It is a fact that every manufacturer of outboard engines installs different gear ratios while we see that as the horsepower decreases the gear ratio gets higher (numerically), which means the revolutions per minute of the propeller decrease.
As we have seen in previous articles, everything is a matter of Torque and combination of propeller’s diameter and pitch.
Let's see the gear ratios that Suzuki and Mercury use on some of their 4-stroke outboard engines:
We note that at the same horsepower, Suzuki’s gear ratios are higher (numerically) than Mercury’s. That means e.g. on a Suzuki 300hp, at the same engine’s rpm, the propeller shaft rotates more slowly than a Verado 300hp.
At 4000rpm the prop shaft of Suzuki 300hp rotates with
4000 / 2.08 = 1923.07rpm, while of Verado 300hp rotates with 4000 / 1.75 = 2285.71rpm.
At 6300rpm, the prop shaft of Suzuki rotates with
6300 / 2.08 = 3028.84rpm, while on Verado at 6300 / 1.75 = 3600rpm.
This means that the propshaft of Suzuki is slower but the torque which is produced is greater, while the propshaft of Verado is faster but the torque is less.
Of course, both Suzuki and Mercury are able to install the same gear ratios so as to have the same propshaft speeds. Obviously their different choice has to do with the overall philosophy of each company.
Suzuki prefers higher (numerically) gear ratios because it is more interested in achieving greater torque and therefore greater thrust, while Mercury prefers lower gear ratios, at the same horsepower, because it is more interested in achieving greater speed.
Suzuki by following the rule: ‘the greater the reduction gear, the greater the torque at the propeller shaft is and the greater the diameter of the propeller can be turned’, focuses on the use of larger diameter propellers to take advantage of achieving greater thrust.
The larger diameter propellers:
- get the boat more easily and quickly on plane
- keep the boat on plane at lower speed
- give greater acceleration at low and medium rpm
- respond better in rough seas, producing greater thrust which is necessary when our boat is trying to climb on a large wave
- have less slippage at low and medium rpm and thus achieve higher cruise speeds
- are more «economical» and therefore we have greater autonomy at low and medium engine speeds in most of the cases
- work better on heavy loaded boats or in hulls witch have a lot of drag ( more wet surface) during the ride
- lift the stern more out of water, which compensates the large weight of the four-stroke engines which are mounted on our transom
In general, the combination of a large diameter propeller with a short pitch is more efficient than the combination of a small diameter propeller with a long pitch.
It is therefore obvious that Suzuki focuses more on all around uses, in a range of 20 to 35 knots, in which we usually operate our ribs at a percentage of 90%.
On the contrary, Mercury is much more focused on achieving higher speeds and prefers the combination of smaller diameter - longer pitch propellers, which are most efficient at speeds exceeding 35 knots.
After these (35) knots, the large diameter propellers disadvantage, presenting great drag just as well as the bigger (because of the larger reduction gear) lower units of Suzuki. At high speeds, therefore, Mercury has the advantage, while at lower speeds not.
Mercury tries to compensate this weakness at low rpm with the design of a wide range of propellers which are made to work with fast propshafts, reduce slippage, increase thrust at low speeds and perform the maximum in high mounting heights as well as in excessively angles of trim.
So Mercury has designed a highly number of i.e. efficient 4-blade propellers with great overall blade surface area in order to have better response in rough seas or in heavily loaded boats as well as 4-blade propellers with less blade surface area and intense features so as to work better in lighter boats and fast hulls.
The practise verifies the theory and shows us that the high horsepower engines with lower (numerically) gear ratio perform better on lighter boats and fast hulls while the higher gear ratio performs better on heavier load, heavier boats and hulls with a lot of wetted surface during the ride, in combinations which overload the engine.