of Seagoing Rowing Boats
Guide for users
by Nick Newland - www.swallowboats.com
Seagoing rowing boats of high performance are difficult to design
because of the multiplicity of conflicting factors to do with
ergonomics and seakeeping, combined with the low power available
to drive the boat.
The type of use is also important.
A boat designed for 2/3 hour excursions might be quite different
in hull form compared to a flat out racer designed to race over
a 45 minute course.
The diagram below illustrates
some of the factors that go into making a good rowing boat. As
can be seen, almost every factor conflicts with another. For example,
speed dictates a narrow hull, whilst for stability a wider hull
is required. Because of the limited power available the compromises
inherent in a good seaboat design are very finely drawn.
It is important therefore to understand
the real requirement and then balance the conflicting factors.
The computer aids help, but judgement and personal experience
are also important in achieving a balanced design.
This note is intended as
a non technical aide memoir to help people make the right decisions
as to their boat. The note is split into 3 sections. The first
discusses some of the contradictory factors in the diagram above,
the second looks at hull structure, and the last attempts to summarise
what leading particulars should look like. The issues are discussed
using a coxed double sculled skiff as a reference, mostly because
it represents a frequent requirement from the marketplace.
It is hard to overstate the
importance of weight. It fundamentally effects speed, and contributes
very much to the difficulty or otherwise of handling the hull
ashore. All the calculations that follow assume that each rower
can develop a quarter horsepower (about a couple of light bulbs
worth !). For short distances this is pessimistic, with powers
of as much as half horsepower being developed, but after half
an hour lung function begins to dominate and most mortals taper
out at about quarter horsepower Typical weights for a 17 foot
seagoing double scull with cox might be
Why worry about such a small proportion
of the weight? For a 150 lb hull with two people rowing it and
some gear, adding 30lbs to the hull might drop the speed by about
0.1 knots. This might not seem much, but in a race it is a lot.
Weight is the place that costs
money in production and in design. Minimum weight is a principal
reason why aeroplanes cost so much.
The later section on structure
looks at typical arrangements in current boats and their effect
on weight, durability and other factors. Suffice to say here that
weight increases the resistance of a boat by deepening the draft
increasing surface area and hence skin friction, and by increasing
the size of the waves generated. The latter is more important
in general as it is the waves that limit rowing boat speeds.
Length is an important issue. The
length of a boat determines it's maximum speed, so generally speaking
the longer a boat the faster it is. However, that is not the whole
story, and if you haven't the power then maximum speed will not
be reached. Taking the earlier skiff (including gear) as a starting
point, and increasing the length might give the following speed
The weight increases with length,
as you can't increase length without increasing weight in the
real world. Length is giving you speed, by reducing the wave making
drag at the expense of skin friction drag. The 17 footer has about
equal skin friction and wave making, whilst for the 21 footer
the wave making is half the skin friction (but the skin friction
has gone up due to the length increase).
Whilst skin friction drag increases
constantly with speed, wave making drag increases extremely rapidly
with speed after a threshold determined by boat length. This threshold
begins when the speed of the boat (in knots) equals the square
root of the waterline length in feet (root L). For heavy hulls
it becomes so punitive that it effectively limits speed to 1.4
x (root L). For lighter hulls (in proportion to their length)
slightly higher speeds are possible. This is because the size
of the waves produced depend in part on the weight of the boat.
There must be an optimum length
for the double scull depending on how fit the crew are. For example
looking at smoothwater competition double sculls will set an upper
limit as to length of about 27 ft. Seagoing boats need more beam
than such boats, thus increasing the wetted area so that the skin
friction component will be greater. To reduce the skin friction
the length needs to be reduced, but this will increase wavemaking
resistance. Thus begin the compromises!
In the end length must also take
cognisance of practical things like transport problems sea-keeping
(longer hulls need more freeboard) and the Recreational Craft
2.3 Beam and Stability
The next most important factor
after length and weight is beam. Beam has a crucial effect on
drag, as any beam over the minimum will increase surface area,
and hence drag. So for speed you want minimum beam.
For stability however you want
beam and a low centre of gravity. Since two crew and a cox is
probably the predominant weight, the height of the crew is very
important in assessing stability, and as will be seen later this
has an important effect on seakeeping since it directly affects
The diagram below illustrates
the effect on circumference and hence surface area of different
boat beams. The minimum value is for a perfectly circular bottom
as is adopted for racing sculls. It is unstable, and requires
the oars and body weight to stabilise the boat.
We have built a 24" beam boat
(12" ellipse in above diagram) and it feels unstable (tippy).
The Teifi Skiff at 36" waterline beam feels fine so somewhere
in between 24" and 36" is probably the ideal waterline
beam for a practical rowing boat.
Looking through boat design references
in relation to rowing boats you will find that recreational sculls
seem to have waterline beams of about 23" to 28", whilst
more practical rowing boats have waterline beams of at least 2
ft 10 ins, and generally much more.The American Naval Architect
Phil Bolger has designed many rowing boats and we can't find a
design of his below 3 ft waterline beam.
The next diagram illustrates the
effect of different cross section shapes on surface area. Again
for a given beam the minimum is an arc of circle, but the other
shapes are remarkably close, and even the hard chine is only 3%
worse than the best.
It is easy to be simplistic
about beam and section shapes. Stability is determined by beam
and shape, and shapes with a flatter bottom are much more stable
than those with a high rise of floor. The pure vee bottom in the
diagram for example would probably be unstable.
For a given weight and length of
boat, an increase in beam of 20% might increase the surface area
(drag) by 13% and the stability by more than 60%. The stability
numbers can therefore change radically with quite small differences
This compromise between beam
and stability is at the heart of good rowing boat design. To get
it right is difficult. Much depends on the skill of the crew,
and the kind of conditions they habitually row in. Experience
and a good set of references to extant designs is essential to
reaching this compromise, since no one has managed to create a
computer model for rowing skill as yet !
2.4 Ergnomics - Layout
of the works
The layout of the seats, gates
(oarlocks), and footrests, is extremely important. Experience
is a hard won prize, and most people with experience will have
their own views on what parameters constitute a good layout. A
minimum list would be
Height from heel to
Height from gate to
Distance from gate
to rear edge of seat
Distance from rear
edge of seat to foot stretcher
Angle of foot stretcher
to the vertical
Distance between gates
fore and aft
In relation to seating positions
different areas have their own different traditions. Here in West
Wales we use central seating, in Cornish gigs they use offset
seating to get a longer oar for the same beam. In Australian surf
boats they use offset seats with a semi-sliding seat. All the
arrangements have advantages and disadvantages.
Stroke rate has a major influence
on the layout as it determines for a given length of oar what
sort of speed can be achieved. The length of oar in turn determines
the beam required at the oarlocks or gates. Or to put it another
way the beam determines the length of oar which determines the
speed via stoke rate.
The diagram below shows a
representation of the oar stroke. The oars are rotating through
90 degrees in time t, with an oar length of r1 outboard of the
The distance swept approx = 90
x 2 pi x r1 /360 = pi x r1 /2
Speed approximately = Distance swept/ time = pi r1/(2 x t)
All oars have about 70% of their
length outboard give or take a few percent. Thus knowing the beam
of the boat (and allowing a little for overlap) the oarlength
can be calculated and thus the outboard length r1 above.
Rowers aim to achieve a cycle of
power stroke to recovery of about one third power to two thirds
recovery. Hence given the rate R (usually in strokes per minute)
the time for the power stroke t can calculated. Rates vary from
20 to 40 plus depending how close you approach Olympic standards
Substituting and simplifying a
Speed approximately equals 0.1
x R x B
Taking the 17 ft boat and fitting
in the values of 4 ft for the beam and say 30 for the rate gives
a speed of about 12 ft/sec or 7 knots with oars of about 7.5 ft.
This matches the theoretical hull speed for the 17ft double scull
quite well at about 6.5 knots. The point to note is that narrow
hulls, whilst capable of being driven fast, cannot achieve the
speed because the oars will be too short, necessitating unrealistic
Some boats do overcome this problem
by rowing with hands crossed to the elbow. The most notable example
of this layout is probably the American guide boat used in the
Aidirondak lakes, but all accounts suggest that this is an ergonomic
nightmare that is an acquired art.
The other obvious solution is outriggers.
Outriggers are a good solution if you have the need for a narrow
hull. The 24 inch beam scull mentioned earlier was fitted with
stubby outriggers and it works fine. Rowing it at sea required
some skill with the oars as stabilisers and it would probably
have benefited from more waterline beam, which would have very
likely removed the need for outriggers So in real seagoing boats
outriggers may not be of much advantage, the speed being limited
by the length of the boat, the beam being dictated by stability,
and the beam being adequate to support oars of adequate length.
Certainly outriggers are a pain when coming alongside, putting
it on the roof rack etc.
For boats used at sea this is an
important aspect. It is dominated by stability and freeboard (height
of the sides of the boat above the water) considerations. Stability
is required to keep the boat upright and freeboard to stop the
waves coming inboard. Seagoing boats have to have more stability
than smooth water boats. In rough water too little stability gives
problems in handling the boat, the uncertain motion giving rowers
difficulty in positioning their oars.
On the other hand too much stability
is also bad news in that it leads to a jerky motion, throwing
the crew around and creating almost as much of a problem as too
little stability. Too much flare above the waterline can have
similar results as too much stability. Stability is largely determined
by waterline beam, and the position of the centre of gravity and
needs good judgement if a reasonable performance is to be achieved
since beam adds surface area and hence drag.
In small rowing boats with light
hulls the centre of gravity is largely determined by the position
of the crew. Ergonomics put a minimum height to the crew centre
of gravity and this in turn determines the position of the oarlocks
or gates and hence the freeboard. Seakeeping requirements might
then increase the freeboard depending on use. Additional freeboard
drives the rowlock/gates up and hence drives the crew centre of
gravity higher and requires more beam for the same stability penalising
Freeboard also has a major effect
on wind drag and particularly ease of steering in cross winds.
For speed in windy conditions, a very low freeboard is required.
The place where freeboard is most useful is near the bow and stern,
so low freeboard at the rowing position can to some extent be
offset by a strong sheer and good arrangements to lift the bow
and stern quickly to meet oncoming or following waves. Such arrangements
might include a strong rake to the bow/stern in profile, and additional
flare at the ends.
Crew skill also comes into the
equation since very skilled crews can handle an otherwise difficult
hull using the oars and their body weight to overcome the hull's
limitations. Heavier hulls also help in giving a more predictable
motion and a lower centre of gravity, but always at the expense
of speed in smoother waters.
Whatever is done to improve seakeeping
therefore is almost invariably in conflict with the parameters
to give speed. Experience and a good database of designs is an
essential requirement to get the compromises right.
After the hull shape, the important
features that are necessary to a good rowing boat are:
Light weight - because weight
is a major component of resistance
Stiffness at the pin- because
this wastes effort in flexing the gunwale
Stiffness at the foot stretchers
- because this flexes the hull locally increasing resistance
Stiffness in torsion of the
whole hull - open boats are particularly vulnerable to twist
A recent examination of materials
by the American naval architect Dave Gerr (The Nature of Boats
) gives a ranking for material efficiency as follows (1.00 is
It can be seen that wood
is an excellent choice for structures like gunwales where bending
and column strength predominate. If it is so good why are racing
boats built with the other higher tech materilas such as graphite
and Kevlar ? The reason is that these materials use a double skin
structure or deep stiffeners to effectively increase the thickness.
The minimum weight hull is probably a double skin hand laid up
glass/carbon structure with a high tech core and utilising systems
like resin transfer moulding or vaccuum bagging to minimise resin.
It's expensive. It's vulnerable
to damage due to core/skin separation and minute leaks into the
core through grounding on hard jettys etc. Any such damage is
difficult to repair.
The next lowest weight is probably
cold moulded wood. For a one off it is competitive on costs, but
not for a production run. The most competitive system in wood
is probably plywood in either single or multi chine construction
using epoxy resins and taped seams or glued lapstrake construction.
Modern plywood protected by modern paints is an excellent material
with low maintenance costs, and high durability.
Whatever the merits of other materials
glass Reinforced Plastic (GRP) has become the industry standard
in terms of cost. Options possible are double hull GRP, single
skin GRP and some form of composite construction.GRP is not inherently
light, it's strength/weight ratio being only marginally better
The most common structures are
the conventional inner and outer moulding (double hull) that turn
up in boats all over the country. The double hull is there to
provide the consumer with a clean modern looking hull interior
with in-built buoyancy. It is hopeless when it comes to weight,
since it essentially adds another skin for little structural benefit.
This is because the core between the inner and outer moulding
is not strong in shear or compression and so cannot provide much
in the way of support for the outer skin without difficult and
expensive bonding operations between skins.
A rough comparison can be made
by dividing the boat weight by the (length x beam) to give a factor
for weight per unit area Some typical values for open boats of
double hull construction give a weight sq ft of plan area of about
4.5 to 5.
By omitting the interior skin, a single skin GRP structure with
GRP stiffening reduces the weight per unit area to about 2.5.
By adopting a more sophisticated approach to hull design and material
selection, it is possible to get robust GRP hulls down to a weight
per unit area of less than 1.8 using wood, aluminium, and plastics
in a composite shell, selecting each material to be best for it's
The other point to bear in mind
is that modern materials do allow designers to move away from
some features of traditional hulls that were there because of
the material rather than because it was needed from an a hydrodynamic
reason. Long vestigial keels, and deadwood aft were needed to
attach planking to. In terms of hydrodynamics they add drag and
though they achieve something (directional stability for one)
there might now be better ways of achieving the objectives. Keep
an open mind.
Torsional rigidity is particularly
problematic in open boats. The ideal approach is to deck it all
in and make it a closed boat! Lacking this way forward, stiff
gunwales, well connected Port to Starboard through the seat structure
and with adequate knees at the bow and stern have been the commonest
solution. Real care is needed in designing such boats to address
Details such as the strength and
stiffness of the seat knees are important. The feature that has
come to dominate material selection for the leisure market is
ease of maintenance. GRP is perceived to be lower maintenance
than other materials, but modern paints and glues have changed
the position of wood though not yet the public's perception. If
timber is used, correct choice of timber (only durable species)
and its treatment is important.
Where does all this lead?
Speed can be determined easily enough by calculation, the oar
length for that speed can be estimated and hence a shot at the
beam overall. The needs of stability and seakeeping will depend
on users appetite for excitement and the areas they expect to
use the boat in. Outriggers might be an option if extreme performance
Looking at the coxed double sculled
example. The power of two rowers in a long race will limit the
speed to around 7 knots even for the longest practicable boat,
and this speed can be attained with 7.5 to 8 ft oars with a reasonable
stroke rate. Such oars can be handled in a 4 ft beam boat without
recourse to outriggers. So the length might be 18 to 24 ft and
the beam about 4 ft.
The waterline beam is more problematical.
Three feet seems on the higher side of our experience, and maybe
some reduction might be possible here. Not much since remember
that the 20% change in beam gave a 60% change in stability
If the longer length's are aimed
at then more freeboard will be necessary to allow for the longer
length and practical considerations of storage, transport and
handling need to be carefully considered before the final decision
is made.Not surprisingly if designs in the 15 to 20 ft region
are considered there are many boats not far from these numbers.
In particular a beam of 4 ft seems a very common feature.
This points to a truism of sorts,
in that the boats that have evolved over time will generally show
the way to what works. Modern structure and a better understanding
of hydrodynamics will refine the solution but the evolutionary
ghosts will probably shine through in many places.