Steering nozzle vs. rudder

[Skylark]

--------------------------------------------------------------------------------
I have a 28ft. Monk wood boat (1956),that I am presently in the process of re-building the transom, and fiberglasing the hull and decks. The propeller on this boat is only one ft. from the stern, and the rudder is actually bolted to the transom. I am thinking of drilling a pilot hole over the prop, and installing a steering nozzle around the prop. and illiminating the rudder.I wonder if there is anyone out there who has tried this and what would the pros and cons be? I know it would make the boat more manuverable,but am hoping it will increase the top speed as well, and not make the boat impossible to steer a straight course.

[badges65]

have a look at levi's surface drive system and they have a steering bucket over the prop. it is off set to the rear of the prop.
i have it in my 32' launch and steering is not a problem but not so good in reverse.
how it would work underneath the hull i am not sure but someone else may have some thoughts.
don

[Mike D]

Skylark

This might help you in deciding
http://boatdesign.net/forums/showthr...=5224#post5224

Look at the diagram. The base is a scale of Taylor's coefficient Bp and it is a formula that reads
Bp = RPM x power^0.5/speed^2.5
RPM is the propeller rate of rotation
power is the power delivered to the propeller in horse power
speed is in knots, take the boat speed less 5%

You'll see that a nozzle is useful at the lower end but anything over about 45 shouldn't be used.

Michael

[Frosty]

The sony surface drive does not go all the way around. But if you did make some thing that did, you might get lift from the bottom side of the nozzle. this might be to your advavtage--however you might get the opposite--ie a sucking downward, pulling the stern of the boat into the water. This uppward thrust or downward thrust will be hard on the shaft bearings and supports-- may even give stiff steering.
Vertical allignment is critical and is probably why you do not see them on fast craft. I have only seen them on net pulling trawlers.
if you build a tunnel rudder as the Sony drive has you will experience problems in strength as you have really two rudders hanging from the one shaft. As the prop is meant to be inside the tunnel you are then limited to the lenght of the rudder shaft meaning the stength of the rudder will be in the blades them selves. if they are not striong enough they will touch the prop tips when turning ( personal experience)
Tunnel rudders and nozzles have a greater wetted area, ie-- drag. they do however opperate in green water and not the turbulant water at the back of the prop.