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M.V. Sea Nass

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Dave Harrison10/01/2021 11:52:10
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29 forum posts
23 photos

Hello everyone, I have really enjoyed reading this forum for a while now, I have always had an interest in boats, ships and scale models. I haven’t built a model boat for about 30 years, so I would be very grateful for any comments and suggestions. I am certainly going to need help along the way.

My chosen subject is the “M.V. Sea Nass” formally “M.V. Mini Luck”. The Mini Luck was the first in a series of ten small coasters built in japan. She was completed in 1970, at 65 and a half metres, around 3000 tons, a top speed of 10knots and a range of 4300 nautical miles. She was shallow draft wide beamed and designed to travel into rivers and estuaries. Originally designed to carry general cargo such as Ore, Coal, timber etc, she could also be loaded with 32 twenty foot containers.

I am using the James Pottinger plans at 1/75 ( model shipwright 119 ) which I have started to redraw digitally so I can cut the frames on a CNC router. I am still deciding on scale but leaning towards 1/48 to give a OAL of 1364mm or 53 and a bit inches.

The hull is hard chine, the frames and keel will be cut from 6mm ply and the hull skin will be whatever I have available in my workshop either 1 or 1.5mm ply. I will probably use the same for the deck and then plasticard for the superstructure etc.

sea_nass-6922262-general_cargo-8-169701.jpg

Dave Harrison11/01/2021 20:16:55
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29 forum posts
23 photos

Over the weekend I have been thinking about the displacement and ballast requirements of this hull in 1:48 scale. When the full size ships sailed unladen they sat quite high. ( see picture ) I quite like the idea of ballasting my model with water. I am thinking of a large baffled tank in the centre of the hull. First ideas are to construct a vented tank that could be filled and emptied with a small electric pump. This would allow me to control how the model sits in the water for any given day.

ballast tank.jpg

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Tim Rowe12/01/2021 06:56:33
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454 forum posts
458 photos

Hello Dave

Ballast tanks should either be empty or completely full. Anything in between and you have what is called the "Free Surface Effect" which can play havoc with the stability.

A way to reduce this would be to divide the tank longitudinally into three, ie one tank on the centre and two wing tanks. You then have the combinations of:

All tanks full
Wing tanks full centre tank empty
Wing tanks empty and centre tank full
All tanks empty.

It's then just a question of plumbing.

Tim R

gecon12/01/2021 08:47:57
309 forum posts
271 photos

Hello Dave, my first thought is that all water should always be on the outside of a hull!

If you must 'pump water' as ballast, the deck-to-superstructure joins must be 100% watertight. You will need a high 'lip' all the way round inside of the superstructure to make sure no MORE water gets into the hull. That freeboard looks menacingly low to me!

Maybe you could fit an automatic bilge pump too?

Tanks should be very well baffelled so that water can only 'flow' very slowly from one section to another.

I see on google that many build sinkable/refloatable boats for warfare and iceberg-bashing simulation sessions, so the necessary equipment is certainly readily available.

Have fun and good luck with the project.

George

 

Edited By gecon on 12/01/2021 08:48:56

Tim Rowe12/01/2021 09:54:59
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454 forum posts
458 photos

Hello again Dave and Gecon

If the tank is empty or completely full then baffles will have no affect at all. In a fuel tank where it cannot operate either empty or full, baffles are fitted to prevent the fuel sloshing around.

The danger of allowing water to flow slowly is that if there is any imbalance or wind heeling, the water will flow to the low side and tend to stay there. If the heel moment is reversed the boat can flop to the other side in a condition know as loll. This is the situation where the boat / ship is not actually stable in the upright position. It's a bit like trying to carry a tray full of water. It requires effort to keep it level.

Tim R

Ray Wood 212/01/2021 10:28:31
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2232 forum posts
777 photos

Hi All,

Sounds like a challenge, but I would use the 2 pint plastic milk bottles filled at the lake, I imagine you will have 2 fair sized holds either side of that massive crane.

The hull doesn't actually look that deep on your drawing and loses some displacement with the chine design versus tradition round bilge design, rather like a TID tug.

Many of the guys in my club use the lake water for ballast and put it back when you have finished boating.

Too simple??

Regards Ray

Tim Rowe12/01/2021 10:37:44
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454 forum posts
458 photos

Good practical solution from Ray as usual.

Tim R

Dave Harrison12/01/2021 13:32:39
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29 forum posts
23 photos

Hi Tim,

Your comment "Ballast tanks should be empty or completely full" instantly reminded me of filling sailplane wings with water ballast.

It is easy to understand why water sloshing around in a void would cause instability, it was slightly harder to grasp why a well baffled tank ( slow flowing / moving water ) would create as many stability problems as it solves, but I think I've got it. So in simple terms the problem is not how quickly the ballast moves, it is the fact that it moves uncontrollably in the first place. Meaning that Ray's idea of sealed containers ( milk containers ) filled completely with water provides a almost static ballast, hence stable ballast. I have the sudden urge to read up on the design of oil tankers.

George, I agree that the freeboard looks menacingly low and one of the reasons why i wanted the ability to adjust the ballast for the conditions ( run the model low in the water on a completely still summers evening, higher in anything other than perfectly still ). I don't know why but I have always liked the look of a ship sitting low in the water. You are right about the hatch lips stopping water ingress and something I will have to remember.

Thanks for all of your comments, I think I'm going to move my attention to cutting some wood and come back to the subject of ballast once I have a hull !

Tim Rowe12/01/2021 15:18:47
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454 forum posts
458 photos

Good plan Dave

And keep the build progress going on here.

Tim R

Ray Wood 212/01/2021 16:19:19
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2232 forum posts
777 photos

Hi Dave,

My 20" TID tug made of balsa to Vic Smeeds design enlarged slighty is the same sort of hull shape as your ship and took 4.5 lbs of steel plate ballast to achieve the scale waterline.

Somebody technically minded could probably work out the displacement, not me I put it in the bath

This picture reminds me what we are missing !!

Regards Raytugs bw 020717 001.jpg

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