sailboat bows

Sharp straight bows have long been favoured by the Scots for fishing the North Sea and beyond (rough, rough waters). In 1879 the Zulu appeared off Fraserburgh (North East coast above Aberdeen). She had the sharp straight bow of a Fifie and the 45 degree raked stern of a Scarfie, which the designers considered the best of both worlds for North Sea, Atlantic conditions. Up to 80 feet long, double masted, double ended (are you listening Fast fred) they were among the fastest, safest sailing work boats of their day. Fifty years ago I was lucky enough to crew one for a season around Iceland. Sure I was cold - but fully confident of her capabilities (after all she was a century old then).

 

All this talk of "reserve bouyancy" for keeping your bow about a wave is rather BS. If you wanted to keep the bow up, you'd have AS MUCH BOUYANCY AS POSSIBLE, as low as possible, to keep the bow from diving in the first place.

So where do we end up with swept bows? Racing boats, of course. When the rules measured the LWL, those long overhangs could beat the rule by only coming into effect when the boat was heeled. Also, in light air where nothing will reach its hull-speed, less wetted-surface rules the day.

Where does the anchor rode get stored? In the bow, of course. Where is the worst place to concentrate the weight of all that chain? In the bow, of course. Now, take all that weight, keep it placed up high, and give it two feet of leverage. That is what a swept bow does for you.

With a plumb bow the weight of that chain can be placed much lower, and there is bouyancy directly underneath it to support it. If you want to keep your anchor from banging the topsides of the boat, you'll have to use a short sprit with a roller to launch and retrieve. Your boat will also have a higher hull speed, you can always reef if this bothers you.

Taking this to the "extreme" are the new racing catamarans. Their (ever-so-slightly reverse-swept) bows are wider at the bottom than the top! The bouyancy comes on even quicker this way. The shape allows the bow to punch through waves at high speeds very well, and rise up quickly when the bow is submerged. Sure, they are a wet ride, but so are the more conventionally shaped catamarans when going over 20 knots.

 

En boca cerada, no entran moscas.

 

Things really are not that simple Seafarer , the red comments are mine.

Overall their seems to be some confusion on the reserve bouyancy issue. The longitudinal restoring moment increases more gently and with far more reserve volume in the raked stem than the plumb bow for a given WL length. Lightweight boats benefit form a plumb stem as it keeps the VCG lower with less deck and topsides fwd. Marina requirements also favour a shorter overall boat so for a given LWL the plumb stem wins.

Also consider the freeboard and total required reserve bouyancy, a low windage hull will need to make up the reserve bouyance with overhanges, overhangs also add to the usable interior voume. Overhangs cost very little in terms of materials in the construction , they give a much more usable deck area fwd for a fine bowed craft, and offer a much more sensible collision zone for many floating hazards.

Also consideration needs to be given to construction material and method.

 

So, Learpilot - ningunnas moscas en usted....

 

No se. Como se dice? Yo no hablo espanol.

 

Then we're both faking it my friend Learpilot

 

That's what I've always said, but I have usually said it during an argument over weight in the ends (more weight in the ends = less pitching) and no-one believes me

Steve

 

LWL ... no penso es LOA ... no ... oh yeah, it's .......


LOL

 

Weight in the ends doesn't reduce pitching. It does resist pitching at the start more than light ends but continues to pitch more after it starts. In smooth water weight in the ends is not bad in a seaway that weight will make the boat want to hobby.

 

Skinny boy,
Weight inthe ends increases pitch inertia, making the boat slower to start the pitch. In reality, it means that the boat is already out of the wave (in most cases) at a lower pitch angle than it would have been with light ends. At this point, the pitch acceleration flips to the other direction, thereby reducing pitch. The added advantage is that the rig can continue to work efficiently in air that is not greatly perturbed. The downside , of course, is that the hull will have added resistance as it goes through the wave rather than bobbing gayly over it
Steve "compromises, baby, compromises......"

 

Moments of interia

It's interesting to see how this thread has morphed. It's all been good stuff, more the most part.

Talking about weight in the ends is delving into the subject on moments of inertia. A greater influence on pitching, than anchors in the bow, is the distribution of ballast.

A vessel with ballast distributed along the lenght of the keel will have a greater MOI (Moment Of Inertia) than a vessel with the ballast concentrated at midships/CG (and/or heavily ballasted). In real world terms, this means that the high MOI vessel will tend to plow though waves more than the other. In the extreme, the bow will bury itself before the righting moment can take effect to raise the bow to the oncoming wave. And yes, once the pitching starts, it takes more energy to stop it. Generally, the hind quarters offer more bearing than the bow and contribute (a great deal) to stopping the pitching once it has started. At any rate, a craft like this may actually be too stable along it's longitudinal axis. The worst case scenario, I think, would be to get into a wave train that was harmonic the the vessel's pitching. A situation that is easily alleviated though.

On the other hand, a vessel with a low MOI, ballast concentrated at it's CG (and/or lightly ballasted), but not as with a deep fin keel, will pitch easily. The bow will respond easily and quickly to wave action, possibly creating vessel that is irratic and uncomfortable to sail on. Not to mention the additional stresses put on rigging and equipment.

Ballast on a fin keel also serves to increase the longitudinal MOI though it is technically located about the CG. (The lawyer clause.)

Once again, it's all about trade-offs and compromises.

 

SailDesign,

Funny, you posted while I was writing. Hmmmmmmmmmm. :idea:

 

Hable más despacio, por favor. No hablo espanol. Habla usted inglés ?

 

If you are unlucky (and you sometimes will be!) the boat with distributed weight (= slow period of pitch) will have a natural frequency that happens to be the same as the waves, so the pitch amplitude will grow and grow and grow....
On the other hand, traditionalists say you should have a heavy mast and the ballast in the bilges (as far out as possible) to make the boat roll slowly...