USS Blueback Sub Build

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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.

[Guest]

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

[Guest]

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!

[Guest]

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.

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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.:judge:

PS Thanks for the much appreciated feedback mate!

[Guest]

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.

[Guest]

............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.

Very good description of the cavitation effect Bunk! Much better than the ones I've read in the past. Most interesting. There are a lot of very cool subjects involving boats/ships etc.

The last paragraph re arthritis is particularly interesting to me as I have Rheumatoid arthritis myself. I thought the clicking you referred to that kids do, was just bones clicking. My joints are all very clicky but that's mostly due to large gaps and worn cartliage etc from the RA erosion. Some of it may also be cavitation as you said.

Regards

Jason

[Guest]

Many years ago I was sat in a class at college doing a subject called Tribology, which is the science of how surfaces react and respond to each other and how lubricants affect this relationship. The science of lube oils is not far off the mark.

Anyway the guy asked the class what people popping thier fingers had in common with erosion of a propeller and, I have no idea why, I saw the connection, stuck up my hand and said "cavitation".

The fact that I was correct has stuck in my mind ever since! (Maybe it was rare!)

I agree with your thoughts on the cool subjects. Propeller design is an unbeleivably complex subject and yet most of them are, at the best, a compromise and, at the worst, such as controllable pitch props, bloody inefficient!

As with a lot of things though the really interesting, more traditional stuff is dying out as more and more technology takes over and the intervention of the engineer with his thought process is less and less of a requirement.

Only the other day we had a problem to solve whereby we wanted to remove a section of main seawater pipe, 4 foot diameter, and fit blanks but we could not get an effective isolation. The Deputy Chief suggested we run a pump on the line to vacate the line while we fitted the blanks. It worked and we saved ourselves a very big job. It is just this sort of thinking that we are loosing as technology takes over and we become isolated from the hardware that is working for us.

I agree though, I still enjoy playing around with traditional Marine Engineering concepts and the appliance of things I learned at college that I never thought I was going to use.

[Guest]

I still haven't received the battery for her yet, but put a 100gram lead bar in the battery bay and trimmed her in the dog bath with foam and lead. She'll start her sea trialls with her decks awash at full flood to save losing her before I get her sorted out. I'll add more lead later on.

The pic is with half the air in her.

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Very nice Magpie - I thought it was a real one when the pic appeared!

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Looks the business. I'm looking forward to seeing her on trials though.

Are there figures on the bridge? I can't quite make out the detail from the picture but it looks like there is a nice level of interest there.

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Thanks guys!!!

The pics below show a bit more of the bridge. It’s not perfect, but fairly close to the real one I think. It has a hatch in the back of the cockpit and a snorkel and several masts/scopes etc. Simulated ones anyway. Like I said, “It’s not perfect”. I think it’ll look good out in the lake anyway.

Jason.

[Guest]

Of course the downside of having the prop in all that lovely uninterupted water flow is that the rudders are nowhere near the wash. Hence the two rudders to try to compensate for this.

Compare this with a U-Boat where both rudders and both aft planes are directly in the prop wash making them very effective. I was actually very surprised at how well my Revel U-Boat steers with no modifications to any of the control surfaces.

It will be interesting to see how well this boat manoeuvres.

[Guest]

Of course the downside of having the prop in all that lovely uninterupted water flow is that the rudders are nowhere near the wash. Hence the two rudders to try to compensate for this.Compare this with a U-Boat where both rudders and both aft planes are directly in the prop wash making them very effective. I was actually very surprised at how well my Revel U-Boat steers with no modifications to any of the control surfaces.

It will be interesting to see how well this boat manoeuvres.

I'm not sure of the theory behind this but the tightest turning sub sub I have seen anywhere is my 1/96 scale Bluback. Its about 28" LOA and can turn around inside 4" underwater (and those verical surfaces on the end of the rear planes actually slow down the turn) I have never seen a U Boat get anywhere near than ratio underwater. Maybe the props behind the rudder in Uboats are optimised for surface running?

[Guest]

Interesting stuff. Are all three vertical surfaces fitted with rudders? I assume on the surface the central fin is doing nothing but underwater it obviously has an effect.