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apdsmith
Post subject: Re: Democratic Stratocracy of NorthernlandPosted: February 20th, 2016, 8:08 pm
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Hi Fox,

From what little I've read on contra-rotating props you're generally looking at a +10% boost compared to a single prop of the same size, so if your single prop would get you 30Kn, you're probably looking at 31.5Kn on a contra-rotating prop. Also, as per Ezgo, I'd recommend a line to separate the parts of the prop - it looks a little like a single casting at the moment.

Regards,
Adam

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ezgo394
Post subject: Re: Democratic Stratocracy of NorthernlandPosted: February 20th, 2016, 8:29 pm
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I did a little more looking on the matter and I found this page that has some info:http://marinewiki.org/index.php/Contra- ... Propellers.
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To reduce shaft vibrations the number of blades of both propellers is different, so that not all blades pass each other simultaneously. The diameter of the front propeller is often slightly larger than that of the rear propeller, to account for the contraction of the propeller wake and to avoid the rear propeller to hit the tip vortex of the front propeller.
As you've drawn it, the propellers are the same diameter and have the same amount of blades.

I have not noticed anything regarding the extra output, but APDSmith's 10% sounds correct. In fact, I have heard it used the opposite way, in that CRP propulsion will reduce the power required by 10 to 15%.

-EZ

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erik_t
Post subject: Re: Democratic Stratocracy of NorthernlandPosted: February 20th, 2016, 9:49 pm
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Evaluated as a propulsor, I recall the efficiency being something like 5-10% better. However, you're going to have a more complex gearing and shafting setup that will definitely eat into those gains, as well as be heavier and more voluminous, and more prone to failure, than a simpler setup.

You are correct that you do not want the same number of blades on the two screws.


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apdsmith
Post subject: Re: Democratic Stratocracy of NorthernlandPosted: February 21st, 2016, 1:08 am
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Hi erik,

I'd understood that even after losses you'd be looking at approx. 10% gain on a large ship, but, yes, with significant concerns as to cost of operation and reliability. I've only done a little reading around it, mainly to use on the stupid H45K I did for April Fool's (and I missed the bit about different numbers of blades on the stages of the prop) - but there it was used to increase the power delivered per prop disc area, reducing the size of the props and making the stern geometry easier.

Ad

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acelanceloet
Post subject: Re: Democratic Stratocracy of NorthernlandPosted: February 21st, 2016, 9:33 am
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apdsmith wrote:
Hi Fox,

From what little I've read on contra-rotating props you're generally looking at a +10% boost compared to a single prop of the same size, so if your single prop would get you 30Kn, you're probably looking at 31.5Kn on a contra-rotating prop. Also, as per Ezgo, I'd recommend a line to separate the parts of the prop - it looks a little like a single casting at the moment.

Regards,
Adam
do note that ship resistance (and especially not wave resistance!) is not a linear formula. a 10% boost in power does not give an 10% boost in speed. for 35 knots you will need at least 136% of the original power (keeping wave resistance as quadratic too, wave resistance gets exponential in reality which is why there is an at least there, so most likely 150% or more is not out of the question) which is keeping into account that the propulsion train and hull shape are updated for the higher speeds. (adding to that, if you go over hull speed, you will need an even larger amount of power)
the shape of the propeller blades is also different between 2 contra- rotating props, because the second one works in a more turbulent, higher speed waterstream then the first. do note here that I think you are forced to fit an fixed propeller here instead of the controllable pitch propeller that is on most warships, because there is no space to really turn the propeller blades without hitting the second one.

I am wondering what the power levels would be for this setup. you basically fit 2 propellers of the same size as the original one on the same powerplant, so while more efficient, will the propellers still work? or should 2 propellers be powered by twice the powerplant?

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apdsmith
Post subject: Re: Democratic Stratocracy of NorthernlandPosted: February 21st, 2016, 11:53 am
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Hey ace,

I know it's not linear! ;) You'll note 30Kn * 1.1 != 31.5Kn - I threw a SQRT on the power to get an approximation of the ratio for the speed.

The way I'd read the documentation ( And, obviously, I'm not trying to tell you how ships work!) that 10% was for the assembly of both props - seems a little excessive to add a whole extra prop and only get 10%, but it seems even more counter-intuitive to add an extra prop, get the thrust from that prop and an additional 10% on the original one.

Cheers,
Ad

Gaaah stupid phone, posted the comment midway through the typing.

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Fox
Post subject: Re: Democratic Stratocracy of NorthernlandPosted: February 21st, 2016, 2:08 pm
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Will adjust the blueprint asap, wonder if I could fit an bigger engine, or perhaps adjust my engines to something more powerful, also anything on supercavitation?

Because without supercavitation you make it harder for subs to detect you. So perhaps if this reduces output of supercavitation it is a good tactic against subs

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apdsmith
Post subject: Re: Democratic Stratocracy of NorthernlandPosted: February 21st, 2016, 4:36 pm
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Hi Fox,

As I understand things everybody - except speedboat designers - avoids cavitation like the plague, due to the damage it will inflict upon the drivetrain - best case is the shockwaves caused by the cavitation bubbles collapsing damages the propeller, worst case is the the entire drivetrain overspeeds, wrecking it.

Regards the engine, you've got a couple of options - the cheap one is to accept better fuel economy and bump the range of your ship. The more expensive option is to use the increased potential power to speed up the ship, but this could require a redesign of the power plant and the hull, not a trivial undertaking.

Regards,
Adam

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Fox
Post subject: Re: Democratic Stratocracy of NorthernlandPosted: February 23rd, 2016, 9:57 pm
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I understand, however cavitation always happens to a certain degree, with differentiating speeds,
Just wondering if anybody had any knowledge on this area.

Also I'm going to keep the vessel this way, with the increased efficiency and longer maintenance.
Currently 5 Noorland class have been built. This makes it possible to keep one in the dock all times and have 4 ready for escort duty.
Perhaps in the future more can be built.

Anyway updated the Navy Section!
-Updated Noordland Class
- Fixed template
- Fixed screw/prop
- Added flag and designation

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Fox
Post subject: Re: Democratic Stratocracy of NorthernlandPosted: February 27th, 2016, 12:58 am
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Update No 2.

Updated Noordland Class
- Fixed Shading

Added WIP Antwerpen Class

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