high thrust speed range

Hi there. Is the only point of high thrust outboards "power" at low speeds? I rarely run my cat at anywhere near full power (i have 2 8hp high thrust yamaha's), so under way would I be just as happy with a normal outboard; power is power after all?? The down side as I see it is that I would loose some of the "power" manoeuvering, but surely just giving it a bit more throttle would compensate for this. Is there a speed at which the transition from high thrust to normal outboards happens in terms of efficiency??

Cheers, Al.

 

In general terms, yes. In short, and without using professional propeller designer's vocabulary, this is it:

To begin with, we must define efficiency. In this context it is the relation between how much work per unit time you get from a given power input, ie how much thrust x velocity you get per power unit. There are two efficiency factors to consider:

First: The "jet" efficiency, which is inversely proportional to the thrust developed per unit area of propeller disc.

Second: The hydromechanical efficiency of the propeller (or pump) itself.

The total efficiency of "the wet parts" is the product of these two efficiencies. There is an optimum configuration, where the total efficiency is maximized.

As the product of force (=thrust) times velocity is power, this means that for a given engine power, the thrust force is increasing with decreasing speed. The consequence is that the disc pressure for the given propeller is also increasing.

Finally, at a certain speed, the disc pressure becomes too high to give a resonable efficiency, and the original prop has to be replaced with a bigger one (rotating at lower rpms), and alas, pressure is down and efficiency is up again.

So, using a standard, small propeller outboard to push a heavy, low speed vessel may be possible to a certain level (see below), but the engine has to work harder to produce the required thrust, ie consume more fuel, and there will be no margin for thrust increase.

The ultimately limiting factor, if increased fuel costs are not....., is cavitation. With the propeller spinning at high speed, and with restricted inflow (low advance velocity and high thrust), the local pressure on the forward side of the blade tips will be reduced. It can become close to the boiling pressure of water at sea temperature, and the flow over the blade is disturbed. When this happens, the propeller thrust will not be increased by increasing rpm's further.

The difference between cavitation limited thrust for the big and small prop is very substantial, and may often be the deciding factor.