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Author Topic: Salt Water Darnell U37  (Read 54915 times)

unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #200 on: April 20, 2016, 05:09:24 pm »

With a good forty second fill on the tank the model was almost decks awash. Despite this, it was not keen on submerging. I have an auto-leveller on the aft planes, set to completely automatic mode at present, with active bow planes. I will try making both fore and aft planes RC with auto-leveller providing levelling inputs when the stick is in neutral, see how that works.
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unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #201 on: April 20, 2016, 05:11:43 pm »

Then the rudder linkage worked itself loose and that was that for the evening. Last shot shows the sub on its bespoke trolley with its 'hydrophone on a stick', just in case!
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spooksgone

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Re: Salt Water Darnell U37
« Reply #202 on: April 20, 2016, 08:00:00 pm »

The shots of her on the water are realy good, she looks very sinister indeed :-))
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Jack D

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Re: Salt Water Darnell U37
« Reply #203 on: April 20, 2016, 11:07:28 pm »

She does look very good on the water indeed!
Hopefully she'll have her crew reunited with her soon(tm).
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unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #204 on: April 21, 2016, 09:39:04 am »

Cheers guys :-))
It will be back to the test tank on Sunday, for final trimming and searching out the last of the air pressure leaks. Rudder is now re-connected, all I need to change is the set up of the Auto-Leveller plus see how many amps I sucked out of the battery on its maiden run.
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salmon

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Re: Salt Water Darnell U37
« Reply #205 on: April 21, 2016, 07:24:03 pm »

Great photos and congratulations on a good run! Sitting in the water it looks like it has been on patrol for awhile! It tells a story without words. Nice.
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unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #206 on: April 22, 2016, 12:45:18 pm »

In its own way it is a story. The model has passed through three people that I know of before me, plus a murky past before that. It is pleasing to bring an old model back for a new life. It will be joining our U-Boat arm as part of the PMBDTs World War II Convoy Display during the summer...once I have polished out one or two little issues that is............ ok2
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petesubman

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Re: Salt Water Darnell U37
« Reply #207 on: April 22, 2016, 05:28:43 pm »

I built a darnell  u boat in the  early 90s and i liked the look of the boat more so than everything that came afterwards , a lovely boat ,regards  pete
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unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #208 on: May 03, 2016, 06:03:53 pm »

On the second run it submerged, after a bit of a fight though! I am still drawing air out of the RC compartments when running the pump to surface, not as bad as before but still an issue.
So, the following mods are planned; I shall replace the 4oz fore and aft trim tanks with bigger, bespoke tanks with acrylic tube, they did help with diving the boat by re-trimming slightly bow down. Doubling the size should prove more useful.
I will modify the Tx to give me a two position surfacing function, one, moving the servo just off of the pinch valve, utilizing the pressure held inside the dive tanks to vent and surface slowly. Two, full deflection of the servo plus activation of the pump to surface, in case of emergencies only.
I may consider hacking off some of the lead keel at the aft end of the boat. This is mounted externally and should not cause any damage to the water tight integrity of the boat.
Back in the test tank for a re-trim :-)
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unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #209 on: September 21, 2016, 01:24:29 pm »

With the display season behind us (mostly!), it was back to the test tank on Sunday. All the external keel lead was hacked off with chisel, hammer and crowbar. Then sanded flat with a big Black and Decker hand held belt sander, nice!
With the lead removed it promptly turned turtle in the test tank as expected...then Colin arrived. From now on he shall be referred to as 'The Sub Whisperer'!
Following a couple of hours of subtle trimming, removal of foam, more trimming, the model sat upright, but a little deeper in the water.
Following some suggested mods, including cutting a 5" long by 3/4" wide slot through the keel at the front of the free flood compartment, into which a hammered lump of 250g of lead was fitted, the model is now ready for a final test tank test next Sunday.

Once again, thank you to The Sub Whisperer :-))
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spooksgone

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Re: Salt Water Darnell U37
« Reply #210 on: September 21, 2016, 02:38:58 pm »

Great stuff, can't wait to see it, roll on Sunday :-))
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unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #211 on: October 06, 2016, 01:21:56 pm »

Well, it was one step forward and two steps back on that Sunday. I added some more ballast in the form of lead bullets, drilled into the keel. Probably .38s or 9mm, not a bullet expert, however, I know they are heavy, and 9mm across. So a series of 9mm holes were drilled in the keel areas adjacent to free flood areas within the model. did not want to be putting 9mm holes into water tight compartments!
The bullets were tapped in and then epoxied into place..
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unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #212 on: October 06, 2016, 01:30:40 pm »

To explain the next part, I need to give you a brief history lesson.
The Portsmouth model Boat Display Team have occupied the Weigh Bridge Offices at the site of the Pump House museum at Eastney for the last nineteen and a lot years. Unfortunately, due to issues beyond our control, we will be losing this building in the next few months. This has resulted in a bit of a fire sale on some of our assets, including our much loved used test tank ( two baths glued together for the un-initiated!). So cutting a potentially long story short, Colin The Sub Whisperer offered to buy it from us. Unfortunately, Colin was in a bit of a rush to acquire this asset, forcing me into a bit of a rushed series of tests.
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unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #213 on: October 06, 2016, 01:48:06 pm »

The trimming, foaming and ballasting was going mostly to plan, except for a problem with the acrylic water tank, which has sprung another leak. A crack has appeared again, at the join with the turned nylon end cap. This, I think, is due to the end caps being fractionally too big, putting strain on the 3mm acrylic tube. Not helped when the tube is pressurized when full of water. This is an issue, as I rely on not losing any air pressure from within this tube for surfacing actions. I epoxied and bandaged this joint as a quick fix, knowing the test tank would be gone by the end of the day.
Back to the water and the sub settled slowly down as the compartments filled up with water. The latest foam placement had cured the trim issues, with the boat settling on an even keel. Suddenly, the model started to settle rapidly, with many bubbles emerging from the rear compartment. It took about three seconds to comprehend the model was sinking!
Out of the water and back inside double quick, I already knew what had happened, I had left the aft screw cap off the model. The screw cap being directly over my Microgyros Auto Leveller, 7 Channel Futaba Receiver, Microgyros Speed Control, three Futaba Servos, two Pitmann Motors and a 9.6v NiMh pack, so no panic then!!!
Its funny, when in a workshop environment, as opposed to a salt water lake based environment, I had 30 seconds of 'what do I do'! Unscrew the big lid, got to be three minutes to do that. Nope, grab the sub, turn it upside down and pour the water down my leg!
With the test tank due to be gone within a couple of hours, I had to make a decision. So, once the water had been removed from everything, bolt it shut again and continue with the ballasting.
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unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #214 on: October 06, 2016, 01:49:39 pm »

So, sub now successfully ballasted, but ballast tank not reliable under pressure, it was time to rip everything out of the model again.
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unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #215 on: October 06, 2016, 01:57:54 pm »

Fast forward four days...following a bit of the old magic Graupner Wet Protect, and much drying out, I powered everything up.

 :-))

All electrical items working, thank you fresh water!

So, this leaves the pressure vessel. It will need to be replaced, I cannot trust it under pressure. This week I have looked at 68mm down pipe. 110mm soil pipe, plus I have some 86mm black tube of dubious origin. But probably best option, I have some 5mm wall thickness acrylic tube which is a sliding fit over the current tank. This will require a bit of a midships refit, plus new end caps, but promises to be the best way forward.
Still, the model looks good with its big lead lump removed.
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tsenecal

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Re: Salt Water Darnell U37
« Reply #216 on: October 06, 2016, 04:18:36 pm »

i would not recommend using acrylic for your ballast tank.  as you witnessed with the past one, acrylic cracks relatively easily when under pressure.  by replacing the original acrylic tube with another acrylic tube, you are just setting yourself up for doing this all over again when the new one cracks.


call these guys and see if 1) they will ship to europe, or 2) the have a european warehouse...  http://www.mcmaster.com/#polycarbonate-hollow-rods/=14ha1pg

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unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #217 on: October 06, 2016, 05:51:40 pm »

Advice Taken, just spoken to Mike at Microgyros also. Its either polycarbonate or PVC. Watch this space.
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Davy1

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Re: Salt Water Darnell U37
« Reply #218 on: October 06, 2016, 05:54:01 pm »

Plastic pressure vessels are not very easy to arrange. (Presumably you are going up to 2 or 3 Bars?) Polycarb would certainly be better than Acrylic but is expensive in the UK or US, I believe.
I have had success in the past by making up the tank in PVC (even thin walled drain pipe is OK for this) to be leakproof and then sheathing it in 2 or 3 layers of glass and polyester resin.

However I never liked these pressure tanks much and so moved on to (home made) piston tanks using aluminium tubing.

David
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unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #219 on: October 06, 2016, 06:16:33 pm »

Well, this model is a test bed for all and sundry, a bit of a crash course in every option for a model sub within practical limits. I don't want to tear the RC compartments out and fit a tube as I have spent much effort improving the model structurally ( all except the most important dive system! ). I have a couple of subs waiting in the wings, both with Craycraft modules waiting for a 21st century spit and polish, however this one is staying bolt down. Space is at a premium, as is usual with subs, which focuses the mind!
The Red October had pistons, which were great, but being Engel units were pretty big bits of kit. I have idly toyed with the idea of piston tanks in the past, however the precision engineering side has frightened me off. Aluminium and salt water certainly don't get on, so that is out. Other materials, I discount nothing %)
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Subculture

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Re: Salt Water Darnell U37
« Reply #220 on: October 06, 2016, 07:51:58 pm »

Have you considered going for a vented system?

It would allow you to use all your existing stuff, and you would just need to modify the tank slightly. Because the tank is no longer under pressure it doesn't need to be particularly strong, and the pump will have an easier life.

Chief snag, you have to trim your boat so it remains slightly positively buoyant. Also you can't produce a big slug of upthrust if you get caught in weeds etc.

You could add a smaller dive tank inside the main tank for going negative or for adjustments to different water densities.

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Re: Salt Water Darnell U37
« Reply #221 on: October 06, 2016, 09:13:40 pm »




 Sorry to read of your trials and tribulations in getting this boat sorted, amazing how quick we can react sometimes when prized model gear is at stake!! One important thing to make sure of with acrylic tubing is to make sure that the ends are radiused both around the inner and outer edges and then polished to remove any saw and file marks, ideally to the same level of finish as the tube surfaces. This is quite important in preventing crack propagation. Polycarbonate tube though stronger, should receive the same level of care  and finish after cutting the ends.
     I buy my polycarb. tube from http://www.theplasticshop.co.uk they are a good supplier.


    Trevor

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microgyros

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Re: Salt Water Darnell U37
« Reply #222 on: October 06, 2016, 10:35:23 pm »

I mentioned pvc for the replacement of a pressure vessel and polycarb tube for a complete cylinder. But that was after hearing about the high pressure windscreen pump and pinch valve to add to the desire to retain the existing layout and system.
 
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unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #223 on: October 12, 2016, 05:15:32 pm »

Have you considered going for a vented system?

It would allow you to use all your existing stuff, and you would just need to modify the tank slightly. Because the tank is no longer under pressure it doesn't need to be particularly strong, and the pump will have an easier life.

Chief snag, you have to trim your boat so it remains slightly positively buoyant. Also you can't produce a big slug of upthrust if you get caught in weeds etc.

You could add a smaller dive tank inside the main tank for going negative or for adjustments to different water densities.

Sorry about not responding sooner, not paying proper attention to my own thread!
By a vented system I assume you mean one that relies on outside air to pump dry and surface? I am not keen on this option in a salt water pond. If I loose signal or bouyancy, its going to be difficult for the model to recover back to the surface, or near to it, to pump out water, it just plain makes me nervous {:-{ .
I have a smaller 65mm x200mm tank to go in the model now, PVC etc, do I mount high as before or low as I now can, with the smaller diameter pipe? High, I assume will aid stability, while low will sit the model deeper in the water?
I have been doing too much thinking lately, so I am sub-contracting any thinking out to others....
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Subculture

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Re: Salt Water Darnell U37
« Reply #224 on: October 12, 2016, 06:03:30 pm »

Provided the tank sits below the waterline, its height will make no difference to the overall displacement.

I understand the requirement for reserve buoyancy. unfortunately subs are like all forms of engineering- a compromise.
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