Propeller: steel or aluminum — which is better?
«Steel propeller versus aluminum» is perhaps one of the most burning topics for discussion at boat parks and water-motor forums. The disputes do not subside, although the question, as they say, «with a beard» — the problem of such a choice has been facing waterfowl humanity for many decades. Being supporters of a professional approach, we once again turned to our permanent consultant, Honored coach of Russia Alexander Belyaevsky, one of the best sports «rifles» of the country, for clarification.
Alas, the level of the arguments cited does not always rise above the «intuitive-emotional»: some attribute absolutely miraculous properties to steel screws, extolling them to the skies, while others go to another extreme and prove with no less fervor that this is nothing more than an expensive «fetish» — capable, in addition, of destroying over time motor or gearbox.
We usually do not look at the sparkling polished beauties out of idle interest (although this also happens when aesthetic preferences or simply prestige considerations come to the fore). It often happens that you just need a screw, and in this case the problem of choosing «steel–aluminum» arises willy-nilly.
For example, the «stock» screw (in the vast majority of cases, light-alloy), complete with which you bought the motor, may not match the characteristics of the boat on which it is installed. On very small vessels — for example, collapsible inflatables — such a discrepancy is not particularly critical (especially since, due to the lack of a tachometer, it is impossible to objectively assess whether a relatively weak motor is operating in an optimal mode), but with an increase in the size of the vessel and the power used, possible operational losses are becoming more and more significant: put up with a shortage of speed and the inexplicable voracity of the engine does not want to be on the «Kazanka» with a «thirty» or «forty». Especially dangerous is the «twist» caused by an overly «light» propeller — it can literally shorten the service life of the motor at times. (We have already told you more than once about how to choose the characteristics of the screw correctly — for example, in the material «We squeeze the speed» published in No. 195, see also K. Konstantinov's articles «Propeller in rubles and liters» in No. 173 and «Why does a screw need four blades» in No. 177). For a minor «fit», you can slightly modify the existing screw, but in the vast majority of cases you have to purchase another one.
The motor can be sold without a screw at all — many companies do this in relation to medium and high power models. This is quite justified: the propeller is a purely individual thing. Why pay extra for something that you may not use?
But even if the existing screw suits you perfectly, do not forget that it is in some way a «consumable», albeit not cheap. Having a spare tire on a boat is a very reasonable measure, especially in shallow rocky waters. (By the way, if a regular screw acts in its role, which does not quite correspond to the boat in its characteristics, in some cases it can be used not only in emergency situations, but also on a permanent basis: for example, a «lightweight» screw for normal operating conditions should be used for transitions with a heavy load, and a «heavy» one for high-speed «scoundrels» light).
In general, let's assume that for one reason or another you need to purchase a propeller. And if you have already decided on its main characteristics (by choosing them experimentally or using the dealer's advice), then the answer to the question in the title of the article will be the next one.
Let's try to compare steel and aluminum propellers impartially by a number of basic indicators that determine their operational qualities, and let's start with what, as they say, lies on the surface.
This is usually the main argument of the opponents of «steel». In fact, a steel screw costs twice or three times more expensive than an aluminum one with similar characteristics (in relation to, say, a sorokovka motor, this is about 7-9 thousand rubles against 2700-3600 rubles). Here we are talking primarily about mass «consumer» models — in the case of specialized screws (say, sports), the difference may be much more significant. What do they charge such money for? About this — just below.
The difference in this indicator is also easy to determine even for a layman. A steel screw is indeed at least twice as heavy as a similar aluminum one. Having estimated both screws in their hands, like the famous crow from the plasticine cartoon, many begin to put forward all sorts of hypotheses, some of which are true only theoretically. Let's look at them in more detail.
There is an opinion that a more massive steel screw «unwinds more slowly» because of its greater inertia, which makes the boat lose in pick-up.
The theorizing of pure water! Yes, from the point of view of the laws of physics, such an assumption does not seem to be devoid of common sense, but in practice — for example, when going to planing — you will not feel any difference. It's one thing when you turn a screw in your fingers, and quite another when an internal combustion engine rotates it, even with such «undignified» power as 20-30 hp at the present time. In general, the difference in weight in this case turns out to be so insignificant that, as scientists put it, it can be neglected.
Other factors have a much more significant impact on the speed of the screw's set of revolutions — first of all, the water flow resistance created by its blades, which is noticeably less in a steel screw than in an aluminum one. We will still «decipher» this provision, but so far it is an indicative fact: racers who fight not only for tenths of a kilometer per hour, but also for tenths of a second of acceleration speed, use only steel screws.
Some are convinced that due to the greater weight of the steel screw creates an increased load on the propeller shaft bearings, which causes their increased wear.
This assumption is even more «theoretical» than the one above. The safety margin of the gearbox bearings is incommensurable with the weight of the screw itself, and there can be no question of any increased wear — even if the screw is not made of steel, but of lead or gold. Let's not forget that the thrust created by the propeller on the move is tens or even hundreds of kilograms — against this background, the weight of the propeller itself is almost invisible.
The cause of accelerated wear of the propeller shaft bearings can only be an imbalance that causes vibration during rotation. Steel screws are usually distinguished by impeccable static and dynamic balancing, while in the case of aluminum, this indicator may depend on a particular instance. For example, in the manufacture of aluminum casting, shells often remain, and even with high casting accuracy, the blades may differ slightly in weight from each other.
Many have noticed that when a steel screw is installed, the forward stroke or reverse is turned on more «rigidly» than in the case of aluminum.
The skeptics are right about something here — a more significant inertia of the steel screw can manifest itself in such a situation. As you know, unlike an automobile transmission, the reverse gear of an outboard motor does not have a slip clutch or friction synchronizers — the connection of the front and reverse gears is rigid here, through a cam clutch-a «ratchet», and only a rubber hub damper is designed to soften the «shock» when switching, which is equipped with any propeller, regardless of the type used material.
However, it should be added that such a phenomenon becomes really noticeable only if the switching is carried out at an increased engine speed. (Experienced water-enginers are probably aware that in such a situation, a safety key or a screw pin can be cut off on domestic suspension systems). There is only one way out of the situation — keep the idle in the norm (usually within 650-850 rpm), and then you will not feel much difference between steel and aluminum screws.
If the idle speed suddenly «jumped» somewhere far from the base, and how to fix the situation yourself, you do not know, in the case of both types of screws, it is better to start the engine with the gear turned on — however, on any modern suspension, you will have to somehow disable the lock that prevents starting on the front or in reverse. In addition, when using this temporary measure, do not forget about caution — check whether the space is free on the course, whether the steering wheel is not put on board and whether passengers are sitting securely in their seats.
Hardly anyone will dispute that a steel screw looks more attractive than a painted aluminum one. Probably, for the reason that the sparkling mirror gloss is the first thing that catches the eye, many tend to believe that its only advantage lies in cleaner polished surfaces, which is expressed in reducing resistance to water flow. But, in the end, you can also polish the aluminum screw...
Yes, smoother blades do create less resistance, but this is only a detail, and far from the most significant. Much more important is such an indicator as the cross-section of the blades — roughly speaking, their thickness. Here the difference between steel and aluminum screws is much more serious — the first blades are about twice as thin in profile. Which knife — sharp and thin or blunt and thick — is easier to cut off a piece of smoked sausage? With water, about the same principle.
The point here is not only in the greater strength of steel, which makes it possible to make the blades thinner compared to aluminum. An important role is played by the difference in manufacturing technologies due to the characteristics of both materials.
Aluminum screws are made by a fairly simple method of coquille casting, which imposes certain restrictions on the configuration of the finished product and does not differ in precision accuracy. Steel is cast according to moldable molds, which not only provides higher accuracy, but also allows you to give the casting almost any shape with any thickness of sections. The disadvantage of this method is only one — high cost, hence the rather significant price of steel screws, but the costs are justified: the technology used allows developers to implement in a series even the most intricate design solutions obtained during calculations and field experiments.
A modern propeller is an exceptionally complex thing, and in order to noticeably improve (or worsen) its final characteristics, sometimes literally the smallest thing is enough. We will not go into details here, but as one example, we recall that the same step in inches is an average indicator in some way: the angle of the blade installation can smoothly change from the hub to the outer edge, and rather high bends-interceptors can be found on the output edges... All such «subtleties» aimed at making your boat move faster and more economically, and allows you to implement an expensive technology for manufacturing steel screws.
Most manufacturing companies promise about a 10 percent increase in speed when using steel screws, and, as the results of editorial tests at least show, this is far from an empty phrase. Naturally, we are talking about comparison with aluminum screws that have the same indicators for a number of basic parameters: diameter, pitch, number of blades and disk ratio (in other words, the area of the blades). By the way, such a comparison reveals another important feature — a steel screw, as a rule, is «lighter» than a similar aluminum one, so that by selecting the light-alloy screw most appropriate for the boat and motor, a steel screw can be taken in increments of an inch or two more (provided that other basic characteristics coincide).
Strength and maintainability of the propeller
As already noted, the propeller is a «consumable material» — who often goes out on the water, at least once, but flew with the underwater part of the motor on a stone, an underwater pile or a firewood.
It is unlikely that there will be those who want to dispute that a steel screw is stronger than an aluminum one, but supporters of the latter assure that in an emergency, increased strength is only harmful. They say that, although the aluminum screw is more damaged on impact, but at the same time, «sacrificing itself», it takes on a shock overload that can destroy the transmission or even the engine itself...
A common misconception. (By the way, there is a well-founded assumption that in some situations a steel screw somewhat softens the blow due to its greater weight and, accordingly, inertia — roughly speaking, it stops the transmission not as instantly as aluminum, although this can also be argued). Another thing is indisputable: the role of the «weak element», designed to weaken the impact, is played by the rubber hub sleeve, which, with certain efforts, simply turns. The principle is the same for both steel and aluminum screws. Of course, this is not the good old «key», to which older generation water-enginers are accustomed — if they collide with an underwater obstacle, any screw will definitely be damaged, but in most situations, the rotating sleeve reliably protects the transmission.
If after the incident it turned out that, for example, the splines of the «spring» (deadwood shaft) were twisted, then you probably ran into an obstacle at full throttle, and which screw was on the motor at the same time — by and large it doesn't matter. Only from aluminum at the same time, usually only a hub with pathetic stubs of blades remains, and on steel the blades simply get stuck. And even if you don't have a spare screw, not everything is lost.
In the event of a collision with an underwater obstacle, the blades of the steel screw (on the left) usually just bend, and in field conditions it is possible to «first approximation» to return it to work, armed with a hammer or at least an axe. With an aluminum screw (on the right), such a trick will not work — even if the blades did not break on impact, this will inevitably happen when trying to «cold» straightening. Possible damage to transmission parts is the same in both cases — after the incident with the aluminum screw depicted on the right on the Mercury 60, the deadwood shaft had to be replaced, on which the slots were twisted.
To bring the damaged steel screw into a more or less working condition, allowing you to get to the base, you will need only an anvil — for example, a suitable stone on the shore, and a hammer (an axe will do, which is found in almost any boat). An attempt to rule «in nature» the blades of an aluminum screw will inevitably lead to the fact that you finally break it, the material is fragile.
Later, of course, you will have to turn to a specialist armed not only with knowledge and experience, but also with a walking slide. A good master is able to easily return the steel screw to its original condition and at the same time give a one hundred percent guarantee for his work. In the case of an aluminum screw, especially if the damage is serious, the responsible person will refrain from the warranty and immediately warn about it. (Read more about such repairs in the conversation «The second Life» of the killed «screw», published in No. 194). How much will it cost? Of course, everyone evaluates their work in their own way, but, as a rule, the repair of a steel screw costs no more than 30%, and aluminum — no more than 50% of the price of a new one.
Let's briefly summarize the results. What distinguishes a steel propeller from an aluminum one similar in characteristics?
The disadvantages can only be attributed to its higher price and sensitivity to idle speed when the front or reverse gear is turned on. The advantages, in our opinion, are much more. First of all, this is a lower resistance, achieved due to a smaller blade cross-section and surface cleanliness, as well as a more «advanced» design, which implements the latest achievements of hydrodynamics. The result is higher speed and better fuel efficiency. Manufacturing accuracy and balancing are at a height; in terms of maintainability, including «emergency», they also win.
We have already mentioned that a steel screw is «lighter» than an aluminum screw similar in basic characteristics (meaning, of course, not the weight in kilograms). The ability to use an increased pitch while maintaining optimal engine speeds is an additional reserve for speed connoisseurs, but there is another important point that will surely interest this category of consumers. The fact is that the design of steel screws allows them to be used in semi-submerged mode — and here the increase in speed is much more significant... However, this is a topic of a separate conversation, and we will try to cover it in detail in the very near future.
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