In article ,
Rodney Myrvaagnes wrote:

On Wed, 20 Apr 2005 05:25:11 GMT, Jere Lull wrote:

In article ,
DSK wrote:

Usually making hulls non-rigid makes them slower for given power.

One notable exception: a PortaBote, but that's not practical for most
hulls. The hull deforms unbelievably, which is disconcerting.

It is hard for me to believe that is an exceoption.

It would be notable if so. Indeed, it would be a miracle.

Obviously, you haven't run around on a PortaBote. 4 HP gives a solid 10+
knots speed with lots of load.

When the boat goes on plane, the "floor" under the helmsman's feet drops
a bunch of inches. Unlike a solid hull, the PortaBote expands into areas
of low pressure to limit the drag (suction).

Rodney Myrvaagnes NYC J36 Gjo/a

"Religious wisdom is to wisdom as military music is to music."

I *LIKE* military music, aka Marches.

--
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/

On Thu, 21 Apr 2005 05:40:38 GMT, Jere Lull wrote:

In article ,
Rodney Myrvaagnes wrote:

On Wed, 20 Apr 2005 05:25:11 GMT, Jere Lull wrote:

In article ,
DSK wrote:

Usually making hulls non-rigid makes them slower for given power.

One notable exception: a PortaBote, but that's not practical for most
hulls. The hull deforms unbelievably, which is disconcerting.

It is hard for me to believe that is an exceoption.

It would be notable if so. Indeed, it would be a miracle.

Obviously, you haven't run around on a PortaBote. 4 HP gives a solid 10+
knots speed with lots of load.

When the boat goes on plane, the "floor" under the helmsman's feet drops
a bunch of inches. Unlike a solid hull, the PortaBote expands into areas
of low pressure to limit the drag (suction).

If you say so. The age of miracles is not past.



Rodney Myrvaagnes NYC J36 Gjo/a


"Curse thee, thou quadrant. No longer will I guide my earthly way by thee." Capt. Ahab

On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote:

In article ,
Stephen Trapani wrote:

Hull speed is the absolute maximum that boat can travel through water.
All your examples have the water moving forward also so the boat is not
exceeding hull speed through the water.

I thought I mentioned this before. Hope I'm not repeating myself.

Hull speed is a suggestion for our boat, not the law. Though our
theoretical hull speed is 6.65 knots, we regularly exceed that with
aplomb, close hauled, close reach, broad reach, whatever point of sail.
Spent a wonderful afternoon with 6 other sailors last season. As long as
I was on the tiller, pushing her to where she likes to be, we were well
above the theoretical hull speed. As we pinched to get back into the
harbor, she insisted on doing over 7 knots directly into the wind (okay,
about 15 degrees off). That last was our lovely lady showing off, of
course, as what we did was clearly impossible.

1.34 was derived from observing boats about a century ago. Depending on
the hull, that constant can be quite a bit different. As I recall, some
multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp
transom keeps her driving towards a 1.7 or so constant.

Jere,

It sounds like your speed-length parameter is higher than 1.34 - a
testimonial to your hull designer.

The 1.34 comes from the fact that speed-squared of a wave = g/2*pi
times wavelength.

If your hull's stern really places the stern wave a distance back from
the bow wave equal to your design waterline length, then 1.34 is
pretty accurate as the point where the curve of additional HP to yield
additional speed for a displacement hull becomes almost vertical.

However, with sweet butock lines, stern reflexes and other
sophistications of hull design, the stern wave can actually be moved a
bit aft of your transom. The wavelength thus becomes greater than your
DWL.

Since speed-squared is proportional to wavelength, and since your boat
speed and the wave speed must match, you get a speed-length parameter
that's higher than 1.34 as the effective multiplier times the square
root of your DWL (since DWL is now less than the wavelength).

At least that's my simplified understanding of a very complex subject.

Al
s/v Persephone
Newburyport, MA

In article ,
Albert P. Belle Isle wrote:

On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote:

In article ,
Stephen Trapani wrote:

Hull speed is the absolute maximum that boat can travel through water.
All your examples have the water moving forward also so the boat is not
exceeding hull speed through the water.

Hull speed is a suggestion for our boat, not the law. Though our
theoretical hull speed is 6.65 knots, we regularly exceed that with
aplomb, close hauled, close reach, broad reach, whatever point of sail.
Spent a wonderful afternoon with 6 other sailors last season. As long as
I was on the tiller, pushing her to where she likes to be, we were well
above the theoretical hull speed. As we pinched to get back into the
harbor, she insisted on doing over 7 knots directly into the wind (okay,
about 15 degrees off). That last was our lovely lady showing off, of
course, as what we did was clearly impossible.

1.34 was derived from observing boats about a century ago. Depending on
the hull, that constant can be quite a bit different. As I recall, some
multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp
transom keeps her driving towards a 1.7 or so constant.

Jere,

It sounds like your speed-length parameter is higher than 1.34 - a
testimonial to your hull designer.

Full agreement.

The 1.34 comes from the fact that speed-squared of a wave = g/2*pi
times wavelength.

Yes, I agree with the derivation of the formula -- as long as we include
that wavelengths can differ. Swells have wavelengths 100s of feet and
periods many seconds from crest to crest, while wind-driven waves have
quite a bit shorter wavelengths and periods. And wind-driven waves have
different periods and wavelengths.

If your hull's stern really places the stern wave a distance back from
the bow wave equal to your design waterline length, then 1.34 is
pretty accurate as the point where the curve of additional HP to yield
additional speed for a displacement hull becomes almost vertical.

However, with sweet butock lines, stern reflexes and other
sophistications of hull design, the stern wave can actually be moved a
bit aft of your transom. The wavelength thus becomes greater than your
DWL.

Our resting WL is 24'. To maintain a 1.34 constant and get the speeds
we've verified while definitely not surfing, our effective waterline
would have to be greater than 35'. That's a LONG way behind our transom!

Since speed-squared is proportional to wavelength, and since your boat
speed and the wave speed must match, you get a speed-length parameter
that's higher than 1.34 as the effective multiplier times the square
root of your DWL (since DWL is now less than the wavelength).

It feels like you left a bit out and mixed a couple of things here.
Again, I agree that it probably has something to do with the wave speed,
which has a certain value when the constant is 1.34. Change the wave's
speed and you change the constant, and wave length.

At least that's my simplified understanding of a very complex subject.

Al
s/v Persephone

After having chased several NA's explanations for a few years, I finally
gave up trying to explain and simply accept that it's more complex than
1.34, since other hull shapes have much higher observed constants.

--
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/

On Mon, 25 Apr 2005 04:06:45 GMT, Jere Lull wrote:

In article ,
Albert P. Belle Isle wrote:

On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote:

In article ,
Stephen Trapani wrote:

Hull speed is the absolute maximum that boat can travel through water.
All your examples have the water moving forward also so the boat is not
exceeding hull speed through the water.

Hull speed is a suggestion for our boat, not the law. Though our
theoretical hull speed is 6.65 knots, we regularly exceed that with
aplomb, close hauled, close reach, broad reach, whatever point of sail.
Spent a wonderful afternoon with 6 other sailors last season. As long as
I was on the tiller, pushing her to where she likes to be, we were well
above the theoretical hull speed. As we pinched to get back into the
harbor, she insisted on doing over 7 knots directly into the wind (okay,
about 15 degrees off). That last was our lovely lady showing off, of
course, as what we did was clearly impossible.

1.34 was derived from observing boats about a century ago. Depending on
the hull, that constant can be quite a bit different. As I recall, some
multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp
transom keeps her driving towards a 1.7 or so constant.

Jere,

It sounds like your speed-length parameter is higher than 1.34 - a
testimonial to your hull designer.

Full agreement.

The 1.34 comes from the fact that speed-squared of a wave = g/2*pi
times wavelength.

Yes, I agree with the derivation of the formula -- as long as we include
that wavelengths can differ. Swells have wavelengths 100s of feet and
periods many seconds from crest to crest, while wind-driven waves have
quite a bit shorter wavelengths and periods. And wind-driven waves have
different periods and wavelengths.

If your hull's stern really places the stern wave a distance back from
the bow wave equal to your design waterline length, then 1.34 is
pretty accurate as the point where the curve of additional HP to yield
additional speed for a displacement hull becomes almost vertical.

However, with sweet butock lines, stern reflexes and other
sophistications of hull design, the stern wave can actually be moved a
bit aft of your transom. The wavelength thus becomes greater than your
DWL.

Our resting WL is 24'. To maintain a 1.34 constant and get the speeds
we've verified while definitely not surfing, our effective waterline
would have to be greater than 35'. That's a LONG way behind our transom!

Since speed-squared is proportional to wavelength, and since your boat
speed and the wave speed must match, you get a speed-length parameter
that's higher than 1.34 as the effective multiplier times the square
root of your DWL (since DWL is now less than the wavelength).

It feels like you left a bit out and mixed a couple of things here.
Again, I agree that it probably has something to do with the wave speed,
which has a certain value when the constant is 1.34. Change the wave's
speed and you change the constant, and wave length.

At least that's my simplified understanding of a very complex subject.

Al
s/v Persephone

After having chased several NA's explanations for a few years, I finally
gave up trying to explain and simply accept that it's more complex than
1.34, since other hull shapes have much higher observed constants.

The physics says that extra-long-wavelength swells just propagate more
slowly. The c-squared = lambda*g/2pi is pretty fundamental for surface
waves (as opposed to deep pressure waves, like tsunamis).

There's a passage in John Craven's "The Silent War" where he gleefully
chants the formula from the sail of a nuclear submarine as he watches
her bow and stern waves demonstrate the validity of something he had
drilled into his head in grad school. Maybe you have to be a geek to
appreciate it g.

However, the oversimplified nature of my "push-the-stern-wave-aft"
explanation is, of course, quite true. IANA.

If you haven't already read it, Jere, van Dorn's "Oceanography and
Seamanship" has a pretty good discussion of the speed-power curves for
planing, semiplaning and displacement hulls - as well as, among other
things, a nifty nomograph for predicting sea-state from duration or
fetch of sustained winds.

Good sailing.

Al
s/v Persephone

On Mon, 25 Apr 2005 20:16:09 GMT, Albert P. Belle Isle
wrote:


Our resting WL is 24'. To maintain a 1.34 constant and get the speeds
we've verified while definitely not surfing, our effective waterline
would have to be greater than 35'. That's a LONG way behind our transom!


Jere -

As a quick calculation, 24ft DWL would yield a hull speed of about
6.6kt with a speed-length coefficient of 1.34. To get to 7kt, the
effective DWL at 1.34 would be a little over 27ft - not 35ft.

My previous boat had a DWL of 28ft, for which a speed-length
coefficient of 1.34 would predict 7.1kt.

I easily got 7.4kt on beam reaches, which would say my real
coefficient was almost 1.4 (or that my effective DWL at 1.34 was a
little over 30ft - a two foot "push-back" of the sten wave, which was
roughly how the peak of the stern wave looked from my cockpit.

Regards,
Al

On Mon, 25 Apr 2005 04:06:45 GMT, Jere Lull wrote:

In article ,
Albert P. Belle Isle wrote:

On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote:

In article ,
Stephen Trapani wrote:

Hull speed is the absolute maximum that boat can travel through water.
All your examples have the water moving forward also so the boat is not
exceeding hull speed through the water.

Hull speed is a suggestion for our boat, not the law. Though our
theoretical hull speed is 6.65 knots, we regularly exceed that with
aplomb, close hauled, close reach, broad reach, whatever point of sail.
Spent a wonderful afternoon with 6 other sailors last season. As long as
I was on the tiller, pushing her to where she likes to be, we were well
above the theoretical hull speed. As we pinched to get back into the
harbor, she insisted on doing over 7 knots directly into the wind (okay,
about 15 degrees off). That last was our lovely lady showing off, of
course, as what we did was clearly impossible.

1.34 was derived from observing boats about a century ago. Depending on
the hull, that constant can be quite a bit different. As I recall, some
multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp
transom keeps her driving towards a 1.7 or so constant.

Jere,

It sounds like your speed-length parameter is higher than 1.34 - a
testimonial to your hull designer.

Full agreement.

The 1.34 comes from the fact that speed-squared of a wave = g/2*pi
times wavelength.

Yes, I agree with the derivation of the formula -- as long as we include
that wavelengths can differ. Swells have wavelengths 100s of feet and
periods many seconds from crest to crest, while wind-driven waves have
quite a bit shorter wavelengths and periods. And wind-driven waves have
different periods and wavelengths.

If your hull's stern really places the stern wave a distance back from
the bow wave equal to your design waterline length, then 1.34 is
pretty accurate as the point where the curve of additional HP to yield
additional speed for a displacement hull becomes almost vertical.

However, with sweet butock lines, stern reflexes and other
sophistications of hull design, the stern wave can actually be moved a
bit aft of your transom. The wavelength thus becomes greater than your
DWL.

Our resting WL is 24'. To maintain a 1.34 constant and get the speeds
we've verified while definitely not surfing, our effective waterline
would have to be greater than 35'. That's a LONG way behind our transom!

Since speed-squared is proportional to wavelength, and since your boat
speed and the wave speed must match, you get a speed-length parameter
that's higher than 1.34 as the effective multiplier times the square
root of your DWL (since DWL is now less than the wavelength).

It feels like you left a bit out and mixed a couple of things here.
Again, I agree that it probably has something to do with the wave speed,
which has a certain value when the constant is 1.34. Change the wave's
speed and you change the constant, and wave length.

At least that's my simplified understanding of a very complex subject.

Al
s/v Persephone

After having chased several NA's explanations for a few years, I finally
gave up trying to explain and simply accept that it's more complex than
1.34, since other hull shapes have much higher observed constants.


It isn't a different constant. It is just that many boats nowadays are
light enough so they aren't limited to the length of the wave they
make.

If you observe a tug traveling without a tow, you will see very easily
what wave we are talking about.



Rodney Myrvaagnes NYC J36 Gjo/a


"Curse thee, thou quadrant. No longer will I guide my earthly way by thee." Capt. Ahab

Hi All,

i post this update after changes to the trimming of the boat.

I said the boat is poorly trimmed and seem to "sit" too much with the
stern in the water.

So i put around 3/4 of tons of water (750 liters) in the anchor room
(the small cabin in the bow which contains the chain - is quite large
and sealed).

This had the effect to sink the bow around 8 inches and lift the stern
around 3 inches. The boat is still NOT level but at least is getting
closer.

Now the effect on travelling speed:

I could reach 5.5 knots at 900 rpm instead of 1100 rpm ! And my fuel
consumption went down 40% at once (for the same speed). The benefits
would be less and less trying to drive the boat faster and faster -
basically the speed improvement at full throttle would be
insignificant. Also waves were greatly reduced at 5.5 knots (almost
reduced ripples).

My theory: with the bow "lifted" too much the physical boat lenght
starts quite some distance from the actual entry point in the water.
Adding weight at the bow would push it down and effectively lenghten
the WL.

Now i have a long-range tank in the bow (1.5 ton) which is empty now
(i normally use other two tanks, in the middle of the boat, each 1/2
ton). Next week i will bunker diesel and fill ALL the 3 tanks to the
top - this should push down the boat another bit and eventually set it
level - then make new trials.

I also looked for green slime and the like on the hull - not much
there.

I will post the results in another 2 weeks or so.

Matteo

(Matteo) wrote in message om...
This is the situation: My 40' LWL boat (15 ton displacement) has a 150
PS engine. From the formula for speed I calculated a hull speed of

sqrt(40)*1.34 = 8.47 knots

*but*: when i did trials last week (absolutely calm water, almost no
wind) those are the results:

800 rpm 5 knots no noticeable waves generated
1100 rpm 5.5 knots small waves
1800 rpm 6.5 knots (flat out) - huge waves generated, stern deep in
the water, boat "running uphill".

1100 rpm is around 50/60 PS (from the engine rpm/PS table).

Question: what could be the cause of the "slowness" of the boat ? I do
not pretend to reach 8.4 knots cruising but at least 7 knots should be
in.

I'm thinging of dirty hull (green slime), incorrect weight
distribution (bow tends to "point" upwards even when crossing small
waves).

Any experience ?

Thanks
Matteo

In article ,
(Matteo) wrote:

i post this update after changes to the trimming of the boat.

I said the boat is poorly trimmed and seem to "sit" too much with the
stern in the water.

So i put around 3/4 of tons of water (750 liters) in the anchor room
(the small cabin in the bow which contains the chain - is quite large
and sealed).

This had the effect to sink the bow around 8 inches and lift the
stern around 3 inches. The boat is still NOT level but at least is
getting closer.

Now the effect on travelling speed:

I could reach 5.5 knots at 900 rpm instead of 1100 rpm ! And my fuel
consumption went down 40% at once (for the same speed).

That's a significant difference! Sounds like you need to make some light
stuff in the bow area trade places with heavy stuff in the stern, even
if I interpreted correctly that you can add fuel forward. There are
other things I can say, but I don't even know if you're cruising or
day-tripping.

Something else to look at is the prop. Friends of ours have had their
Marine Trader for many years and maxxed out at the same speed all those
years. In preparation for their first Bahamas trip, they re-propped.
They gained a couple of knots of top end, cruise at a lower RPM and
better economy, but higher speed than they used to. Nothing else was
changed.

At a minimum, send the blade off for a balancing when you get a chance,
as a simple mis-alignment of blades can make a big difference. We did
that for a two blade prop with only 100 hours on it and got a noticable
improvement. The two blades were pitched a full inch differently. Who
knows what an old, heavily used prop looks like?

--
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/

In article ,
Albert P. Belle Isle wrote:

As a quick calculation, 24ft DWL would yield a hull speed of about
6.6kt with a speed-length coefficient of 1.34. To get to 7kt, the
effective DWL at 1.34 would be a little over 27ft - not 35ft.

We've sustained much higher speeds, but don't feel like opening myself
up for someone saying that it's impossible, that I'm surfing, my
knotmeter's off, there's a current, or some such.

I would agree except that I eliminated all those things. We really have
done "impossible" things. Would love to know how, but gave up and simply
enjoy.

--
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/