USS Blueback Sub Build - Page 2

Mankster · 13-08-2006

Thanks Jason, I am going to gie that a go.

magpie · 13-08-2006

She’s all done externally now. Once the WTC bits get here, I’ll make her a goer.
Note: The sail/tower isn't attached yet. It looks crooked. Will be straight!

**Bunkerbarge** · 13-08-2006

Nice job Magpie. That waterline treatment is very interesting and something I have been thinking about for my steam coaster.

I was thinking along the lines of floating paint on the surface of water and putting the hull into it but i think there will be a fair amount of experimentation first!!! Is is a very reall effect though and one that I haven't seen attempted before. It does look the part on the sub.

I would like to try to recreate a number of differrent lines overlapping at differrent draughts and trims but that might be getting a bit ambitious!!!

magpie · 14-08-2006

Thanks mate
Here's a couple of pics of the lazer beam.

magpie · 19-08-2006

I started on my WTC today. I’m using a Subtech motor-belt gear setup and I fitted that today and I made a bracket for the 2 servos from square plastic tube and flat ABS sheet. It also supports the belt drive. The servos run the rudder and dive planes. The speedie also goes in the back section and the APC. The air pump goes in the middle one and the battery (Li-poly) 7.4 2500mah and receiver and regulater go in the front one. The 2 front parts are adjoined for air pressure storage and the aft one is separate to avoid positive pressure on the Subtech seals from the inside when the bladder is empty.

magpie · 23-08-2006

I got a bit more done on her.
I bench tested the driveline and it's all cool. I'm working on the ballast pump now.
Jason

magpie · 25-08-2006

I test assembled the WTC today after drilling and bonding the caps last night. It’s a bit different to the standard kit layout in that I’m using a Subtech switch and clippard valve. I mounted the receiver in the central tube instead of the rear one. The Li-poly battery will be mounted flat in the front tube as it's a bit bigger than Dave's in length and won't fit up against the front cap. I put the air hose outlet in the front instead of having it go through the back cap and back out again at the top. This way I only need to seal one exit point and it gives me a bit more room. My apc is stickyback taped to a servo and the SSS switch for the pump is on the back of the receiver which fits nicely in the middle tube where I can easily get to it and see what crystal it has through the lexan. I'm putting the power switch bolts through the upper back of the front cap as I'd need to remove the whole WTC to access them if they were on the front or put a hatch in the fore deck etc. There's room for them there so it's cool.

This is a prop guard I made that clips on and off. It’s a precaution for when I’m learning to drive her and for my Scope guys who may back her into the bricks!

**Bunkerbarge** · 25-08-2006

I like the guard, the prop does look very exposed.

When you look at the tapering off at the back end though what a lovely uninterupted flow through the prop you've got there.

magpie · 26-08-2006

Yes I've been reading up on them a lot and they are very quiet with regard to cavitation for listening hydrophoned etc. That's why all the modern ones before turbo props (US atleast) have single fully aft props. If it's close to the hull, the blades go through "dead water" low pressure areas and the pressure change causes micro cavitation which is noisy and even erodes the prop blades apparently! Dave M makes nice white metal props to scale, but they are a bit fragile so the guard will be on it most of the time. Once removed there's only the top hole visible and it's right where the real one has a stern light so I can stick a removable one in the hole if I get all rivet counter happy later on or the club members are too judgemental as they can be at times.

PS Thanks for the much appreciated feedback mate!

**Bunkerbarge** · 26-08-2006

Cavitation is a very deep and involved topic and has been the subject of many an investigation and study over the years. It is also a very interesting thing to look into and one that affects the life of a marine engineer in many ways.

The basic concept is this, if you take a liquid and suddenly reduce the pressure it is under dissolved gasses will come out of the solution and form gasses. Under normal circumstances this would migrate towards the surface and escape but when it is done very quickly the bubbles of gas formed, being created by a sudden reduction in pressure, collapse again as they move away from the low pressure area.

Propellers, be they aircraft or marine, are only a moving aircraft wing in so far as they are an aerofoil shape moving through a fluid and creating lift. In a marine propeller you have the high pressure face and the low pressure face just as you do with a wing. The trouble with a marine propeller is it is very much a compromise. The ideal aerofoil shape only exists for a certain speed so the distance from the shaft coupled to the varying propeller speeds all combine to ensure that the propeller does not get to its peak efficiency. In reality you have varying prop speeds and loading conditions so a propeller can only be designed to suit the vessels characteristics under the majority of its operating conditions.

It is when you get away from these ideals that cavitation becomes more of an issue. People tend to think that cavitation exists only when the slip has become 100% and thrust ceases, which will happen when the prop it turning too fast. Cavitation though happens at all revs and is evidenced simply by the formation of gas bubbles constantly forming and collapsing in the propeller wash. What these bubbles do as they are collapsing is to impinge on the surface of the propeller blades and actually cause erosion. To see a propeller that has cavitated a lot is quite an eye opener. It looks like corrosion/erosion at first but it is more of a pitting effect and can completely wear a propeller away.

I have been on a ship where we had a slight bend in the leading edge of one of the propeller blades, which caused cavitation as this bend sliced through the water. The low pressure area following the bend could be heard inside the ship every time that blade passed the hull and it sounded just like someone was banging on the hull with a hammer on the outside. Hence the significance of the most efficient possible flow through a submarines prop and why you find shrouded props and very clean efficient hull forms at the back ends of subs.

This effect is not only limited to propellers though, it can occur in any environment. One of the most common is inside centrifugal pumps where I have seen cavitation erosion wear an impeller away from a 100kg lump of brass until it looked like a piece of lace!

By the way did you realize that when you crack your finger joints and make them ‘pop’ the noise is actually cavitation inside the joint. The ‘pop’ is the gas bubble collapsing within the joint as you cause a sudden reduction in pressure by opening the joint. The more often you do this the more damage you do to the joint and cavitation erosion will eventually cause arthritis.