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Brutus

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Dave Milbourn03/12/2013 11:46:29
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Posted by Andy Stoneman on 02/12/2013 19:39:28:

Paul, I can see where your coming from regarding propshaft angle, If it is too steep when driving it will force the boats bow down and you wouldnt want that.

Andy

Andy
You might think so BUT, just as an example, the shafts on the real Fairey Huntsman 31 are at 15 degrees to the waterline and yet the craft needs transom-mounted trim tabs to keep the bow down under full power. Fairey powerboats are among the best sea-keeping types ever designed.

Paul
I really think you are making things difficult for yourself here. I don't believe that the relatively shallow inclination which the shafts will need in Brutus (in order to accommodate the motors comfortably) would have any noticeable effect on its performance. Aesthetically it would be difficult to see from the outside anyway, given that the props are only of the order of 30-35mm in diameter and therefore the "tilt" from top to bottom would be very small. My final argument for the defence is that the power loss in a belt or gear drive would likely be a lot more than that attributable to an angled thrust-line using direct drive.
I rest my case, m'lud.

Dave M

Paul T03/12/2013 14:07:10
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Dave

Whilst I agree that the Fairey boats represented a excellent benchmark in hydrodynamic design I do feel that the position of the engines and choice of power train were a compromise arrived at to suit the technology of the period.

If current technology were applied to the designs then I suspect that the layouts would be different, for example on the Huntress the motors might be in the stern and power transmitted via Z legs or jet drive. In this particular craft repositioning the engines to the stern and using a Z drive would not only correct the propellers angle of attack but move the centre of gravity toward the stern thus reducing or negating the need for trim tabs

We can argue the merits and failings of the Fairey designs backwards and forwards all day and I am quite looking forward to my learned friends rebuttal.

On a serious note you might have a valid point on power loss when employing belt drives and I will try to find some relevant data on the subject

Paul

Edited By Paul T on 03/12/2013 14:13:23

Bob Abell03/12/2013 14:45:28
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How about a cross section?

Bob

Paul T03/12/2013 14:46:03
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7148 forum posts
1208 photos
2 articles

Andy

Yes, and when you think about it in terms of Newtons third law it becomes blindingly obvious, as the propshaft angle increases then more energy is required not only to propel the boat forward [because the effective area of the blades is reduced] but also to overcome the push of the propeller trying to lift the stern of the boat Ipso Facto for an efficient propulsion system the propeller has to be positioned as close to the horizontal as possible.

In short any propeller shaft that isn't truly horizontal is always the result of compromises having to be made between form and function.

Bob

A cross section?

Paul

 

Edited By Paul T on 03/12/2013 14:47:17

Paul T03/12/2013 15:17:44
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Bob

Is this the section that you were curious about?

brutus cross section at f1.jpg

Section taken through image 1 showing problem of mounting the motors within the confines of the hull at F1 / F2

Paul

Dave Milbourn03/12/2013 15:27:25
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We can argue the merits and failings of the Fairey designs backwards and forwards all day and I am quite looking forward to my learned friends rebuttal.

Nope - not this time. We'll have to agree to disagree on this one, Paul. At the end of the day it's your design and you set your own parameters. I can recognise a brick wall when I see one and I learned a long ago that there's no mileage in trying to argue with one.

DM

Bob Abell03/12/2013 15:28:39
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Thanks Paul.....

How about two over steep prop shafts, but with a bottom plate to deflect the inclined thrust....along the horizontal?????....And gerronwithit!.....lol

That`s a new one!

Bob

Paul T03/12/2013 15:42:04
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Bob

Not so new...........you are describing the principals of a jet drive.

Paul

Bob Abell03/12/2013 16:02:21
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Sounds OK to me, Paul

A Jetdrive matches your project nicely?

Bob

Telstar03/12/2013 16:38:47
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Paul

Is something like this what your looking for

**LINK**

Cheers Tom

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