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[bbdave]

Hi chaps just a question about my u-47 i tried the hull in the pond today just to see if all was ok she sits a little overwaterline and when i started to fill the tank she smartly sank to the bottom 18-24" well befor the tank had filled which is fine but if i were in a lake this would not be good as obviously she would vanish for good.
 So my question is if i install some foam bouyancy inside to bring her upto waterline will this make the dive more controlled and enable me to suspend her at around the 18-24" mark or is what i'm getting perfectly normal?

dave

[portside II]

Hi Dave  not sure if i can give an answer to your question with your u47 ,but i have the trafalgar which uses a positive pressure ballast tank so she will come back to the surface if all else fails . I had to put pollystyreen foam in the bow to allow the sub to static dive and surface level .
Is your u47 a bladder tank if so then the amount of ballast(water) you take on is not critical as long as the sub is at the right waterline when surfaced ,if not then some polly foam in to trim it to the desired level .
I hope this is helpfull
daz 

[bbdave]

Thanks Daz i have an 750 engeltank which has proportional control i'll do some experimenting tomorrow with some pipe lagging and see what the results are.
 My problem isn't submerging she does that fine but i thought she was maybe a little to efficient :-\. would prefer a little more control i suppose alot of playing around in dads pond is required.

[Sub driver]

Dave.

You normally only take on enough water to get neutral boyancy, your capacity is more than that which is good, you just don't fill the tank.
You will need to take on more water from time to time as the water temp changes so what you have is good, if you trim it with foam to get what you descibe above you may find that you sometimes cannot completely dive when you have a full tank you should always have some capacity in reserve.

Adding foam to trim the model when finished is ok and to get the water level right, just make sure it is below the waters surface when surfaced, and you will also have to trim it when dived too.

Hope this is of use.

[Subculture]

Can you post some pictures of the 'waterline'. Most people who have converted the U-47 say the 750 tank isn't enough to give scale waterline (1 litre required), and as you're using a chunkier conning tower, the waterline should be lower still.

Andy

[Mankster]

If you are using an Engel TA tank you want your sub to be at neutral buoyancy when it is in the the 90% full position. So add foam just below the surface waterline till you achieve this and your ready to go. Yep, most people need at least a 850ml tank to get a scale surface waterline with the U47.

[bbdave]

I need to do some experimenting with ballast etc. but i need to complete the topsides first and get all the weight where it's supposed to be then start trimming as it's not really a scale boat i'll be happy with the w.l. near ish. I could reduce the kit supplied ballast as i have been using that as it comes so may knock a few grams off that but i'm sure i have many happy hours ahead with bits of lead and foam  ::)

[bbdave]

Hi chaps i've been playing today and have discovered how sensetive these things are i had her trimmed yesterday so i could suspend her about a foot down but today she hardly submerges! so i'm adding some more ballast the one thing i don't understand is that when she resurfaces she doesn't return to the same w.l. but when i move her fwd and rev she then returns to her w.l. what causes this?

Dave

[Spyderman_uk]

Surface tension?

[bbdave]

Had a very frustrating evening  >:( i added foam to the stern to level her up when submerged now she doesn't submerge so i need to add more weight :( which means she's almost decks awash! wish i'd known she needed the bigger tank at the start the cost is about the same i feel i've wasted my time and money and should have just kept it dynamic diving as the w.l. is no different.

 On opening her up i have a slight vacume inside due to the propshaft not being sealed yet and leaking a small amount of air this may be a silly question but is a vacume less boyant? i can't see it as it still dissplaces the same amount of water.

[Youngat65]

Don't give up Dave keep taking the pills see you in a week or so
                                                          Cheers
                                                                Bob  (Could do with a cup tea)

[dougal99]

 On opening her up i have a slight vacume inside due to the propshaft not being sealed yet and leaking a small amount of air this may be a silly question but is a vacume less boyant? i can't see it as it still dissplaces the same amount of water.

You are correct in that it is the volume displacement and weight of an object that causes it to float or sink. Theoretically a vacuum would weigh less than air and therefore have more buoyancy. In your case it isn't going to make any measurable difference.

HTH

Doug

[bbdave]

been in the pond testing again today i've decided to use a few strategicly placed balast weights to balast her up dependant on how much surface or submerged running i'm going to be doing weed and visibility etc. so crossed fingers i may get her out in the big lake this week.

[Mankster]

On opening her up i have a slight vacume inside due to the propshaft not being sealed yet and leaking a small amount of air this may be a silly question but is a vacume less boyant? i can't see it as it still dissplaces the same amount of water.

No your right, a vacuum is equally buoyant as its volume that affects buoyancy not weight. Though a heavier a sub requires more buoyancy than a lighter one to float. The weight of the lost air is insignificant.
Seeing how a 1:1 sub with its numerous trim tanks cant remain static at a given depth without forward motion, you really have no hope of achieving this with a model. The discrepancies you note is probaby due to trapped air under the top hull. Those small slots in the deck is not sufficient to break the surface tension and let air escape adequately. Most air has to leave through the larger opening around the conning tower.

The proper way to trim your sub is first to ensure that the centre of buoyancy is directly above the centre of gravity. Centre of gravity is normally set below the periscopes. You should aim that when the ballast tank is full, only the top of the conning tower is level with the watersurface, with just the periscopes above the water. Now you should be able to maintain your required depth with the minimal of forward movement and a tad of downwards deflection on the front planes.

[Subculture]

You have a Dream arts conning tower, don't you? The deck slots in the conning tower deck aren't open on those kits (the ones I've seen anyway).

Probably a silly question but you do have a vent in the conning tower, don't you? If not leave the main hatch open or off, this will ensure any air is vented from the boat.

Trapped air will cause you all kinds of headaches, as the air bubble will move around the hull and the boat will be in a different state of trim everytime.

Regarding your disappointment that the tank doesn't provide a scale waterline. Well you haven't wasted your money, a dynamic diver would require a considerable turn of speed to dive, you boat will be able to cruise along at a much more scale pace.

You could increase the size of your tank by ordering a longer tube and threaded rod from engel. Exisitng owners report you'll need 1L capacity to achieve scale waterline with the U47.

Andy

[bbdave]

i spent a long time with little bits of lead trimming her so i have her sat only just decks above surface if i fill the tank she dives slightly bows first and surfaces bows first as i fill the tank propotionaly i can have her sat just below surface and bring her slowly up in steps. I have drilled some 2mm holes in the decks in an effort to free the trapped air more easily.
 Yes Andy the dream arts tower deck slots arn't free but i have left the hatch open and drilled a few 1mm holes it seems to loose the air quickly it's the bows that seem to hold it but that gets released early on. i am amazed how sensetive the sub is under water the koi swim past and she moves as they waft past and they are now so used to her they nudge past! but i have to say i have layed out in the sun this weekend messing about with different trim ect. and thoroughly enjoyed it i have drained two 3300 mha packs diving and surfacing so my frustrations were ill founded thanks guys for the advice'

P.S the resin perriscope didn't last long DOH!!! need to make a new one out of brass i think but light

[Subculture]

Okay, sounds good. Aluminium is about a third of the weight of brass.

The boat is almost weightless when submerged, so longitudinal stability is no better than across the beam.

Andy