MGB propellors

[Bruce MumfordParticipant @brucemumford61966]

I am building a Dark Class MTB but can only get LH 42mm propellers would she be manageable like this?

[Bruce MumfordParticipant @brucemumford61966]

[Chris FellowsParticipant @chrisfellows72943]

PropShop will be able to supply a pair of LH and RH props at 40mm diameter.

[John W EParticipant @johnwe]

Hi there,

As far as props are concerned Cornwall Model Boats online do a variety of brass propellers – left and right hand at 40 mm diameter and they will help you out. I am unsure whether you will obtain two 42 mm props off the shelf – but you possibly could get two made by prop shop (but that might cost an arm and a leg)

As far as your original question about 2 props running in the same direction is concerned, during World War II and after the War, there were a lot of fast patrol vessels built in the UK where the props all rotated in the same direction – for 2 reasons.

. during the War – Companies did not have the time/finances to build reverse drive gear boxes.

– they found some boats performed slightly faster in a straight line but were slightly more difficult to turn in one direction, depending on the rotational direction of the props.

Getting back to the dark class there is a possibility that both props did rotate in the same direction as they were powered by 2 Napier Deltic 18 cyclinder marine diesels. As far as a I can see I am unable to see any evidence of a reverse drive gearbox.

If you google British Military Powerboat Trust and also Vosper Thorneycroft collection, you will find information regarding these vessels on both sites.

John

[Dave ReedParticipant @davereed72029]

Ooo, the Napier Deltic – that was an ingenious bit of kit. A supercharged 18 cylinder 36 piston 2 stroke diesel. Worth looking up if you're not familiar with it.

[Bruce MumfordParticipant @brucemumford61966]

Yup, and I can't get enough power out of the motor and props yet to get her up on the plane.

[Richard SimpsonParticipant @richardsimpson88330]

Posted by Dave Reed on 26/02/2023 20:35:10:

Ooo, the Napier Deltic – that was an ingenious bit of kit. A supercharged 18 cylinder 36 piston 2 stroke diesel. Worth looking up if you're not familiar with it.

One of the highest, if not the highest, power to weight ratio IC engines ever designed. Just have a look at a cross sectional drawing of the engine and be amazed! Of course basically the same engine that went on to power a number of railway locomotives. An absolute pig to work on in situ so tended to follow the cartridge design philosophy whereby the entire engine was removed to work on it and a refurbished replacement was dropped in to minimise downtime.

I've sailed with an opposed piston engine, the Doxford, which was fascinating in itself, but never seen a Deltic, which was basically a marrying of three of them together.

[Richard SimpsonParticipant @richardsimpson88330]

Posted by John W E on 26/02/2023 18:37:05:

Hi there,

As far as props are concerned Cornwall Model Boats online do a variety of brass propellers – left and right hand at 40 mm diameter and they will help you out. I am unsure whether you will obtain two 42 mm props off the shelf – but you possibly could get two made by prop shop (but that might cost an arm and a leg)

As far as your original question about 2 props running in the same direction is concerned, during World War II and after the War, there were a lot of fast patrol vessels built in the UK where the props all rotated in the same direction – for 2 reasons.

. during the War – Companies did not have the time/finances to build reverse drive gear boxes.

– they found some boats performed slightly faster in a straight line but were slightly more difficult to turn in one direction, depending on the rotational direction of the props.

Getting back to the dark class there is a possibility that both props did rotate in the same direction as they were powered by 2 Napier Deltic 18 cyclinder marine diesels. As far as a I can see I am unable to see any evidence of a reverse drive gearbox.

If you google British Military Powerboat Trust and also Vosper Thorneycroft collection, you will find information regarding these vessels on both sites.

John

 

 

 

Interesting when practical and financial restrictions have to be allowed to take over from theoretical ideals. Also interesting to note that all four Rolls Royce Merlin engines of a Lancaster and the Mosquito all rotated in the same direction, for exactly the same reason.

Edited By Richard Simpson on 27/02/2023 09:23:42

[Tony HadleyParticipant @tonyhadley]

There were a couple of magazine articles about the Dark Class, Model Boats 2011 and MMI 2014. If they would be of any help, send your email in a message and I will send a copy.

In the Model Boats 2011 the propellers are 35mm 3 blade brass and in the 2014 MMI they are 40mm 2 blade plastic.

[ashley needhamParticipant @ashleyneedham69188]

Bruce. In answer to your question (which has, as far as I can see, NOT been answered), yes, of course the boat will be manageable. It may TEND to pull one side or another, but a bit of trim set on the rudders will sort this out.

In reverse it will definitely go one way or ‘t other….but is that a real issue? Single prop boats will do the same. I don’t spend a lot of time reversing, usually just sufficient to stop or move backwards out of the way.

Obviously it will handle better with handed props, and if you are buying brass ones it's not a problem to get left and right hand ones. The difference in 42 or 40mm for the props….worth worrying about?

Ashley

Edited By ashley needham on 27/02/2023 11:10:29

[John W EParticipant @johnwe]

https://youtu.be/KNveRCGPMCw

 

(remember to copy and paste the link above and maybe skip the ads if you wish)

 

Hi there I hope this link works for you – its a model of the RTTL which I built and both props turn the same direction and if you watch the video you will see that she tends to twist with the torque of the motors. Also, this may help give Ray a bit of inspiration to build the RTTL from Vic Smeed plan. The one in the video is partially built from Vic Smeed's / original builders plans plus a lot of help from a gentleman called Christan   John

Edited By John W E on 27/02/2023 13:31:12

Edited By John W E on 27/02/2023 13:32:40

[Ray Wood 3Participant @raywood3]

Thanks John,

I have a long list of future builds, and the RTTL is in the top 5

Regards Ray

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