Darnell Type XX1 - Graham's Dad's boat for refurbishment

Graham the Commodore of my club (Barrow in Furness MBC) has very kindly given me his Dad's Type XX1. I am honoured to be seen as a "Good Home!"

His Dad made a very good job of this but it is the original Darnell equipment fit and it is need of some refurbishment/updating. (His Dad got it to the test tank phase but had problems with leaks.)

I thought it would be useful to post some occasional (I have many jobs on the slipway!) photos to help others. I have sent the first batch to Richard for posting here and they show all the original Darnell fit (Steam engine, air bottle, air bags etc.)

My initial thoughts are:

- Leave the hull pretty much as it is.
- Replace the twin Darnell boxes with a cylindrical WTC.
- Single drive motor for both props via reduction gears. (I have not found twin motors to be very effective on subs.)
- Shaft seals rather than grease filled prop tubes.
- Single proportional piston tank where air bag is. (Home made tank and AMS software/hardware)

Any one who has been through this got any comments/ideas or suggestions?

David

PS Don't laugh at me building a U boat (It can happen to any one of us!)

Good grief David, that's a rare beast...but it looks nicely made, nice and neat flood holes too.

Anyway, here's the pictures to accompany David's new build...

Thanks for posting so promptly, Richard!

I didn't realise it was rare. I have just looked up the figures for kits sold in the article in tribute to John Darnell in the last magazine:

Type XXI - 78 sold.

as compared with 277 TypeVIIc sold.

The Type XXIII is the rarest at 7 sold.

Thanks ,

David

Vic Felton built a gorgeous Type XX1, it really was one of the best I've ever seen...there's some photos of it somewhere on the forum. You've a nice boat there, David...good luck with it.

I have been checking things over in the boat and I have resisted the temptation to just rip everything out and modernise. Our Chairman Nigel points out that this boat is the most original Darnell he has ever seen. So I may have the responsibility of looking after some tiny marine model heritage!

I have had a look at the Darnell dive system (oscillating steam engine pumping air out of an air vessel into a bag) and after fixing a small leak in the ports of the steam engine, it works well. Pump in and pump out in less than a minute. John Darnell designed a very good, simple system for his day.

The steam engine is in a good state but my mate Tim has told me how to put an O ring on the piston as a piston ring to reduce leakage, if needed. (The present piston is grooved but has no rings.)

The flat hatches need a much better seal. I have removed the rubber strip. which had gaps. I replaced this with an O ring cord to see how effective this would be.

I have just had a set back. On tightening the screws there was a "crack" and I can now see a split in the lip of one of the boxes. D***n! Does fibreglass get brittle with age?

Any flat hatch experts out there got any suggestions  - can I replace/repair the lip? - preferred sealing methods?

David

PS Nigel phoned during this post - rapid feedback! I have just checked the source of the "crack" and it is a split in the gelcoat - so working some resin in should fix it. He also advises against using O ring cord (puts a twisting force on the lip) and against O rings as piston rings (too much force on the big end in what is a basically an un-lubricated steam engine.)

Sounds like good advice to me. So over to you flat-hatchers what sealing material do you use?

Morning David...cracked rims are a right pain the proverbial, been there and done that.

You can repair the lip, providing that the bolts are still secure and they have sufficient length. Try making a new lip from something like 2mm styrene sheet and having drilled the bolt holes through, bond it to the radio box lip with either 24hr epoxy glue or even a good silicone sealant(Dow Corning aquarium sealant is a good make). Once it's dried, put your O ring seal on the inside of the bolts, and hopefully that will have cured your problem.

As for tightening the bolts/nuts down, Bernie always told me to screw the nuts down to finger tightness, then one quarter turn only with a box spanner, that should be more than enough to seal the lid completely.

Ramesh is the man to talk to about O ring seals, he used them on his Typhoon, and he's proud of the fact that his has never leaked a drop of water in six years.

Gosh Richard - your advice was coming in as I was typing the edit to the last lot. Thanks for this!

The lip is cracked only and I will take your advice re: a quarter turn.

Another thing is that the hatch is fixed with screws. I will need to make these into studs to avoid having to put seals under each individual screw.

David

I hadn't noticed that they were screws and not studs..how strange. M4 brass bolts smothered in epoxy from the inside will soon solve that problem.

Have you seen the price of brass bolts and nuts? I wanted some yesterday for the box in my Type XX111...frightened the life out of me!

Another problem is that the nuts underneath are 4BA.

I already have a problem of Metric and Imperial subs!

Perhaps I should tap one end of the stud 4BA and have M5 on the other end?

Really I can see why flat-hatches have become less popular - I could have made a PVC cylinder WTC by now!

Any suggestions on suppliers of brass nuts and bolts (BA ones even) would be welcome.

David

Nuts and bolts in BA sizes are getting as rare as ten pound notes in my wallet nowadays...I guess eBay may be a good place to look?

Won't the lip be under a twisting force through water pressure bending the lid anyway?

http://www.ba-bolts.co.uk/

Thanks Dagon. Yes it will be to a smaller extent.

Nigel's point is that the O ring cord will be on one side of the bolt. A flat gasket surrounds the bolt so no bending force.

I haven't given up on O ring cord completely and I will try it later once I have modified the fixing to use studs. (And used Richard's suggestion to put less "grunt" in!)

On the plus side I have found that I don't need BA bolts (Many thanks for the link, though) I have found that I can drill and tap the existing 4BA nuts to M4.

I have also been in touch with Dave J who I know uses flat hatches a lot, very successfully. His tips are:

- He taps the studs into 5mm aluminium plate incorporated into the structure (so it is very rigid)
- He uses thin, smooth rubber sheet as a gasket. (He cuts it out of a sheet - so no seams. It is quite thin not much more than a 1mm.)

Thanks again for the suggestions.

David

I've hunted through my archive material, and found this in an American model boat magazine.




The alloy flange is epoxied to the top of the radio box, then the bolt holes are drilled right through the box top and the flange. Bolts are inserted from the inside and then a good coat of resin applied over them. The O ring is then secured to the alloy flange with cyano glue on the inside of the bolts (don't stretch the O ring cord too tightly when glueing it down, and try not to twist it as you apply the cyano).

Use one washer per bolt, and tighten the nuts to firm finger tightness, then using a small socket, apply one quarter turn only.

Tighten in the following sequence..

Oh, and if you need new lids, this guy is excellent...I've just bought an offcut of 3mm acrylic for the Type XX111 box for £3.10.

http://stores.ebay.co.uk/Essex-Signs-and-Displays?_trksid=p2047675.l2563

Thanks Richard.

That looks like a very useful article - I do like the idea of using O ring material.

One thing that seems to be in common on all the methods is a rigid flange as base. Sadly the Darnell has quite a thin, flimsy fibreglass lip. Anyway lets see - the stainless m4 bolts are on order.

I'm not sure I could remember the bolt tightening sequence by the pond side!

David

That was always one thing that let the Darnell boxes down was that thin lip, it was so easy to pull bolt heads right through, or crack and split the lip. The Models By Design ones seem to be much thicker than JD's were, I know my hands are weak compared to yours, David, but it seemed to take me ages yesterday to drill my box out for the bolts to go through.

I suppose you could always use 2 or 3mm styrene sheet for the flange instead of alloy, that's probably what I'll do, I couldn't cut the flange out of alloy nowadays to save my life.

To assist the little grey cells in remembering the nut tightening down sequence, why not mark the lid with those small self adhesive numbers, or even a good waterproof marker pen? Or write them in marker pen on the inside of the deck? As that pesky meerkat on the tv adverts says...simples!

I managed to find this photo, David...it's Ramesh's Typhoon fitted with an O ring seal instead of a rubber/foam gasket.

Bolts all fitted now and about to try some tests on how effective the seal is. Just trying one compartment at this stage.

I have also just received my copy of "The Type XXI U Boat by Kohl and Rossler. I originally saw only copies at more than £100 on ebay but I managed to bid on one succesfully for about £15.

It contains a fantastic level of detail. I wish all submarines had such books!

David

PS Thanks to Ramesh/Richard for the photo looks very nice.

Good luck with the tests, David...finger is crossed.

This might interest you, David...

Thanks Richard but I can't see anything!?

Things progress with this boat. I have removed both Water Tight Boxes (WTBs ?)
They were very easy to remove because they were held in by resin and filler - no glass.

I have glassed over the ends of the boxes - too many holes! (Rule No. 1 of submarine design - minimise the number of holes.

This means that I can test two sealing methods in my small test tank (and with my inflated balloon method!).

Both boxes have had the M4 bolts as studs method put in. Permanently Loctited in (Thanks for the suggestion, Nigel.)

One box has been tested with 3mm O ring cord. Looks promising since it passed the 24 hour balloon test and it has also passed the expansion test (In and out of the fridge - my other half was out at Bridge!)

The other one will be tried with 3mm silicone rubber sheet.On order and I will report back.

But all in all, I'm feeling more confident about flat hatches.

I'm looking to improve the original propulsion system - Type XXIs were fast.

One thing I will be dropping will be the "grease filled prop shafts as seals". My usual Simrit oils seals will go in. (Rule No. 2 Don't tolerate any water leaks!)

David

That's odd, David...the link works perfectly for me. Anyone else having trouble viewing the video?

Sounds as though things are progressing well with the initial testing, I'm glad to hear that...looking forward to seeing some more pictures of the refit. Keep up the good work!

This was the r/c box set up on my X 22, David...note the original rubber gasket seal, courtesy of the Great Man himself.

Hi Richard,

I can now see the videos fine using Internet Explorer under Windows.

(I was using a hacksawed down Linux browser! Ouch!!)

David