I noticed at the last messabout that my boat sailed much better on the port tack than starboard tack. I have been aware of this before, however in the strong winds we encountered Saturday the difference seemed more pronounced. Does anyone else notice any differences with their boat similar to this? What could be the difference? My boat has a cat ketch rig with spit booms. The mainsail has the sprit on the port side and the mizzen has the sprie on the starboard side.
I had a similar problem. My PC wanted to do a 180* on the Starboard tack in strong winds but no problem on the port tack :? . Keath said it was weather helm, I may have to learn to live with it.
Sailing with a lug rig does the same because the sail is on one side of the mast. Don't know if this applies to your rig. Under water stuff all good and equal, I can't see it being anything other then the rig but willing to learn if it isn't.
The only thing I can think is that it has to do with the sprit booms being on opposite sides of the sails. It gives each sail different shapes on different tacks. Much like the lug rig you mention. It is not something that can't be overcome. I guess it is just a characteristic of the boat.
Ed,
Sorry I missed you at the messabout. I didn't get there till Saturday. Seems I missed the ribs too.
A sail can be well off center and not have a dramatic affect on the turning moment. Catamarans are an obvious example, when "flying" their windward hull the sail plan is well to windward of the leeward hull, but it makes no marked difference. Some PD Racers also have their mast in the front corner of the boat, as well as do a few of Phil Bolger's designs, none display any bad habits on either tack.
Weather helm wouldn't cause this to occur on just one tack. It would occur on both, regardless of sail plan type (lug, gaff, sprit, etc.).
Typically these types of problems are appendage related, usually something isn't lined up or is out of plumb. An example would be a fin that is canted to one side a little or has a slight twist in it. On one tack the cant or twist would serve to advantage, maybe providing more lift, but on the other, it would be doing just the opposite, decreasing lift, increasing drag or maybe lifting to leeward, which decreases righting leverage. The same would be true of a rudder that isn't plumb, having less steering affect on one tack then the other.
A quick way to find out would be to level you boat athwartship. On a flat bottom boat, just use a bubble level on the bottom athwartship, maybe jacking one side until it's level or letting air out of one tire. When it's level, then check the keel for plumb and the rudder too (dog the rudder down with the tiller arm dead amidship).
You can check twist by placing a straight edge horizontally along the keel.
I've raced sprit and lug rigged boats and found no noticeable difference between tacks. From a technical stand point there should be a difference, but only a fraction of a knot, which you can lose or gain with the slap of a single wave and I think this is the reason why it's not an issue and also why wishbone (or bowed) booms didn't have a big advantage in these races.
The center of gravity can have a huge impact on boat handling and is a big consideration in powerboats. I've seen some, small powerboats that had CG issues, that caused them to violently swap ends (yep, do a sudden and neck snapping 180 turn) after reaching a specific speed. This is a function of the CG being too far forward in the hull and at certain speeds (determined by boat length, type, etc.) it drifts aft of the CB (Center of Buoyancy) as a result of it's forward motion on plane, where it acts like a fulcrum and the prop forces the boat around it's new pivot point. I've been aboard when this happens and it's not a fun ride, it's damn scary in fact.
Sailboats are just as subject to this problem, though usually don't develop enough power to cause the 180, unless the CG if way off for some reason.
How does one determine where the CG on a boat is? Is it indicated someplace in the plans or is this something that is common knowledge and I am being uncommonly dense?
On every set of plans that I do, I include the CG and on every set of plans I've seen others draw, the CG is indicated too. The Stevenson's plans are unlike anything I've ever seen and don't include any of the typical information about the boats specific static, dynamic or volumetric properties. This can prove a very difficult problem, if major changes or deviations to the plans are incorporated into the build. They encourage change and innovation with these boats, but don't provide the information to get it done properly, nor offer the wisdom of their engineering skills. They don't even participate in a site devoted to their efforts, which I find unique and a bit strange.
I wonder how succefull Stwart Benbow's schooner would have been, had he the design's real CE, CG and CLP locations to work with on the plans.
There are two reasonably easy ways to find the CG without going through the usual and tedious "center of masses" calculations, which is the only true way to determine the CG, unless the lines and structure, including equipment are in a CAD file.
The first method is to balance the boat on a piece of pipe. This means she'll have to be off the trailer or at least raised up, off her keel and then have a roller placed under it. The pipe is moved back and forth until she balances. This is the longitudinal CG. Assuming the boat isn't heavier on one side, then the other, this can serve as the actual CG.
The other method is to launch the boat in a calm waters area, then place a substantial weight on her centerline (say 50 pounds or more). This will cause her to rotate around the CB, which fortunately aligns with the CG when she's floating (technically it's the other way around, the CG aligns with the CB which remains constant). If the weight is forward of the CG the bow will sink a wee bit. If it's aft of the CG the stern sinks. If it's on the CG, then both the bow and stern sink an equal amount and this would be the location of the CG.
Of course, the static test on a roller is more accurate.
If a Weekender is built as designed, then her CG should be 94" back from the bottom to topside planking joint at the stem (there abouts). In this position she'll have about 2" of bury at the transom and the bottom planks will be just kissing the water. This is about the trim most Weekenders start with when first launched and represents around 920 pounds of displacement. If the boat is trimmed down at the bow with batteries or weights to solve some handling issues (a common choice among Weekender owners) then she's about 2.5" buried at the transom and 1" at the bow bottom planks. This represents around 1,100 pounds of displacement.
Before going through all the trouble of trying to find the CG, I'd check the keel to see if it's truly square to the bottom planks, check the rudder for the same thing, check for warping in the keel or rudder and carefully examine the boat afloat to see if she's "listing" to one side. Also check to see that the stem and forefoot are on the same plane as the rest of the keel, ditto the rudder and keel.
Thanks Paul. I think I understand more about my boat now. More than likely, the keel or the centerboard is going to be the culprit.
Greetings Paul,
The following is all supposition, and wild ass guessing, but more than likely true â¦
I get the sense that the Stevensons were surfer dudes first, and boat designers second. Surfboard design is very much a exercise in trial and error ⦠make the board a little wider here, and little longer there, try it out and see what happens. The best boards in the world are made in peopleâs basements. I got fifty bucks that says the first weekender was made the same way. After they got something that sailed pretty well, only then did they think about documenting what it was that they built, and to try to turn out a set of plans.
So the weekender plans are more a record of the parts that were cut, and some additional instructions about how they all go together. I noticed this fact the first time I tried to make some form of CAD drawing from the plans. Station #1 on the deck has nothing to do with station #1 on the hull bottom or station #1 on the keel, and none of the stations line up on the same vertical plane. The plans only make sense if you have a finished working weekender that you are taking apart to see how it was made. So naturally, there is no CG, GM, CB, or any of the other alphabet soup abbreviations.
Is this wrong? Probably not. For the five thousand years we know about boat design, this is how it was done. Get a block of wood and carve a shape that looks like it would be fast. Then slice it like a loaf of bread, take measurements, scale it up on a lofting floor, make the full size parts (using scantling rules determined by whatâs worked in the past â more trial and error), assemble the whole thing, and go sailing to see how it all turned out. After a while, you understand what works and what doesnât.
That Naval Architecture is moving from a black art to more of a science or an engineering discipline is only a very recent development. Most of it is still Art. That plans exist at all are only a later twentieth century development. Most of what we know about the great wooden boats of the âgolden age of sailâ we know not from any plans that were drawn but from the efforts of people who came later and either measured the original half hull carvings, recorded lines of the finished boat, or traced the loft lines off the floor some years after the fact (think Howard Chapelle). I find this absolutely amazing.
Ditto for sail design. It took until the late 1980s before there were any computer designed sails the could compete with what Ted Hood could just dream up out of his head from trail and error â a little more fullness here, a little more length there, maybe the draft should be a little more forward.... And even those fast âcomputer designsâ were found by trail and error with the computer only predicting which of many competing designs should be faster â computerized trail and error.
So that the Stevenson plans are not really plans, or that the Stevensons tend to not pay as much attention to this builderâs blog as would be useful to the rest of us ⦠well, they are just being the surfer dudes that they are; probably out actually on the water enjoying a new boat that they have cobbled together in their spare time. Build it, float it, sail it, repeat.
I am personally glad that you are here to step into that void and take up the slack â answering our questions with a rare combination of engineering knowledge, humility, humor, and some good natured jabs at our foibles and follies.
By the way, your description of moving a fifty pound weight along the center-line of the boat until the trim no longer changes sounds a lot like the inclination test I suggested a few months back. 50 lbs sounds like two or three cases of beer. Turned on edge and stacked one atop each other would lead to fairly accurate position measurements. So, how about the entire fleet using this method to find out the CG location of each weekender â as built â and reporting the results. What happens to the three cases of beer afterwards would be up to the individual crews. :wink:
Any volunteers?
Cheers
Tom
My understanding is the elder Stevenson was is possible still is an aircraft engineer of some sort. In this same vain, ship board "bilge rats" are titled engineers as part of the "black gang", but they know nothing about engineering, though they do know which valves to turn when they need to shunt steam some place.
I can only speculate about the origins of the design and it's development process. It's my personal opinion, not based in any first party information I should add, that the keel profile was an experimental concept, the senior Stevenson was toying with. From an aeronautical view point I can see why it would appear a possible avenue of pursuit to an aircraft engineer. From a hydrodynamic point of view it has clear issues.
I also should add it's not my intention to insult, attack or offend either the Stevenson's or builder and owners of their creativity.
I started out as a surfer dude too, in the 60's and quickly realized I needed to get into collage or make friends with a US congressman if I was to avoid the war. Had I known Carter would pardon everyone a few years after the war, I would have likely made the "extended beer run" to Canada like many of my friends did. I grew up on the water, literally and it was clear early in life, that the farmers and fisherman I knew, worked way too hard for me to accept as a life, so I started fixing boats and going to school, were I took up engineering.
Actually, "documentation" of vessel design was started by the British Admiralty, possibly as early as the 15th century, but wasn't widely practiced until the 16th (China may have been before this). They were pretty anal about it, pulling lines off of "prizes" they thought had some merit and incorporating the newly acquired design evolution into similar hull types of their own.
In this country we have a reasonably clear lineage from pre-revolution days to present, though much has been lost, but nothing compared to what the English have. Howard Chapelle did much to advance our understanding of American types and classes. He was a brilliant fellow and one I knew and called a friend. There have been others, but none with his impact on the historical aspects of sail.
I have a text outlining inclination procedures, which involves weights and a timing the return to even keel precisely. Of course I can't find it, but it boils down to inclining the boat, with tackle or weights, then releasing her to time how quickly it's rights from that angle of heel. Armed with this information you can make calculations on other angles of heel and develop a stability curve.
Personally I'd rather not participate in these inclination tests, but would happily help any and all crews dispose of their burden of inclination ballast.