By Rob Rohde-Szudy - Madison, Wisconsin - USA

Measuring & Drawing Hulls - Part I

To Part Two

Last time we looked at my first real boat design, and a little bit about how it evolved from the Michalak AF4B. Let’s take a closer look at the latter.

Measuring hulls and drawing them is every bit as much of a time-consuming pain in the butt as it sounds like. But I found the need to do just that when I realized I’d sufficiently bastardized the AF4B to give it its own name. While the time commitment is rather large, the tools are simple and cheap. Mostly string, a tape measure and some squares. A helper is a wonderful thing for this job.

Chapelle and others have already covered this topic in great detail, but here I am focusing on a simplified version. Many fewer steps are needed to measure a nail & glue “Instant Boat” with a constant chine bevel.

First let’s make a sketch of the side view with bulkheads and stem. This is not to scale, but it does establish the location of some important measurements.

To add to this, I also measure the distance aft of the stem of each of these bulkheads. The point of reference at the stem is the very top forward corner of the plywood planking. Conveniently these work out to be mostly even multiples of 12 inches. This makes sense because these measurements are holdovers from the original Michalak design, and he tries to stick with round numbers.

Base Lines

Now we need to get heights and a few more lengths filled in. To do this we must establish a reference line at the keel. In my case the trailer got in the way of running a line right under the center skid, so I had to run two separate lines 19” outside this line. Note how I used masking tape to make visible markings on the hull.

After driving stakes into the ground it takes some jockeying to get the lines in place. First, they have to match up to marks 19” from the centerline fore and aft. Then they also need to just touch the bottom of the transom, and run straight forward not following the contour of the bottom. This assumes a planning hull with a flat run aft. For a sailing hull, the stern will sweep up, so the strings won’t touch it. There you will probably want to try to keep the baseline about parallel to the load waterline. This is fiddly work in either case.

Now that we’re cold and damp anyway, we may as well get some measurements.

Baseline Measurements

I started at the stern, since it’s easiest in this case. A square tells us how far the transom top is aft of the transom bottom. In this case the string and the bottom are in the same plane, but for a sailboat hull the square would have to meet the string. It is convenient to fill in the transom height at the same time. I also captured the position of the rubrail at the transom, as it is a continuation of the forward sheer.

With a simple planing hull I get a break here, as the bottom is dead flat for about six feet. I don’t need another measurement until the string departs from the bottom. There I make measurements every foot—aligned with the bulkheads above—and record the height.

It is handy to mark the string with masking tape at these one-foot divisions.

Be sure to measure square to the string fore and aft, and square to the bottom athwartships. This is very tedious and takes a steady hand. I couldn’t even get a picture of it. You’ll want two small squares and a ruler cut off right at the zero mark.

This gets harder when the string emerge from under the bottom. Now we need to connect the one-foot markings on both strings with a crossing string. More tedious positioning.

Now the measurements are harder to square up. Ideally you need a square aligned with the transverse line, then its vertical part aligned with another square touching both the transverse and longitudinal strings. This is impossible without at least one helper or a lot of duct tape. Fortunately eyeballing it is close enough for short distances, if you’re careful.

Sometimes you can’t get a direct measurement because something is in the way. The winch post, for example, block direct measurement of the stem height. Here Pythagoreas can help. Measure the distance from the longitudinal string to the top front corner of the planking, making sure the tape is square with the string. This measurement is the hypotenuse of a right triangle, and the bottom side is the offset of the longitudinal string from the centerline is minus the stem half breadth. The square of the hypotenuse minus the square of the other figure equals the square of the height.


I also took a bevel gauge measurement of the bow angle as a cross check. Just be aware that this is more prone to inaccuracy, since you can easily be measuring a local unfairness rather than the overall angle. I don’t trust these measurements a lot, but if any are very far off I do some re-measuring to figure out which measurement was in error.

Finally, remember to deduct the thickness of the bottom from all of these measurements. This includes any skids your baselines are outside of. Enough of this rolling around on the ground. Time to get in the boat.

Inside Dimensions

Here’s the easy part. On a fresh sheet of paper sketch the bulkheads and start filling in measurements. Sometimes this is easier than others. Occasionally we need to estimate the measurement to a corner when an epoxy fillet obscures the actual corner. I put these estimates in brackets so I know it wasn’t exact.

This can be a real problem, actually. Sometimes it is easier to get cleaner measurements at stations not near a bulkhead, as there is often less of a fillet obscuring the measurement. On the other hand these measurements are more prone to error from the hull being twisted or hogged while the bottom is getting nailed on. The bulkheads are more reliable, assuming the building cut them correctly, because they force the hull into a predetermined width.

There is a way around this doubt. We can get around the fillet and project the actual measurement. But first we need to measure some heights.

Heights can be interesting too, though more precise. Since most cabintops are cambered or otherwise peaked, it is often best to run a string from deckline to deckline, then measure to that. Make sure to measure square to the bottom.

As you probably noticed, the string here is not at the deckline at the wales. This one was at the hatch opening.

You will probably measure the width along that same string, but if you measure outside the cabin, remember to deduct the thickness of the sides and cabintop frames. The same goes for exterior width measurements on the transom. If you have fillets at the cabintop the same trick can work upside down.

Here is the resulting sheet.

Let’s get back to our fillet problem. You might very well find a cleaner bulkhead-to-side joint a few inches above the bottom. If so, mark the bulkhead at that height on both sides.

Measure the width at that height and write down both width and height. In this case I had to go 7.5” high to avoid the fillet on the forward bulkhead, and the width was 15.75” at that height. For the rear bulkhead it was 6 and 55.5”.

Actually, getting accurate inside measurements can be a trick even when there is no fillet at all. I did this mostly with a tape measure, but it is not very accurate because you have to bend the tape and guess where it would have ended if it were straight.

A better way is to use a piece of wire or non-stretching twine. Tape it in place, mark it, then measure between marks. Better yet is to use two straight wooden sticks. 1x2s or yardsticks work well with a point at each end. Overlap the sticks so you have a point in each of the corners you want to measure between. Make sure your sticks are in line and mark the overlap point. Remove the sticks and clamp them together lined up at this line. Then it is easy to get an accurate exterior measurement of the stick with a tape measure.

This is accurate enough that I have managed to set the tie rods on my car within tolerance (checked by my mechanic on the laser jig he uses).

In any case, we will want to put these starting measurement on an offsets table.

To the drawing board.


The first thing to do is create a table of offsets. Basically what we are trying to do is fill in all the values in this table. I put in bold the values I actually measured from the hull. Regular type I use for values I generate through drafting.

The very first thing to do is finish figuring out those chine half breadths where our fillets were causing trouble. This is easy, and a good warm-up for the real drafting and gets you used to accurately using the drafting triangles. We’re going to draw half of a simplified bulkhead. Start with a baseline and a vertical centerline—I draw two or more of these on the same perpendicular. Then with the scale rule add horizontal lines at the height of the sheer (or cabintop) width, and the “fillet avoidance measurement” we made.

Then fill in the two half breadths (remember to divide your actual measurements in half!) on the appropriate heights, and connect them with a line. Where this line intersects the baseline, measure the real chine width for that bulkhead. Put this in the bottom half-breadth blank for that bulkhead on the offsets table.

Now onto real drafting.

Michalak covers all you need to know in his wonderful online series on designing hulls. Regrettably, this is not in his book. (It is to be hoped that there will eventually be a sequel containing more on drafting. No pressure, Jim.) I’m using the same techniques, but more or less as a way of checking the offsets. Filling in what we know, we can draw a baseline and verticals for bulkheads, transom and stem. On these we can mark the measured heights above the baseline and half breadths below. Note that in my case the bottom height measurements do not line up with stations or bulkheads. That’s OK.

Then our friend the spline shows us how easy it is (or isn’t) to connect these points. If you find yourself needing to reverse the curve or put wavers in it, check for errors. Also remember that you are measuring an imperfect hull, so you will need to decide whether your purpose is to document the exact shape as it exists, or to document your guess at the designer’s intent. My purpose is the latter in this case, or I would have taken many more measurements.

If the unsplinable curved persist on closer examination, you will have to start guessing at which measurements you trust more. I tend to trust a measurement at a bulkhead, since it is harder to squeeze the sides in or bow them out during assembly. Fillets can be a problem, but we discussed that above. A bulkhead can also be crooked, say if its line on one of the side panels is an inch off. It happens. Measure from the stem to both sides of the top of the bulkhead – they should be the same. With a crooked bulkhead all bets are off, as the hull will be a bit narrower at that station, and this will probably induce twist as well. Also be aware that “hard spots” introduced by butt blocks can affect the fairness of the curves in the plywood. The closer a measurement is to a butt block the more suspicious I get, unless there’s a nearby bulkhead to pull things into line.

In any case, here is what the spline tells us about the bottom.

Looks reasonable. Next we start a section view and draw what we know about the bottom on it. Now let’s fill in the bulkheads we know for sure in these views. Now we have enough to spline those lines. Take note that we needed to draw the section view to figure out the actual half breadths of the rubrail, since we measured the bulkhead widths at the coaming or cabintop.

Now we can add station lines.

Now we have all points fully defined. So let’s transfer this information to the section view. First the bottom.

Then the rest.

Well, this can’t be right, can it? All of the section lines on the sides should be parallel, otherwise there is twist. Nail and glue “Instant Boat” hulls can’t have side twist, or you wouldn’t be able to rip the chines to a constant angle. Well, have a look at station #1. We have twist in the bow, and I’m pretty sure the actually hull doesn’t.

Furthermore, Michalak designs all outboard-powered boats with a 15-degree transom rake, and this doesn’t measure 15 degrees. Let’s figure out where I went wrong.


Well, the transom adjustment was simple. I just plain measured wrong! I didn’t have the square firmly against the bottom for its whole length. This changed the fore and aft distance between the bottom and top of transom from 4” to 5.25”. This in turn made the transom angle way closer to the intended 15 degrees. Still one degree off, but probably close enough for any motor to work fine. I bet Jim drew it at 15 degrees, since he says he always does for powerboats, and it wouldn’t take much error to end up with 14.

The bow was nearly as simple. I had not taken into account the width of the stempost. My splined lines should not have crossed the centerline at the stempost, but rather ended a touch under 1” from it. This straightened out the bow, and now we have this.

About this time you start wondering how it can possibly be that you’re not done yet, and realize what an impressive achievement it is that Michalak has drawn over 100 boats. Drawing one is an awful lot of work—about like recording an album. But how many artists record 100 albums? Even if he’s four time as good at it as I am, how many artists record 25 albums?! Impressive.

OK, back to the drawing board.

Notes on Drafting

I should mention something about these splined lines. My rather extreme sheer causes some quirks with the spline. The sheer flattens out forward—almost a “powderhorn” sheer, but not quite. This means you need four or five spine weights, where Michalak’s easier lines normally need only three or maybe four. But still this line goes into place without too much coaxing, and so it is on the full-sized boat.

Aft is even stranger. The upturned stern makes the rubrail line reverse its curve in the plan view. It is still splineable, but you need at least four weights and they had better be heavy. Even then the spline tries to shove them out of place. This accurately reflects that it will be difficult to shove that piece into place, which is why I had to laminate it.

If you think about taking on a hull that isn't nail and glue Instant Boat construction, remember that you don’t have these handy cross-checks available. Knowledge of the rules used to design the hull is the only reason we could isolate some of these errors. Far more care is required for measuring a hull designed with fewer constraints (e.g. fiberglass or carvel planking).

Drafting limitations

The beauty of drafting is that it is a relatively quick and easy way to fill in the unknowns in the offsets table we are building. The trade-off is in accuracy. I am making these drawings at a relatively large 1/12 scale using a 0.5 mm pencil. Assuming a pencil mark of the same size, my markings translate to a little under 1/4” wide when blown up to full size.

Practically you can do a little better than this by using magnification and estimating the center of the line, but you can see why offsets tables made from full-size lofting are much to be preferred! You can also see why guys like Bolger would sharpen their pencils on fine sandpaper and make marks that a lot of us can’t even see when working at ridiculous scales like 1/64. The best I can do is 1/16 before the errors start getting way out of hand.

However even when drafting by hand we can still use computers to fix some of this error. We’ll dig into that next month.

Rob Rohde-Szudy
Mazomanie, Wisconsin, USA


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