pitch reduction. The benefits in this regard are measurable and therefore inarguable. 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, on smaller craft we observe that 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, "Running vs. Heading Up."

Have I used bow bulbs on my designs...?? Yes, by customer request the Free Spirit 76 and the North Coast 80 have prominent bow bulbs, sized for maximum effect at slightly above the most efficient voyaging speed. These are about the smallest vessels that can effectively use a bow bulb without falling prey to the above mentioned hazards.

Propeller Nozzles

Propeller nozzles at least are not dangerous...! Nozzles are known to provide substantial efficiency advantages on slow speed vessels engaged in 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 efficiency advantage at a fixed 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, possibly 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, 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 rationally consider a modest hydraulic drive system for a get-home installation, primarily to save weight and to provide flexibility of machinery location. For example, a PTO on the vessel's generator might 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.

On smaller craft, there is probably better justification for the use of a 'wing' engine with conventional drive line, which can be used as the vessel's generator... Examples of this kind of layout can be found on our Valdemar 53, and Chantage 64 designs.


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 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.

If the expense and added complexity can be tolerated, and if it is not carried to extremes in terms of location, on larger vessels it can be a useful strategy to gain interior space in the belly of the vessel. Examples of this more moderate approach can be found on our Chantage 64 and North Coast 80 designs.

Sea Chests

Many builders and designers favor using a sea-chest on relatively small vessels, but I think it is an irresponsible recommendation. In my view a sea-chest does not belong on a small / medium sized yacht – say under 200 or so feet long! Of course this is only my opinion, but it is based on sound reasoning and experience.

For good access, a sea chest requires a very large hole in the bottom of the boat. Unless it is carried well above the deep load line its basic rationale is defeated, which is that of being able to pop the lid off under-way and un-foul an intake. This un-fouling rationale to me is scant justification for having to create extra-long hose-runs off one big central source. Multiple long hose runs introduce added vulnerability to chafe and damage.

The whole sea-chest takes on the aspect of a giant multi-armed monster in the middle of the engine room – a place where space is always at a premium. Further, unless a sea-chest is of extremely generous proportions inside, it will not be at all easy to maintain, nor for that matter to correctly build, clean and paint in the first place. At that size, the presence of the large opening will provide additional unwanted underwater drag. If you suck a big jelly fish or plastic trash bag into the sea chest, you've shut down ALL of those intakes. If left alone in fresh water in an un-heated boat during winter, a sea chest can freeze and burst, creating one truly enormous leak.

If it is desired to limit the number of through-hull penetrations, then an over-sized flanged sea-cock can be positioned as desired, and two or more intakes teed off that. Note though that some engine makers will not allow this per their warranty terms. Other equipment such as water maker, wash down, salt water intake for heads or for galley, etc. can share a single larger intake valve if desired.If located intelligently, each intake valve will be close to the equipment that it serves in order to limit hose runs, and will additionally be sited so that each valve is easily reached for shut-down when leaving the boat.

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. However 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 our article on Roll Attenuation.

Similarly, we have observed much hype put forth favoring the use of a tri-keel arrangement, unconventional LCB locations, and propeller nozzles. The "tri-keel" keel arrangement in this case refers to the vessel having a rather deep fore-body, combined 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 need good protection and the vessel must take the ground upright. These were not created as efficient "trawlers" but as high speed chase boats, and the shapes were optimized for planing performance rather than for efficiency at 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. Twin engines will mainly be used for the sake of engine and drive line redundancy, and to achieve shoal draft while maintaining good directional stability and in-harbor maneuverability.

Double Bottom

Among all the "features" mentioned here, 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.

Further, it should be obvious that the bottom tanks cannot occupy the entire region below the intended waterline, since there would no longer be sufficient floatation to achieve that waterline... Therefore, with a true double bottom that extends throughout the below-WL region, there must necessarily be a generous amount of "void" space that is permanently sealed from access or storage. In terms of maintenance and / or repair, this strategy can hardly be recommended.

In other words, a double bottom can be a good thing as long as it does not compromise the vessel's handling characteristics or stability, or limit one's access to the hull. For an example of how we have implemented a semi-double bottom, please see our North Coast 80 design. For an in-depth review of double bottom strategies, please see our article on Integral Tanks.

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, this degree of survivability requires water tight integrity of all openings such as windows and hatches, etc. Despite this fairly obvious requirement it is rather common for 'ocean going' trawler yachts to have relatively large windows for the sake of enhancing the 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 combined 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, adequate thickness, and robustly installed, this degree of survivability and peace of mind need only amount to 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.


Inevitably, all boats involve compromise!

Many of the above 'features' 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 a prior compromise...!

Such 'gimmicks' can in fact become a real obstacle to good design. Many who are fairly new to boating will be intrigued by and thus attracted to these offbeat "features." There is the tendency for the novice to become fixated on some unusual feature or other which may have recently been read about, often being unwilling to abandon it until much effort (and consequent design 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 ploys foisted upon an inexperienced and unsuspecting public. For every claimed advantage, one must relentlessly ask what the possible disadvantage may be.


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...!

For more information about what we do encourage for passagemaking 'trawler' yachts, please see our article on the Ideal Passagemaker.

Michael Kasten

Metal Boat Quarterly #21 - Winter 2000 Editorial - Updated 2003, 2006, 2010


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