Hull speed

[mariner2k]

An old man once told me "there are no stupid questions unless you ask them twice". So I'll ask once. If you have a sailboat with a water line of 35 feet, then you remove ballast which lightens the boat but decreases the water line....Using the same boat, Which configuration will be faster, the hull with the heavier,longer water line or the hull with the lighter shorter water line? I'm guessing there is no simple answer.

mariner

[Ray Frechette Jr]

Well, my guess is that with lower ballast the boat would have faster acceleration from a stop, and would be faster in light airs due to less wetted surface.

However, technically if the boat is riding hoigher in the water, and less waterline, then theoretical hull speed would be lower fractionally.

Of course without ballast it would likely be rather tender and not stand up to much of a breeze.

Tenderness without ballast may mean you never get up to hull speed.

[John B]

This is what I've been thinking about recently too. I thought I'd take some lead off this winter to make the boat more reactive ,after sailing on one that is.We have a pretty high ballast ratio as near as I can work out.
Only I've learnt from past experience that trimming the boat aft to put more counter in the water earlier has been faster.........
My current line of thinking is to take lead off AND alter the trim aft so I don't interfere with the speed making aspects of the stern/ waterline length factor.

[Hwyl]

Hmmm, I would not worry too much about water line length as most boats are designed to increase their water line length as the speed increases (given a non planing hull).So if your boat is ballasted correctly, forget the waterline issue. Some rules, for instance the Metre boats seem to encourage this with their long overhangs. Another good example would be the Luders 16 in the most recent WB. Yes their w/l is 16 feet, but once you add the bow wave and the stern wave it's more like 22 feet. The international 14 boys were working on putting a horizontal flap on their rudders to fake the boat into thinking it was longer than 14 feet, but they soon figured out that they could use the flap to enance planing (and reduce pitchpoling) but that's another story.

[brad9798]

Not scientific, but from the couple of 'extensions' (read that cockpit extensions on a motoryacht) I've know, hull speed was increased AND fuel economy was increased with the added displacement and length ...

[mariner2k]

Hmmm...makes you think. However what hwyl states appears to be true. With my waterline lenght my hull speed should be about 7.5 knots, but I have had her up to 9.5 knots and the waterline length during that period was greatly increased, and quite exhilarating.

John B, Pardon my ignorance but What dou you mean by trimming aft?

mariner

[ 04-15-2004, 10:29 PM: Message edited by: mariner2k ]

[John B]

Just the fore and aft trim of the boat.( a counter sterned yacht) When we first bought the boat she looked stern heavy to me so I quietly redistributed any gear a bit forward. Then an exasperating year came when the motor and all its peripherals proved too much for me and we hauled it out and threw it away. The boat looked pretty ,riding more evenly to the waterline ( I mean , one of the waterlines she's had) and the stern a few inches higher than it was with the tank and motor etc aft. She sailed well but not that fast. I clicked that what I'd done by lifting the stern was in effect,to shorten the w/l in any given wind plus also alter the angle that the counter made to the water. IE not only was the W/L physically shorter, but the bonus W/L from the counter was not getting immersed as fast.

[ 04-16-2004, 12:32 AM: Message edited by: John B ]

[John B]

a gratuitous hull wave pic....Iorangi overtakes us.
http://www.imagestation.com/picture/sraid102/p0b9fe5837b2ed438d0c7bbc98280009a/f9cd169f.jpg

we got a tow on that wave for 100 metres or so.

[mariner2k]

Thanks John, pictures like that make me glad spring is here.
kevin

[ 04-16-2004, 06:18 PM: Message edited by: mariner2k ]