Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[Robin smith]

I have a OTW  Vanguard which dives betetr than my type 7 but there is a problem with cavitation of the propulsor. I accept when not fully submerged the real thing suffers from the same problem. Even with the propulsion unit under water it does not seem to push the thing along. The motor is fine and the prop spins fast if required it just does not seem to move sufficient volume of water to move the thing forward. I have tried all sorts of ideas but no go. When it does move it is slow that it takes ages to turn. I have fitted a false see thru rudder which clips on to the lower rudder to get it to turn better which does work. My thougts are that the blades are set too fine a pitch. I have an Engel Lafeyette which performs really well, complicated but good dive perfromance, turns well, and will shift quite fast when you want it too. Any ideas on my Vanguard?

[Subculture]

The problem you are experiencing is ventilation rather than cavitation. The propeller is sucking air down from the surface when at very low depth. Once you get below a few inches the ventilation problem should disappear. There isn't a lot you can do to prevent this, other than keep the revs down when near the surface. It is a common problem with spindle sterned boats. Subs which were designed to spend the majority of their time on the surface tended to have hulls shapes with the props mounted underneath the hull (e.g. submarines of WWII) which prevents ventilation, but is less efficient when submerged.

There are two versions of propulsor fitted to the OTW Vanguards.

The earlier version had a Prop Shop propulsor that looks like this-



The later boats have a propulsor that was made by Dave Merriman, and is now cast by Prop Shop for OTW. This has a more blades, but the blades are smaller in cross-section and lower pitched.

Either should provide adequate thrust if well matched to a motor's RPM, but I think it's fair to say in model form conventional props tend to work a lot better. The Vanguard will never come close to a Lafayette in agility. The boat has a high aspect ratio, and the fixed shroud tends to fight the rudder, a swivelling shroud would give the boat very good turning capability, but it isn't scale.

[Robin smith]

Thanks for you reply. The picture you sent me the blades look a much coarser pitch. I have the Dave Merriman type which as you say look to be a finer pitch. The water inlet or first stage fixed stator blades I presume should act like inlet guide vanes on a gas turbine engine which guide the air onto the fist stage compressor blades. Asuming water is no different to air the pronciple is the same. Or have I got this wrong? I did think about removing soem of these front blades but the housing would be weakened. I have even tried moving the propulsion unit further backwards with spacers but to no real effect. Even thought of taking the thing off and fitting a conventional propeller for use and fit the other thing on for display!!

[Robin smith]

The other thing is your picture the baldes look more square whereas mine is more scimitar like?

[Subculture]

The fixed stators/vanes are there to provide counter torque to the single propeller. If you remove them you lose that counter torque effect, so not a good idea.

You have Dave's later version of propulsor which is supplied as standard with the later kits. Propulsor technology is based on jet engine fan jet design (Rolls Royce make the props for the our boats), but the design and configuration are highly secret, so modellers give it their best guess.

Dave's propulsor is pitched about right for a model. The original propulsor I pictured is very heavily pitched, and shouldn't be run past 2000RPM unless you want to draw some serious current.

If you find the ventilation is a serious impediment when running surfaced, you could do what fullsize skippers do and trim the boat a little stern heavy. This gives the propulsor a bit more bite in the water. If that's not enough for you, I'd say you ought to aim more for WWI and WWII boats which usually had hull shapes more conducive to surface running.

Fitting a normal prop, or even fitting a more conventional prop isnide the shroud should net some improvement in performance, but you shouldn't really expect a boat of this type to handle like an attack boat.