sailboat bows

Now you bring into play the height of freeboard (or whatever the correct name is for the distance from LWL to deck) having a longer freeboard with a plumb bow may not plunge the bow under the waves as by the time the freeboard is submerged the wave is sufficiently far enough beneath the hull for it to displace the water and not cut through it. I think it would enable the boat to track better in this instance.
Although having a short freeboard without a plumb bow may look as if it will ride over the waves better - but that may be only for a given wave length in relation to the freeboard. Anything more might be able to swamp the bow.

Personally I find it easier to visualize this design feature on a canoe as the movements and entries are so responsive it is easy to achieve an idea of how the design affects the boat.

 

Most od my designs have a straight stem bow for the obvious reasons of speed but it does detract from cruising boats for silly reasons like banging your anchor on the hull when hauling it back aboard. Generally, the straight bow slams a bit more in a sea way and the long over hang bow can be a bit softer motion in a seaway because the entry is softer and the reserve bouyancy is more gracefull.
Saying this, I am sticking with the straight bow as with most of my designs
www.sayerdesign.com

 

"Thanks for the replys,I understand the extra waterline for more speed but what about the effects on a cruising design.Why do you not see ships with that design?"

Ships are designed for a particular cruising speed ., not top speed.

Large stuff , oil tankers ect MUST travel with a midship wave , they would snap in two at "hull speed" .

Its cheaper to just build bigger and go at a moderate speed than accept the weight of a stronger hull and the attendant fuel bill of fast travel.

Different for the Navys of the world , they have no fuel bills to worry about.

FAST FRED

 

One advantage of the long overhang bow is when mooring the boat in the natural harbours of archipelagos.

 

overhangs give more reserve bouyancy and a softer ride at sea for a given WL length but there are many factors to consider here , suffice to say it is a complex issue and depends on the rest of the hullform.

 

But the marina charges by LOA, and the plumb bow gives more reserve buoyancy per foot of LOA. Your call
Steve "just another sample of the complexity of the whole argument"

 

Bows

Another consideration in design the bow of the craft is in the shape of the forward sections. Are they exhibiting "flare" or "flam"? See the attached JPEG. Flam sections have an initial rapid increase in reserve bouyancy with a reduction in the rate of increase with increased submersion. i.e. They respond to wave entry sooner and the the response remains fairly constant with increased submersion. A flared has a slow increase in reserve bouyancy so it takes longer to respond to wave penetration. Also, there is a rapid increase in reserve bouyancy as submersion approaches the sheer which accentuates the upwards accelations of the bow as it responds to the wave. Flammed sections start their response sooner and at a more stable rate than flared. Flared forward sections will be less comfortable in heavy conditions when compared to flamed sections. As to whether a flammed section is less likely to submerge than a flared section, I'll leave that response to the experts.

I'm going to go out on a limb here. Flared are generally associated with larger deck areas forward. I which case, flared forward sections may have more difficulty in responding to submersion than flammed sections. For the same deck area, flammed forward sections will carry more bouyance (and more interior volume) than flared sections.

I hope this wasn't too much of a tangent for you, but the boat as a whole has to be considered. I like plumb stems and many a classic design carries them. Overhanging stems can be overated. Herreshoff took them to the extreme. They are beautiful to look at, but they are a rules beating design and I don't mean to imply anything bad by saying it. It was quite the design coup. As the craft heeled, his craft would submerge a bit and actually increase the effective waterline.

 

There's been a bit of mention of the desirability of keeping the deck above water. I won't argue that every sailor wants the deck to stay dry. But in the ocean, there will be waves that will come over the bow. When this happens you want that water to get off your deck quickly and have your bow rise back up quickly. A sharply swept bow with a lot of flare is not exactly conducive to this. Personally I prefer a narrow bow, either swept or plumb, with a cambered deck and a deckhose shaped to allow the boat to "submarine" without harm when things go bad. If the bow stuffs into a wave and can't easily get back out you are in serious trouble. So my vote goes to fine bows with a lot of reserve buoyancy.....

 

Marshmat is on the right track, just leave the transom open so the wet stuff can get off the boat.

Yoke.

 

keeping the boat above water is always a good idea...

 

Flare in the bow sections permits the stem to drop quickly in a wave and then suddenly gain a lot of reserve. In rough weather this can cause damage, so excessive flare should be avoided. Flam on the other hand brings on reserve quicker as the bow plunges and increases the amount of buoyancy gently, with much less risk of "canning" or stoving in the forward section in a rough sloshing. Flare can have a dramatic effect on usable deck space in the eyes of the boat and can be easily taken to extremes, with the boat having to tolerate server slamming loads in some conditions. Flare is a fair weather shape, in my opinion, good at slapping down chop and spray, but easily over matched in harsher seas. Neither of these (flam and flare) has much to do with the bow profile.

It is particularly true that the underbody profile of the hull will have considerable influence on the sectional shapes in the ends of the yacht. Racing rules force decisions in some of the shapes used, but aside from this, client needs in concert with designer's ideas, generally generate the wide array of bow profiles used currently, as it has since the first boat was designed for a customer.

The clipper bow was once the most scientific available and many words bestowing it's virtues were written. In fact, each profile will have it's fans and opponents, both equally convinced their ideas and rational are correct. Bow profiles, just like the vast number of other compromises made in the design process, try to meet the requirements of it's intent. An ice breaking research vessel's bow will be decidedly different then what may be asked of a ocean cruising passage maker's. These two bows, may have boats of similar shapes, displacements and other important comparisons, but the passage maker is relieved of the ice breaking burdens, which may have limited the research vessel's bow shape.

So in answer to your questions Dman, there is no truly supreme bow profile that will be the best thing to have at sea. Shedding water off a recently dunked deck is a function of many things, bow shape being one, entry, hull form, freeboard, deck crown and rail height are a few of the other considerations also.

For what it's worth, a 400' LWL vessel steaming along at 27 knots is doing hull speed, and it is regularly done. I personally clocked the USS Nimitz doing over 50.

 

i dont mean to dispute you, but from what i have read and seen the top speed for a Nimitz class carrier is around 36 knots. Where did you see this?

 

Setting aside generalities about different solutions for different problems, this is a good discussion and an interesting topic. My personal interest is mostly dinghies and cruisers, but it's also nice to have a feel for what the variables are over a wider range of applications.

One relationship between bow width and vertical fullness is how the ratio between them affects helm balance when heeled. Back in the 1960s, Francis Chichester's Gipsy Moth IV had a fine, plumb bow with a deep, square forefoot. Any time the boat got pushed over more than 30 degrees, the bow lost lateral surface area and developed a wicked lee helm at the worst time possible. Newer designs seem to keep the two dimensions more evenly matched.

 

A note on the big-ship side of things... with nuclear power there can be quite a big difference between "full speed" and "flank speed", the latter being absolute maximum possible engine output, no holds barred. As for the tankers/freighters- there's a big difference between a 115,000 tonne aircraft carrier and a 475,000 tonne supertanker. Such tankers, if taken to hull speed, would actually crumple in half in the middle for lack of support due to the bow wake. So they travel slow with a midship wave or two. Carriers are strong enough to reach hull speed, but never do because none of the surrounding battle group can keep up.

 

The actual section shape further aft, combined with the waterlines has a significant affect on the bow profile. If you are planning to draw the waterlines parallell you tend to end up with a raked spoon bow. With a vertical bow the waterlines tend to finer at the bottom than the top. If you have flared sections you tend to end up with a clipper bow and so on...