For some reason, my post on this question this morn didnt get posted.


Does towing my hard shell dinghy effecively increase my waterline
length thereby increasing my hull speed?

Could you add a lightweight inflatable extension to the waterline of a
boat to increase its waterline length thereby increasing its hull
speed?

I believe it would have to be rigidly attatched and fair to the hull to
improve hull speed. Hull speed is a function of a moving body
displacing water and the length of the wave it produces. Two bodies
make two waves. A dinghy being towed adds nothing but drag.

Parallax wrote:
For some reason, my post on this question this morn didnt get posted.


Does towing my hard shell dinghy effecively increase my waterline
length thereby increasing my hull speed?

Could you add a lightweight inflatable extension to the waterline of a
boat to increase its waterline length thereby increasing its hull
speed?

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

I agree. I think the difference is that the added 'hull' would have to be such
a part of the original vessel as to contribute to its displacement.

Glenn Ashmore wrote:

I believe it would have to be rigidly attatched and fair to the hull to
improve hull speed. Hull speed is a function of a moving body
displacing water and the length of the wave it produces. Two bodies
make two waves. A dinghy being towed adds nothing but drag.

Parallax wrote:

For some reason, my post on this question this morn didnt get posted.


Does towing my hard shell dinghy effecively increase my waterline
length thereby increasing my hull speed?

Could you add a lightweight inflatable extension to the waterline of a
boat to increase its waterline length thereby increasing its hull
speed?

On 14 Oct 2003 14:24:02 -0700, (Parallax)
wrote:
Does towing my hard shell dinghy effecively increase my waterline
length thereby increasing my hull speed?

Could you add a lightweight inflatable extension to the waterline of a
boat to increase its waterline length thereby increasing its hull
speed?

If the wave pattern from the dink was out of phase with the wave
pattern of the tow boat then the total wave drag of the tow boat plus
dink system might in some condtions be less than the wave drag of the
tow boat alone... See
http://www.maths.adelaide.edu.au/App...ydro/hydro.htm

In your fist questin the hull speed of the tow boat does not change,
in the second it does. In both cases you will likely have more total
drag most of the time. An inflatable transome extension for power
reaching is a neat idea.

--- Tom

Parallax wrote:

Does towing my hard shell dinghy effecively increase my waterline
length thereby increasing my hull speed?


no

Could you add a lightweight inflatable extension to the waterline of a
boat to increase its waterline length thereby increasing its hull
speed?


probably not, as it has to support the boat. What you're describing
would probably just deform and not change your effective LWL. Its drag
would probably erase any benefits.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/

Not quite. If you get a chance, watch carefully when a tug or pilot
boat approaches a big ship at, say, 15 knots. The tug is hurrying
along like mad, throwing a huge bow wave, while the big ship is just
gliding effortlessly through the water. When the tug gets within a ten
or twenty feet of the big ship, suddenly it, too, is gliding along
effortlessly as it has become part of the big ship's wave pattern.
Tugs have to be careful at this moment because the power required goes
down dramatically.

Your dinghy may not do it, but it is possible to fool the water -- the
Kiwis tried it in the last America's Cup

Now for a standard rant of mine. There is no such thing as "hull
speed". If you look at the math, at tank tests, at rpm curves, or at
actual fuel consumption of displacement vessels, there is no
inflection, change, bump, or anything else in the power versus speed
curve at S/L 1.34. Sure, the power required and fuel consumption are
going through the roof, but it's a smooth curve. If you apply more
power, you will go faster, to the point at which the power applied
exceeds the integrity of the boat or the size of your wallet.

If you doubt this, ask any destroyer sailor. They regularly exceed
S/L 1.34. An Arleigh Burke destroyer is 466' LWL, 32 knots maximum
speed, for an S/L of 1.48. Of course it takes almost 12hp per ton to
get there.

Jim Woodward
www.mvFintry.com



Jere Lull wrote in message .. .
Parallax wrote:

Does towing my hard shell dinghy effecively increase my waterline
length thereby increasing my hull speed?


no

Could you add a lightweight inflatable extension to the waterline of a
boat to increase its waterline length thereby increasing its hull
speed?


probably not, as it has to support the boat. What you're describing
would probably just deform and not change your effective LWL. Its drag
would probably erase any benefits.

Jere Lull wrote in message .. .
Parallax wrote:

Does towing my hard shell dinghy effecively increase my waterline
length thereby increasing my hull speed?


no

Could you add a lightweight inflatable extension to the waterline of a
boat to increase its waterline length thereby increasing its hull
speed?


probably not, as it has to support the boat. What you're describing
would probably just deform and not change your effective LWL. Its drag
would probably erase any benefits.

I agree with you ppl, the dink just adds drag, however, the inflatable
transom..............unfortunately hull spd only goes as sqrt of LWL.

Jim Woodward wrote:

Not quite. If you get a chance, watch carefully when a tug or pilot
boat approaches a big ship at, say, 15 knots. The tug is hurrying
along like mad, throwing a huge bow wave, while the big ship is just
gliding effortlessly through the water. When the tug gets within a ten
or twenty feet of the big ship, suddenly it, too, is gliding along
effortlessly as it has become part of the big ship's wave pattern.
Tugs have to be careful at this moment because the power required goes
down dramatically.


That's a special case! Most cruisers don't drag a boat behind them that
is bigger than they are. (but I regularly use other boats' wakes to
increase our speed when possible. Most times, I'm cursing powerboat
wakes from the wrong direction, though.)

Your dinghy may not do it, but it is possible to fool the water -- the
Kiwis tried it in the last America's Cup

As I recall, that attempt didn't work particularly well....

But our little boat with a wide flattish transom considers hull speed
(1.34 x sqrt(LWL)) a suggestion rather than the law. It seems the AC
boats and other newer sleds are more advanced that way.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/

Jere Lull wrote:
Jim Woodward wrote:


Your dinghy may not do it, but it is possible to fool the water -- the
Kiwis tried it in the last America's Cup

As I recall, that attempt didn't work particularly well....

But our little boat with a wide flattish transom considers hull speed
(1.34 x sqrt(LWL)) a suggestion rather than the law. It seems the AC
boats and other newer sleds are more advanced that way.


Two points he

1. Hull speed only relates to displacement hulls. Planing hulls such as
the aforementioned sleds are another story.

2. The jury is still out on the hula skirt as used on Black Magic in the
most recent America's Cup finals. Remember it was other problems such
as taking on excessive water, having the jib pull out of its track and
losing a mast because of standing rigging failure that lost the races.
When the boat was able to sail it was plenty fast. The hula was always
a tradeoff of LWL versus increased drag caused by the rule-mandated gap
between the skirt and the hull proper. I'm sure that it will be
outlawed in the next go around.

Joe Wood

On Wed, 15 Oct 2003 23:17:29 -0400, Joe Wood
wrote:



Jere Lull wrote:
Jim Woodward wrote:


Your dinghy may not do it, but it is possible to fool the water -- the
Kiwis tried it in the last America's Cup

As I recall, that attempt didn't work particularly well....

But our little boat with a wide flattish transom considers hull speed
(1.34 x sqrt(LWL)) a suggestion rather than the law. It seems the AC
boats and other newer sleds are more advanced that way.


Two points he

1. Hull speed only relates to displacement hulls. Planing hulls such as
the aforementioned sleds are another story.

It also only applies to displacement hulls with a L/W of less than
around 3 or 4. When you get to the 8-10 range, you can easily exceed
hull speed, as catamarans (also displacement hulls) do.

Steve