sailboat bows

I wrote an article on this question for Professional BoatBuilder a decade or more ago, advocating plum bows on "performance cruisers". Some have traditionally argued for raked bows in terms of "reserve bouyancy", but I believe that analysis to be flawed. It depends very much on what you assume is fixed, and what is varied, so it's a difficult issue to frame properly (and I'm not sure my article was altogether successful at explaining it).

The America's Cup question is related. Like the 12 meter (or "universal") rule before it, the IACC class essencially limits waterline length. This is taken a little above where the boat floats (half a meter above if I remember correctly). No hollows are allowed, so you can't just have a dimple there (Though New Zealand tried to get around this in the stern). You can, however, have a flat spot. Early in IACC class development there was "spoon bow" vs. "destroyer bow" competition. That was settled by the success of Laurie Davidson's solution in 2000, which was a compromise -- two knuckles with a flat spot between.

If you left displacement and sail area alone and, using the same deck, dropped a plum bow, it would be both faster AND have more reserve bouyancy (though the boat's center of gravity would be a little aft of optimum if you didn't move weight forward, which would cut the gain in reserve bouyancy down some). But the modified boat would measure too big to be a legal America's Cup boat.

Ted Brewer's objection to a plumb bow on a cruiser (in a letter to the editor taking exception to my article) was that bow overhang makes it easier to recover the anchor without damaging the hull!

 

Stephen Ditmore: Some have traditionally argued for raked bows in terms of "reserve bouyancy", but I believe that analysis to be flawed. It depends very much on what you assume is fixed, and what is varied, so it's a difficult issue to frame properly

That problem seems to be very common. Obviously you cannot add extra buoyancy by cutting away flotation! The raked bow would have to be longer to compare more equally on the basis of overall size. In that case, speed, interaction with waves, and lateral resistance when heeled or yawed become more important differences.

 

True, but there are ways to begin to quantify reserve bouyancy that make sense. I see it as a ratio between longitudinal gyradius and height of the center of effort in the numerator, and in the denominator the longitudinal moments of inertia of the waterplanes forward of the center of gravity about an axis taken at the center of gravity.

What that means in practice is something Bruce Farr observed years ago (and it's helped him design winning boats): boats with low displacement/length ratios need less reserve bouyancy forward than boats that are short relative to their weight.

 

What we have here in many cases is the guy who drives a sport bike comparing his bike to the guy who rides a Harley.

 

I have just completed major bow surgery on my circa 1967 20foot trailer yacht.
Attached is before and after drawings from an older thread
http://www.boatdesign.net/forums/showthread.php?t=7997
Also a link to a Delft paper on bow shapes.
http://www.rina.org.uk/rfiles/HISWA...nce of the bowshape on the performance of.pdf

 

It looks better with a vertical bow. Enjoy

 

A sharp, vertical bow does not give yoy much reserve buoyancy, and will allow more pitching. Off the wind, at high speed, it cuts more, maybe ending on a broach. The waterline is longer, and will provide a lower friction coefficient, but it also adds wet surface. It all depends of the water you will sail in, heavy waves, you are better off with a falling bow.

 

hateka, you have not been listening...
A vertical bow has MORE reserve buoyancy (for its length on deck)
Steve

 

hateka,maybe if you read the paper in my link above.....

 

How about the freeboard of the bow? Would 5' seem outrageously high for a nearly pumb bowed 33 footer?? Would it be too much windage?

 

My reference is the waterlinelength. If you take the decklength as a reference, you are subtracting reserve bouyancy with a raked bow. With the waterline as reference (What I am tought as common practice, i.e. length between perpendiculars), a raked bow adds buoyancy. The decklength is a strange criterium, the hydrostatic values, friction etc, are all based on waterline length.

 

I agree that Hateka's way of looking at this is the conventional way. It's also the way my article in Professional BoatBuilder argued is flawed. Deck weight raises the boat's center of gravity, requiring more ballast be added to offset it. If you were to start with a sail area and work backwards, deck weight would drive displacement more than waterline length would.

The thing one has to wrap ones mind around that what we're talking about is longitudinal stability relative to pitching inertia, which generally translates: volume in the ends relative to displacement.

 

I see your point. Maybe I am too old. Back to the bowsprit. Shorten the mast at the 7/8 point and go for a carbon gaff. Hateka

 

Sorry for making a joke, last night. I have indeed always been thinking starting from a given hull, and not from the sail area or the rigging. This makes me agreeing to the vertical bow and the negative transom, only from the weights in the ends. Not from the lookings. Hateka.

 

Many important things are based on or related negatively to decklength, including dock fees, and the weight and price of the boat. A heavier boat can be harder to trailer (small) or singlehand (large), and hydrodynamic advantages won't be enough if the person can't afford the boat or store it in their yard.