Part 1.

Above; Sensori 58 Sedan Cruiser

(i) Asking questions

In early 2021 I was involved in a design project for a production power cat to be promoted as a long range live aboard cruising boat. The pace of the project was largely driven by boat show deadlines and the need to create marketing material. Little if any consideration was given to what would be the ideal hull form to use and the design work defaulted to some existing hull forms already employed.


I thought this was a missed opportunity and in mid 2023, at the time of updating the Sensori designs I decided to take a deeper dive into the issue of fuel efficiency in power cats. What are the factors we need to consider and what is the ideal hull shape for a long range cruising power catamaran? I started digging into my folder of research papers to see what I could find.


(ii). Design parameters under the microscope- a deeper dive into detail

Some of these papers were fairly rudimentary in nature, validating and putting numbers to concepts that most of us who love boats already understand reasonably well. No harm in that. We have to start learning somewhere. On the other hand some papers were thoroughly researched and well written, providing valuable data and sometimes delving into areas not so well understood. Above all two things stood out.


The first is that the vast bulk of the papers relied on tank testing or mathematical analysis to arrive at conclusions. Very few reported on measured results on actual boats or ships that could be compared with the theoretical data or the tank test results. This understandable. To compare hull shapes in the real world at full scale would be a very costly exercise and for the most part we have to rely on theory derived from fluid mechanics and tank testing.



The other thing that stood out was that each paper studied just one aspect of hull design and none of the papers that I came across studied how variations in any one design parameter interact with the other design characteristics of a particular vessel.


This area of design involves a highly complex set of relationships and interacting design parameters, however it seems to me to present an exceptional opportunity to refine hull design for optimal performance in a pre determined speed range. 


In order to find a relatively simple pathway to investigate these two aspects of hull design I have put them under the headings of hull hydrostatics and hull hydrodynamics.


To this purpose I use the term hydrostatics refer to the basic hull form and how the basic geometries of the form, each on their own, create resistance. Hydrodynamics on the other hand describes what is happening when the hull is in motion, making waves, and all of the hull characteristics are interacting to determine speed and motion. This aspect of hull design will be discussed in Part 2 which is currently being put together.


Of course this division of the two areas of design is a simplification. A lot of things that come under the first heading overlap with the second but lets get back to the original purpose of this study, and that is the quest to optimise power cat hull design for fuel efficiency and seakeeping.  I'll keep it as simple as possible.

(iii). A Considered Approach

To proceed with the design work I needed a starting point.

From the outset I determined to take a close look at the choice of hull form as well as looking at other aspects of the designs that might help to reduce drag including a more streamlined wind profile, optimum hull spacing to minimise wave drag in the tunnel, and a low centre of gravity to minimise pitch and roll, both of which are enemies of fuel efficiency and comfort.

For fuel efficiency my instinct was that it had to be primarily about wetted area which is the highest form of resistance at displacement speeds. If you're living aboard and doing lengthy passages you'll be travelling mostly at displacement speeds.

Our initial series of hulls for the Sensori range was built on the canoe stern concept developed and made popular by Malcolm Tennant in the 1980's. The canoe stern has its advocates and for good reasons. In particular the fine hulls have excellent sea handling characteristics and wave interaction in the tunnel is minimised thanks to the narrow hull and fine entry at the bow. But are they the right choice for fuel efficient operation?


(iv.) So which is best? Canoe Stern or Semi Displacement Hull?

 In order to be able to make an informed decision on the preferred hull form for the new Sensori cats I built a CS hull and a Semi Displacement hull of equal displacement for each of the cats in the range and examined the hydrostatics for each in turn. For the semi displacement hull I used a relatively deep V entry with the maximum draft well forward. The V form extends well aft of amidships and gradually rolls into a flattened bottom with a modest transom immersion at half load.

For the CS hull I stayed very close to the form we had initially employed on the Sensori cats. The two hull forms can quickly and easily switched in our design software and examined on each of the models. 

On studying the hydrostatics of the two hull types four characteristics are immediately apparent.


 1. The semi displacement hull provides greater internal volume for engines and sleeping accommodation.

 2. The reduced sinkage rate for the semi displacement hull (cm immersion per kg weight) allows for greater load carrying capacity and there is less change in the hull characteristics such as wing clearance as fuel and water load changes.

 3. The Canoe Stern Hull will inherently require greater transom immersion and hence turbulence in the wake.

 4. There is a significant increase in wetted area for the CS hull. Around 19% to 20% higher on average compared to the semi displacement hull.




The first factor can be compensated in the CS hull by providing a step above the waterline but this is not very helpful in providing access around the engines if they are inboards and the step can produce slapping on anchor if it is too wide. Increase the hull beam too much and you're compromising one of the primary attributes of the CS hull form.


The second factor also favours the Semi displacement hull for load carrying ability. 


The third factor at least to some extent cancels out the purpose of having the canoe stern.


The fourth one is the big one and it's the one that swayed me toward using the semi displacement hull as the default choice for the new Sensoris. I had expected an increase in wetted area for the CS hull but was surprised at the extent of increase.


Wetted area is the primary drag factor in the low to mid speed range and this is also the speed range where most owners will choose to operate if range and economy are important. 


Another factor in favour of the Semi Displacement hull form is that they are likely to take advantage of some lift from the hull form in the higher end of the speed range, something the CS hull is very unlikely to be able to achieve.


One thing that I don't think is debatable is that if the CS hull is your weapon of choice then a clean bottom is imperative. The CS hull is already paying a penalty in additional area and if that area is encumbered with growth then you're probably probably paying a higher price for fuel than you really need to.


The Issue of Transom Immersion

Transom Immersion causes turbulence. We don't want that. We want the water separating from the hull as cleanly as possible. This presents a delicate exercise for the designer. The CS hull relies on a certain amount of the box section at the stern to be immersed to maintain trim and resist pitching. The same applies to some extent to the semi displacement hull but its hull form has minimal transom immersion and more forgiving to trim adjustment.

The two Curve of Area Plots are a graphic representation that show how the hull form changes shape quite abruptly at the stern on the CS Hull (upper image), while the semi displacement hull encourages a much smoother, more fluid release at the stern. 

The Issue of Transom Immersion

The Canoe stern hull earns its cred for seaworthiness by way of its deep and narrow hull form with minimal rocker. The canoe shaped body allows minimal turbulence where flow separates at the stern, but the CS hull relies on adequate immersion of the box section above the canoe to provide buoyancy aft and dampen pitching. This can be a problem for a cruising boat which has to allow for variable loading to account for fuel and water. Not enough of the box in the water and you're out of trim. Too much and you have excessive turbulence from transom immersion.  



The two Curve of Area Plots are a graphic representation that show how the hull form changes shape quite abruptly at the stern on the CS Hull (upper image), while the semi displacement hull encourages a much smoother, more fluid release at the stern. 

Above: Curve of Areas Plot for the Canoe Stern Hull

Above: Curve of Areas Plot for the Semi Displacement Hull