Rudder or thrusters

Hello,

Total amateur’s question - keeping in mind offshore, displacement, non-commercial boat around 40’ length, cruising speed below 8 knots, single convention shaft with propeller, will it be wise to use 2 tunnel thrusters in the bow and stern instead of rudder? The background for the question is to concentrate on the underwater hull design for fuel economy / speed.

If I am not mistaken, there are some specific cases when rudders are not used, f.e. work boats with azimuth thrusters.

Can you please comment the question and / or provide some more examples when rudders are replaced.

 

Generally speaking: Rudder

-Tom

 

Thank you, Tom, but why not thrusters?

 

On a small craft, how will you keep the forward thruster in the water as the vessel moves thru a seaway ?

 

Michael, thanks for the first reason.

In the situation you mentioned I assume that stern thruster is still under the water

 

I'd go for rudders, too.

Keep in mind that transverse thrusters don't really work when the vessel is under way and their transverse flow is overlaid by the flow along the hull, resulting in no or almost no transverse thrust.

In addition, the thruster tunnels will produce quite a lot of drag which might be larger than the resistance of a conventional rudder blade of proper size. According to Holtrop & Mennen* the drag coefficient of a bow thruster opening is somewhere between 0.003 and 0.012 (so you have some numbers to play with).

Just my two cents...

*J. Holtrop and G. G. J. Mennen: "An Approximate Power Prediction Method", International Shipbuilding Progress 1982, Vol. 29, pp. 166-170.

 

On a typical 40' boat, you would only need one prop to move her around and if straight shaft or sail drive a single rudder. This is less resistance then a few pods scattered about the bottom of the boat. The only reason you'd want to tolerate the increased resistance of pods, would be to satisfy some operational requirement, such as precise station keeping or better maneuverability on certain hull shapes. This falls back or your satisfying SOR goals, established in the preliminary design stages.

 

Olav and PAR, thank you

I will keep the link for home assignment, with the current knowledge I hardly can swallow it
http://www.marin.nl/web/News/News-items/An-approximate-power-prediction-method.htm

The very first idea was to close both our reasons with jet thruster (regulated thrust, only 5 holes in the hull 1 x 90 mm and 4 x 45 mm), but cannot tolerate the ideas of letting the water inside the hull and complicity of the entire system.

I take time out, the question might be changed for a kind of "how to integrate a rudder into fuel efficient hull and find low resistant and simple maneuvering solution"

 

Thrust vectoring is simple and efficient (no rudder too), but you only need one unless there are special circumstances. An outboard is a classic and simple thrust vectoring device.