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Go Back   SailNet Community > General Interest Forums > General Discussion (sailing related)
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  #11  
Old 01-18-2010
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Lightbulb

I would second most of Jeff's excellent post.
Personally, If I had to pick one feature in boat design that can enhance speed and ease of trim to weather, I would suggest a boat with more symmetrical waterlines, fore 'n' aft.

I have always found that the one saving grace of the IOR hulls (the best designed ones, anyway) was driving them to weather. Next best and very close would be boats with moderate beam and a clean run aft in the hull form like all the descendants of the Cal 40, the Cascades, the Niagara's, and others.
Worst are the fat-and-flat stern boats that bury the bow in a puff and then rise up on one of their fat stern quarters and try to round up.
That design allows room for king sized beds aft, though, and are quite popular for "gracious dockside living."

Happy boat shopping, and fear not -- there's a perfect boat out there for you, somewhere.

LB
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Old 01-19-2010
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To the OP, you might want to pick up a copy of Bob Perry's recent book... say what you want, but it's a pretty darn good current overview of sailboat design.
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  #13  
Old 01-19-2010
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Narrow fins with bulbs tend to generate more leeway than wider fin sections; although they carry more ballast deeper which helps them stand up and carry more sail area. The trade off is efficient in moderate winds when the boat can use the ballast weight as righting moment; but in heavy air this type of keel does not track as well as a wider fin section. (canting keels are outside the scope of this discussion)

A smaller headsail that can be sheeted closer to the centerline of the boat; sheets run inside the shrouds is better for pointing ability than a large headsail; but at the cost of reduced area. It would depend on the boat and the windspeeds that you would expect to normally sail or race in to determine if a close sheeted smaller headsail is better than a larger headsail and lower pointing angle.

If you are looking for a boat that points high and sails fast to windward you might research the PHRF ratings for upwind performance; there might be some variation in the numbers from the 'base' rating. A lower PHRF equates to a faster hull.

Nowadays most racing hulls are optimized for downwind sailing (Spinnaker) with a hull form that will plane while sailing downwind (ULDB); while still providing adequate upwind performance (with plenty of railmeat). That's not to say the VMG upwind is poor; it just is not optimal.

When a boat sails upwind at it's hull speed the apparent wind angle does move forward. It's how well the sails can produce lift (and how easily the hull is driven) at that shallower wind angle that determines how well the boat points. Heavier wider beam boats with full keels will not point as well as a lighter, narrow beam hull with a fin keel and close sheeted low stretch sails. But; the heavier built boat may be more comfortable in rough conditions and more able to withstand heavy conditions without any failure of the hull.

Last edited by KeelHaulin; 01-19-2010 at 04:23 AM.
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Old 01-19-2010
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I want to touch on a couple pounts raised in the posts above. Olson34's post is somewhat outdated regarding boats that are balanced fore and aft and his comment about IOR boats going to weather.

In the late 1970's and into the 1980's designers began moving towards boats with finer entries and more powerful sterns. At first these boats tended to develop a tendancy to jack the stern out of the water and force the bow down causing the boat to develop increased weather helm, which forced the helmsman to use more helm which in turn meant greater drag and so poorer windward performance in a breeze. But by the late 1980's and into the 1990's designers learned to model the hull shape so that there was not a large shift in fore and aft trim with heel. They also determined that a fine bow helped with tracking and leeway, and that more powerful stern sections permitted greater stability without the inherent motion comfort and limit of positive stability liabilities of an over dependance on form stability.

When it came to rising up on one of their fat stern quarters and try to round up, there was nothing worse than the later IOR era boats. But also while it is true that the IOR boats generally make their best VMG pointing slightly closer to the wind than more modern designs, they tend to make a lot more leeway and do not have the upwind speed, and so the are not especially good up wind compared to a more modern design. And as a broad generality IOR boats do not handle a chop as well as modern designs.

Next worse and not all that close would be boats with moderate beam and a clean run aft in the hull form like all the descendants of the Cal 40, the Cascades, the Niagara's, and others. These boats have a lot of drag generated by their large amounts of wetted surface and their inefficient keels and rudders.

It is also a mistake to say, narrow fins with bulbs tend to generate more leeway than wider fin sections. At very slow speeds in moderate winds, as would occur immediately after a tack, a deep, narrow fin will make slightly more leeway than a larger surface are, lower aspect ratio fin, but the deep aspect ratio fin has way less drag and so accellerates more quickly, and almost immediately generates enormous lift as compared to older style keels. That said, these more efficient keels take a little more skill to sail well, since they do not tolerate pinching as much as older keel types. But while it takes a little more skill to sail with these modern high aspect ratio keels, they make way less leeway than earlier lower aspect ratio keels.

The other thing about a bulb is that it acts as an end plate, tricking the keel into acting like a higher aspect ratio than it is, delaying stalling and reducing drag due to the tip vortex albeit a partial trade off since a bulb adds small amounts of drag due to more wetted surface and frontal area.

I also disagee that most racing hulls are optimized for downwind sailing. These days there are still designs optimized for particular venues, but the majority of modern race boats are optimized for all around performance, meaning the have their performance optimized for all points of sail, but especially upwind and deep reaching. The fine bow, eliptical hull sections and moderately full stern sections that were popularized on IMS typeforms and carried over into the newer IRC boats, go upwind with a more comfortable motion, minimizing disturbance of the flows over thier sails, keel and rudder, pointing higher, making less leeway, and exceeding hull speed and with boatspeeds that sometimes exceed true wind speeds producing tremendous VMG's with smaller crews than earlier designs. And while ULDB's (ultra light displacement boats- typically L/D under 115) are still popular for Pacific downwind racing, but for the rest of the world, ULDB's are an outdated design concept that neither makes sense for cruising or racing since they are one trick ponies and modern racing requires boats that go upwind and can reach well.

Both the Compact 16 and the Westerly 25 are classic cases of boats with a lot of wetted surface and inefficient sails and underwater foils. That said, I have always liked the Compact 16 for messing about in shoal draft waters.

Respectfully,
Jeff
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