The potential effects of
Types of marine engines
Motorboats use a wide variety of different
engines and transmission systems. This makes an assessment of
the generic impact of emissions from such craft particularly
difficult. The emissions themselves also vary in their importance.
Some, such as carbon dioxide, are harmless to the local marine
environment, although they may make a very small relative contribution
to global environmental effects, such as climate change. Others,
such as hydrocarbons, may impact upon the water column and sediment.
To compound the difficulty in assessing emissions, there has
been little research carried out specifically into the impacts
of marine engine emissions on the environment.
This section attempts to identify the different
emissions from motorboat engines and the potential impact of
these emissions on the marine environment. It begins with a
description of the different types of engine and drive systems
It should be noted that emissions
are gases exhausted from engines. They should only be classed
as pollutants when they have an actual direct or
indirect impact on the environment.
Types of Marine Engines
The vast majority of marine craft use one of
the following engine types:
- 2 stroke engine fuelled by a petrol/oil mix or converted
- 4 stroke engine fuelled by petrol or propane
- 4 stroke engine fuelled by diesel
Recreational craft engines come in two broad
categories - outboard or inboard. These different engine types
have markedly different emissions characteristics as outlined
Outboard engines are usually mounted at the
rear of the vessel and have a self-contained drive unit. The
small size, weight and low purchase price make outboard engines
popular for smaller craft. Most outboard engines are two stroke
petrol units using an oil/petrol mix although four stroke outboards
are growing in popularity and the use of propane gas for both
4 and 2 stroke engines is also becoming more popular.
As the name suggests, inboard engines are mounted
inside the craft and drive is relayed through various different
transmission systems such as shaft drive/propeller, stern drive/propeller,
sail drive and water jet drive. The latter are used on personal
Inboard engines tend to be used within larger
craft and are usually fuelled by petrol or diesel. These engines
are generally more powerful and are used where the weight and
size of the unit are not the overriding consideration. Many
inboard engines are marine derivatives of commercial automotive
engines. Although diesel inboard engines are becoming increasingly
popular, four stroke petrol engines are usually chosen where
installation space is more limited or where higher power to
weight ratios are required, as with watersports craft.
Electric engines are becoming increasingly
popular on inland waterways, but due to limited power and range,
they are not generally used in coastal areas.
During the combustion process, internal combustion
engines of all types generally produce, in varying quantities,
the following substances:
- Oxides of Nitrogen (NOx), a potential contributor to photochemical
smog and to ozone layer damage
- Carbon Monoxide (CO), a toxic gas
- Carbon Dioxide (CO2), the most significant cumulative 'greenhouse
- Hydrocarbons (HC), a constituent of photochemical smog
- Sulphur Dioxide (SO2), an element in acid deposition
- Lead, a toxic heavy metal
- Particulate matter, a potential carcinogen.
4 stroke engines generally produce higher CO,
CO2, and NOx, but lower HC than 2 stroke engines. 2 stroke engines
emit relatively high levels of HC in both burnt and unburned
form, but low levels of NOx. Diesel engines are more fuel efficient
than 4 stroke or 2 stroke petrol engines and therefore emit
lower overall CO and CO2 However, they produce greater quantities
of SO2, Nitrogen Dioxide and particulates. In terms of marine
features, it is the effect of hydrocarbons and lead in the water
column and sediment that are of key significance.
The impact of marine engine emissions on the environment has
been the subject of much discussion over the last twenty years.
Despite this, there has been relatively little original scientific
research conducted on the subject in recent years, particularly
concerning the potential impacts of emissions on specific habitats.
There are studies which examine the implications of engine emissions
on non-designated marine species and plants (e.g. the Institute
of Applied Environmental Research in Sweden reviewed the potential
effects on fish). However it should not be assumed that the
findings of such reports have significant consequences for the
assessment of the designated features within mSAC areas. Features
other than the specific Annex I and II features designated within
mSAC areas largely lie outside the remit of this report.
Legislation concerning marine engine emissions
is to be taken forward in the forthcoming European Directive
on marine engines which will specify common European emissions
standards. The standards will extend to emissions such as benzene,
about which very little is known in the context of impacts on
the Annex I and II habitats and species in mSAC areas.
Hydrocarbons - modern outboard engines exhaust
below the water surface and, as a result, all hydrocarbon emissions
pass through the water. Most remain in the gas phase and are
released directly to the atmosphere; the remainder condense
and become suspended in the water column or form surface film
until they degrade or are released into the atmosphere. Evaporation
is the most significant factor in the removal of exhaust products.
Hydrocarbons reach the sediment slowly due to the efficiency
of the degradation process.
In response to proposed legislation on the
Bodensee in the early 1990s, a study on the contribution of
outboard engines to marine pollution was carried out by Barlett
(FSRC, 1990) in conjunction with the International Council of
Marine Industry Associations (ICOMIA). With the exception of
one site, the highest concentrations of total hydrocarbons were
found to be close to large municipal centres, or near the mouths
of major rivers draining industrialised and urban catchment
areas, rather than in heavily used boating areas. The report
suggested that, in general, residues from outboard motor oils
are likely to be negligible in relation to other sources of
the hydrocarbon burden in the sediments.
This conclusion is also supported by the TNO
Road Vehicles Research Institute (1991), which found that, at
present, there is little environmental impact caused by marine
engines. Hydrocarbon contamination of sediment is not, to any
great extent, caused by the use of 2 stroke marine engines.
Butcher (1982) also concluded that outboard engine use was not
a major cause of hydrocarbon pollution of the water column,
and although hydrocarbons in surface films were more problematic,
evaporation was rapid.
However, recent research carried out by the
State of California Air Resources Board (1998) found that two-stroke
engines can discharge as much as one-third of the oil/petrol
mixture unburned into the water. It concluded that there is
potential for considerable impact from such emissions. Clearly,
such contradictory evidence emphasises the need for further
research in this area.
Lead - Butcher (1982) found that lead concentrations
were increasing, but that at that time it did not create quality
problems in the water column. However, long term build-up of
lead in the sediment was considered to be of potential environmental
concern. He concluded that sublethal and long term biological
effects of leaded fuel cannot be entirely ruled out. The increased
use of unleaded fuel has led to a decrease in, although not
a total elimination of, the amount of lead in engine emissions.
Two stroke engines use a method of combustion
which results in some unburned residual oil and partially burnt
oil entering the marine environment through the exhaust. This
leads to the familiar sight of small patches of oil forming
on the water where 2 stroke marine engines are being run.
Whilst the environmental effects of these emissions
are uncertain (see above), legislation has been implemented,
or is being considered, in many countries with the aim of severely
restricting such emissions. In response, the oil companies are
putting increased emphasis on the use of biodegradable lubricants
for 2 stroke engines.
Although commonly used mineral oils eventually
degrade, the process is very slow and such oils would not normally
be defined as biodegradable. Therefore biodegradable lubricants
are usually formulated from non-mineral oil fluids, both from
vegetable sources such as rapeseed, and from manufactured esters
with similar properties.
Biodegradable lubricants are designed to break
down into carbon dioxide, water and biomass by the biological
activity of micro organisms. The quantity of biomass formation
depends on the extent of biodegradation while the speed of the
biodegradation process depends on water temperature. In deeper,
colder waters biodegradation takes longer than in warmer waters.
Whilst mineral oils remain the dominant lubricant
in the marine market, increased use of biodegradable oil in
2 stroke outboard engines could reduce the magnitude of any
hydrocarbon impacts on the marine environment. However, the
biodegradation process produces a number of toxic by-products
which have an unknown impact on the environment. Furthermore,
by-products of the 2 stroke combustion process will continue
to be emitted with the biodegradable lubricant.
The case for the use of biodegradable lubricants
in 4 stroke engines is even less clear. It is also important
that consumers are not misled into believing that the lubricant
can be disposed of in a more casual manner than common non-biodegradable
lubricants. In particular, the used product may contain non-biodegradable
components and contaminants which, if disposed of incorrectly,
will impact on the environment. There is also uncertainty over
the recyclability of the used product.
In many ways, accidental discharges of oil
and fuel present a potentially more serious environmental risk
than regular discharges from running engines. New European boat
safety standards address the issue of fuel equipment in some
detail and this should ensure that most boats at least have
safe equipment. The handling of the oil/fuel by the boater and
fuel dock attendant is another matter. Fuel docks are inspected
to ensure safe functioning but incompetent handling of the pumps
can cause problems if there is no sealed system in operation.
In most inland waterways the pumping of bilge
water into the waterway is prohibited. However, although technically
illegal in coastal areas, detection of bilge water pump out
is notoriously difficult where discharge does occur and it is
uncertain how much oil enters the marine environment in this
The discharge of oily bilge water can be minimised
by the use of absorbent pads placed underneath the engine installation.
These can be regularly cleaned or replaced to ensure the bilge
water is not unduly contaminated.