THE PLANING HULL

Changing the dynamic behavior of a dinghy is possible thanks to the hull appendages (flaps, brackets, jack plates, trim tabs, etc.), many common in competitive competitions, a little less in pleasure craft. 

But are they really useful or are they just a palliative? 

It depends on what you want to achieve by carefully evaluating the contraindications (starting with the costs!)

That, inevitably, every choice entails.  In the automotive field, in recent years there has been the development of modular design platforms that can be adapted to several models, even of different types and brands: this synergy is used to create vehicles that, with few sacrifices in road behaviour, are able to adapt to fairly suitable uses.

Here’s one of last invention in the nautical field:

THE NAUTICAL FIELD

In the nautical field, things do not work exactly in the same way since the marine vessel and the inflatable boat in particular have a much more variable displacement due to the greater quantity of cargo that can be carried (passengers, luggage, fuel, water, equipment, food and so on), as well as for the variability of the propulsion with consequent oscillations of the static and dynamic centres of gravity (which does not happen on the road, since the car does not glide). 

It follows that a well-designed hull must already withstand an extreme diversity of loads in different weather conditions;  if this were transplanted into a completely different medium, the results would certainly not be the same. 

The infamous “design deck”, imaginable as the set of all the requirements that the dinghy must meet, is always too “short” and no marine vessel can excel “in every aspect, load and condition”: the compromise requires that it the hull must go well in all conditions, can excel in certain aspects which are those that the requirement framework imposes, to the detriment of others. 

One of most efficient hull (actually effective over 23ft rhibs) is a step hull:

To give an example, a racing hull will have to excel in speed and perhaps in stability at high speeds, while it may leave something to be desired in stability at anchor because this is not a required prerogative.

A recreational hull will certainly be fast and keep the sea well, but it will not exhibit an exaggerated pitch and roll when the boat is stationary. 

Once you reach the limit of balance then there are some tricks to extend a hull to improve its performance towards a further variability of conditions or to optimize it towards some specific needs: these are the appendages that can be installed on an existing hull.

STEP 1: HOW TO BALANCE THE RHIB

Before proceeding in any alteration of the hull, it is important that the following rules are followed:

  1. PUT WATER TANK AHEAD, lots of rhibs have the issue that all the weight is astern, well if you are sailing alone a good thing would be to have the water tank on the bow, just behind the anchor.
  2. PLACE PEOPLE IN A BALANCED WAY, the smallest the vessel, more the passengers can modify its balance. Place them in a safe but balanced way
  3. PUT BAGS UNDER THE CONSOLLE AND AHEAD, this will help the planing and stabilise the vessel
  4. FIND THE PROPER SPEED, once all above is done, finding the proper planing speed is important to maintain low fuel consumption and be more reliable at sea
  5. TRIM THE ENGINE, a good setup is what is needed, find then the most safe and fuel efficient speed

STEP 2: PLANING DEVICES

LOWER UNIT HYDROFOILS

These devices are very useful on small vessels which tend to “wheelie”.

They improve the planing timing and makes the engine cavitate less. This product is produced by many manifacturer, in EU and abroad.

Features

  • Lower fuel consumption
  • Straight tracking
  • Excellent steering at low speeds
  • Smoother ride
  • Maximum lift
  • Reduced planing time
  • Increased stability

FLAPS AND TRIM TABS

On bigger vessels you might need to correct stability at sea, especially

These devices accordingly to my experience are more reliable on boats rather than on rhibs, because of two factors:

  • higher draught, boats have a deeper hull therefore (and especially single propelled ones) they need stability corrections
  • tubes, they are permanent flaps themselves

JACKPLATES

Let’s start saying there is no ideal engine, all transoms are different, and while a L shaft fits perfectly on a boat, on a rhib with same heigh, same lenght, the same shaft could not be as good.

The jackplate is an oleo-pneumatic system which allows the captain to decide the heigh of the engine. And here are the benefit you can encounter in installing this system:

  • towing in&out
  • adjust to any sea condition
  • improvement of prop efficiency
  • control the ride attitude
  • Better fuel efficiency and lighter handling

HULL HYDROFOILS

This system allows the hull to come completely out of the water level.

KEYWORD: EFFICIENCY

The biggest issue any hull need to face is planing. And especially on rhibs for above mentioned reasons the planing requires lot of power and fuel consumption, as the “wheelie” effect, creating significant hydrodynamic drag and affects the planing speed.

But once the hull is lifted you work on foils only and the immerged surface is down to 20%!

PROS

  • reduce the disturbance of waves
  • smoothest ride, but only up to a top speed
  • when not lifted improve stability or handling and in some circumstances

CONS

  • high expense for fixing if cracked
  • high speed limit due to structural manifacturing
  • longer shaft or (ideally) jackplate needed