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  #1  
Old 02-05-2004
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how much beam?

I belive (but could bewrong)

These days an "easily driven" boat is basicly a plaining boat with a narrow entry and big aft end.

Apparenty a few decades ago an easily driven hull was built to be as narrow as possible, but still deep, which had to be a bit more convient for giving some headroom below, but also had to heal over a bit.

So, why the shift? What was the logic behind each, and what are the biggest pros and cons of each aproach?

Thanks

-- James
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Old 02-05-2004
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how much beam?

You certainly have a way of asking questions that are not particularly simple to answer. The answer starts with the hack phase that all well designed boats are designed as a system with each part making sense one to the other and for the environment in which they exist. If we look at the design of boats in the early 20th century they were limited by the technologies of the day. The were built of wood, with metal fastening, and galvanized iron standing rigging, and natural fiber running rigging, and cotton sails. Built lightly, boats built this way are very dynamic, hulls flex, and rigging and sails stretch far more than we can even begin to imagine. When you think about sails and rigging that stretch as much as they did back then efficiency to windward was not in the cards so the best you can do is optomize for reaching. Yachts in the early 20th century generally had huge sail plans carried low in the boat which is fine for reaching. These large sail plans really placed a lot of stress on a hull and rig, so everything was builtg proportionately heavier.

In the wake of first World War a lot happened. Fabric weaving became better, sail shaping improved with the lessons on aerodynamics that came out of the aircraft industry and there was an improved understanding of structural designs. What that meant was that sails could point closer to the wind, rigs could get more efficient and that put pressure on boat designers to come up with boats could be built to take advantage of that improved sail efficiency.

That meant less drag and more efficient foils. This lead to a quest for boats designed with lighter hulls structures and rigs, less wetted surface and more efficient keel shapes. Boats became narrower, rigs became taller, ballast went outside the hull at the bottom of the keel, forefoots were cut away and rudders moved forward. What resulted looked like Dorade or Stormy Weather, which were fine, easily driven hulls with efficient Bermuda rigs.

And while there was some fluctuations in the design patterns that resulted from designing to cheat some racing rule, in a general sense boats did not advance much until the late 1950''s and early 1960''s when there was a whole rash of new materials that hit the yachting world. Aluminum spars, s.s. standing rigging and Dacron sheets and sails, were revolutionary because they allowed a big leap in sail efficiency and once again it was the keel and hull shape that limited the performance of a boat.

And once again there was a lot of attention directed at keel shape and wetted surface, and as keels got smaller in area, the norm of the mid-1960''s being boats with fin keels with attached rudders, boats became much harder to steer. It was at that point that fin keels and spade rudders went from being a design concept that had been banded from racing some 70 or so years earlier to beginning to become the norm.

At that point it became a simple matter to keep going through a rapid reiterative process. Better sail cloths needed better sail shapes needed more efficient keel shapes. At some point the hull became the limiting factor but with the early IMS designs of the 1980''s, hulls became semi-displacement hull forms with tremendous stability. These easily driven semi-displacement hulls are still comparatively narrow and tend to have fine entries. Their centers of buoyancy moves aft to provide bearing as the boat speeds have gone up.

That''s the short form rambling explanation on a night when I am too tired to write this any more clearly.

Jeff
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Old 02-06-2004
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how much beam?

one more related question. The race boats that are the exaduration of the type, with the flat bottoms and huge keels (in depth, not length) do they even have bildges? Where does the water go?

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Old 02-06-2004
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how much beam?

What water? Boats today should be accentually dry. With the new technology of construction modern boats should only have one source of seawater and that is the stuffing box at the prop. And even if it is maintained properly there should only be enough water to pick up with a sponge, if that. The only water that I get on an old IOR design is what runs down the keel stepped mast. My bilge is only 3" deep and that is just to accommodate a flat cabin deck. Even so, I still have a large volume manual pump plus an auto activated pump, as well as other options of fast overboard discharge. Safety first!

Del
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Old 02-06-2004
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how much beam?

Actually many of the newer IMS types do have more of a bilge than you might expect. While it is not big enough to stick tanks in, it often results in a reasonable sump. While my boat is not a current design, I have roughly the same bilge capacity as the Pearson Vanguard, which is an obviously older design of roughly the same displacment as my boat.(I only cite the Vanguard because we had one in the family when I was gowing up and I am quite familiar with the bilges)

Jeff
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