Unbelievable hulls that «flatten» heavy seas!!!
How to buy a RIB (Rigid Inflatable Boat)
How to Set-up our Rib properly
It would not be an exaggeration if we said that it is wonderful and sometimes even fascinating to have the required basic knowledge so as to be able to setup a boat properly, thus making it possible to maximize its performance in every condition.
Being aware of the fact that many rib-friends are interested in learning more about the proper setup of their boat, we will seek a concise and a simple approach to such a very interesting and complex issue.
If we attempted to give a definition of the term «proper boat's setup», we would say that it is the suitable combination of all those factors that affect the boat’s ride in order to achieve its optimum performance for the specific use we want. In other words, our aim should be finding the particular engine / hull combination able to run as efficiently as possible.
Setting up a boat properly is the key to maximize performance.
Still, the boat’s performance is influenced by many factors, which indeed interact with each other, and of course many of them are not possible to be controlled because we don’t know exactly what is going on behind our transom.
If we consider that the interactions of the water, hull, lower unit and propeller with each other are very complex and constantly changing according to our speed, loading and sea condition, and that we also need to take into account a plethora of factors - such as engine’s gear ratio and mounting height, propeller’s distance from the transom, weight distribution, and so many others- that play a significant role in the performance of a boat it is obvious that we need to have a general knowledge of all the above parameters so as to be able to setup our boat properly.
Definitely, for many people, all the above mentioned seem like trying to move an iceberg!
However, we will try to make it as simple as we can.
And of course, when we refer to the «maximum performance of a boat», we mean the best numbers in terms of top speed, cruising speed, slip percentage, fuel consumption or time to plane combined with the best possible boat’s behaviour in terms of riding attitude, handling, and generally the quality of navigation.
Achieving the maximum speed does not mean we get the maximum performance of our boat
We have pointed out many times that we can not have a specific set-up for every use. We can not achieve the maximum top speed and, at the same time, the shorter time of plane or get on plane at the lowest possible rpm. We can not get the best possible ride quality in tough weather and at the same time gain the ultimate final speed. A setup for maximum top speed sacrifices the time of plane, increases the slip percentage at low and medium rpm degrading thus the ride quality in rough waters. On the other hand, a setup for strong acceleration and a good handling in rough waters sacrifices a part of the maximum top speed.
Therefore, it becomes clear that first of all we have to decide which use our boat is intended for and then focus on its proper set-up.
A completely different set-up is suitable for a boat intended for long-distance trips rather than for a boat intended for fast rides/races or for one pulling a skier.
After all, we realize that it is impossible to give all the required instructions that should be applied in each case separately in a few article’s lines. We can focus however on some steps that are useful to follow as a guide for setting up any boat.
Setting up in action:
First of all we have to know that the boat’s setup starts from the choice of the engine which will be mounted on our transom. One of the main factors that will determine the choice of the engine, along with the use our rib is indented for, is the hull itself.
The length and characteristics of the hull determine the amount of horsepower and the kind of the engine to be chosen.
So after choosing the proper engine and deciding on the use of our boat, we load the usual loads we've figured out that we will carry and come out for trials in the real world of the sea.
Follow these helpful tips, experimenting with:
- The mounting height of your outboard engine.
The engine’s mounting height is one of the most critical elements of set-up. That is why the use of jackplate is absolutely necessary. Only with the jackplate’s help will you be able to adjust easily the engine height and find quickly the best possible one for your use.
So, lower the engine on the transom and start your measurements.
Concerning the rigid inflatable boats and their usual hull’s type, we can start our sea-trials by aligning the A / V plate 4 cm above the bottom of the transom.
Record the speed, the fuel consumption and estimate the slip percentage in the whole rpm range, measure the acceleration and the time it takes to plane, try all trim levels and feel the hull: its behavior, handling, how it stands on the water at every speed, the throttle response, the bite of the propeller. Write down your measurements and impressions and try several times.
When you are finished and sure about your measurements and impressions, raise one hole - two centimetres - higher the engine and follow the same procedure by writing again measurements and impressions.
Continue to raise the engine until you reach the highest possible position by always taking measurements and observations.
To be objective in your measurements, make all of the above tests the same day, in the same sea area, in the same direction.
When you're finished, put your notes on the table and compare the performance of your boat to any engine’s height. Do not trust anyone except the GPS indications and your own feeling.
Trust mathematics, not the unsubstantiated theories that some people may probably tell you.
Maths never lies.
The key is to find the exact engine’s height that gives you the best combination of measurements and hull’s behaviour for the use you are interested in.
Always remember that the higher we raise the engine, the less the amount of gearcase in the water and the higher the final speed gets, although the thrust decreases as well as our ability to raise the bow. However, if we raise the engine too high, the propeller will tend to ventilate losing its bite on the water and the bow will fall down lowering thus our speed.
On the other hand, the more we lower the engine, the more the amount of gearcase in the water is and so the less the final speed gets, while we gain thrust and bow lift. But if we set the engine too low, the gearcase creates excessive drag, reducing a lot the final speed and fuel efficiency.
For any boat/engine combination and use there is an ideal engine height.
The key is to find the perfect balance of thrust and speed for all-around use. To have, in other words, the engine as high as possible without the lack of getting on plane and generally at the low rpm.
- Various propellers.
Another factor to experiment with is the type of prop you will match. Nowadays, there is a great range of high performance propellers on the market. Still, only a few of them will fit the combination of your boat / engine and will perform very well. So you have to choose from a variety of propellers, the one which will give you the maximum performance of your boat. The propeller you will choose is defined by a number of factors, such as its distance from the transom, the engine’s gear ratio, the hull characteristics and the use of the boat. However, one of the most important factors that determine the proper propeller is the mounting height of the engine. Each propeller is designed to work at a specific mounting height. Other propellers are designed to work deep in the water, others on the surface, and others half in and half out of the water.
Therefore, the choice of the propeller should always be made in conjunction with the engine mounting height.
In other words, each engine’s height affects the propeller's choice. We can not get the maximum performance even if we theoretically have the ideal engine’s mounting height without the proper propeller. Engine’s height and propeller work together, hand by hand. Therefore, we should never look for the appropriate engine’s height separately, but always in relation to the proper propeller.
The right combination of engine’s height / propeller is part of the ideal setup.
Generally, keep in mind that the propellers with a higher rake angle, taller cup, and larger diameter but also with more blades or blade’s surface, work better at elevated mounting heights.
A three-blade propeller usually runs faster and gives a higher final speed than a four-blade one of the same pitch. Conversely, the more the propeller's blades - under certain conditions - the quicker the planing time, the better the bite in the water and the handling of the boat in rough water.
For every hole we raise up the engine we gain 150-200 rpm, which means that for each engine’s position higher we can add one inch of pitch or one blade more, remaining at the same engine’s rpm.
- The weight distribution
The proper weight distribution is another very important factor that affects the boat performance and defines the right setup.
Achieving the right distribution of weight is crucial and it takes several hours of testing and observation to be able to find out in which parts of the boat we need additional weight and in which ones not.
An improperly loaded and balanced pleasure boat can lead to an excessive increase of the hull’s wet surfaces requiring thus more power and leading to increased fuel consumption, speed reduction, inappropriate ride attitude and poor boat control, especially in bad weather conditions.
The secret here is the boat’s Center of Gravity.The center of gravity is a specific point that we must not ‘disturb’ in any way. If this is not possible, at least we shouldn’t switch it far away from its sweet spot.
As a general rule, keep always in mind to place the heavy objects as near to the console as possible, on almost i.e. the aft third of the deck, where is usually the center of gravity of the boat.
The lightest things have to be placed on the stern the load of which is already heavy, especially in recent years due to the almost permanent mounting of four-stroke outboard engines. Also make sure to keep your bow lightweight so as to be able to respond to the engine’s trim, allowing you to control effectively the ride attitude which is the best indicator of the ideal setup and is a crucial factor of handling in both calm and rough seas.
Remember that even only 10kgs on the bow can affect the boat’s performance and they are equivalent to multiples kgs around the console.
Things become much more complicated in lightweight racing boats where even a load of 30kgs play a huge role in the boat’s balance. An improperly balanced racing boat may lead to dangerous instability and porpoising or chine’s walking phenomena. In this case, besides the battery, oil’s and fuel’s tank, there is no other gear to distribute properly achieving the right balance. So we apply other solutions such as the addition of integrated ballast systems of water or metal plates in order to achieve the most efficient ride attitude.
The best combination of engine’s mounting height / propeller / weight distribution will give us the ideal setup and the maximum performance in each condition.
Unfortunately, however, there is no any formula that gives the best combination and suggests the right setup. Each factor influences the other and finding the best combination requires knowledge, rationality, hard work and of course many sea trials.We also have to remember that in an ideal setup our engine should under no circumstances exceed the manufacturer’s maximum rpm, whereas it can reach at least in the middle of its recommended maximum rpm range at WOT.
* All the above principles mentioned are also valid to any multi-engine installation.
