Understanding how your hull shape responds to waves will keep you and your crew safe and comfortable in a blow, says Julian Woolfram
How hull shape affects comfort at sea
There are many desirable attributes sailors want in their cruising yacht.
Stability is obviously a crucial factor, but there are other important factors that should be considered when judging a sailing yacht and the balance of these will depend on the type of cruising envisaged and whether there may be some club racing involved.
Comfort, or seakindliness, is high on many people’s list and a boat that bounces around or slams in a choppy sea is generally shunned by all except hardened racers who will put up with any discomfort in the pursuit of speed.
When the wind picks up a bit, a yacht that responds with a good turn of speed is a delight and can make the difference between arriving in daylight and tying up in the dark after a long passage.
Finally, there is course-keeping and manoeuvrability.
A yacht where the helm can be left unattended for a minute or two can be a godsend to a short-handed crew, but the same crew will welcome good manoeuvrability when going into a crowded harbour or marina.
A yacht regarded as being ‘well-found’ is usually a boat that responds slowly and gently to the waves and doesn’t slam when sailing or motor-sailing up wind in a bit of sea, but she may not be the fastest boat out of the blocks.
Comfort depends on low accelerations, and research has shown that the travelling public feel ‘general discomfort’ when accelerations exceed 0.2g (more than 20% of the acceleration due to gravity).
To put this in context, the acceleration at the top of a big, very steep wave can reach 0.5g.
There are international standards on comfort, based on acceleration figures, that have to be met by operators of public transport systems and these are used in the design of manned and unmanned rail and road vehicles.
Whilst we can maintain the smoothness of rails and roads you can’t do the same for the sea, so ferries have ‘operability criteria’ that limit the sea conditions in which they can operate.
Of course bigger vessels are able to operate in more severe conditions than smaller ones. There are also Motion Sickness Indices based on accelerations that predict the number of passengers likely to be ill.
This decision should depend, at least in part, on how comfortable the yacht is in a seaway.
Now the comfort of yachts of the same size can be remarkably different. Take yachts of around 35ft or 10.6m long – a common yacht length.
Three examples show how much the displacement can vary for this size of vessel. The Heard 35, a traditional long keel working boat type design, weighs 12.7 tonnes; the Hallberg Rassy 352, with quite a long keel but separate skeg-hung rudder, weighs 6.7 tonnes; and the Pogo 10.50, which has a very modern wedge-shaped hull with a narrow but deep lifting keel and spade rudder, weighs 3.6 tonnes.
Hull shape: Rolling
Consider these three yachts moored in a line when a fat motor boat goes past with a big wake.
The heave (lifting) force produced by the waves will depend on the yachts’ waterplane area and these will all be roughly the same for all three.
So, initially, they will all experience the same heave force. Now remember Newton’s famous law: force = mass x acceleration. This can be rewritten for boats as:-
Acceleration = Force/Mass (displacement + AV M)
Of course Newton’s Law is usually applied to objects in air, and when a boat moves in water it moves the water around it as well, and this must be accounted for.
This is known as the added virtual mass (AVM) and it will be broadly similar for all these yachts when heaving, and has the beneficial effect of reducing the acceleration.
When it comes to rolling, weight is also an advantage, given careful design and a metacentric height (GM – the vertical distance between a boat’s centre of gravity and its centre of buoyancy) that is not unnecessarily large.
The sail-carrying capacity of a yacht depends on the product of its displacement and GM, so if the displacement is large the GM can be smaller.
A lower GM will lead to lower angular accelerations and a more comfortable rolling motion.
Let’s assume that all the yachts will heel to the same angle when setting full sail in a light breeze on a close reach.
The Heard sets 92 sq. m, with its topsail up, the Hallberg Rassy 64 sq. m and the Pogo 71 sq. m which means the Heard can have a lower GM than the Hallberg Rassy and both can be lower than the Pogo.
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The result is that the Pogo 10 will accelerate nearly twice as quickly as the Heard 35 and about 30% more quickly than the HR352.
So the Pogo 10 motion will be lively whereas the Heard 35 will respond slowly and the HR352 will be somewhere in the middle.
A single wave breaking against the side of the hull will be more noticeable jerk in the Pogo 10 than in HR352 and less noticeable again in the Heard 35.
Interestingly the late Ed Burnett, in discussing the design for his long keeled yacht Nomad, decided against a carbon fibre mast and in favour of a wooden one so as to keep GM smaller and the rolling motion more comfortable.
The reasoning behind this decision is reflected in the diagram above, which shows the estimated roll angles and corresponding angular accelerations for our three yachts whilst on their moorings with waves on the beam.
In a harbour with a 6-knot speed limit the waves generated by passing craft will be between 2m and 6m long, enough to get resonant rolling of the Pogo 10.
But it needs a passing vessel to go at 9 knots to get the Heard 35 to roll.
More significantly, although the maximum roll angles are reasonably similar, the acceleration is much higher for the light Pogo 10.
No wonder racing crews tend to sleep ashore whenever possible. (It should be mentioned that these curves are based on simple linear theory and it is assumed the damping is the same in each case, however these approximations do not affect the broad thrust of the results.)
So in general, for a given length of boat, a greater displacement will result in a more comfortable motion in the sort of waves you might encounter in an anchorage.
Of course the bigger the boat the greater the comfort, and large vessels don’t move much when the fishing boats tear out at 0400!
Hull shape: Pitching
When sailing, of course, the sails will damp out the rolling motion but not the heaving and pitching.
The diagram below shows our yachts going into head seas at 5 knots. Here the wave height is 30cm with a wavelength of 5m.
This goes up to 1.3m when the wavelength is 35m, which is roughly what may happen when the sea rises as the wind increase to Force 6 (25 knots).
The natural period of pitching is larger for a heavy yacht and resonant pitching will occur when driving into longer and generally higher waves than for a lighter yacht.
This is the one occasion when heavy displacement can be a negative in terms of comfort.
The more symmetrical hulls normally associated with heavy displacement have less damping and tend to ‘hobby horse’ in these conditions.
However, they can drive through a short chop comfortably with small accelerations when the light boat is having a more torrid time.
Of all the yacht motions, slamming is often the one that produces the most discomfort and accelerations of over 3g have been measured by Southampton University on racing yachts going upwind.
Slamming was first studied analytically by Theodore Von Karman in connection to seaplane floats in 1929.
He showed how important it was to have a ‘V’ shape to the bottom of a seaplane float to reduce the slamming impact on landing. Naval architects call the angle the hull makes with the horizontal at the centreline the ‘dead rise’ .
So a flat bottom has 0° dead rise.
Von Karman showed if you doubled the deadrise angle the impact force should half.
Impact pressure = 1/Tangent of deodorise angle
So, approximately if you increase the deadrise angle from 5o to 10° you halve the impact pressure and significantly reduce the slamming. (This formula doesn’t work when there is no deadrise, i.e. a flat bottom, and Von Karman produced a more complex analysis for this case).
Unfortunately, a trend was started in the 1970s to give sailing yachts flat-bottom bows following the introduction of the International Offshore Rating rule.
The rule wanted to get an estimate of displacement for each yacht without actually weighing it. That was considered too difficult at the time.
So they introduced a complex series of measurements to various points around the hull beneath the waterline including some at the bow.
However, yacht designers soon started to distort the hull shape at the measurement points to kid the rule that a light hull was actually heavier, which gave the yacht a better handicap than would otherwise be the case.
The diagram above shows what was typically done in the bow region to get a lighter boat with lower wetted surface area.
My first personal experience of this trend came in the 1980s when I changed my Trapper 500 for a Westerly Fulmar.
The Fulmar was quicker and had much better accommodation, but it was much more prone to slamming.
I remember motor-sailing, to catch a tide, down the Irish coast into a short chop when it seemed to slam on every other wave.
The Trapper, on the other hand, rarely slammed even when my kids decided to take a shortcut through overfalls at the top of Skye while I was having an off-watch nap below – it was the pitching that woke me, not any slamming.
The photos show clearly the difference in bow shape.
A ‘V’ or well-rounded bow shape is the best to avoid slamming and if there is any flat area near the bow it should be small to ensure the deceleration on a slamming impact is minimised.
If you look at lifeboats and RIBs you will see they have deadrise angles of at least 20o, even at the stern.
Going at high speed in a flat-bottom boat through a steep chop can be an ankle-shattering experience!
Drawing a chalk line around the hull mid way between the bow and the keel from the waterline on one side to the waterline on the other side and viewing from the bow will give a good impression of how seakindly, or otherwise, the bow is.
If you have the body plan for the hull look at the shape of the sections near the bow.
Hull shape: Wave slap
Now look at the stern, if it is wide and rises at a shallow angle through the waterline to accommodate a spacious aft cabin, it will probably suffer from ‘wave slap’.
Ideally the angle should be about 17o.
Much steeper, especially on a wide stern, and water flow separation will increase drag; much shallower and there will be wave slap (see below).
I watched a fat little fishing boat going out at a fair lick from an inner berth in Le Crouesty marina last year and nearly half the sailing yacht sterns it passed were slapped on their undersides by its wake.
Not a great way to wake up (excuse the pun) in your sumptuous aft cabin at 0530.
Above and below decks
The other factor to consider in how your crew will experience the seakindliness of any boat is where they are on the boat.
We all know that sleeping in the bow cabin at sea can be like a fairground ride compared to the saloon.
A good sea boat will have this factored into its cockpit and accommodation arrangement, with everything that will be used at sea in the middle third of the boat’s length – heads, chart table, galley and sea berths all need to be here, close to the point of minimum motion just aft of amidships and close to the waterline (see below).
The same is true on deck.
Ideally the helmsman and crew should be able to operate the boat from the forward part of the cockpit and not have to go forward of the mast, or to be perched right at the stern.
While many larger boats, and some boats as small as 30ft, now have twin wheels shoved out to the aft quarters to create more cockpit space and a better view forward, this isn’t the most comfortable place to be.
Tiller steering on small boats is no bad thing as it allows the helm to be much further forward, where there is less motion and more shelter.
However, another person’s ideal sea boat may be very different to mine.
It’s just important to understand the difference that their design makes to the conditions you are happy to set out in, understanding when it’s going to be uncomfortable, and how to handle your boat when the wind does get up.
Source: Yachting Monthly