Re: which gadgets?


"Ronald Raygun" <no.spam@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:9ZtCf.10333>>>

>>>> It is a physical reality that a boat will yaw first, then
>>>> only
>>>> later change it's direction of movement (COG if you will).
>>>
>>> The above point is false. As soon as a boat yaws it is
>>> immediately moving in a different direction.



>> When the boat rotates, the fin/keel
>> increases its angle to the water flow (a temporary leeway).
>> This
>> temporary leeway creates the lateral force that will accelerate
>> the boat sideways.
>
> Yes, but *this* provides the desired acceleration immediately.

Agreed.

>> Acceleration takes time to create a lateral velocity (a lag),

> No, there is no lag. Acceleration creates velocity immediately.
> Of course it doesn't "create" (not really the right word, but we
> know what we mean, perhaps "achieve" woudl be better) all the
> velocity we want straight away, it takes time to achieve that,
> and your "2 or 3 seconds" is a reasonable figure I do not care
> to argue with, but I'm just pointing out that the boats *begins*
> to change direction as soon as yaw has happened, a situation
> which
> your original wording appears to contradict.

OK, we're nearly in full agreement. The barrier is perhaps we
define 'lag' differently, and the cause of that misunderstanding
may be that we're thinking of different boat speeds or different
keel configurations.

I'll accept that, in fast boats, there's no detectable lag between
changing heading and changing track. I'm defining fast as 12kts,
and assuming we're using around 2 degrees slip to create the turn.
(I'm sure there's a knowledgable guy out there who'll put real
figures on this). And I'd agree that a 2 degree difference (if it
existed) is not an easily detectable change.

This small amount of slip arises because the lateral force from
the keel (if the flow angle to the keel is constant) rises as the
square of the boat's speed. Or, looked at another way, to achieve
a given lateral displacement (track change) a boat travelling at
half the speed of another must use 4 times the slip to get there
in the same time.

Trivial cases are the half knot sail boat which could be rotated
360 degrees without any measurable lateral displacement. Or,
(thinking keel efficiency, OK, a separate issue), the canal barge
which rotates 20 degrees about its axis while you count the tens
of seconds for the track to change. Mesure by collision with the
bank. Or, in extremis, the keel-less hovercraft - just like
driving a supermarket trolley with fully castering wheels!

My point here is that a compass will detect these heading changes
immediately, while it will take time before these heading changes
convert to a detectable displacement - track change. OK, they're
trivial cases above, but the line joining them to the 'detectable
response' is a square law, so at half that speed, you'll need four
times the keel angle. On my maybe foolish assumptions, 8 degrees.

I'm going maths here. If I instantly slapped on 8 degrees (or 0.8,
or 0.0008), I would instantly have a force and an acceleration. We
agree here.

However, I would not, in that microscopic instant (the beloved
delta t of my maths master), have a lateral velocity, nor a
lateral displacement. The reason is that acceleration has to be
integrated over time to create the velocity, and in turn, the
velocity has to be integrated over time to create a displacement.
Every integration creates a lag (as my dynamics tutor drummed into
me).

Now we can leave the maths gurus arbitrate that one.

But, if it's true, we've got a compass showing 8 degrees of change
(or whatever), and no track change yet. That's what I mean by lag.
And why I think it's significant on a slow boat (to China or
wherever). And amazing on a supermarket trolley.

Thanks for making me think.

JimB




.

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