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Darnell T class submarine

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Anthonyw
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Post  Anthonyw on Wed Sep 14, 2016 7:06 pm

Dear members

I'm posting here on behalf a of a friend who is having issues with the darnell t class submarine. He is not a stranger to model submarines or boating but this issue has him in a pickle. The issue is his submarine keeps rolling over, more so when he turns. He says that the Centre of buoyancy and centre of balance are correct and therefore is rather bemused by the problem.

Does anyone have advice I can pass on to him, Anything would be helpful at this stage.

Yours hopefully

Anthony
david f
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Post  david f on Thu Sep 15, 2016 2:16 pm

It depends what  "correct" means 8-) (If it rolls over something is wrong!)

The first thing  he needs to do is make sure that the waterline is about right - add weight as low down as possible until most of the rounded hull is underwater with the deck superstructure clear. (Ballast tank empty) If that fixes the roll problem - stop there.

If there is still a problem - I wonder if there is too much superstructure weight?? Lovely looking metal guns, working periscopes etc. can all add too much top weight.

Still a problem, then he may need to start adding foam in the top of the hull with more weight low down to again get a reasonable waterline. Placing foam needs care - see this earlier thread on a Darnell which also describes the ballasting process:


http://www.theassociationofmodelsubmariners.com/t917-ballast-and-trimming

David
John Wrennall
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Post  John Wrennall on Thu Sep 15, 2016 10:10 pm


I interpret "rolling over" as upside down. Is this correct or is it rolling on turns (say 30 degs) and then returning to vertical?

John
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Anthonyw
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Post  Anthonyw on Thu Sep 15, 2016 10:37 pm

david f wrote:It depends what  "correct" means 8-) (If it rolls over something is wrong!)

The first thing  he needs to do is make sure that the waterline is about right - add weight as low down as possible until most of the rounded hull is underwater with the deck superstructure clear. (Ballast tank empty) If that fixes the roll problem - stop there.

If there is still a problem - I wonder if there is too much superstructure weight?? Lovely looking metal guns, working periscopes etc. can all add too much top weight.

Still a problem, then he may need to start adding foam in the top of the hull with more weight low down to again get a reasonable waterline. Placing foam needs care - see this earlier thread on a Darnell which also describes the ballasting process:


http://www.theassociationofmodelsubmariners.com/t917-ballast-and-trimming

David


Thank you for your advice David i will pass this onto him and see if he can resolve his issues. I know he has little in terms of super structure and has mentioned lead balast and the waterline. The foam might be something for him to explore.

I think he wants to take it to a popular show he's going to this weekend, I'm sure he said it's the Mecca for model submarines.
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Anthonyw
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Post  Anthonyw on Thu Sep 15, 2016 10:42 pm

John Wrennall wrote:
I interpret "rolling over" as upside down. Is this correct or is it rolling on turns (say 30 degs) and then returning to vertical?

John

I'll have to find out for sure but my understanding is that it was rolling inverted.

One thing he did mention was possible torque from the propellor. But surely the balance is very questionable if that was the case ?
salmon
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Post  salmon on Fri Sep 16, 2016 6:29 am

A sub can roll when the metacentric height is small. As an example lining all your weight and buoyancy in a longitudinal fashion along the mid-line and centerline. The sub can easily roll because of that. A larger metacentric height would be weight as low as possible and buoyancy as high as possible for its function. This causes a dynamic that will allow the sub to right itself and maintain that proper position. The bigger the dynamic, the more stable the sub.
Torque from the prop is a possibility too! Too large or overpowered of a prop or too light of a boat and it can cause a sub to list, combine that with low metacentric height and you have a sub that will corkscrew through the water.


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david f
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Post  david f on Fri Sep 16, 2016 9:05 am

I think he wants to take it to a popular show he's going to this weekend, I'm sure he said it's the Mecca for model submarines.

That is Bournville presumably. I am not going to this one myself but there are plenty of people there who should be able to help and it is a very good pond for trials! (I think I remember seeing a test tank in the club house too.)

(Hands on is always better but photos and especially videos posted on here help with "diagnosis" no end.)

I hope it goes well for him!

David

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