Re: [PSUBS-MAILIST] Re: high speed subs

["Marsee Skidmore" <heyred@email.msn.com>]

----- Original Message -----
From: <VBra676539@aol.com>
To: <personal_submersibles@psubs.org>
Sent: Sunday, February 27, 2000 4:49 AM
Subject: Re: [PSUBS-MAILIST] Re: high speed subs


>
> In a message dated 2/27/00 1:35:08 AM, heyred@email.msn.com writes:
>
> << Vance,
>
> My old dusty books describe squat (aka "settle butt") as a surface
> disturbance phenomenon. You're saying this occurs in mid-water, so that's
no
> help. Does it become any less pronounced with depth? Please tell me more
> about the velocity relationship. Your butt settles as you start from a
dead
> stop? And settles more the faster you go?
>
> Joe
>  >>
>
> The subs squat from the start, and recover somewhat at power so they can
be
> trimmed for controllable forward motion. All of them do it to some degree,
> even the JSLs. The subs with the biggest props are more pronounced, and
the
> subs with the shortest lengths are the least. It has to do with the
distance
> of the thrust from the CG/CB. The longer that lever is the more pronounced
it
> can be. Even WW II Fleet Boats did it. The point is that this is a common
> thing. Not theoretical. Side thrust subs don't suffer from it because they
> are being pulled through the water, rather than pushed. Anything that uses
> dive planes and a rudder changes attitude underway to some extent, I
expect,
> and that motion is multiplied a bit by the push.
>
> The numbers you didn't like on the K had to do with its resistance through
> the water when we towed it around with the scale in line. It burns up
thrust
> due to unnecessary losses due to the ballast shell and so on. Your crib
notes
> say I'd be better off with tubular tanks and no attemped streamlining of
the
> bow at such and such a speed. We didn't do anything with the boat under
power
> that day, although the stern settled a bit when I was surface maneuvering,
> just like they all do.
>
> Vance
>
I agree that settle-butt at start-up results from the lever arm between the
centerline of thrust and the CB/CG. Settle-butt at start-up is very
pronounced in dirigibles (Goodyear Blimps), where the same rules of
hydrodynamics apply. In a dirigible, the centerline of thrust is waaaay
below the CB.

I'd bet the reason that you've observed more settle-but with bigger props is
that a bigger props are mounted lower on the boat, to minimize ventilation
at the surface. If I'm wrong on that score, then Cap'n Nemo has some
compelling arguments.

Even though settle-butt happens at start-up, I'd guess that it can't be
entirely divorced from flow affects. If the boat has less flow resistance to
pitch than it does to forward motion, then settle-butt will be more
pronounced.

As I mentioned previously, "squat" is a surface thang that is related to
wave making, Critical Speed, and all that classical Naval Architecture
stuff. Many smart people have written many longwinded treatises on squat. If
you have trouble with insomnia, you, too, can become a squat expert. I do
have one dusty book that talks about submarines in mid-water, and I quote,

"When submerged, changes in power or the use of large rudder angles has a
direct effect on depth control. If the engines are stopped or reversed, even
though the boat is perfectly trimmed, there is a marked tendency for the
stern to squat. If a large rudder angle is applied, the boat, in addition to
turning, responds as though the Stern Planes had also been depressed.
Because of this, the Diving Officer must be especially alert when power
changes or course changes are ordered.

The reason for this unusual behavior is obscure and is buried in complex
hydrodynamic phenomena. Some submarines have a greater tendency to respond
to power changes and rudder angle than others. The reaction to power changes
is due in part to the impingement of the helical propellor discharge
currents on the stern planes, and the reaction to rudder angle is connected
with the disturbance of the flow over the after part of the hull caused by
the bridge fairwater. Is is sufficient from the point of view of the
shiphandler to understand that speed and rudder control hav an effect on
depth control."

"Naval Shiphandling," Crenshaw, Cdr. R.S., United States Naval Institute,
1955.

In 1955, he's talking about Guppy Class boats. It's a textbook for Naval
Officers, hence some of the
it's-too-complicated-you-wouldn't-understand-just-take-it-on-faith tone.

Joe