The DIY Spinnaker Sprit

Posted by Darrell Nicholson at 12:49PM - Comments: (0)

December 23, 2013

PS testers used a retractable mini-bowsprit for a 2013 test of top-down furlers for cruising spinnakers.

Our recent test of the latest generation of top-down furlers for cruising spinnakers brought up some questions from readers about the type of add-on sprit used for our test boat, an Ericson 41. For some insight into the selection and installation of an add-on sprit for a cruising sailboat, I pulled up excerpts and links from several related Practical Sailor reports for this week’s blog.

Already de rigueur with many performance-oriented sailors, easy-to-handle, lightweight sails are gaining popularity with cruisers. And setting a gennaker, asymmetric spinnaker, or a new rendition of an old-fashioned drifter/reacher is easier than ever before. The big question is: Will the cost of a mini-bowsprit actually be worth the effort and expense involved?

After a series of sea trails with sprits, spinnaker socks, and free-luff furlers, we’ve come to the conclusion that the technology works well, but whether this is a valid investment depends on how important it is to you to keep sailing in light conditions. With fuel prices scraping the stratosphere, efficiency in light wind may be a bigger priority than it was in the past.

In our recent look at top-down furlers, as well as our previous report on endless line, foil-less furlers used to roller furl straight-luff, light-air drifter/reachers and code sails (PS, March 2008 and April 2011), we proved that an efficient “no foil” furler greatly simplified hoisting and dousing a large, light-air sail. We also quickly recognized that setting the sail a couple of feet ahead of the stem, on a pole, sprit, or other projection improved both performance and the sail’s handling characteristics. Our detailed examination of contemporary bow and bow-roller designs showed how some manufacturers are more aggressive than others when it comes to engineering the mini-bowsprits for these light-air sails. We also have seen several do-it-yourself installations that appear not able to withstand the loads some of these spinnakers can impart.

With this in mind, we decided to delve deeper into the issue of sail-tack projection, and evaluate the differing approaches to tacking the light-air sail forward of the bow of the boat.

Much of the credit for mainstreaming this renaissance in bowsprits goes to Rod Johnstone, designer of the J/Boat line and creator of the metric Js (130, 120, 105, etc.) that hit the water with a built-in, extendable, free-standing pole meant to replace traditional spinnaker-handling gear. The new approach simplified foredeck gymnastics and significantly streamlined the chute-handling routine. Even so, cruisers and many racers rightfully balked at the prospect of having a big hole near the bow of their boat and a noticeable proboscis built into the topside. At the same time, shorthanded round-the-world racers were flocking to triple sets of furling headsails with light-air sails the size of circus tents set on an articulating tubular bowsprit.

This approach to sailing efficiency has now launched into the mainstream as Seldén, Forespar, Forte, Sparcraft, and others have designed aftermarket kits suitable for a wide range of sailboats. All these kits have one aim: Move the tack point of any light-air sail ahead of the stem. Each design faces similar challenges, specifically the need to handle side loads on a tubular structure and the ability to adapt to a wide range of deck geometry and pre-existing obstacles.

Bowsprit engineering

In order to better understand the forces associated with modern sprit technology, a bounce or two on a playground seesaw offers some insight. On both the seesaw and the bowsprit, the location of the fulcrum and the load applied at one end of the lever arm determines what happens at the other. It’s important to note both the strength and the direction of the pull when contemplating the force vectors involved. Like any lever, the longer the stress arm becomes, the more load is imposed on the fulcrum—and the stronger both the bowsprit tube and its support members must be.

Traditional bowsprits incorporated a bobstay and even whisker stays to reduce the bending loads at the fulcrum. For our most recent test, we used an aftermarket sprit sold by sparmaker Selden. The Selden sprit is set up with a part-Dyneema and part-stainless-steel bobstay that allows the sprit to be easily retracted. Without this bobstay, there is a need for a very strong tube section and deck reinforcement at the bearing points. A bobstay or strut attachment is best if the sail is to be carried in anything but light-air, off-the-wind conditions. Our tester, Ralph Naranjo, found the releasable bobstay a good option.

Just as rigging turns side loads on a mast into compression loads, a bowsprit’s bobstay and other wires do much the same. However, many modern mini-sprits actually behave more like a free-standing mast, resisting sail loads through stiffness rather than being kept in column by a bobstay. This requires thicker or higher-modulus materials able to withstand the point load induced at the fulcrum, usually a collar-like fitting mounted on the deck.

Our furler tests revealed that close reaching with a Code 0-type sail (a specialized lightweight sail designed for sailing tighter angles than an ordinary asymmetrical spinnaker) or even the use of a more conventional light No. 1 genoa, tripled and even quadrupled the tack load. In Practical Sailor’s view, this sort of use mandates the need for a bobstay. Those planning to use the sprit solely for reaching purposes with an asymmetrical spinnaker or gennaker only need a bobstay if they intend to use the rig in heavy air conditions. Most manufacturers set extension length and/or wind speed limits for their sprits.

On some boats, the advantage of a sprit is offset by the amount of clutter it adds to the foredeck. Working around a windlass, bow roller, cleats, and the anchor well can turn an easy installation into a real challenge. Fortunately, these sprits come with versatile hardware kits and installation guidelines that make sense. There’s usually a need to add topping and backing plates, as well as address the concern about spreading the loads.

The installation is best handled by a pro rigger, but a skilled do-it-yourselfer should be able to handle the job. It is critical that the deck core be sealed at any new penetrations, and any new points of stress in the deck or hull are conservatively reinforced to cope with the loads of the sprit kit. For more details on the various sprits on the market, subscribers can see our most recent test of aftermarket mini-sprits.

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