Interesting things seen in a boatyard

[DogCavalry]

Interesting. Although the packing on the port side is the stuff of nightmares.

 

[DogCavalry]

Here's a double rudder on a bc built multi use fishing boat. Don't see much of this on open wheels.

Indeed not. The multirudders in the yard are all on nozzles.

 

[DogCavalry]

More of 542

 

[comfisherman]

What a 58 Wahl looks like with only fuel and water aboard.

Sitting a solid 10 to 12 feet out of water.

 

[DogCavalry]

Correct me if I'm wrong, but doesn't water have mass and viscosity, and therefore will resist making 2 abrupt turns over a distance of ( let me get my tape) zero?

 

[fallguy]

I'm gonna go on a limb and say those are married to each other. Actually can see the bar, but can anyone explain why this rudder arrangement? My guess is that each engine can steer any direction and if a single rudder; it would potentially defeat the other if both pointing to same side, but I also can't dream why that would ever be desirable. Unless...one forward and one reverse and trying to keep clean flow??

 

[DogCavalry]

Well, actually I'm talking about the 2 sharp angles in the hull bottom. I didn't even look at the rudders on the kort nozzles.

 

[philSweet]

Somebody needed to cut costs and decided to leave six feet out the middle. Here's your boat, fella. Wait. what? Where's the rest of it?

 

[DogCavalry]

Yeah, that brutal angle in the middle can't be intentional, can it? But whowould build a ship that size without a competent NA involved?

 

[baeckmo]

No guys, your’e on the wrong track here! This NA has performed a perfectly relevant balancing act between cost/benefit, honour to thee who deserves it!! This example highlights the danger in using intuition in fluid dynamics contexts!

When “tugging” (pushing or pulling) the speed of advance is very low, ie the dynamic pressure around the midships section is low, and there is little static pressure to regain by act of diffusing the flow. The nozzles/propellers are acting as giant hydraulic sinks, increasing the fluid velocity. The complete volume aft of the “step” is a low-pressure region; ie the flow is accelerating from the step and there is no major flow detachement. There is a limited volume of recirculating fluid in a “bubble” just aft of the step, assisting in the directional change, but the energy to keep it running is negligible. It might even benefit from a small vertical threshold (a “cusp”) that will stabilize the circulation.

The design driving condition for a tug is getting enough (undisturbed) water into the nozzle inlets, everything else is secondary. The losses that matter occur in the vicinity of the nozzles, where velocities are high. Your logical mistake is that you look at the isolated tug hull and draw conclusions. Imagine the hull in its working condition with a set of square barges or a timber float ahead; then the potential losses around the tug hull is a spit in the ocean; propeller inflow is all that matters.

 

[DogCavalry]

Thanks @baeckmo ! I hadn't thought of it from that point of view. Still an interesting thing seen in the boatyard though. You learn something new everyday.

 

[philSweet]

Boats with a cusp bottom have been in production since 1800. It's sort of a proven design even before engines. Flat bottom sharpies and smacks often had a sharp cusp, but with them, the big centerboard, a foot thick or so, smoothed the displacement curve somewhat. As Baeckmo said, the priority is to supply those low-advance-ratio wheels with well-conditioned inflow.

 

[DogCavalry]

Well, since it's a physical impossibility for water to cross that cusp smoothly, I'm not sure that's a success. I can't find a single image of another hull like that one. It's a certainty that it's meant to work slowly, because quickly isn't a possibility at all. This vessel must be more or less permanently associated with it's tow, because it won't be relocating between jobs.

 

[comfisherman]

Perfect example of the balances of boats. No doubt it would be better without a crease mid hull, but apparently the designer and builder didn't want to take the time to bend some metal.

Kinda a funny paradox that exists, boats can be sub optimum but still work for many decades. I'll post a Pic tonight of a boat recently built. It's going to have an abysmal keel shoe design. Instead of taking the time to do it correctly and deepening the keel and building a tunnel, they are taking the worse way....

I have a similar hull and spent weeks building forms laying new glass and doing it "right". We will have slightly different performance numbers when done. But theirs will be done weeks before and take less work.

 

[DogCavalry]

Unsinkable logging boat | He's having a great time | By UNILAD | Facebook https://fb.watch/lRbbsQf1cp/?mibextid=6aamW6

A great video of a log boom tug.