Can we alter the properties of the water surface to change hull speeed.
What I have in mind is like spreading oil on water where oil is spread
from the bow. I assume that what this does is to decrease the
amplitude of the shorter period waves. Even if it didnt increaqse hull
speed, would it reduce the energy going into the shorter period waves?

No.

--

Roger Long



wrote in message
oups.com...
Can we alter the properties of the water surface to change hull
speeed.
What I have in mind is like spreading oil on water where oil is
spread
from the bow. I assume that what this does is to decrease the
amplitude of the shorter period waves. Even if it didnt increaqse
hull
speed, would it reduce the energy going into the shorter period
waves?

wrote:
One of these days I'd like to look into the derivation of hull speed.

Check out the work of William Froude.

Can you suggest a very basic explanation (a source thereof). I'm just
curious as to when it applies. Like:

Does it apply to non-rigid hulls (hulls that might flex in the middle)
Does it apply to totally submerged objects?
Does it apply to towed objects, like dinghies?

Yes, yes, & yes.
Usually making hulls non-rigid makes them slower for given power. There
are some interesting studies of marine mammals that can swim faster than
science suggests they should, and apparently they can change their shape
to promote laminar flow and possibly reduce wave pressure areas.

For totally submerged objects, the rules are a bit different and they
don't have the same limits as displacement-supported objects on the surface.


What happens when an object exceeds hull speed?

That depends a lot of the hull & the conditions. Some "objects" will
plane, ie will transition from being supported by displacement to being
supported by the dynamic lift of water flowing under the hull. In other
cases, the cross section density is high enough that wave resistance is
less (ie very narrow hulls).


Is there any way to "fool the water" into acting as if the boat is
longer than it is?


Sure, that's what bow bulbs are all about.

Fresh Breezes- Doug King

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.

But, in general, it's true.

--
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 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.



Rodney Myrvaagnes NYC J36 Gjo/a

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

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/