Propellers
If you believe
that engines are a distraction on Drascombes then you may have no engine at all
or perhaps a 2.5hp engine. In which case you will have adequate performance in
calm weather but little reserve power for adverse conditions and there are no
alternative propellers available to improve matters for you.
The picture
shows just some of the propellers available for the Mercury, Mariner, Tohatsu,
Nissan range of 4hp to 6hp single cylinder 4 stroke outboards which are the
engines of which I have most experience and which are discussed here.
Most people
don’t give a second thought to the propeller on their outboard, essentially
because they just work. But the standard propeller doesn’t give optimum
performance, particularly on a Drascombe. Unless the engine was bought as a “
The choices to
optimise propellers are usually about pitch, diameter and blade design. For our
engines however, all available propellers are eight and three eighths of an
inch in diameter which simplifies the decision process no end. To simplify
things further there are just two basic blade designs and no folding or
variable pitch options.
One design has
large blade area and is known variously as “high reverse thrust”, “high thrust”
or “saildrive propeller”. It comes in just one choice
of pitch – 6”. That means that if you twisted it through a big block of butter
it would advance six inches for each full revolution. This propeller is
normally only fitted as standard on “Saildrive”
engines. These usually come as a package with a longshaft
and battery charging too. I shall refer to this propeller as the 6” saildrive.
The other
design has swept blades and is often referred to as “weedless”
because it is less susceptible to weed fouling. It comes in 6”, 7”, 8”and 9” pitches.
As standard fit the 4hp engines are often fitted with a 7” pitch weedless. The 6hp engines are usually fitted with an 8”
pitch weedless.
The propellers
in the picture above are -
Top row:
Standard weedless 8” pitch from 2002. An optional weedless of 6” pitch
Bottom row:
Standard weedless 8” pitch from 2009. An optional saildrive of 6” pitch
How do you
find out what pitch your propeller is? That is usually easy enough since the
pitch in inches is cast into the back of one of the blades as on this 8” one
below. Sometimes the casting is a bit rough making the number difficult to
read.
What
difference does it make? There are three issues affected by propeller choice. Boat speed, economy and responsiveness. The standard 8” weedless is not best for any of these issues since it is
designed to make a small lightweight aluminium US
fishing boat or inflatable tender with one adult on board plane as fast or
faster than other makes of outboard of the same rated horse power. If you look
at forums when performance is mentioned this is the kind of top speed data that
is discussed. With such a light load the engine will reach its maximum rpm and
power and the propeller will not “slip” too much. In fact on the 6hp engine the
8” prop is recommended for boats having a top speed of 12 to 18 mph!
Drascombes do
not plane and cannot reach anything like these speeds. For Coasters a realistic
maximum under power is about 6.5 knots. The result is that with the standard
prop the engine will not reach its rated rpm or horsepower. Like driving up a
hill in too high a gear, the throttle is wide open but progress is slow. In
theory every 1” reduction in pitch should allow the engine to rev 250rpm
higher. Testing is not easy since water conditions have a significant effect
with even small waves but my tests reveal a near 10% improvement in the maximum
speed of a Coaster laden for cruising when an 8” is replaced on my 6hp engine
by a 6” weedless. The wide open
throttle rpm for this engine should be in the range 5,000 to 6,000 rpm. With the
6” weedless it manages to get up to 5,450 rpm. With
the other propellers it does not get into the correct range. I would expect a
corresponding improvement in economy since it should be possible to achieve the
same cruising speed with a smaller throttle opening. Unfortunately, identical
conditions would have to apply for rather a long time to be able to accurately
test economy (which is excellent in any case). Luggers
with their shorter waterline length and therefore greater resistance at
cruising speeds have an even greater need of a smaller pitch. However, if the
6” weedless propeller is replaced with a 6” saildrive up to 3% of the speed improvement is sacrificed
but the small sacrifice is in my opinion worthwhile as
I will explain.
After speed
and economy the other issue is responsiveness. What does that matter? Well,
when you are picking up a mooring and there is wind or tide running or when you
are coming alongside a quay or pontoon - particularly if there are boats ahead
and astern of you, you will often want to “put on the brakes” or swap forward
for reverse motion. The difference when you engage reverse to stop the boat
when manoeuvring is remarkable. The weedless design
has a relatively small blade area. This reduces friction at speed and allows
the prop to slip and churn when fitted to a heavier boat. The downside of this
smaller blade area means that it has less “grip” on the water. The big blades
of the saildrive however, give a higher thrust. It is
like having beefier brakes and more power to punch through the waves when the
going gets rough.
In conclusion,
should you change your propeller? If you are about to buy a new engine it is a
“no brainer”. Specify a saildrive package or just a saildrive prop. If you are not about to buy a new engine it
depends on how much and what kind of use your engine gets. These propellers are
not cheap. I paid about eighty pounds a few years ago. So if you are going to
get a lot of use out of it go ahead. If not then look forward to your next engine.