To get the most out of a heat engine, you have to respect certain settings such as the height and inclination, the choice of a propeller adapted to the boat, etc. Many fishers simply center their engine on the transom, tighten the fixing screws, and roll youth. With small powers, it happens in principle quite well, but as soon as one exceeds the ten hp problems can occur, and in any case, the performances are rarely optimized with a pose also empirical. We lose the top speed, we consume too much, or even we observe a bad behavior of the boat when we do not jeopardize the survival of the engine (over-speed). Here are the basics of what you need to know to exploit the potential of an engine fully. If you want to change your outboard motor and to buy the outboard motor for inflatable boat read our experts test.
Table of Contents
The right length of the tree
The first precaution is to make sure, at the time of purchase, that the engine shaft length is adapted to the height of the transom of the boat (see table opposite). These figures, although generally accepted, are given here only as an indication to rough the choice, but in practice, one often observes differences of several centimeters according to the manufacturers of engines.
The safest method is, therefore, to measure the height of the transom, i.e., the vertical distance between the top of the table (on which the engine caliper is supported) and the bottom of the hull (measure at the exact center Of the boat). The length of the shaft, also known as the length of the ground bolt, is usually indicated in the engine manufacturer’s documentation. In the case of a second-hand purchase and the absence of this documentation, you can take the measurement yourself. This is the distance between the underside of the mounting bracket (the one that rests on the transom) and the anti-cavitation plate.
With these two measures you are fixed and there are several cases:
– the shaft is shorter than the transom, the motor is unusable (except cutting the top of the table to shorten it).
– The length of the tree is equal to or greater than 5 cm at the height of the table: it’s perfect.
– the length of the shaft is 5 to 10 cm higher than the height of the switchboard: possible use with motor lift.
– the length of the tree exceeds the height of the table by more than 10 cm: tree too long.
The use of a tree too long is theoretically possible since the propeller and the water intake of the cooling system are in the water, but this generates serious drawbacks: poor performance (high resistance to advancement), vibrations, spray of water flooding the back of the boat, too much effort on the transom, etc.
Adjusting the right height
Once in possession of a motor of suitable length, it is necessary to carry out a fine adjustment of the height, it is the phase of optimization.
How is it possible to adjust this height?
The lowest setting is simply to leave the mounting bracket resting on the transom. It is not possible to lower the engine more, but it is, however, possible to raise it by inserting a shim.
Hence the interest, as explained above, to have a tree rather too long a little too short: it leaves more margins of adjustment. Lower than the back of the shell a few centimeters. Indeed, this plate has the role of preventing the propeller can suck air, which would pack the engine and reduce the thrust.
By placing it lower than the hull one is sure to have a powerful and uninterrupted flow of water on the propeller. But at the same time, the anti-stacking plate (2) must be out of the water when the boat is sailing at full speed.
This plate has the role of folding down the shower of water caused by the wake of the submerged part of the engine. If this plate is too low, it no longer fulfills its mission and a shower of water occurs, which in some cases can flood the back of the boat.
It is therefore understandable that before definitively fixing his engine it is necessary to make adjustments and tests on the water, possibly with wedges of different thicknesses.
We find the height that gives the best performance but without disadvantages (cavitation, water spray). Some works recommend to align the anti-cavitation plate on the hull, the notices of the engine manufacturers bend rather for an adjustment 5 cm under the hull (for safety to avoid any risk of runaway by cavitation). In practice, an adjustment to 2-3 cm under the hull is often a good compromise between performance and engine safety, but again each hull is a special case according to its shape, the distribution of masses, so tests are desirable.
Once the correct height is found, one usually proceeds to bolt the engine on the transom, at least for powerful engines and not likely to be dismantled regularly because too heavy. In concrete terms, this concerns engines above 25 hp in two stages or 15-20 hp in four stages.
The bolting is safety, it prevents the engine can move, and it distributes well the effort on the transom. It assumes the drilling of the latter and therefore a sealing system (putty) even if the holes are above the waterline to prevent the infiltration of water by projection.
Once the correct height is found, one usually proceeds to bolt the engine on the transom, at least for powerful engines and not likely to be dismantled regularly because too heavy. Bolting also allows, in some limiting cases (tree too long), to raise the motor more than would allow the butterflies are clamping.
The result may seem daunting because the butterflies are no longer in contact with the transom, but the fixing by bolting is much more robust and reliable, so there is no risk.
Some motors have several height adjustment holes, which makes it possible to avoid drilling too close to the edge of the board. We understand that before drilling it is necessary to be sure to be at the right height. Hence the importance of carrying out severe tests in navigation.
Once this paramount setting is done, there is still room to get the best performance from his engine and his boat: the inclination of the shaft (trim), possibly the drift, the choice of the best pitch of the propeller, l addition of accessories (flaps, hydrofoils). One of the most common problems and the most troublesome is the difficulty or even the impossibility to plan, that is to say, to hover the hull on the water.
A boat that does not plan to crawl because it pushes the water instead of sliding on the surface, the engine forces and consumes a lot.
It is evident that to plan you need enough power given the size of the boat. We can not hope to weigh a 400 kg hull with a 6hp; it’s just impossible. But even when you have enough power, in theory, a boat may have difficulty in sizing, especially when it is heavily loaded (passengers, equipment, etc.).
Spread the charges
The first point to check is the good load distribution, which is all the more important as the boat is light and the engine limits low. When the throttle is on, the front of the boat lifts and the stern sinks. This is a natural phenomenon, but if the boat is unbalanced with too much weight on the back, this greatly aggravates the problem and requires extra power in trolling motor battery.
Besides, it is sometimes enough to ask the passenger to sit at the front of the boat to plan immediately. An unbalanced boat is easily recognizable by the fact that it rises a lot when you put the throttle, but also that it has a tendency to porpoise when you have gone to the water: the front lifts and falls in a continuous movement.
It is essential, once the imbalance is identified, to distribute the loads in a way that compensates. The main heavy and easily moveable elements are the fuel nurse, the battery (s), the passenger (s), the onboard bags and crates. Often it is enough to place the batteries and the nurse at the front (sometimes even in the middle) to greatly improve the performance of the lift.
Importance of trim
There is another point to check is the inclination of the motor shaft relative to the bottom of the hull (called trim or angle trim).
There are three possible inclinations:
– Neutral trim, shaft perpendicular to the hull.
– negative trim, inclined shaft towards the boat.
– positive trim, shaft inclined outwards.
To understand the impact of the trim you have to remember that the thrust axis of the propeller tends to get closer to the horizontal when you put the gas, which plays on the inclination of the hull (plate). The negative trim facilitates the planing by plating the front of the boat on the water, avoids the pitching and increases the bearing surface which pushes the hull upwards. It may happen that the tilt of the transom does not allow the negative trim (poorly crafted boat).
In this case, we can cheat by inserting a bevel wedge that is placed between the engine and the transom. These wedges make it possible to gain about 2 cm of inclination and do not cost much (less than 10 €). Do a search on the internet with the term “stern plate” or “transom plate.”
The negative trim does not achieve the maximum speed once sailed. If you have an adjustable electric trim in navigation, the idea is to lower the engine to the maximum to plan (negative trim), then raise it gradually (neutral or positive trim) to relieve the front to what the boat starts to porpoise or the propeller to cavitate (suck in air).
We go down slightly and we are at the best of what the engine can provide with the current load. If you do not have an electric trim, there is a choice to make. All motors allow several tilt adjustments that can only be done at a standstill. If the boat is struggling to climb, the lowest setting is used. Otherwise, a higher setting, determined by tests.
Aids to the lift
There are accessories to prevent the “bottom” of the boat from sinking too much acceleration: foils and flaps. The foils (also called hydrofoils) are fixed on the engine while the flaps are fixed on the hull. Both have the effect of opposing, by leaning on the water, that the rear sinks. The help with the clearance and the improvement of the attitude for a hull tending to pitch up are quite spectacular. They also improve stability in navigation by limiting roll effects. They do not increase the maximum speed (on the contrary), but the foils make it possible to hover at lower engine speed.
Which system to choose? Foils cost less and are easy to install yourself (drilling and bolting on the anti-cavitation plate). In addition, coupled with electric trim, their inclination is adjustable. Their use on large, heavily motorized boats (> 70 hp) is however not recommended because in the tight curves and at full speed these fins exert fundamental forces on their fixation.
There is a risk of rupture of the anti-cavitation plate with which more is a loss of the manufacturer’s warranty.
In other words, these foils are suitable for medium engines, their efficiency being maximum between 15 and 50 hp.
Flaps are preferable for large engines and heavy boats, but they require drilling the hull for their attachment and the cheapest models are not adjustable in inclination (or only when stopped) which does not allow to modify the plate in navigation. In addition, they especially allow lowering the nose of the boat. Their action in the opposite direction is negligible, unlike foils.
If despite all these solutions, the boat is still struggling to dive with sufficient power, there remains only one solution: change helix.