T&T: Why do longer boats go faster?

[John Blackburn]

Hi Tom, Try this, get a friend with a boat, go out with him and run side 
by side, increasing speed, in steps, from about 3 knots to max speed.  
Have your friend take profile shots of your boat at each speed step.  
Now, look at the sequence of the pictures as speed increases.

Keep your eye on your bow wave.  You will notice that the first wave 
peak will move aft as the speed increases.  Hull speed is where the wave 
peak stops moving aft, and it is at this point your boat is trying to 
climb out of a hole.  Your stern does sit down a little when you put the 
spurs to her does it not?  Therefore, the max speed of a displacement 
hull  is related to waterline length (i.e. that is where the wave peak 
stops), because the place where the wave peak stops moving aft is always 
the same, and the value for this relationship is Hull Speed = 1.4 x sq 
rt of lwl in feet.

You are right that other things have a BIG relationship on how fast we 
go.  BUT, if you have the correct prop, clean hull, etc.  You can get 
close to the theoretical hull speed.  Remember we are talking about a 
pure displacement hull.  Any hull form with flat rear sections isn't 
included in this discussion.  If I remember correctly, (If you talk to 
the Admiral, that statement is up for challenge!) the initial 
experiments that established the relationship (among others), towed a 
flat vertical plank with a load transducer that measured resistance.  It 
gets a lot more complicated after this.  :-)  Trust me, it works.

What you refer to as a porpoise nose is called a bulbous bow.  The first 
use was as a ram in the galleys that you see in the old movies, You ever 
heard the expression "Ramming Speed" while some sweaty over-weight guy 
in a toga pounds on a drum?  Well, ....., battleships even into the 
1900's were built with rams, out of habit.

When someone realized that current tactics didn't have a ramming option 
in the play book, Navies started talking the rams off.  Guess what, the 
boats slowed down!  Come to find out, the Rams had been creating a wave 
on their own, so when two wave forms (BOW AND BULB) meet and combine, 
some things get canceled out and other enhanced.  The effects of the 
bulb are now pretty well understood and one can be designed for any 
hull., however, it will add some resistance and benefits only overcome 
the penalties when speeds pick up.  From what I remember (see above) 
unless you get above 50 feet lwl or so.  Maybe someone on the list can 
chime in here.  I'd love to see if a bulb would work on my boat.

John Blackburn
44 DeFever "Yak Rack"
Deale, MD

Thomas Averill wrote:

>This whole issue confuses me.  I have seen a simple formula which goes
>something like this:  Hull speed is equal to 1.4X square root of the length of
>the vessel's waterline. Maybe by chance, it pretty well explains why my CHB34
>will only go about 7 knots.  Also, I have been told that the  porpoise nose on
>some larger vessels uses this device to increase the waterline length to
>increase the hull speed.
>Maybe the formula for hull speed is just a rough estimate, but it seems to
>ignore the shape of the hull, particularly its beam.
>
>I discussed this at my local lap pool and concluded that disstending my
>stomach would probably not reduce my lap times.
>
>Can you explain this issue to me please?
>
>Thanks Tom
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