I have had the same question. Peter has addressed how flair and

tumblehome could affect sheer. But I have been interested in the

geometry and design of (and designing) instant boats (built with no

backbone) where there is a (virtually) constant flair from stem to

stern, with a constant width side plank, such as Windsprint and

Birdwatcher - but also boats like Black Skimmer (or Skillygalee) ,

with a varying plank width. The section view of these boats have the

flair lines parallel. Getting the plank ends cut at the proper angle

ensures that the plank will lie correctly and come together just

right. Just recently I have worked it out on paper like this.

At the bow (and the stern of double-ended boats) of a straight

stemmed boat, you can work with three related right triangles in a 3

dimensional figure. Since the fore and aft distance is so short, and

there is so little curve to the sides in this short forward section

of the boat, we just assume that there is zero curve to the plank.

Take a look at the plan view of Birdwatcher or Windsprint and you can

see this straight line forward.

The first right triangle is in the profile (side) view and lies on

the centerline, where the stem is the hypotenuse, the bottom corner

is

where the stem meets the boat's bottom, the other two sides are

perfectly vertical (side Y) and perfectly horizontal (side Z), all on

centerline. One of the angles of this triangle is equal to the bow

profile angle, the angle above horizontal.

The second right triangle is in the section (end) view. It describes

half of a vertical bulkhead (if there were one) and sits on its point

right at the base of the stem. Its has a vertical side on the

centerline (side Y shared with the first triangle), its upper side is

horizontal running athwartship to the sheer (side X), and its

hypotenuse lies on the side plank. Its bottom angle is the angle of

flair.

The third right triangle is seen in plan (top) view, and its sides

are the top edge of the vertical bulkhead (side X with triangle 2),

the centered horizontal line to the peak of the stem (side Z shared

with triangle 1), and its hypotenuse is the sheer (or really close to

it). The forward angle of this third triangle is the entry angle.

Note: as in Carlson's HULL program, X is width (or 1/2 beam), Y axis

is vertical, and Z is horizontal along the length. If I explained

this well enough, and you were able to draw this, you would have all

three right angle sharing edges or sides with another right angle.

We can make these comparisons,

Tangent of the bow angle (angle above horizontal) = Y/Z

Tangent of the entry angle = X/Z

Tangent of flair = X/Y

Assuming a given entry angle and given flair, there can be only one

bow angle. As you said, you can play around with the possibilities.

Less flair and a wider entry angle will force the bow angle up from

the horizontal. And conversely, more flair and a narrower entry

angle will lower the bow angle towards horizontal. If you have Build

the New Instant Boats, it is quite evident when you compare the Light

Schooner with Windsprint.

I have been "working" on a Bolger-inspired, instant-type box sharpie,

that has much in common with Skillygalee, has the leeboards of

Blackskimmer, and uses planks 20 to 22 feet long. That's how I've

gotten into this. But I have know idea how Bolger or Michalak come

up with the angles. They probably are drawing the lines in a cad

program and they all come out because they are being calculated

constantly.

Seems "on topic" to me. ;-)

Phil Lea

Russellville, Arkansas

> From: Matthew, Agnès & Fletcher Peillet-Long

<matthew.long@l...>

> Date: Thu Nov 2, 2000 10:59pm Subject: Another design question

>

> Here's another question for all the other would-be small boat

designers in the

> group. [snip] Say you want to make a boat out of straight cut

sides, like Teal

> or Zephyr or many others, just cutting off the ends at an angle,

spacing the

> sides with a frame, and joining the ends at stem and stern (or bow

and stern

> transoms). For a fair, untwisted side panel there is a set

relationship between

> angles of the end cuts and the width of the frame that determines

the flare and

> the sheer. Can someone explain this to me? I can visualize it, I

can

> demonstrate it, but I'd like to be able to calculate it.

Basically,

I want to start

> with rectangular side panels of a given length (8', 16', 24', you

get the idea)

> and height, and play around with the possibilities.

> Thanks,

> Matthew