When, on April 3, 1845, the steamer Rattler towed, at a speed of as much as 2.5 knots, her sister Alecto, which was pushing in the opposite direction, she empirically demonstrated the effectiveness of the propeller.
The shaft line was born.
In an age of steam engines and wheel propulsion, the propeller was a revolution.
And what better way to turn it than through an axle that, entering the hull, connected it directly to the propulsion apparatus?
After all, it was the simplest solution.
The remarkable thing is that it still is today.
This feature makes it the most popular drive system for moving anything that floats and needs thrust.
It is installed on pleasure boats, small and large, fast and slow, on workboats and on sailboats, on yachts and on fishing boats, on cruise ships and on container ships.
Shaft line: what it is
The shaft line imparts the rotary motion of the crankshaft to the propeller, whether we are talking about a small automotive-derived 1300cc or a 100,000-horsepower Wartsila 2-stroke (yes, you counted the number of zeros right).
Yet its luck seems to be running out, and not just in the boating arena, now cannibalized by increasingly powerful outboards and IPS transmissions when the wallet is no limit.
Even passenger ships are increasingly adopting azimuth pods, to be independent in maneuvering in ports that, compared to their gigantism, are getting smaller and smaller.
Shaft line or outboard
A few days ago, a reader published his ode to outboards., stimulating an interesting exchange of views between supporters of the two parties.
There were essentially two issues most mentioned: safety and affordability (in all its forms).
Speaking of numbers, and cost, there is little room for discussion: the measurable parameters most often taken into account-weight and cost-are all significantly in favor of the outboard, given the same installed power.
https://www.barcheamotore.com/fuoribordo-entrobordo/ Separate discussion, however, is consumption.
As our reader rightly pointed out, the bare figure of hourly consumption must be related to the hull and transformed into the much more useful and comparable consumption per mile.
Therefore, this parameter depends very much on the type of boat, and use, we make of it.
A small, light, fast recreational hull, such as an open or a dinghy, will consume as much, if not less, than a corresponding inboard diesel engine due to weight savings.
And the significantly lower purchase cost will more than amortize the higher cost per liter-now less than 10 percent-of gasoline.
Different is the case for a larger boat that needs a lot of power and must grind out several hours, perhaps for work.
Inboard engines that run at lower RPMs will report lower fuel consumption.
And traveling many miles, for which substantial fuel supplies are needed, even a 10 percent savings can make a big difference.
Does the perfect boat not exist?
So here we come to the real crux of the matter: what one is looking for.
The perfect boat does not exist, but the best boat for our needs does exist.
And here a number of factors come into play, many of which are not only unmeasurable, but to which each of us, whether by tradition or personal taste, places a different value.
Therefore, they are, by definition, incomparable.
One example is the issue of reliability, a close relative of safety.
A diesel-powered inboard inline engine runs, as mentioned earlier, at much lower RPMs than a counterpart outboard gasoline engine: less mechanical stress, not only on the engine itself, but also on all transmission components.
Not only that: lower revs allow, where you have the space, to fit larger, and therefore more efficient, propellers.
This is a fact, but one that is difficult to quantify and therefore publicize and “monetize.”
Also because the Sunday boater doing 50 hours of motoring per season might prefer another type of safety.
For example, that of having a completely watertight hull, with no through holes below the waterline.
The shaft line requires at least 2: the one to pass the shaft through (whose “seal,” the packing, is always to be monitored) and the sea inlet for the cooling circuit.
I have seen more than one boat laid on the bottom of the Marina because of a leak in the latter.
This I think is the real “judgment” that emerged from the above discussion: you cannot generalize.
Each solution has advantages and disadvantages, and the best one is chosen based on how much weight we give to one or the other.
In short, to paraphrase a famous saying “tell me what yachtsman you are and I will tell you what engine you have.”
The shaftline still has a lot to say: let it speak! Stefano Monfroni
