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Pleasure crafts and the market

Marine propulsion is steadily, albeit slowly, evolving into a new experimental phase: the choice of a propulsion unit is more and more dictated by the craft type and its future use. Propellers of various type, shape and throughput face today a robust competition from the jet propulsion


Propeller, like the wheel, ranks as one of the most important man's discovery: no similar mechanism able to transform energy from a rotary movement into a linear one exist. If however is true that the wheel is not found in its natural sate, the same can be said for continuos rotary movements (a body constantly rotating in the direction around its axis).

Every rotating body is always tied up to a fixed one, either by a pin or a hinge or in the case of the human body, by tendons and muscles preventing more than one full rotation on the same plane. nature could easily dispense with the wheel and the propeller without loosing the possibility to exert a linear movement and perform them alternatively (walking, jumping, fin movements). Man, in his continuous research for a mean to produce rotary energy, has overlooked nature and has continued to study the wheel bypassing some transformations that in some instances could have been more effective to improve what nature itself has provided in the motion field. Jet propulsion for instance, widely used today in the air industry, originate from the water and can be found in the motion of some invertebrate species: its efficiency is limited since animals lacking the wheel use contraction to produce an uneven and alternative motion in the same way as we use our pulmonary air reservoir to blow the candles on a birthday cake! Having said all this, it still remains to clarify why after starting with a propeller unit, the air industry have opted for the jet propulsion while in the marine field this form of propulsion is relatively speaking a new comer having strongly appeared only recently in military and professional applications.

In order to properly analyze the various aspects of this intriguing aspect, it will be convenient to subdivide the discussion into two different fields: one is the analysis on the HP involved (which is again subdivided into high, medium and low powers); the other is the type of navigation, more generally the trade, where this propulsion is better suited, i.e.: Offshore racing where everything is sacrificed to benefit speed or tugs operation where the optimization of the bollard pull is the ultimate achievement. Before examining the graph at fig. no. 1, one additional consideration must be made for the jet propulsion applied to the aviation trade (something that can be traced in the marine field as well): it is true that the air transportation field as we know it, has almost completely adopted the jet propulsion, but it is also true that its utilization is almost exclusively professional. Sport aircraft with localized trade patterns not requiring long hauls or high altitudes is almost exclusively still dominated by the propeller.

The reason probably lies into the different output and efficiency at different speed performances. So what is the difference between prop and jet propulsion? The propeller "screw into" the fluid, it climbs inside like a revolving ladder, its blades compress the fluid transferring the resulting pressure onto its shaft and depending on the fluid's density result in advancing force. Theoretically the speed of advance is the resultant of the revolutions in the time unit (RPM) multiplied by the pitch; in practice the slip has to be accounted for. A fluid like water, although incompressible, is not solid material where a screw advancement is the sole product of the pitch at each turn.

There is a common theory indicating that without the slip the propeller would not advance. This is a bad explanation of the proportional relationship between propeller, trust and slip. In other words, when the difference between the theoretical vessel's speed (calculated on the basis of the pitch and the RPM) and the maximum actual speed, absent cavitation, is at the peak. Cavitation can be compared to a screw slipping trough a material not hard enough to sustain the screw bite; similarly, the propeller will tend to cavitate especially when starting the engine: because of air or, more generally, vapors presence, RPM will increase without producing any thrust (a plastic bag around the propeller will be sufficient to create cavitation). As the vessel's speed nears the theoretical speed, the propeller looses progressively its thrust and efficiency; an example by comparison would be trying to climb a moving stairway at the same speed with which the same stairway descends: the fatigue exerted will be greater only to remain in the same place.

The propeller's use is therefore recommended when the initial thrust is required, when the requirement for high speed and its variance is not of paramount importance and engine stops are frequent, when a strong initial thrust is required and the available IHP are relatively small and, finally, when the "project" can not absorb the jet propulsion installation cost. This is generally more expensive with a more elaborated installation.

As for the air industry, variable pitch propellers have been tried in the pleasure craft industry; however the results have received a lukewarm support. On the contrary, the adoption of contrarotating propellers on stern drives as introduced by Volvo Penta in the eighties has been first criticized and later copied by Mercruiser and lately adopted by Yamaha for their outboard engines. There is no doubt that these plants deliver better performances due to the enhancement of the rake couple tending to produce a heel moment and a pronounced vibrations abatement and a better performance of the thrust tube. The thrust changes and the efficiency remain the same: there may be a flattening of the efficiency curve which positively cross the single propeller's with the end result of improved speed efficiency and initial thrust. On the negative side one has to mention a poor slewing control made more difficult by the tendency of contra rotating props to resist any course change.

This latter characteristic is not necessarily always negative: a steady navigation is, among others things, fuel efficient. The contra rotating propeller is only used on a single stern drive; it would result expensive and assembly complicated otherwise due to the installation of the inverter. Technicians' attention has focused precisely on this latter element coupled with electronic control could lead to those innovations that certainly lie ahead on this field. For the time being, all of this do not involve the propeller as such except that propeller shape refinement and efficiency is certainly part of it. Part of this study is the application of the inverter's logic coupled to the engine's logic and, sure enough, Mercruiser has presented a two-speed transmission. It would seem plausible that the next step will be the co-ordination of the engine control system with the trim sensors so that the entire propulsion control chain is devoted to the boat efficiency and passengers comfort and safety. Propellers are supplied in two versions: submerged or at sea level and the material used is bronze, stainless steel, aluminum alloy or plastic material; they are also available in the variable pitch version, with collapsible blades, built in composite material and with interchangeable blades.

Let's now see what goes on the jet field: to create a useful thrust, the jet engine does not need a support as it is based on the reaction principle (because if this the jet propulsion is the only one allowing high altitude flights and the launching into space of ballistic missiles). If you try to jump ashore from the bow of a boat of consistent displacement you will safely land on the jetty; on the contrary, if you try the same jump from an inflatable rubber dinghy you will mast likely fall in the water while the inflatable will be kicked from the dock due to the principle of action/reaction in force of which the reciprocal thrust has a bigger effect on the smaller mass and if masses are equal then the thrust is the same for both. Empirically we can then say that 100 kg of water passing through a nozzle of jet engine will generate a 100 kg and its initial impact will be inversely proportional to hull mass. As speed is increased the thrust gradually diminishes but it efficiency improves with a trend to reach value 1 (one) when the hull speed approaches the speed of the water passing through the jet nozzle.

We have spoken at length with Castoldi's engineers over the problems posed by the requirement to design efficient propulsion plants at both high and low speed: positive, negative even neutral intake pressures stand prominently among the problems to be accounted for; another task to be kept in mind is the variable geometry at the intakes and the nozzles: something similar to the octopus' ability to shape its form by muscle contraction and thus changing intakes and exhausting water speed. This phenomena, difficult to be mechanically reproduced, can only be achieved by using additional water intakes to be employed as required. Also on the agenda was the discussion on the future of the jet propulsion by using electronic logic aimed to exploit the jet's initial efficiency and obtain the optimal thrust avoiding cavitation.

It is clear that the jet propulsion has great advantages over the conventional propeller not only as propulsion plant in general but also for safety considerations or when navigating through shallow waters. It is true that jet also can suffer contact damages in shallow waters, or that mud or sand can reduce its efficiency, but it is also clear that jet is less accident prone compared to the propeller. Safetywise, jet has also an advantage: if nothing else, it does not have the murderous effect that even a small propeller at low speed has in case of collision or worse, when hitting semi submerged bodies (alive or not). The braking effect by which speed can be quickly reduced and reversed is another jet's typical characteristic together with the maneuvering ease: a minimum of experience is required when maneuvering in confined waters but the ease of going alongside with one engine only or turning within the boat's length are pleasant surprises when compared to the sluggish behavior of conventional propulsion. Maneuvering is made further easy and precise by the smoothness provided by the hydraulic and electronic controls.

All of the above has not been able to secure a proper place in the pleasure craft market for the jet propulsion; military applications in the field of fast ferries and emergency support crafts, speed records with fuel saving advantages have all contributed to a successful new era in the marine propulsion where high HP output are required without the installation of a reduction gear. It may be that at medium HP the tendency is still to hang on the more traditional stern drive but surely at lower HAS, the jet still suffer the negative impression left by a poor and superficial attempt to market it few years ago: all this has deprived the jet propulsion of its merit in the pleasure craft field where preconceived ideas still remain even in the field of water bikes whose success directly derives from the jet concept.

In the outboard engine field, the jet is retaking its position especially in the USA where due to their particular configuration they are the only authorized craft to make landfall on public beaches. A few moments at the helm of a jet powered boat would be sufficient to convert the most faithful propeller fan; its simplicity, the smooth response and not to be forgotten, the convenience to have all the HP concentrated in one power plant with sensible economies.

One further handicap imposed on the jet must be traced to the building yards: either because the jet convenience and adaptability is unknown to them or for lack of specialization to design a proper hull form and install them, this propulsion type is not fully exploited. It goes without saying that jet is not for those performing water ski it is designed for towing purposes for which another system based on the cycloid characteristic similar to the helicopter propeller has been in use for several years.

Jet is neither designed to serve the fishing industry or the small coastal trade but, and here I express my surprise, those medium and large tonnage yachts whose trade is to alternate long stay at moorings with long navigation spells at high speed, could certainly made a good use of it.