On 16/01/2024 17:35, Bob wrote:
> A low speed ship propeller needs to generate a lot of torque at low
> speed, hence it requires a much higher solidity to achive this. This
> could be done with a small number of long chord blades or a large number
> of blades with a modest chord.

Some warships seem to use the latter option. I filmed the launching of a
naval vessel at Vosper Thornycroft years back and the prop had around 7
or 8 blades looking a bit like a modern aircraft prop. Incidentally I
was forbidden to actually film the prop as the exact details could be
used to deduce the sonar signature of that particular vessel allowing
positive identification by a hostile submarine.

--
Graeme Wall
This account not read.

In message <uo6db2$1hgtf$2@dont-email.me>, at 18:10:58 on Tue, 16 Jan
2024, Bob <bob@domain.com> remarked:
>On 16.01.2024 17:26, Roland Perry wrote:
>> In message <uo689c$1gfij$1@dont-email.me>, at 16:44:44 on Tue, 16 Jan
>>2024, Bob <bob@domain.com> remarked:
>>> On 16.01.2024 14:19, Roland Perry wrote:
>>>> In message <uo5pkf$1dep3$1@dont-email.me>, at 12:34:39 on Tue, 16
>>>>Jan 2024, Bob <bob@domain.com> remarked:
>>>>
>>>>>>  I assume the blade can't me made much wider to physically push
>>>>>>more  stuff  backwards for each rotation at a given rpm or they'd
>>>>>>do it.  But why?
>>>>>
>>>>> The simple answer is increasing the blade chord (the width of the
>>>>>blade) will not have that effect. The more detailed answer involves
>>>>>taking a 3rd year undergraduate engineering course in turbomachinery
>>
>>>>  Or you used to be able

Dates are important (see below).

>>>>to take a general engineering degree at Cambridge, which would have
>>>>covered that in the second year.
>>>
>>> Not really, the 2nd year course doesn't really go any further than
>>>the Euler work equation and velocity triangles.

>> I wasn't aware you did the course too, and presumed you had that
>>level of insight into the syllabus. Because that's probably "No" and
>>"No".
>
>I both took the course, to MEng level,

There was no such thing as MEng in my time. It was BA/MA.

>as an undergraduate (taking all of the aerodynamics and turbomachinery
>elective options),

Options were only 3rd year, I was talking about the 1st/2nd year where
everyone took every element of the course.

>and as a postgraduate student, supervised the IB and IIA thermodynamics
>and fluid mechanics modules. Since then my day job has been as a
>compressor aerodynamic design engineer.

What year did you attend the 2nd year course? Mine was 1973.
--
Roland Perry

On Tue, 16 Jan 2024 18:35:21 +0100
Bob <bob@domain.com> wrote:
>On 16.01.2024 17:00, Muttley@dastardlyhq.com wrote:
>> Fair enough. Why does the same thing apply to props in air though which are
>> all much thinner that could potentially be?
>
>Speeds are the relevant difference. For a propeller to be effective, it
>needs to operate in a relatively narrow flow coefficent range. Flow
>coefficient is the ratio of blade speed to incident flow speed (ie
>vehicle forward speed). Aircraft are fast, so the propeller blades can
>spin fast. Ships are slow, so the propellers spin slowly.

Isn't it the other way around? Aircraft propellors spin fast so the plane
goes fast, slow ship propellors so the ship goes slowly.

On Tue, 16 Jan 2024 18:46:02 +0100
Bob <bob@domain.com> wrote:
>On 16.01.2024 18:13, Muttley@dastardlyhq.com wrote:
>> Rubbish. Move a book at a 45 degrees angle and the air will be pushed out of
>> the way. Its the exact same thing as an angled blade moving through the air
>> and the fact that its going around in circles doesn't change that.
>
>That analogy breaks down because ships and aircraft are not stationary.
>The air/water flows through the propeller. Due to this velocity,
>propellers do not act as paddles, but as airfoils.

But the movement is caused by the propellor itself. Its not something thats
happening to the propellor, its something the propellor is causing by dint of
it chucking fluid backwards.

>> Just like the lift of a wing doesn't come entirely from the bernoulli
>> principle (otherwise no plane could ever fly upside down and paper planes
>> would immediately plummet to the ground) neither does the force pushing the
>> air back from a rotating blade.
>
>The lift from a wing comes from the sharp trailing edge of the wing
>causing the boundary layers on the two surfaces to separate at an angle
>to the incident flow, causing circulation vorticity to be embedded
>within the wing, turning the flow. This is commonly referred to as the

Except if I blow on the underside of some paper it moves away. There's no
boundary layer involved there, its simple vector forces. You have enough fluid
hitting the underside of something and that something will move.

>Kutta condition. All the "Bernoulli" stuff is a plausible sounding half
>truth that gets told when actually dealing with the real answer is too
>time consuming and involved to explain.

And neither will apply when it comes to aquaplaning boats yet the effect is
the same.

On 17.01.2024 10:28, Muttley@dastardlyhq.com wrote:
> On Tue, 16 Jan 2024 18:46:02 +0100
> Bob <bob@domain.com> wrote:
>> On 16.01.2024 18:13, Muttley@dastardlyhq.com wrote:
>>> Rubbish. Move a book at a 45 degrees angle and the air will be pushed out of
>>> the way. Its the exact same thing as an angled blade moving through the air
>>> and the fact that its going around in circles doesn't change that.
>>
>> That analogy breaks down because ships and aircraft are not stationary.
>> The air/water flows through the propeller. Due to this velocity,
>> propellers do not act as paddles, but as airfoils.
>
> But the movement is caused by the propellor itself. Its not something thats
> happening to the propellor, its something the propellor is causing by dint of
> it chucking fluid backwards.

The propeller turns circumferentially, so any "paddle" effect is
undesirable. In the process of driving flow backwards, the flow is
moving across the propeller blades along the surface, not being pushed
aside as in a paddle.

>>> Just like the lift of a wing doesn't come entirely from the bernoulli
>>> principle (otherwise no plane could ever fly upside down and paper planes
>>> would immediately plummet to the ground) neither does the force pushing the
>>> air back from a rotating blade.
>>
>> The lift from a wing comes from the sharp trailing edge of the wing
>> causing the boundary layers on the two surfaces to separate at an angle
>> to the incident flow, causing circulation vorticity to be embedded
>> within the wing, turning the flow. This is commonly referred to as the
>
> Except if I blow on the underside of some paper it moves away. There's no
> boundary layer involved there, its simple vector forces. You have enough fluid
> hitting the underside of something and that something will move.

Paper is thin. The back end of the paper causes the boundary layer to
separate, imposing the kutta condition.

>> Kutta condition. All the "Bernoulli" stuff is a plausible sounding half
>> truth that gets told when actually dealing with the real answer is too
>> time consuming and involved to explain.
>
> And neither will apply when it comes to aquaplaning boats yet the effect is
> the same.

Hulls designed for aquaplaning have a sharp step at the stern, designed
to force the flow to separate cleanly at a specific location, imposing
the kutta condition. Hulls not designed for aquaplaning have a smooth
rear profile designed to keep the flow attached and recover pressure.

Robin

On Wed, 17 Jan 2024 19:42:31 +0100
Bob <bob@domain.com> wrote:
>On 17.01.2024 10:28, Muttley@dastardlyhq.com wrote:
>> But the movement is caused by the propellor itself. Its not something thats
>> happening to the propellor, its something the propellor is causing by dint of
>
>> it chucking fluid backwards.
>
>The propeller turns circumferentially, so any "paddle" effect is
>undesirable. In the process of driving flow backwards, the flow is
>moving across the propeller blades along the surface, not being pushed
>aside as in a paddle.

I think there's some hair splitting going on here. The blade is physically
pushing air molecules backwards whether you want to call that "flow" or
something else entirely because when the blade goes past the air has to move
out of its way as it can't go through it. Its not some magic effect.

>>> Kutta condition. All the "Bernoulli" stuff is a plausible sounding half
>>> truth that gets told when actually dealing with the real answer is too
>>> time consuming and involved to explain.
>>
>> And neither will apply when it comes to aquaplaning boats yet the effect is
>> the same.
>
>Hulls designed for aquaplaning have a sharp step at the stern, designed
>to force the flow to separate cleanly at a specific location, imposing
>the kutta condition. Hulls not designed for aquaplaning have a smooth

Oh right. So how do water skis work then? Are you saying if something had
a tapered stern it would immediately sink no matter how fast or what angle it
was going through the water?

In message <uoaqjt$2hfhu$1@dont-email.me>, at 09:22:05 on Thu, 18 Jan
2024, Muttley@dastardlyhq.com remarked:

>I think there's some hair splitting going on here.

In other news, the Pope shits in the woods.
--
Roland Perry

On 18.01.2024 10:22, Muttley@dastardlyhq.com wrote:
> On Wed, 17 Jan 2024 19:42:31 +0100
> Bob <bob@domain.com> wrote:
>> On 17.01.2024 10:28, Muttley@dastardlyhq.com wrote:
>>> But the movement is caused by the propellor itself. Its not something thats
>>> happening to the propellor, its something the propellor is causing by dint of
>>
>>> it chucking fluid backwards.
>>
>> The propeller turns circumferentially, so any "paddle" effect is
>> undesirable. In the process of driving flow backwards, the flow is
>> moving across the propeller blades along the surface, not being pushed
>> aside as in a paddle.
>
> I think there's some hair splitting going on here. The blade is physically
> pushing air molecules backwards whether you want to call that "flow" or
> something else entirely because when the blade goes past the air has to move
> out of its way as it can't go through it. Its not some magic effect.

While it is a different way of describing it, one approach suggests
behaviours that are representative of what really happens, such as the
relationship between chord and thrust, and the other lends itself to
drawing conclusions that are not representative of real world behaviour.

>>>> Kutta condition. All the "Bernoulli" stuff is a plausible sounding half
>>>> truth that gets told when actually dealing with the real answer is too
>>>> time consuming and involved to explain.
>>>
>>> And neither will apply when it comes to aquaplaning boats yet the effect is
>>> the same.
>>
>> Hulls designed for aquaplaning have a sharp step at the stern, designed
>> to force the flow to separate cleanly at a specific location, imposing
>> the kutta condition. Hulls not designed for aquaplaning have a smooth
>
> Oh right. So how do water skis work then? Are you saying if something had
> a tapered stern it would immediately sink no matter how fast or what angle it
> was going through the water?

Most ships and boats float due to hydrostatic effects, ie displacement
of water, not due to hydrodynamic effects, ie hydroplaning. Most vessels
that can hydroplane will also be able to remain afloat due to
displacement if their speed is insufficient to generate dynamic lift
sufficient to initiate planing.

Robin

On Thu, 18 Jan 2024 15:56:19 +0100
Bob <bob@domain.com> wrote:
>On 18.01.2024 10:22, Muttley@dastardlyhq.com wrote:
>> On Wed, 17 Jan 2024 19:42:31 +0100
>> I think there's some hair splitting going on here. The blade is physically
>> pushing air molecules backwards whether you want to call that "flow" or
>> something else entirely because when the blade goes past the air has to move
>> out of its way as it can't go through it. Its not some magic effect.
>
>While it is a different way of describing it, one approach suggests
>behaviours that are representative of what really happens, such as the
>relationship between chord and thrust, and the other lends itself to
>drawing conclusions that are not representative of real world behaviour.

No idea what that word salad is supposed to mean.

I assume you agree that the blade is physically pushed the fluid out of the
way. If not what is happening, magic pixies tickling the molecules?

>> Oh right. So how do water skis work then? Are you saying if something had
>> a tapered stern it would immediately sink no matter how fast or what angle it
>
>> was going through the water?
>
>Most ships and boats float due to hydrostatic effects, ie displacement
>of water, not due to hydrodynamic effects, ie hydroplaning. Most vessels
>that can hydroplane will also be able to remain afloat due to
>displacement if their speed is insufficient to generate dynamic lift
>sufficient to initiate planing.

Nice dodge. We both know water skiers stand up because of the water pushing
on the underside of the skis. Its a fairly simple vector force calculation,
no Kutta or whatever required.