Sheer?

If you have a look at my blobspot, you will see how easy it is for me to instantly adjust the sheer line using my method of construction. Then for me it is just an asthetic choice. bourneboats.blogspot.com/
Cheers from down under Peter
bourneboats

 

I'm not sure Ted455 has got the answers he was hoping for. As a relative newbie to boat design I would like to learn more, too.

The factors going into sheer design identified so far include workboat needs, natural bend of planks, internal space, seakeeping, behavior at anchor and performance, the desire for a dry boat, and of course the cultural and esthetic considerations that seem to have hogged the limelight so far.

He asked for information on how to measure sheer, plus sheer variations due to propulsion - power/sail, I would add manual and towing. Then there's the environmental influence such as type of waves encountered - short-choppy-breaking waves and power boat wakes, open vs decked craft ...

Could you take some of the more extreme styles of boat sheerlines and explain why? Ti was mentioned, presumably the reverse sheer curve keeps the bow and stern profiles low and the higher sheer midships keeps water off the deck for safety and drag reduction when heeled. Is that correct? Some boats have long, gentle sheer curves whereas others curve up sharply at bow and/or stern; Why? Perhaps an initial discussion could start with open boats to keep the number of variables down.

 

One of my posts at the top explains my method of developing a nice looking sheerline.

The biggest consideration is that it is hard to work out how a three dimensional curve will present in real life when it has been designed by 2 dimensional methods - on a computer screen or on paper.

The methods of trying to make that happen are mentioned in one of my posts near the top.

If you are looking for some guidance as to the "correct" sheer - then there are none.

Alan White in his post near the top does give a rough formula for drawing a classical sheer - but he knows it is only a rough guideline but he trusts his eyes (and knowing the required interior volume of the boat) to get it right.

On most modern designs the displacement is so low and the hull volume so high that any sheer is practical without changing the performance or behaviour of the boat much at all.

With a big heavy boat - long graceful ends and plenty of sheer help it get up and over waves. But a light boat will get up over waves whatever you do with the sheer. In that case you need reserve buoyancy in the ends and sheer is one of the ways to get it - as are overhangs. But if the boat is not likely to see rough water it is a moot point.

There are some other repercussions mentioned too as far as blowing around anchor, but you cannot be definitive about it because it depends on what is happening under the water too.

With most modern designs (even the ones that look like older boats) it probably makes very little functional difference to the boat (except in terms of interior volume - though there is no rule that the sheer has to be equal with the deck level) so just draw something that seems to match the style of the boat you are designing.

Best wishes
Michael Storer

 

Sorry Ted, we have driven the cart of the trail a fair way.

Other then some very general guide lines, typically surrounding classic yacht sheer, there aren't any real set rules.

Styling and racing trends seem to dominate more then anything else in the last 50 years of design.

There are so practical considerations, such as internal volume, standing headroom, decreasing weight in the ends, windage, etc., but these are usually judgment calls on the designer's part. The ideal sailboat sheer would be flat and right on the LWL, but practical issues such as boarding waves, stowage and accommodations would tend to get nixed in the deal.

Sheer types vary with the times, much like sail plans or the currently popular amount of transom deadrise to use. They swing back and forth through the decades, not being ideal for any given design.

This said, most boats will look terrible with the wrong sheer on it and it's quite difficult to anticipate the actual 3D shape as Michael mentioned. Experience teaches one how much to "cheat" here or there and some hull types require more cheating then others.

A fat, well burdened hull will need a healthy sweep to it's sheer or it will appear to droop at the ends. This is why working craft usually have a pronounced sheer. A lean racer would be carrying excess weight if the sheer was "springy" and it would probably have a weird reverse near the bow at some angles. These examples are a result of the hull shapes affecting the 3D shape of the boat.

Narrow boats require less sweep, beamy more as a rule. The actual sweep of the sheer is a pretty complex curve and requires some practice to get a sweet looking one.

As has been pointed out, with practical issues addressed, you can have pretty much any shape you want; classic, flat, reversed, S or reversed S are most of them.

If you're looking to strike a new sheer line then get a long batten, one at least 10% longer then the boat and tack it in place, roughly where you think it should be. Then stand well back (50' or more) and look her over, from every angle, sighting down the sweep to insure it's fair and the curve sweep is what you desire. This sounds easier then it is. I've cut new sheers into boats and it's difficult, requiring many adjustments and subtle batten moves to get it just right from every angle. Once you're satisfied, then call a trusted friend and have them look at the line you've created and have them look from every angle.

One tip is to insure the batten continues past the ends of the boat in a fair line. This is a common mistake. By not doing this you can have a hump or flat spot in the ends of the curve, but you'll not be able to see it because there's no reference to eyeball. On paper we design the sheer the same way, well past the ends of the boat so it's really fair.

So 'ya thinking about cutting a new sheer into something?

 

almost sketched a dozen different sheer lines as i think
the sheer line of a boat is the most eye catching
but its a whole lot more and can have many forms
sheer in wiki gives lingery, form follows function ;-)


here a "bakdek" (what is it called, just S sheer?)
from witch i direved the ladybird sheer (see my galery)

 

Here in Norway, I have heard the name "Serpent line" for that "S" shaped line, but then again, I may be wrong...

 

If you are one of those people who don't have an eye for what looks good , hiring a marine artist to help you get it right is a worthwhile investment.
The classic sheer, the fife sheer was a section of an elipse. To draw an elipse is easy. Pick out two focal points on a board , then put a nail thru each end of a string into the board . Then put your pencil in the loop between them, and draw the elipse, using the string as a compass with two focal points. Changing the length of the string or the spacing of the two focal points , changes the shape of the elipse from narrow to wide. The classic sheer is a section of this elipse, from the middle to one end, the middle being the bow and the end being the stern. How deep and how far back takes an artists eye, but using an elipse in this way gets you closer to home base easily, and eliminates screwups in the middle.
Brent

 

Yipster,

A raised (fore)deck with a broken sheer would be a/the normal description.

the common trick is to put a secondary strake along the side of the boat to provide a continuous visual sheer below the deck edge - exactly as in your example.

Their secondary strake looks a bit straight to me and don't like the steady rise from stern to bow - I would tend to make it more parallel with the actua sheer right near the transom then gradually accelerate the curve up - but that is personal

Not going to argue - everyone can be right with sheerlines as they are non functional for most boats!

MIK

 

I design mostly canoes at present and since the boat ends at the sheer it's functional all right! I get nice sheers with a straight sheer plank angled out slightly so the plan curves create the profile; not much sheer rise to the stems but OK for river use. A circular arc for the sheer plank in development tips the ends up nicely for more choppy conditions. With short decks at the stems in the classical Peterborough canoe style, a sheer plank with a hyperbolic top edge blended into straight ends with a constant sheer plank angle has the benefit that the decks peak at a straight king plank so the planks can be wide; it looks good too.

The conic family of curves are easy to generate on drawings and planks developments and seem to produce a good appearance every time. Not sure if that method would translate to boats with full decks and topsides. Interestingly, some classic designs I have investigated also seem to use conics.

 

Sheer Beauty

...from a reply letter I sent to Soundings a few years ago

Just finished reading your article about Ray Hunt in the June issue of Soundings, I wanted to write and let you know how much I truly enjoyed the article.

You captured not only the factual history, but more importantly, some of the essence of being involved with yachts and yacht design itself….. “the measure of total understanding of the nature of a boat.”

And most importantly, you included the oft forgotten element, the sheer line. What a terribly important factor! Romantically stated, but oh so true, “it is simply her sheer … sheer beauty that is. She enters the harbor like a beautiful woman entering a room. Her sheer is the line we try to get right when we doodle boats.” So many of today’s boats lack this beauty, and correspondingly, some of the essence of yesteryear’s yachting.

 

this deserves repeating these days when modern designs seemed to have often forgotten this element of yacht design....