Seeking Stability, Maneuverability and Speed Feedback on Design

[Submarine Tom]

I believe I have designed the best initial stability, maneuverability and speed possible given the constraints in a model boat design contest.
Before going ahead with the build, I invite feedback from those in the know on the design and how it may perform in initial stability, maeuverability and speed.
Motor, shaft, prop, batteries, rudder, payload (and it's location), length are all fixed variables and standard issue. Draft is limited to 20 cm, length 90 cm. Foam construction with 6 kg of ballast: 3kg above the waterline, and 3 kg below.

Displacement: 7.2 kg (4.1 in pods, 3.1 in hull) Balanced as drawn.
Hull speed: 1.2 knots
Beam: 14cm
Draft: 20 cm
LOA: 90 cm
Freeboard: 4 cm
Variable speed control with variable reverse
Variable steerage via rudder and aft swing pod (proportional)
Pods are Fiberglass with lead shot (compulsory ballast) filling void space around components. Again balanced.

Competition points are also awarded for creativity and appearance.

Just to be clear, none of the components can be changed. I am only looking for feedback on how the vessel may perform in initial stability, maneuverability, and speed in calm pool water.

Thanks,

-Tom

[Mr Efficiency]

Seems remarkable that that amount of draft is allowed, 'cos a full size ship to that scale would be lucky to find a harbour deep enough to enter.

[PAR]

Why design a hull with zero initial stability? The logical choice would be a square or nearly so midship section, flat bottomed and of course easy entry and exit.

[Submarine Tom]

The scale is 100:1. So, it's a 90 meter ship!

It has to be able (in theory) to transit the Suez Canal which is 23 m, but they impose a 20 m limit.

-Tom

[Mr Efficiency]

Originally Posted by Submarine Tom

The scale is 100:1. So, it's a 900 meter ship!

It has to be able (in theory) to transit the Suez Canal which is 23 m, but they have a 20 m limit.

-Tom

Hang on, 100:1 is a 90 metre ship ? (300 feet)

[Submarine Tom]

Originally Posted by PAR

Why design a hull with zero initial stability? The logical choice would be a square or nearly so midship section, flat bottomed and of course easy entry and exit.

Because it would...?

The stabilty test used applies weights at 5 cm off centre athwartship, amidship.

-Tom

[Submarine Tom]

Ah, now I understand the question PAR.

I was trying to maximize volume/surface area and reduce wetted area. With the keel design, wouldn't initial stability stiffen up in a hurry from that huge righting arm?

-Tom

[Submarine Tom]

Originally Posted by Mr Efficiency

Hang on, 100:1 is a 90 metre ship ? (300 feet)

Excellent point Mr. E, my mistake.

Post corrected.

Thanks.

-Tom

[Submarine Tom]

What think you PAR of my reasoning?

By the way, this design is not for the kid. I know it was one of your concerns earlier. This is my design for me.

-Tom

[Stumble]

Since there is almost no initial stability, that thing is going to roll, all the time. The heavy low slung ballast will snap it back, and over correct, and with a rounded bottom there isn't much to stop it. The motion will be exacally like a pendulum, short non-stop rolling in any wind or wave conditions, though it won't heel much. This is not a boat I would ever want to spend time on.

[Mr Efficiency]

Every cloud has a silver lining, we might have happened on a way to harness wave energy, non-stop pendulum rolling harnessed by some internal mechanism.

[PAR]

Giro stabilization? A bit over the top on a model don't you think. If the CG is high enough, the roll moment could be kept reasonable, but why disadvantage the project from the start with a meaningless hull shape. There's no gain with this shape, so why employ it. You want a reasonable entry, reasonable exit and a square midship section, which is pretty much the way large vessels are shaped BTW. The advantages of these shapes address and apply meaningfully to the project, across several areas of concern, making the whole viable, which is likely the point of the project in the first place.

[Submarine Tom]

Originally Posted by Stumble

Since there is almost no initial stability, that thing is going to roll, all the time. The heavy low slung ballast will snap it back, and over correct, and with a rounded bottom there isn't much to stop it. The motion will be exacally like a pendulum, short non-stop rolling in any wind or wave conditions, though it won't heel much. This is not a boat I would ever want to spend time on.

Stumble,

At 90 cm LOA you have a very hard time spending any time "on"it.

-Tom

[Submarine Tom]

Originally Posted by PAR

Giro stabilization? A bit over the top on a model don't you think. If the CG is high enough, the roll moment could be kept reasonable, but why disadvantage the project from the start with a meaningless hull shape. There's no gain with this shape, so why employ it. You want a reasonable entry, reasonable exit and a square midship section, which is pretty much the way large vessels are shaped BTW. The advantages of these shapes address and apply meaningfully to the project, across several areas of concern, making the whole viable, which is likely the point of the project in the first place.

PAR,

Where on earth did you get giro stabilization from?

The hull shape is to reduce wetted area by maximizing volume to surface area.

I believe you have missed the point of the project, but thank you for your reply.

Nothing further.

-Tom

[Stumble]

Submarine,

I was talking about the hull shape, not the model you want to build.

And if you want to maximize volume to wetted surface, make the hull more of a sphere as opposed to a tube. It will further increase the desired ratio, if on the other hand you want to make something vaguely boat like, without tying yourself to some arbitrary requirement (max volume to wetted surface) you will have a much nicer hull shape, that will be able to carry all that cargo you are planning to fill all that volume with.