What's a bowsprit for?

[JeffM]

In the interest of furthering the informal education I'm getting into boat design, I'd like to have answered a question that's been bugging me ever since I laid eyes on Stevenson's Weekender: what is the purpose of a bowsprit? If I wanted to design a shoal draft trailerable displacement hull, I'd make that hull as long as possible both to increase hull speed and to maximize useable interior space. From the little I've learned, the bowsprit seems the worst of two worlds: it increases the length a trailerable rig would need to accomodate, without increasing the waterline length. I'm guessing there are other factors I'm missing, but I can't figure out what they are. Anybody who knows want to straighten me out?

[Frank Hagan]

Mike Stevenson has said that one of the design criteria for the Weekender was to put it under 16' on the waterline ... in some states, that prevents you from having to register the boat.

The Pocket Cruiser comes closer to what you are thinking about. In the Weekender, the bowsprit allows you to increase the head sail size without increasing the height of the mast by moving the forestay farther out.

I think the real reason for a bowsprit on the Weekender is that it is a design influenced by the Skipjack, all of which had bowsprits to increase their headsail size.

[Dave R1]

I think, in addition to Frank's point about allowing a larger head sail there are a couple of other things. If you consider that in the past, the primary purpose for a boat was to haul cargo or a load of fish, oysters, lobsters, etc. The bowsprit allowss for an increased sail area to hull size/displacement allowed for better performance as far as carrying full loads are concerned.

The bowsprit also allows for better geometry to support taller masts.

The hull could have been lengthened as you say but boats were taxed on their overall length. Bowsprits were commonly set up as reeving or running meaning they could be retracted and stoweed in port. This reduced the quayside space they took up and thus the taxes they had to pay while still letting them have the larger sail areas that improved their performance.

The old revenue cutters (from which the term "cutter rig" is derived) set as much sail as possible to make them faster than their prey. At least that was the idea. Of course the prey increased their sail areas as well.

[JeffM]

Thanks, Dave. Interesting history lesson! Is it still relevant in modern boats? I don't know how difficult it is to unship the bowsprit of a weekender, for example. Of course, even in place, I suppose it might reduce the trailer length, since the bowsprit might ride well above the bow chock.

[Dave R1]

Jeff, I considered making my bowsprit so it could be unshipped. I thought this was going to be important since I had added some length to it. As it turns out, the bowsprit doesn't quite reach the end of the trailer tongue and if it did, it would go up over the tow vehicle, a Saturn VUE.

As to its relavance in modern boats, I don't know. I suppose that no matter how long you make the waterline, you might always want to carry more sail area and a bowsprit would help. On the other hand, look at modern boats. Few have a real bowsprit anymore.

Who knows?

[JeffM]

Incidentally, I'm trying to understand the elements of hull speed. It has ocurred to me that it can't be as simple as being proportional to the waterline length; otherwise I could lengthen my waterline by the simple expedient of putting little extensions on a hull. I expect that length must be bouyant enough to help the hull span the wavelength. In that case, would a punt-bowed boat have a greater effective waterline length than a pointy-ended boat of the same waterline? What about a boat like some of the Bolger designs (Micro, for example) that have free-flooding wells at bow and stern? Is their hull speed limited by the waterline length of the buoyant part of the hull?

[PAR]

Bow sprits have been used for a number of reasons, though in the hay day of sailpower they were more a function of hull design or vessel use limitations requiring a Center of Effort well forward. Until the middle of the 19th century, pleasure craft were quite rare and vessels had to work for a living. These well burdened craft, typically had very full midsections and full length deadwood assemblies. Considerable drag was built into the keel for good handling qualities, the beam was drawn out into the ends of the craft and the hull forms suffered from the "Cod's head" mentality of the era. These and many other factors caused a need in large sail plans which were placed forward in the boat, for deck room and to balance the mighty hunk of aft sloping keel. Making the rig "knock about" meant it wouldn't go when fully loaded or in light air and placed the mast farther aft, which may have cut into usable deck space.

In Europe the shippable sprit was and still is the thing, but the concept didn't really catch on over here. In the USA, a ship's rig was installed, tarred and left, very few examples of retracting sprits can be found in the working craft or Navy of this country in the 1800's. In the very confined spaces on the canals of Europe, cocking up the jigger or boom and retracting the sprit meant more slip space could be found.

As the ends of boats became finer, the forefoot carved back, the displacement and CLP more centrally located, the hulls became more efficient. The sail plans came inboard to match the changes in the hull forms and the more easily propelled craft shapes.

High aspect rigs didn't become possible until technology caught up with the industry. Wire rope and new design ideas caused rig heights to soar. In order to get the same power from hemp rigging, the sail plan needed to be a much lower aspect ratio and was spread out across the length of the yacht.

Many modern boats carry sprits, Most use them for down wind work or reaching, but others take advantage of the easy to stow an anchor opportunity and nice fore triangle size with the mast in the sloop location, which rids the cabin of the stick or compression post.

With the exception of a small handful of boats, all the American revenue cutters were schooner rigged. The term "cutter" was a common one, taken from the English, because of the association in the public's mind, of this rig, vessel type, used in the collection of revenue. These vessels were typically designed and built like fast privateers, in the Baltimore "sharp model" fashion.

Hull speed is a set of concepts that can take some time to fully understand. Basically it boils down to a few "modes" a boat may find itself operating in. Some, that because of their shape or available power, can not escape the wave train they create (displacement craft), others that can partly over come the wave train (semi-displacement or semi-plane boats) and craft that can overcome their own wave train (planning vessels)

Displacement vessels lack the power to climb over the bow wave that piles up before the craft as she slices through the water. These boats have a maximum speed limit they can achieve, based on waterline length. A longer waterline length will result in a speed increase potential. Planning and semi displacement craft use a different set of principles to govern some of the speed potential and limits available to them. Basically these boats have a dynamic relationship with the water's surface and the bottom of the boat, which allows them to be lifted bodily from the water, which decreases resistance.

All boats can be operated in all three modes (displacement, semi and planning) An outboard runabout with a 150 HP puttering away from a dock is in displacement mode. A little more power, the stern squats and the boat starts to climb up the bow wave (semi displacement mode), a little more power and the bow wave is overcome and the boat settles into a new trim, piling up the old bow wave under the aft sections of the hull, this is planning mode. Vessels designed for one primary mode of operation will perform poorly in the other modes. A displacement craft could be towed at planning speeds, but will become very unstable and may capsize. A planning boat makes a very inefficient displacement vessel.

When a sailboat reaches a speed-length ratio of about 1.25 the increase in resistance rises such that it may be virtually imposable for her to exceed this speed. Around 1.5 speed-length ratio the trough of the bow wave coincides with the stern wave. At this point the hull settles down into a hole or bed of sorts, the water is tossed to each side and reunites aft of the transom. The hull then can be considered to have a fictitious length, matching that of the "bed" she makes. The boat's bottom and the water surface meet at an angle of incidence which generates lift. The craft is now skimming over the surface and in planning mode.

The formula is 1.35 times the square root of the waterline length for most moderan hull forms, though many can easily exceed this, it is typical for figuring the displacement speed.

[JeffM]

Thanks, Mr. Riccelli! You put a great deal of time into your reply, and I appreciate it! I'm going to take a little while to digest this. The cod's head I gather is a bluff bow. I'm not sure I understand how, at a speed-length ratio of 1.5, the trough of the bow wave can coincide with the stern wave -- how can one motion make two waves that cancel each other out? (The destructive interference of my high school science days involve waves from two different sources, or waves reflecting from a barrier and interfering with oncoming waves.)

I wonder about the trade-offs in hull type: I appreciate that my cs20 hull can, at the right points of sail, wind speed and load, achieve speeds only possible in a displacement hull of much greater length. What do I give up for this advantage? My boat will have more of a "snap roll," handle rough water less calmly and probably have a greater wetted surface than a displacement hull of similar size. Should I have any other concerns?

Thanks again for the lesson!

[brian watkins]

I'd like to add one more bowsprit advantage to "PAR"'s excellent essay. If you look at a plan view (overhead) of the same vessel with and without the bowsprit, you'll notice that the angle of the forestay to the shrouds is different. A foresail can be sheeted in tighter. This and the change in Center of Effort, seem to be two common reasons why existing rigs are sometimes modified by adding a bowsprit. The big modern downside is that many marinas charge by overall length over spars! Of course, a cat ketch dodges the whole issue altogether, hence the popularity of the B and B two-stickers.