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balancing a Robbe U-47

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balancing a Robbe U-47

Postby gawnefishing » Thu Jul 17, 2014 10:03 am

I have a Robbe U-47 with a engel ballast tank and a 6v 10000mA NiMH battery for power. I cut the lead ballast bar 40 mm per instructions for water line level. With the boat sitting in a calm pool not under power and the lead bar as far as it will go to the stern, the boat is almost level. My main problem is that the boat wants to turtle to the left. We tried to ad fishing weights to the right side of the deck to correct the tip but nothing worked. The kit instructions don't say anything about a way to stop the boat from turning over, plus I have changed the boat from a dynamic to a proper submarine (more stuff inside). I am alone in this project up here in northeastern lower Michigan and the local hobby shop (200 miles round trip) is more into planes & cars. While talking to some people around here about round bottom boats, they said that the weight needs to be as close as possible to the keel and below the water line for a level trim. So since you fellows are into this way more than me could someone help a want-a be submariner out? Thank you, Matt
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Re: balancing a Robbe U-47

Postby JWLaRue » Thu Jul 17, 2014 10:19 am

Hi Matt,

You came to the right place for assistance!

In addition to the weight being placed as close to the bottom (keel) as practical, you also want to place floatation foam as high as possible. If you still have fishing weights on(?) the right side of the deck, remove them. Adding weight above the waterline is going to make things worse.

If the boat has a tendency to roll left or right, you probably don't need to add weight but do need to add foam. For a Type VII, I would first try placing foam right at the waterline ( no higher) and in the saddle tanks. Placing it in there will get you the most lateral separation and yield to best roll stability.

...this is a start, let us know if/how any of this has helped and we'll continue to work this with you.

-Jeff
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Re: balancing a Robbe U-47

Postby tsenecal » Thu Jul 17, 2014 7:46 pm

Matt,

on my robbe u47, which was built almost exactly the same way as yours, (4 6 volt nimh packs and a 750ml engel tank, as well as geared 400 motors) i used the entire lead bar in the keel, but i had foam in the saddle tanks, as well as in the bow ahead of the WTC, and in the stern behind the WTC. the stern foam was cut so it would fit around the pushrods for the rudder and diving planes, the forward foam was cut so that it would fit around the forward diving plane pushrods and the "bolt" used to hold the two parts of the WTC together. lead wheel weights were placed strategically around the tech rack to keep the boat ballanced port to starboard. foam was held in place with double-sided servo tape so that it could be moved around/removed when needed. as jeff stated, most all of this was placed below the water line to give it a nice water line when surfaced, but more importantly, it was also arranged to keep the boat on an even keel with about 3/8" of the sail visible when the ballast tank was full. because of that, a lot of the stern foam also existed above the water line.

Tim
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Re: balancing a Robbe U-47

Postby gawnefishing » Mon Jul 21, 2014 1:13 pm

Well I tried adding foam in the hole under the counting tower and any place else I could stuff it. And tried a water test again. She sat upright for about 30 sec. and rolled to port again. I am limited on using any large sizes of foam because my center deck (brass) is glued on and taking it back off will screw up my paint. I placed the lead keel weight as far back as possible, water level is about wright now. Also added some foam in the bow, it helped a little too much so I took some out. For the spaces that I can't get to because of the deck, How about spray in low expanding foam? Has anybody tried it? Thanks, Matt
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Re: balancing a Robbe U-47

Postby Ralph --- SSBN 598 » Mon Jul 21, 2014 2:44 pm

I am just catching up on this project.
You have a hull rolling problem.
The answer is in the posts above but I am not sure it is clear.
There are two center of gravities in a submarine.
Surfaced and submerged. The farther apart you can make these two, the more stability you will have.
Surface CG above Submerged CG.

I will see if I can help make it a little clearer.
Heavy items whether it's the equipment or the ballast weight (lead or steel or?) should be placed as low in the hull as possible.
Some equipment because of it's job can't be place low...Example would be the motor being mounted inline with the propeller shaft(s).

So we depend on ballast dead weight and foam.
One sinks the boat the other floats the boat.
We use these to trim the boat at full surface (waterline) and full dive (just a little bit of the sail is above the water. 1/2")

Heavy ballast as low in the hull as possible.
Foam as high as possible in the hull BUT below the waterline.
If any of the foam is above the waterline it will do it's job until it is lifted out of the water then it becomes a liability by adding weight above the waterline which will require more weight in the keel to compensate for the rollover weight above the water surface.

Once you get the dead weigh and the flotation weight correct for both surfaced and submerged then you can move either or both forward and aft to balance the boat to be level on the surface and when submerged.

I hope I was able to explain what I was trying to get across.
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Re: balancing a Robbe U-47

Postby JWLaRue » Mon Jul 21, 2014 2:58 pm

At this point, some photos would be very helpful to allow us to go from generalized advice to potentially specific guidance.

-Jeff

p.s. and it is *not* required that the motor (shaft) be inline with the prop shaft. That's what universals are for. :)
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Re: balancing a Robbe U-47

Postby Ralph --- SSBN 598 » Mon Jul 21, 2014 3:12 pm

Yes, photos would help tremendously.
-----------------
Yes, universals are a big help.
I use them even when the motor shaft is inline with the propeller shaft.
Helps when motor shaft and propeller shaft are different sizes as well.

In this hobby of submarines, there is more then one correct way to get the job done.

My help will be in a general format because I have not built a Robbe kit.
Others offering help may have hands on experience with the Robbe kits.
Go with their advise. Their information will be far more useful than mine.
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