Expanded sheer lines

[Roger Long]

Long before most of you were born, I spent some time in the mold loft at Goudy and Stevens with the loftsman who laid down the lines for the Schooner America. He explained to me how all yacht designers draw sheerlines improperly and how he always had to fix them on the loft floor so they wouldn't have to either extend the stemhead or reduce the freeboard in the forward quarters when ribbanding off the sheerline for checking the fairing by eye.

The process is describe in Chapelle's Boatbuilding but described as only applying to full bowed motor yachts of the sport fisherman type. It actually has to be applied to every sheer line if the design is to be constructed exactly as built without looking lifeless or having the distressing hump seen on the majority of boats wood and glass.

I've been away from wooden boats for almost three decades now and haven't been looking at many plans. Has this principle become common knowledge in the meantime? I'd be glad to explain it but I don't want to beat on something that has become a dead horse.

[Ian McColgin]

So many rely on computer lofting onto big sheets of mylar that they might not know. Which can cause more problems than just a poor sheer. One boat shop I know got the mylar for a power skiff but the master builder still insisted on doing the right thing on the lofting floor. Turned out the computer had a programming error and the mylar "lofting" put in about 50% extra molded depth.

Fortunatly, most builders still rely on Chapelle and Viatsias and the the majority of NA's I like - from LFH, Culler, Dunbar, Alden (& studio progeney), Stevens, Brewer, Bolger (when he's in the mood), etc - draw a perfectly sweet sheer from the start. The problem is more in builders not fully understanding how to loft, not designers failing to understand.

[Roger Long]

The problem is more in builders not fully understanding how to loft, not designers failing to understand.

You can loft it perfectly fair as drawn and, if it was not expanded on the plans, it will have a hump.

[outofthenorm]

I'd be glad to explain it but I don't want to beat on something that has become a dead horse.

Roger, beat away. It's something I think I understand, but another explanation would be good.

- Norm

[P.I. Stazzer-Newt]

Even if it were a dead horse, it sounds like an interesting one.

[rbgarr]

There was an article in a WB issue, iirc perhaps by Jay Paris(?) Or maybe it was somewhere else. At any rate I'd say it's only applied by the most knowledgeable and experienced builders. The term 'planar' rings a bell somehow.

There was another article about a somewhat similar phenomenon, the need to alter deck crown on a foredeck to avoid a 'hump'. It too was addressed in a WB article.

[Roger Long]

OK.

First, you have to determine if the lines plan you are going to use is properly drawn, few are. Look closely at the forward sheer on this boat:

http://www.cruisingonstrider.us/Boats.htm#27Pinky

(click the lines plan image for a larger view)

Note the slight hook or increasing amount of curvature as the sheer approaches the stem. If it is not there, you will see a corresponding droop or flatening out in the finished boat. Since this is a double ender, the stern is also expended but, since we expect curvature to increase at the aft end of a sheer, it is not so apparent. If you want a double ender to look as you or the designer expect, you have to expand the stern as well. The note on my lines plan warns people not to expand on top of the expansion. Whenever I lofted a boat however, I refaired the sheer line to take out the hook and then put it back through the expansion process.

The reason for this is that the eye sees a boat and the sheer in three dimensions. Let's take the simplest possible sheer and assume that the designer wants it to look dead straight.



This sheerline is defined by rising 3/4" for every foot it goes forward. Putting the designer's eye for a boat aside, there is no question of improper fairing here. It's dead simple. Still, if it is built exactly as drawn, the finished boat will look like this to the eye in three dimensions:



Here is the reason:

Look at the deckline. Distance A is larger than distance B. The eye looks AROUND the three dimensional sheer and sees the 3/4" per foot of rise as being along and around that line.



The 3/4" per foot of rise thus becomes, say, 3/4" per 15". Since A becomes progressively larger than B the effect becomes more pronounced towards the bow and produces the ugly droop or flattening seen on most boats.

Visualize a waterline half model sitting on a table and imagine wrapping a piece of cardstock around the deck perpendicular to the table top. You basically want to lay that out flat, draw the sheer as you would like it to look on paper and then wrap that back around the model. If you transfer those offsets to flat paper, the difference between A and B at each point will create the upwards hook towards the bow that is the mark of a sheer line drawn for construction as designed without intervention by the mold loftsman.

Here's how you do it with hand drawing or on the loft floor.

Draw the sheerline in profile without regard to the 3D effects. This will generally be a sheer that gets progressively straighter towards the bow. Extend the batten well ahead of the sheer continuing the line as if the boat were longer.

Next wrap the batten around the deckline and make a tickmark at each station from midships forward and at the stem fairbody line. These will be the true lengths. Lay the batten out along the profile sheerline with the midships marks aligned. All the other batten marks will be progressively further forward of the station lines.

Draw a line from the unexpanded sheer line at each mark on the batten back parallel to the baseline to the corresponding station line. These will be the actual desired heights for the sheer offsets. Run the batten through these new lines producing the "hook" on the flat lines plan that will disappear in the three dimensional boat.

I've shown a dead straight sheer here. The principles are exactly the same for a proper curved sheer line but the 3/4" per foot becomes a much more complicated mathematical expression that you don't need to worry about. Whatever you want your sheer line to look like, it won't come out as you expect unless you put it through this expansion process.

Sure, you can fair up your sheer by eye on the stocks but, if the lines plan or lofting did not anticipate the expansion, either the stem will be a bit too short or the freeboard in the bow shoulders will have to be correspondingly reduced.

[Phil Jans]

Roger- thanks for bringing this issue up. I was working on my boat a few months ago, setting up the molds and marking the sheer with a batten. I was perplexed by the weird hump in the sheer toward the stem. It looked like a subtle version of the "powderhorn" sheerline that is intentionally designed into some boats. But my boat clearly is not supposed to have a hump in the sheer.

Whenever I am in a jam, I always come back to the "gospel"- Chapelle. There it was on p. 349. I followed his directions on expanding the sheer, and it now looks right, the way the designer's sketches show it. But it puzzles me why I have never heard anyone else talk about this phenomenon. Your post is the first time I have heard it discussed by boatbuilders.

Phil in Wenatchee, WA

[rbgarr]

Very cool, and fun to read the explanation!
It must be a disappointment if someone doesn't do this and finds their stem is too short.

[Roger Long]

But it puzzles me why I have never heard anyone else talk about this phenomenon. Your post is the first time I have heard it discussed by boatbuilders.

The loftsman who told me about this said that naval architects didn't know about it because they worked on flat paper. Most boatbuilders, back in the time when there were was enough work that lofting was a specialized trade, didn't know about it because everything was fixed in the molds and patterns by the time they saw them. It was a mold loftsman's secret so the builders wouldn't come back and say, "Hey, you screwed up the sheer."

Most lofting is on the rough side now that it is not a trade and people just assume that the didn't get it quite right if they spot and fix the hump. It can be done right by someone who understands the projections and the process. When I built the Whitehall boat and the first half of the 17 foot Quoddy boat, the planks went into the rabbets with almost no clean up or adjustment after cutting them to the line.

Look at plans and you will see precious few with the sheer expanded properly. In fact, I can't recall ever seeing any other than my own.

Another secret, speaking of planking rabbets: When you spring the battens to draw the lines to define the rabbets, don't extend the batten out to keep the nice fair continuation of the curve as shown on the plans. Put your last nail right at the rabbet. This will create a flat spot near the end of the plank that doesn't look great on the floor. However, the plank is going to take that shape anyway because there isn't any extension to keep the final few inches in the nice curve. May as well loft and cut the rabbet the way it's going to end up right from the patterns. In places where there is some support for the final few inches, such as a breast hook behind the sheer strake, you can and should keep the curve running right to the end.

If you are lofting from the "bible" (Chappell's), take his fairing instructions with a grain of salt. After you lay out the sheer, fairbody, and body plan, first fair up all the diagonals and adjust the section lines to suit. Run some extra diagonals in critical areas. Then, worry about the waterlines and buttocks. You'll find very little to do. Chappell treats the diagonals as an afterthought and final check but they are the starting point as they lie closest to the flow lines and what the eye sees.

[Jim Ledger]

Thanks for the explanation.

How can you tell if the correct sheer has already been drawn into the lines plan or if the expansion still needs to be done?

I'm referring, in particular, to Brewers Chapaquiddick catboat. There's still half a foot of unshaped stem that needs to be cut down at some point, and now I'm glad it's there.

[Roger Long]

The term 'planar' rings a bell somehow.

I remember seeing an article with that term but don't remember if it was Jay Paris. There is another theory that the sheer in the forward half of the boat should be such that it lies in a plane, in other words, a flat board could be laid on a half model so it contacted the entire sheer line. This will produce the kind of sheer line I am talking about but greatly restricts the style and kind of sheer the boat can have. If your boat wasn't designed "planar", you can't fix it with out much larger shape changes than simple expansion.

There was another article about a somewhat similar phenomenon, the need to alter deck crown on a foredeck to avoid a 'hump'.

That happens at the centerline if you use the same curve for each deckbeam. The fix I was taught by Chet Rittall was to use the bottom of each deckbeam as the pattern for the top of the next in the forward quarter of the boat. I prefer to just loft a fair centerline and layout a new curve for each deck beam. It doesn't take very long.

[rbgarr]

Did you work here in Boothbay for a while (in the sixties)? You've mentioned Goudy and Stevens and Chet Rittall. That's quite a while ago. I was at the first in the mid-seventies for a brief period.

[Roger Long]

Did you work here in Boothbay for a while (in the sixties)?

I got a call from Paul Luke a few days after my graduation from high school in 1969 offering me a summer job in response to a letter saying that I knew how to loft. My father took me out for a haircut so short I expected to see blood coming from the roots and drove me up to Maine.

Paul Luke came out before we could get in the door and said, “We don’t hire any of you long haired fellows.” I showed him the lines for a boat I had designed and he said reluctantly, “Well, I need a loftsman so guess I have to hire you.”

I picked up where the previous loftsman had stopped after what the other workman told me at lunch was an abrupt and noisy departure. After finishing laying down the lines for the aluminum powerboat, it was time to make the patterns. The yard used the traditional method of curved pieces of wood with a notch for the batten in one end and nail in the other. After these were set up along the line, the plywood for the pattern was slid underneath and the line drawn. It was slow and time consuming. I found a box of small brads and drove them in half way every few inches along the line. I then threw the plywood on top, walked on it, connected the imprinted dots, and cut out the patterns. A quick pass with a plane produced a nice smooth pattern.

Luke came in; saw what I was doing and the big stack of patterns. He said, “That’s the damndest thing I ever saw. Keep at it.” I kept at it.

I was a bit afraid of him so I asked some of the other senior workers what to do and they said, “Keep at it.” I cut out the keel, engine girders, and floors for the powerboat incorporating slots so it could be put together like a big egg crate and hold itself together while being welded. One morning, I took all the parts and set it up on the shop floor. It went together quickly and was ready for truing up and welding. I didn’t see much of Luke since he was busy in the other shop with the rather steep learning curve of his first aluminum boat.

I was just finishing up when I heard behind me, “Jesus, Jesus, Jesus, JESUS! Where the hell did that come from.” I turned around and he was standing at the top of the stairs looking stunned and a little panicked (a lot of expensive aluminum was sitting there on the floor). He went down, looked it over, and said, “It’ll do. Keep at it.”

I then noticed that every hippy who stopped by looking for a job go hired. None of them lasted. As the last one trudged up the driveway Luke muttered to me, “I guess you aren’t all alike after all.”

I returned to Boothbay Harbor in the winter of 1972 and worked for Chet Rittal for a few months planking and doing some of the interior structure on Outward Bound boat Number 14 now sitting in front of Jim Sharp’s museum in Rockland. I then got a couple yacht design commissions and quit to do those and spend the summer sailing a catboat that friends owned. My 27 foot Pinky was one of those jobs. I moved to Rockland the following fall and then on to Boston where I went into commercial boat design.

I returned to Boothbay Harbor in 1982 to become Vice President of Woodin and Marean until 1986 when I started my present company in Portland.

[Bill Perkins]

Thanks for the interesting posts Roger.

[htom]

Thank you, Roger. That has always puzzled me, both in my drawings and in looking at real boats.

[Roger Long]

How can you tell if the correct sheer has already been drawn into the lines plan or if the expansion still needs to be done?

Look carefully at the portion just after the stem. It should have a bit of a canoe like hook or increasing curvature as it approaches the stem. If the line drawn gets straighter from aft to forward in about the last 1/10 or so of the boat's length, the sheer is not expanded. I've never seen plans other than mine (although I haven't looked that much in the last 20 years) with the sheer properly drawn so yours probably aren't.

[Siolle]

Hi Roger, thanx for lifting the curtain into some of that old craftmanship.

The boat I´ve started with has flat panels, chine #1 - #4 and sheer. When lofting should I treat all chines the same way as you described for the sheer?

I guess if I´m only building up to sheer #4, I would get the same hump in the sheer line #4, and less in #2 and #3. Or is this phenomen just visable on the chine?

Another thing I´ve looked for is the "diagonals", why are the located as they are? Are they related to the buttoks or the outer hullform, how do you chose the location and angels for the diagonals.

Best Regadrs
Olle

[Canoeyawl]

I think a good builder knows this but may find it frustrating and time consuming to try and explain it to the customer. I also think this may be why the old designers preferred to work with a specific builder.
It is a tricky thing to execute, the sheer - you have to look at it from many angles and in the end it is always an interpretation of the designers intent. Is the view from a beach, the dock, another vessel, is it heeled?
If the builder looks at his sheer batten for a while (like a week!) and then walks up and moves it a sixteenth, you know he is thinking about it. I have no idea how boats get built in a space not allowing one to get back and look at it.
The Greeks knew this...
In order to even create what appears to be a straight column (or even a straight wall) they curved it.
Even the old house builders knew this, if you can find an old enough clapboard house the clapboards will get progressively larger from the bottom up.
I had a drink with Paul Luke one evening; we talked “boat” for a couple of hours - he was a heck of a guy… Some yachts just stand out as the work of a certain yard.

[Peerie Maa]

Hi Roger, thanx for lifting the curtain into some of that old craftmanship.

The boat I´ve started with has flat panels, chine #1 - #4 and sheer. When lofting should I treat all chines the same way as you described for the sheer?

I guess if I´m only building up to sheer #4, I would get the same hump in the sheer line #4, and less in #2 and #3. Or is this phenomen just visable on the chine?

Another thing I´ve looked for is the "diagonals", why are the located as they are? Are they related to the buttoks or the outer hullform, how do you chose the location and angels for the diagonals.

Best Regadrs
Olle

Diagonals are normally placed so that they are perpendicular to most of the plank surface along their length, but so as not to cross each other. Some yachts, especially canoe sterned yachts attempting to achieve good balance, designed with the diagonals rather than buttocks.

[johnw]

Why are Thunderbirds, with their completely flat sheers, so handsome?


Seaborn gave them flat sheers so they would be easier to build.

[Siolle]

Yes I think you´re right Canoeyawl, trying to explaine a lifetime of experiace to someone new is not an easy task. Rogers made a very good attempt to uncover some of the secrets of the art of lofting a mold, explaining some secrets, and there are probably a thousands more yet to be told.

I´m just an amature builder, trying to learn as much as can about the project I´ve started. And yes you might be right...I´m focus on the nitty gritty doesn´t matter...

And Nick: Thanx for the explaining of diagonals...is it as I expected, if you have a curved surface the perpenicular will be choosen at one station and then you look at the faired line for the whole hull.
Eg: how the water sees the hull lines at an heeling angle?

Olle

[Canoeyawl]

Years ago I worked with a man that never lofted anything but he had a good eye and his sheerlines would be exquisite. Once however his parsimony got the best of him and his layout did not leave a wide enough plank for the sheerplank aft. Now with a lapstrake boat, this is a huge mistake - no going back really. He would refer to the boat, and we knew which one, as the one with the "unfortunate sheer"
The customer never noticed, all dazzled by the laps and the shiny rivets, the lovely carved knees, bronze fittings and varnish, but when we looked at it our eyes sort of drifted down towards our feet...

[Tumzara]

Roger,
Thank you very much for sharing with us your wisdom.
Does this apply to a boat with reverse sheer?

[Scot L T]

Thanks Roger for posting this. It helped sort out an issue I was having on a small lofting project.

Also, I've been lurking about on your web site and find the section on vessel stability very interesting. Thanks for that.

[Roger Long]

Why are Thunderbirds, with their completely flat sheers, so handsome?

If they were designed so that the plywood sides had a straight edge before being wrapped around the molds, then the sheer is automatically expanded for a boat in which the intention is for the sheer to appear straight.

[Roger Long]

Does this apply to a boat with reverse sheer?

Yes, if you want the reverse sheer to look as you drew it on the flat paper.

[johnw]

If they were designed so that the plywood sides had a straight edge before being wrapped around the molds, then the sheer is automatically expanded for a boat in which the intention is for the sheer to appear straight.

I can see where that would work with a wall-sided boat, but if there's any flair, as there is on a Thunderbird, as the sides bow outboard, you'd get a curved sheer. That's how I designed the sheer of Black Swan, my sharpie.



Maybe Seaborn did the expansion, or Ed Hoppen, the builder he worked with, fixed it on the loft floor. The dead-straight sheer of the Thunderbird is a feature intended to make the boat easier to build, so I don't think they did anything to fool the eye.

[Tom Robb]

T-Birds handsome?
More like pretty is as prettyy does...like your homely grandma who was a fantastic cook and a great dame who raised good kids.

[johnw]

No hump in the sheerline, though.

[Jim Ledger]

Roger, I took your instructions for expanding the sheer and applied them to the Brewer catboat. In the two following pictures the larger batten represents the expanded sheer, while the smaller batten underneath follows the designed sheer. As you can see with a full bowed boat the difference is dramatic, the sheer at the stem rabbet being four inches higher than the plans show. Even if this difference didn't scare me to death, it would involve a complete re-fairing of the upper forward section of the boat.

Could this be correct?

[htom]

I'm not Roger, but looking at where the bottom of the hull would be, my guess is that the higher sheer is correct (or that the designed sheer was already expanded.)

[Roger Long]

Could this be correct?

That looks just about right and 4" is about what I would expect for a wide boat like a cat. When you re-project the sheer on the body plan, if the first part of the new line is nearly straight, you'll know you have it right.

It may be a lot of re-fairing now but, if you have a good eye, you'll be doing it anyway when the boat is set up and you are running the sheer batten. Easier to do it now.

[David Cockey]

I remember seeing an article with that term but don't remember if it was Jay Paris. There is another theory that the sheer in the forward half of the boat should be such that it lies in a plane, in other words, a flat board could be laid on a half model so it contacted the entire sheer line. This will produce the kind of sheer line I am talking about but greatly restricts the style and kind of sheer the boat can have. If your boat wasn't designed "planar", you can't fix it with out much larger shape changes than simple expansion. ........

There's a theorem which says that for any curve which does not lie in a plane, some projection (view) of the curve will have reversing curvature. The converse of the theorem is a curve which does not a projection with reversing curvature will lie in a plane.

It would be interesting to see how close "expanded" sheer curves are to being planar. If a curve is put into 3D surface software such as Rhino it's simple to determine if it's planar, even if the plane is not orthogonal to one of the conventional views.

[Roger Long]

It would be interesting to see how close "expanded" sheer curves are to being planar.

They are not at all planar. An expanded sheer will show a reverse curve from many angles when projected flat; especially when viewed from below. The issue is not whether any reverse is apparent but rather the eye interprets the sheer it sees in 3D as being on like it sees on the plan, one that starts fairly straight and with an increasing amount of curvature as it runs aft. Because you see the actual sheer in 3D running around the deck, you will see the unexpanded sheer as flattening, or drooping at the forward end. The projected reverse you see from a low angle looking at the shoulders of the bow is a separate visual effect.

Planar sheers can be drawn (I remember the Jay Paris article) and they are naturally expanded but they greatly restrict the kind of appearance that can be obtained and force a boat style all their own. Expanded sheers can be applied to any type and style of boat.