Dedicated to Alessandro Tomei
The admiral engineer G. Venturini, in the quality of Comitato
Progetti Genio Navale MM.II. President, said : 'Tomei always knows more'. By his genius
we have the 'Tabella Tomei' for motor-yacht. NAUTICA magazine technical consultant
during 20 years, great expert of propellers, has contributed to the Turboprop plans,
to its realization and testes.
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Planing boats
Given weight and aerodynamics, it is Known that a car needs wheels fit (diameter, largeness, tread of tyres, and r.p.m.) for giving the maximum efficiency.
Analogously a planing boat, given navigating weight and keel lines, needs a propeller fit (diameter, pitch, area and r.p.m.) for giving the maximum efficiency.
Like an aircraft, the planing boat needs the maximum push at the "take-off " moment, that is at the moment of the characteristic tail-down (take-off beginning).
The true "volplane" is characterized by the progressive and sensible reduction of the tail-down angle till 6° about for the pleasure boats and less for the race boats.
The right evolution of these stages (take-off and volplane) is conditioned by many facts, for example:. barycentre position, navigating weight and propulsion characteristics.
Remember that, like the aircrafts, the planing speed increases with the navigating weight and the weight excess nails the boat at the take-off stage.
Propulsion
We annotate the following:
Sub cavitating propeller not much used these propellers partially inserted inside the keel, for want of specific knowledge with consequent errors.
Super cavitating or surface propeller – permits speed increases till + 20%, but It is under the effects of the navigating weight variation.
Hydrojet : slow efficiency. it is a turbine conveniently usable for a speed over 40 knots.
Marine Turboprop : permits speed increase till + 20%. High efficiency
At all events any propeller has to permit always to the engine the maximum r.p.m. fixed by its builder.
Cavitation
All the propellers cavitate. The cavitation is comparable to a clutch which, skiding inside calculated limits, absorbs the navigatine higher charge and permits to the engine to reach the maximum r.p.m. as foreseen by the builder.
A propeller with large diameter doesn't cavitate, brakes the engine r.p.m., overcharges it !!!
Shipyard builder
The shipyard builder which makes known : L.W.L., real desplacement without any charge (people,
fuel, water, ecc.), graphic with the real speeds truly practicable at engine max r.p.m.
and max r.p.m. - 10% with the desplacement variation (min and max), is a builder who has
no secret and is forward to those that are obliged to dissimulate these data.
Comparison propeller - Marine Turboprop - Hydrojet
| propeller | Marine Turboprop.it | Hydrojet |
| pressure | / | low | high |
| waste | Under water | surface | surface |
| Rudder system | rudder | rudder | spoons |
| steerage | mec./hydraul. | mec | hydraulic |
| Back drive | Revolution reversal | Revolution reversal | spoon |
| efficiency | / | high | low |
| sensitiveness to load variations | medium | low | high |
| Projection over stern | rudder | rudder | Jet with spoons |
| maintenance | 0 | 0 | high |
| Material of construction | / | Like the hull | metal |
| feeding | / | forced | aspired |
| turning | / | normal propeller | Special propeller |
| shaft | large | short | shorter |
| weight | Very low | Very low | high |
| assembly | 0 | 0 | Flange to hull |
EXAMPLE
planing hull - twin Turboprop
| L.W.L. metres | 7,90 |
| Engines 2 x | 163 HP |
| Displac. empty tons | 3,56 |
| Displac. min. tons | 3,90 |
| Speed kts about | 40 * |
| Displac. med. tons | 4,25 |
| Speed kts about | 38 * |
| Displac. max. tons | 4,60 |
| Speed kts about | 36 * |
*) expected
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