Displacement motorboats
A deep-keeled motorboat is one of the most seaworthy vessels, just ask any commercial fisherman
Coming from five generations of fishermen and having been worked aboard a deep-keeled crab-boat during all of my formative years, this is my favoured hull. It saved our lives in big seas on numerous occasions while we hauled crab pots in North Atlantic swells. These boats descended from designs of our Norwegian-Viking forefathers – double-ended and rounded hulls with long, deep keels and large bilge keels to further minimise yawing. Our propulsion was initially a lug sail until the transom was drilled for a shaft-driven engine. Recreational versions echo these features and there are plenty of varieties, especially favoured in higher latitudes where the swells are bigger.
Design
Design of displacement hulls in recreational boating often follows the trawler concept. Beamy, with low centre of effort and designed for the open ocean. These boats can also be ballasted to increase their stability - to 90 degrees or more - so can cope with adverse side-seas and tumbling off wave tops. Propulsion is often a slow revving diesel with a huge flywheel which is frugal when run at full-displacement speed. The modern shapes tend to be square-sterned to maximise internal volume and this also gives buoyancy when running with seas. Steerage is good because they usually have a large rudder, deep in the hull. The most robust hull materials are steel and alloy but a monolithic fibreglass hull is sufficient for most general boaters.
Controlling displacement vessels in confined spaces is easier done with some smarts. Credit: Kevin Green
Characteristics
These boats aren't generally built for high speed, apart from special types such as naval destroyers and sometimes pilot boats – the latter have to race out to meet ships in wild conditions. The ultimate in sturdiness is the British RNLI Watson Class lifeboats that also were self-righting.
So, the philosophy is generally slow and steady, which means not being able to outrun a storm. But then again you don't need to, because the vessel is designed to survive heavy weather. Tall bulwarks, deep scuppers and minimised windage (a low wheelhouse and deck structure). Another feature is deep bilges to withstand a substantial incursion of water without severely affecting stability.
I controlled this Horizon motor cruiser easily on the busy French waterways, thanks to the predictable handling. Credit: Kevin Green
Handling
Slow speed handling is generally very predictable because of the amount of hull in the water and generally the greater weight of the vessel. However, the phrase 'in gear to steer' applies - without controlled momentum rudder will not guide you. The more deadrise and deeper its vee sections, the better the grip on the water and the more predictable the handling. Turning involves slicing through the water, rather than skidding, like a planing boat. A single-engine shaft driven boat can be turned almost in its own length by putting the helm over hard and shifting repeatedly between forward and reverse; using propeller kick.
Propeller kick is the bias a propeller has one side or the other. This should be used to advantage. For example, if the prop kicks to port then come alongside on port so that the propeller will draw the vessel alongside.
Docking is a controlled manoeuvre (not merely slow) whereby you are in control of your direction and speed. The amount of control varies with the strength of wind and tide – the latter having a greater effect on the displacement vessel. The ideal outcome is to gently come alongside the dock with fenders in place, painter ready and hands clear of the gunwales.
At speed a predictable motion through the water, no matter how bad the sea, gives the displacement hull a lot of comfort, albeit at single digit speeds. Trim is also not generally a worry as the design is built to cope with substantial loading and most of the hull remains in the water. However, some fishermen (formerly like myself) do overload their vessels sometimes by putting too much deck cargo (crab pots) which of course compromises stability.
Oncoming seas are not a problem because of the plum and high bows with deep forefoot that slices through the waves. However, avoiding launching off wave tops has to be done as well as fore-reaching which is taking swells at an angle to lessen their downside.
Beam seas are what trawler-style boats are built for. So, they roll with the sea and their good stability minimises this, which allows the crew a good degree of comfort. The hull's slow speed limits the amount of evasive action that can be taken – like running from the wave as a planing hull can do – so the vessel's sturdiness and enclosed topsides are designed to withstand a swamping.
Running with the seas is a common scenario during fishing trips, in moderate conditions.
Shape A is the classic displacement hull, as opposed to planing varieties B and C. Credit: Wikipedia
Broaching can be problem because of the displacement hull’s keel catching (rather than surfing) on the waves. And given the slower speeds of this hull, waves are faster causing the boat to yaw and roll. But that deep rudder is a lifesaver as it usually stays in the water so remains in control and the high stability ratio means the vessel can recover from major rolls. However, going beyond the hull's speed is a common scenario in big seas, and then things get a wee bit unpredictable. Anticipating the waves, throttling back or speeding up in moderate conditions are some techniques.
Overall, this is the best bluewater vessel for those with adventure and the far horizons in their sights.