Problems with Robbe U-47 - RC Groups
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Aug 31, 2004, 09:24 AM
Registered User

Problems with Robbe U-47

Hi guys,
I was wondering if anyone else had experienced problems with their Robbe U-47 kits leaking like sieves ?????

I have been extremely careful with mine to follow all the build directions etc very closely. Im not a sub novice, having built two type VIII's before from premade fibreglass hulls.
But this Robbe model is getting my blood boiling, as it takes approx 3 minutes before she takes on enough water to loose all buoyancy and start sinking - and for the life of me I cant work out where it's getting in !!!

Any suggestions would be much appreciated !!!
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Aug 31, 2004, 10:05 PM
Registered User

The leaks could only be from the two O rings, the vent valve (ie the bicycle tire tube valve), the prop shafts or the bellows. One way to test the source of your leaks would be to submerge the sub completely and pressurize the WTC -- ie pump air into the vent valve and see where the bubbles are emerging from.

One thing I did to my U47 was to replace the stock O ring and use a slightly larger diameter O ring. I also tapered the inside rim of the PVC tube. When tighting the WTC bolt, the O ring mates against a tapered 'cone' instead of a flat surface. The O ring is forced into a 'wedge,' forming a good tight seal. It also helps to lightly lubricate the O ring and the small O ring on the WTC bolt with vaseline.

Sep 04, 2004, 04:19 AM
Registered User

Re U-47 Leaks

Hi Albert,
Thanks for that Info. I like the idea of tapering the inside rim of the pvc tube to obtain a better "wedge type" fit. I too have replaced the O-ring as it didn't seem up to the job to me, (I do confess that I prefer to OVER rather than UNDER engineer a project !!).

But I replaced it with a large bead of silicone sealant - the type of stuff that you use round the edges of a bath to seal the gap.
I am also a little suspicious of the prop shafts - I might not have greased them enough.
Im not a fan of the rubber bellows and might end up replacing them with tube just thick enough to allow the pushrods to slide through. If this is well greased it works fine (Used it on my other two subs and never had any problems at all).
The bolt is also another area I hadnt thought too much about to be honest.

This Robbe U-47 kit reminds me of a quote someone made about the British Navy's K-class steam powered subs of the 1920's. I forget who said it - but after a string of disasters and sinkings someone said, "The problem with these boats is simple - too many bloody holes" !!!

Sep 04, 2004, 05:38 AM
Man from Atlantis
Sub culture's Avatar
I don't see any need to replace the o-ring seal, provided it's a reasonable interference fit, leaking shouldn't be a problem. Replacing with silicone selant is a bad move IMHO. Using a tapered pressure hull is not necessary.
It's always good practice to chamfer the edge of the cylinder to prevent the seal chafing on a sharp edge.

Use silicone grease instead of Vaseline, I have a tube of Servisol grease which I got from Maplins- lasts ages. Vaseline hardens considerably with lower water temperature, plus it does protect the rubber as well.

Leave the bellows alone, they do a fine job of keeping water out.

If you want to improve your boat, replace the greased propshafts with a stuffing box incorporating a simmering seal. This will greatly improve running times, and you will not get any leakage.

Go here for some excellent tips on modifying the standard shafts to incorporate these seals, plus additional tips

You might also consider replacing the direct drive Speed 500 motors with geared speed 400's- this is much more efficient.

If you want find out where your boat is leaking, fit a Schraeder valve (Tyre valve, free at you local tyre fitters) into the end cap of the pressure hull and inflate to a few PSI with a bicycle pump, any leaks will quickly show up when placed in the water as a stream of air bubbles.

Last edited by Sub culture; Sep 04, 2004 at 05:43 AM.
Sep 04, 2004, 06:28 AM
Registered User

re problems with Robbe U-47

Hi Andy,
Interesting points, thanks for those.
I cant help noticing though that in the pictures at the bottom of the page showing the rear diving planes servo, Jason has replaced the rubber bellows with the type of system I mentioned in my post.........

As to the use of silicone sealant it's a personal favourite of mine and I have used it on many previous models with no problems at all. The only difference being that my previous two U-boats had clear acrylic "Lids" that were bolted down onto a bed of this stuff. The lids were secured with about 18 4ba sized bolts.

Just a matter of personal preference maybe - but im going to see how things go with the bellows as I've not used them before, I'll give them a few months trials and if they are ok - leave them. If not I'll replace them.

My boat hasnt seen proper trials yet - just a 7 foot test tank I built in the garden from scrap wood. I always test models in this way first - saves embarrasing and sometimes costly errors at the poolside !!!!!!!

Last edited by English Nige; Sep 04, 2004 at 06:32 AM.
Sep 04, 2004, 09:08 AM
Man from Atlantis
Sub culture's Avatar
He hasn't used the same system as you describe to replace the bellows.

He's used subtech seals, which have a special rubber seal similar to the seal used for the propshaft.

I've used rubber bellows in my subs for years, without any problem whatsoever. There are other methods of sealing, however it's pointless upgrading something if it works.

I'm familiar with the use of silicone rubber for sealing flat hatches, however it doesn't work the same way with a cylindrical pressure hull. Infact you've probably compromised the seal more, and increased the probability of it causing a leak.

Take a look at all the kits out there, and all the scratch built boats that use cylinders- they all use o-rings for a seal. There's a good reason for that.

Sep 04, 2004, 09:14 AM
Registered User
Hi Nige,

I have just built a robbe U-47 and had the same problems.

I followed all instructions. pumped up the WTC via the vavle(with a balloon pump). There were leaks everywhere, you can hear the his.

I tracked these down by wetting my fingersand feeling around for the cold air.

I had a leak in one shaft which was fixed by taking the plastic cover off and resealing, one of the bellows had a small hole in it...silicon fixed this and setting servo so there was no stress on the repair.
My big problem was the other end cap was a good seal at all.
So i had to take the whole front off the boat. I sanded back the end cap at the join while still attached then wiped it clean with spirits.
I then coated the join with a thin band of epoxy, pressing it hard into the join.

I then pumped it back up again and the bellows inflated also. this meant i had good seal.

I have had my boat in the water several times and not one drop of water has come inside the WTC.

Hope this helps

Sep 04, 2004, 07:58 PM
Registered User

re problems with Robbe U-47


Lighten up man !!!!!

Sep 04, 2004, 07:59 PM
Registered User

Hi Michael

Hi Michael,
Thanks for that info - nice to know im not the only one experiencing problems !!
Sep 05, 2004, 12:14 PM
Man from Atlantis
Sub culture's Avatar
Originally Posted by English Nige

Lighten up man !!!!!



You ask for advice, which I give, then either justify your own building methods or mock what I'm saying.

As I see it, you are the one with the heavy, waterlogged boat- so I guess it's you that needs to lighten!!!!!

Sep 06, 2004, 02:53 AM
Registered User

re Robbe U-47 problems

Hey man - look, yes I asked if anyone had experienced problems with their Robbe U-47. Someone kindly replies and has had almost EXACTLY the same kind of problems I have. This person then offers advice and help on how they fixed their particular problems.

YOU then come along and slate everything this other person has done !!!

Comments such as:-

"I don't see any need to replace the o-ring seal, provided it's a reasonable interference fit, leaking shouldn't be a problem. Replacing with silicone selant is a bad move IMHO. Using a tapered pressure hull is not necessary."


"Leave the bellows alone, they do a fine job of keeping water out......"

......are not going to win you any popularity contests. It's not so much WHAT you said it's THE WAY you said it.

I get the impression that you have not built this model, as if you had you would also know that there is a Tyre valve included in the kit - no need to go out and get one.

Help and advise people yes/always, but lecture/talk down to people NEVER.
TONE and common courtesy is everything. As is respect for other peoples abilities.
Sep 06, 2004, 10:12 AM
Man from Atlantis
Sub culture's Avatar
I don't see the problem with what I said in my posts.

My comments were objective and based on experience.

If you want to disagree with my opinions or ideas, that's fine- but attacking my character and writing style will achieve nothing.

Regarding seals, it's not the cross section of a seal that improves integrity- it's the accuracy. Using silicone to seal a cylinder is okay- for a one shot job. However for constant use the o-ring is much better option. If the cylinder and mating endcap are reasonably accurate a thin o-ring should be more than upto the job. With a small interference fit, a taper isn't really necessary.

However don't just take my word for it, take a look at the work of Dave Merriman, Norbert Bruggen, OTW etc. they all use the same approach for sealing their cylinders.

What Albert has done, will not harm his boat, and perhaps in his individual case, a thicker o-ring was required. In most cases though, the o-ring provided should be adequate.

Bellows are a good method of sealing linkages, they don't require lubrication or maintenance, other than the occasional visual inspection for tears, perishing etc.
They're not so good for (very)small boats as they displace water when in use and can affect trim.

Testing their integrity by filling the cylinder with compressed air isn't a fair test- as water pressure will have the reverse effect, compressing the bellows rather than expanding it. This works in our favour as the seal becomes tighter as pressure increases at greater depths.

I enjoy my hobby, and don't take it too seriously. However when I let several hundred pounds of equipment submerge beneath the waves I like to have reasonable odds that it will come back up again.

I haven't built a U47- it's not my kind of boat to be honest. However, I have worked on and modified a friends U47, which as far as I'm aware never had a schraeder valve fitted (perhaps this is a later addition?)as standard.

It doesn't matter much anyway- what we're dealing with here is a cylinder which most working submarines are based on(certainly mine are). With regards to water integrity, the principles remain identical from boat to boat.

Have you found your leak yet?
Last edited by Sub culture; Sep 06, 2004 at 10:55 AM.
Sep 12, 2004, 09:06 AM
Registered User
MC4U's Avatar

your problems with the robbe

hey gang I have built too many of these kits and have only experianced this problem once here is my take on the subject
1 check the fit of the screwjoiner and gromet seal it must be tight a snug fit isn't good enough.
1a check your screwjoiner for leaks if it is not soldered correctly you can get water seapage in the boat through that really it can happen
2 check the fit of the O ring mating surfaces for nicks and scratches
and excess glue you only need 4 small spots glued down try to get them in equandistane posistions (star trek lingo) IE top bottom and sides
3 check bellows for holes and replace if needed a good little hint here is to use blue rtv to seal the bellows to the tubes then apply the clamps.
4 check the schraeder valve (tire) for good seal IE particulate matter in the the threads and clean if needed
4a grease the shafts and tubes very tightly IE use as much grease as you can fit in the tubes the best stuff is Prathers cable grease a blue silicone grease completely water resistant IE will not emulsify in water use a syringe to fill tubes oil is not good enough prathers will withstand heat up to 500 degrees
I have filled the tubes with this stuff for ever and only need to top it off once a season and I don't use filling tubes I mearly remove the shafts and fill the tubes then reinsert the shafts and wipe away the excess.
finally if the equipment tray is not assembled correctly and glued to the aft cap you will have nothing but headaches until it is corrected.
here is a good test to perform best done before the hull is atached to the pressure hull ,cap up the screwjoiner entry hole I use silicone bases clay and fill the pressure hull with water and watch for leaks around the matting surface of the front cap

I hope this has been educational for you robbe builders and there will be a written test in 10 days time
he heh

Last edited by MC4U; Sep 12, 2004 at 09:10 AM.
Sep 15, 2004, 07:22 AM
Registered User

i cut of 10 cm of the rear end of the mid section hul
then cut the thicknes of the coupling of the WTC
then glued on an alu baionet coupling from norbert brugge
then rebuilt the hul (i was lucky my friend is nieuw 7c was at the building area) and i could make an polyester part that i copied from the inner side of the hul and glued it on the innerside of my hul
to give the hull in that spot a bit more stiffnes and support

and i have no problems with leaks in the wtc

greatings raf
Last edited by wittekonijn; Sep 15, 2004 at 07:33 AM.
Oct 25, 2004, 04:30 PM
Registered User
Hi experts!
When I turn my new standard equipped Robbe U47 on, the propellers starts imideatly (even though the RC is turned of).
Do you know what I have done wrong?

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