Twin props

[ManxmanParticipant @manxman]

Hi Folks

I need to syphon a little of your experience –

I am currently building "Pacific Buccaneer", my first twin prop vessle. I have my matching pair of props (left/right bladed) which will run in fixed Kortz nozzles.  I know they run in opposite directions, but which way do they turn? –  Looking from the stern, do they turn inwards towards the rudder or outwards, away from it? – Does it matter?  Is one way better than the other?

Cheers – Kenny

[Paul TParticipant @pault84577]

Hi Kenny

I have never noticed any difference with twin engined models (real boats/ships might be different) and I have built several such models the largest being 4ft long and none of them suffered from steering or cavitation problems

Hope this helps.

All the best

Paul

[ManxmanParticipant @manxman]

Hi Paul,

It’s the cavitation that concerned me. Thinking of water being thrown upwards towards the hull and may be losing power, though I don’t think I would lose much as the motors are big ones – Robbe Power 1000s –

 Cheers – Kenny

[ManxmanParticipant @manxman]

[Neville Taylor 1Participant @nevilletaylor1]

Hi Guys I need to ask a question from you twin prop guys. I am currently building a Vosper MTB 49″ GRP hull model. I intend to power it with two Vertex CY-25 nitro motors.

My questions are:- 1. the hull width varies from 9″ from where the prop shafts will exit the hull to 8″ approx where the props them selves will be situated PLUS two rudders.

2. should the prop shafts be parallel or should they follow the taper of the hull?

 

I have a few plans and articles out of Model Boats but they only show a single prop model albeit electric to boot. There is nowhere I can ask here. If I am in the wrong forum could you please direct me to the correct one.

 

Regards.

Neville Taylor

Australia.

 

[Colin BishopModerator @colinbishop34627]

Twin propshafts would normally be aligned with the keel so keep them parallel. I assume that you lrady have the spacing between them but obviously the props need to be well under the hull and not too close to the sides otherwise you may suffer cavitation and reduced thrust.

 

If it’s not an exact scale model than a good tip is to align each rudder very slightly to one side of the shaft line as this will make it a lot easier to withdraw the shafts for maintenance as you won’t need to remove the rudders first. It will not affect performance.

 

Colin

[Neville Taylor 1Participant @nevilletaylor1]

Thanks Colin.

 

I gather that they have to be parallel that is what I had anticipated any reply to be, as for spacing between them that is what I need help on. The props will be under the hull quite rightly and they are straight shafts not flexys. the plan does not indicate a distance from transom but I have considered that they will be about 3 to 4″ from the transom based on previous models I have built.

 

However the hull width at this point is 8″ and I was thinking 2 and1/2″ from hull side to centre of shaft and 3″ between shafts thus making up 8″ . what do you think? I intended to offset rudders anyway seeing as I will be using two. One, I suppose would suffice centrally but I was thinking that could cause the boat to yaw a little bit.

 

Thanks again for a quick response I have received a quicker answer from you guys than Westbourne Models.

[Colin BishopModerator @colinbishop34627]

Hi Neville,

 

These two photos should help you. they are of a Perkasa model MTB. The size is probably different to yours but the relative proportions should be pretty similar so you should be able to use them as a guide to your model.

 

Hope this is useful.

 

Colin

 

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

Greetings Manxman, faster mie,

 

just to say its usual for twin props when viewed from astern to rotate outwards.

 

Also to add, had a look at you album and “bingo”, you have a vessel with a kitchen rudder assembly on, I spotted my first at Chatham Dockyard. I’ve been pondering how to achieve a small version in very small boats and lo, you have a very simple plan showing it. I admit I’ll copy that layout – have you copyrighted it? Tough.

It seems to me that with a kitchen rudder on I can omit an ESC – I’ve photos of this rudder type on a “put-put” tin boat, and it steers going astern quite well.

 

Kistow from Ramsey, and yes the sun’s shining, Kim.

[Neville Taylor 1Participant @nevilletaylor1]

Hi Colin,

Neville here in Australia. I recognize the photos of the twin props as those of the Vosper Kit review. I have been a while to respond as I have been trying to source propshaft supports or “skegs” as I referred to them in the 60’s. I notice that the P frames shown are to support the shaft and not the tube. Another thing I cannot use left and right turning screws the motors only run anti clockwise.

 

Can you point me in the right direction of company to search or would in be easier if I were to consider exposed shafts as shown bearing in mind that I am using twin .25 nitro motors and not electric. I would still probably have to purchase through the UK as down under we’re still in the dark ages as regards the ‘boat’ scene.

 

Regards Neville

Australia.

[Colin BishopModerator @colinbishop34627]

Neville,

 

You might try Mark Windless at Mark’s Model Bits http://marksmodelbits.com/ He will make you tubes and shafs to order in stainless steel for a very reasonable price. As long as you explain that you are using i/c engines he should be able to confirm that they will be up to the job. I have seen his shafts and tubes and bought a couple of shafts myself and they are excellent quality. They were also carefully packed against a wood batten to ensure thet they would not get bent in transit.

 

He might even be able to run you up the external bearings for the shafts and maybe skegs as well, he is very helpful.

 

Colin

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