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This article first appeared in  RIB International.

Rib International - 'In the balance'

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 greater performance.

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! HMS

Author: Hugo Montgomery-Swan
Contact: www.ribmagazine.com
 

 
 
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