It is a fact that RIBs, like a lot of other small powered
craft, are subject to being very sensitive to weight distribution. Trim, a word
used to describe the angle or attitude of a craft when underway, is greatly
affected by weight and its distribution. It goes without saying, that an
incorrectly balanced craft will perform in an unsatisfactory, and perhaps under
extreme circumstances, unseaworthy fashion.
Some RIBs seen on the water today are prime examples of this,
with their heaviest items such as big engine, stern arch and elaborate seating
consoles, stuck on the back of the boat often causing it to be wholly inadequate
in a head sea. Coupled to this, the problem can be further accentuated in some
instances with those RIBs whose sponsons are set well clear of the waterline.
For example, when large amounts of horsepower are applied, without the lateral
stability of the sponsons to provide minor correction underway, a stern heavy
craft of this type can be even more prone to chine walking etc. The matter of
counterbalancing with fuel tanks or ballast is important, plus ensuring that
internal fittings such as consoles, seating etc. are all positioned correctly to
obtain the optimum balance and performance. When we speak of performance
however, we generally think in terms of speed and handling, but don't forget
that a badly trimmed boat will labour and struggle to perform at its best, with
the result of an uncomfortable ride and costing you unnecessary money in
fuelling expenses too.
Most craft can be controlled sufficiently well to guarantee
economical performance and a comfortable ride in calm conditions. Nonetheless,
the problem comes when the seas get up, it's here that the weaknesses are
exposed, especially if the boat is being driven hard. Here's a tip, especially
if you go cruising; rig your boat for optimum head sea performance. It's very
much easier to mechanically lift the bow with power trim than it is to lower it
this way when powering into steep seas where bow down trim is vital. Difficult
head seas generally cause one to have to run at a very reduced speed, hence,
with the sensitivity of engine trim being largely dependant upon the amount of
power applied, the only other obvious way to achieve correct trim is by means of
weight distribution. Although the shifting of baggage etc. can help, it's
obviously far better to have a correctly balanced boat in the first instance.
A well designed RIB, suitable for deep sea use, will
generally incorporate many of the items seen below:
1. A low central point of gravity, i.e. endeavouring to
locate such things as fuel tanks below deck, to further aid lateral stability.
2. The positioning of helm console approximately bench just aft of amidships
so that crew weight and seating are not pushed too far aft - back to the the
wettest part of the boat!
3. Ensuring that engine weight is considered properly when
power units are fitted or installed. i.e just because an engine manufacturer may
state a maximum horsepower for your boat model, it doesn't always follow that
the largest motor would be best suited to your layout/configuration. Horse power
ratings relate to hull design only. (Remember also that 4 stroke engines are
considerably heavier than that of their 2 stroke counterparts.)
4. When buying a RIB, ensure that stern bench seating or additional pods
situated aft, are really capable of being able to accommodate weight of the
passengers they are intended for.
5. If a water ballast tank is to be fitted it should be f more than adequate
size. Many tanks fitted are too small. Ensure also that its dumping capability
is rapid enough.
6. The weight of any items such as self-righting equipment
should be very carefully considered before installation. If retro-fitted, then
consider that the addition of such may severely affect the performance and
handling characteristics of the craft, unless proper thought is given to the
matter of counter balance etc.
7. To assist lateral stability and help ensure a level ride,
endeavour to fit heavy items such as your battery etc. on the starboard side.
Offsetting the engine too is necessary to counteract the torque of the engine.
Even large RIBs are prone to the problems spoken of here. For
example, many of the RNLI Atlantic 75 crews confess that the good old
"21" was a much better craft in terms of balance and head sea
capability. It doesn't always follow that merely extending the length of an.
already successful hull to take larger amounts of horsepower, will guarantee
As RIBs are invariably used in difficult sea states, though
they may pound for pound have a very high degree of horsepower, speed will not
always be at the coxswain's disposal. A RIB still needs therefore to have
adequate trim control at low speeds when the stem of the craft is "digging
in," as opposed to when fully on the plane when the craft's stem weight is
lifted and is naturally held forward by the effects of momentum and its level
running attitude. In conclusion then, a well balanced craft is a pleasure to
drive, it will look after its crew and gain their confidence, plus in the long
run, it may even save its owner unnecessary expense, which can't be bad!