My Old Boatshop

Weston Farmer

I was rereading my copy of Weston Farmer's book From My Old Boatshop over the week since my last post. I started out looking for a particular item, and ended up rereading the entire thing, it's a very instructive book and I would recommend it to you. If you have that and Skene's you're well on your way to understanding the fascination of small boat design. In that book Mr. Farmer states that in his experience the best riding sea boats come in at 64lbs per square foot of the waterplane, the waterplane is the plane surface where the waterline cuts the hull.

To obtain the area of the waterplane you draw the waterline onto the plan drawing by using the reverse of the process we used to draw the sections. You can then measure that area on the plan drawing and multiply it by 2 to get the area of the whole waterplane. If you're using a CAD program this is easy, else you must have a planimeter or draft in small squares and count them.

In the case of our salmon boat the area of the waterplane is 40.02 sqft and we nominally set a displacement of 2000 lbs. Divide 2000 by 40.02 and you get 49.98 lbs per sqft. Mr. Farmer opines that that would be a very lively boat. If we multiply 40.02 by 64, the per foot weight recommended by Mr. Farmer we get a displacement of 2561.28 lbs.

All this has been pretty theoretical so far. To get a better idea of the displacement as shown on the drawing we need to do a little math. I have spreadsheet that does all the math for me. All I have to do is put in the half breadths and half areas and what are those I can hear you asking?

Halfbreadth – the distance from the centerline to where the waterline cuts the hull at each section.

Half area – the area at each section encompassed by the hull below the waterline

The math is Simpsons Rule, and to read more about Mr. Simpson and his rule you need to get Mr. Farmer's book or google it.

Having plugged in all the numbers we get the following,

Displacement 913.2365 lbs, well that's not going to work so back to the drawing board, well maybe not. The calculations give us a prismatic coefficient of .53 but our earlier calculations were predicated on .66. Maybe, just maybe, our weight estimate was off.

This design is essentially a power dory and meant for a low power inboard such as the RATO 11hp, well that engine only weighs 31 Kg without oil or fuel add the oil and fuel and tops 40 kg or 88 lbs. 11Hp will push this vessel at hull speed, a little over 4kts. It is not a planing vessel there is too little bearing surface at the transom. According to this site, http://www.psychosnail.com/boatspeedcalculator.aspx we could get away with much less horsepower which would reduce the weight even further, the 6.7 hp engine only weighs 16kg plus 9kg oil and fuel 25kg or 55 lbs.

So lets review the weights.

Hull construction roughly 400

Engine and fuel( including spare gas) 75

Prop and shaft 25

Gear 150

Crew (3 adults 225lbs each) 675

Total 1175

Less 913.25

Difference 261.75

Our calculations told us that the pounds per inch immersion were 240 lbs so if we increase the draft by an inch we should be right on the money. But that's the draft over the entire vessel but we don't want the transom to be immersed so lets drop the bow by an inch keeping the same waterline and increase the draft at station 5 by 2inches then we should be alright.

By changing the draft without changing the half breadths our recalculation is much easier and the displacement worked out to 1177 lbs.

Before we stop for the day lets look again at Mr. Farmer's 64lbs per sqft of waterplane, with a waterplane of 40 sqft the displacement would be 2560 lbs we could only accomplish that by greater draft and a longer waterline. Clearly that measure is not suitable to a small open boat.

Next time - where do we go from here/

Curves of the fairer kind

Fair curves

When I was taking the Westlawn course, well worth the money by the way, all the work was done by hand as we have been discussing in the posts until now. Much cursing and erasing was needed to get the curves to fall fair. The cause of all that is the limitation of the medium. A 1/16^th pencil line on your 1/2”=1' drawing is an inch and a half in the real world hence the need for full size lofting when building.

I just want to take a moment here to dwell on the need to actually build some of the boats you're designing. A sound knowledge of how boats go together makes you a better designer. I spend a lot of time building the boat I'm designing in my head. Imagining all the joints, frames, beams and ceiling which would all surround the interior design and really determine how that design would come together.

Back to the drawing, a minute error in transferring a half breadth or height above the waterline causes a cascade of errors in the curves and you'll get a flat spot or a hard spot which has to be smoothed out by subtly changing the lines drawing. All that can be avoided to a great extent by not doing drawings by hand.

When I was about two thirds of the way through the Westlawn course I bought a new computer, it came with a bunch of bundled software one of which was a 1985 or so version of AutoCadLT. I began exploring lines drawings using this program and rapidly learned that the accuracy of the drawings increased exponentially. And with accuracy came better lines.

I used that ancient version of AutoCad for the next ten or more years. Then along came Windows 7 and that program would no longer work. So I began to hunt around for another CAD program within my budget, AutoCad being stratospheric with respect to the budget at hand. I continued to use AutoCad on an old laptop until I discovered QCad. I have been very happy with this program, primarily because it is easy to master the basics and secondarily because it allows for the importation of AutoCad drawings and lastly because it was free.

Why am I enamored by CAD? Because you can do the basic things, like drawing the grid, very quickly and accurately. You will recall in the third post we constructed the grid and I spent some time talking about having to change the spacing because 13' was not easily divisible into ten spaces. With a CAD program you don't need to worry about that. You draw your waterline and then a perpendicular line at one end, tell the program that you want a circle of 1.3 feet radius, apply that circle at the intersection of the two lines and copy the vertical line to the intersection of the circle and the waterline and tell the program that you want 10 copies. Bingo. It takes longer to tell you how to do it than to actually do it. And erasing the circle is just a key click, no muss, no fuss.

Curves are dead easy, you establish the point at the end of the bow, the low point of your sheer and the end point of the transom, click on the curve and connect the three points, voila! a fair curve.

Transferring points from plan and profile to the sections drawing is done the same way as as I described in the last post only now there is little or no room for error. A line drawn from a single point, such as the intersection of station five and the sheer in plan can be drawn accurately perpendicular to the centreline of your transfer drawing, creating an accurate intersection with the 45 degree line and an accurate line from that intersection to the baseline of the sections drawing. Again erasing all these lines takes just a key click and if you erase the wrong line you don't have to draw it again you just undo the mistake.

In the last post I ended by saying that we would discus why the curves fell fair on the first go. It's because I used my CAD program to draw the lines and two fair curves in plan and profile will naturally produce fair curves in the sections.

From here on in I will be using the CAD program to develop our other drawings in this design, however everything I tell you can be done by hand just more slowly.

Next week – what are those other drawings?

The Tricky bit

The Tricky Bit

When we left off we had our boat in plan and profile. The job now is to see if those lines will match up in the third dimension and still have fair curves. This is done by drawing the sections.

In the cartoon the sections are drawn in the middle of the drawing about station 5 which is used to represent the centreline (CL). You can in fact draw it any where, the reason I chose that particular position was that I was doing this without most of the tools and no drawing board. So to lay off the sections I was using tick strips. You can read more about this method here (http://www.lackeysailing.com/daysailor/sidebars/tickstrip.htm)

In that application the person is using a large piece of wood as he is doing it full size, however you can use strips of paper to do the same thing on your scale drawing. In order to do this you must first draw in a waterline (WL) where you are going to put your sections drawing, in this case I put it mid way between the profile drawing and the plan.

In the following picture a small slip of paper is laid along Station 5 and ticks are made at the WL, the chine (the curve of he bottom) and the sheer.

This then transferred to the new WL and CL by lining the strip along the CL with the WL tick at the WL. Then the strip is laid out on the plan to take off the widths and the widths laid out in the same way along the WL. Lines are drawn from the ticks parallel to the WL and CL and where they intersect are the sheer and chine points. You draw lines through those points and you have a section. This is time consuming and prone to error.

There is another way. To the right, or if you prefer left, of the drawing extend the WL and draw an new CL perpendicular to the WL. Extend the plan CL to intersect with the new CL and from that intersection draw two 45 degree angles.

Then using your triangles transfer the plan and profile measurements. First the plan,

You will probably have to extend the line of the second triangle using a ruler, make a tick mark on the diagonal slide your triangle down and do the same for the chine line in plan.

Then slide your triangle along the yard stick and draw two perpendicular lines through the marks.

Now go to the profile and draw Two lines parallel to the WL from station 5 at the sheer and chine.

Join the intersections, erase all the unneeded lines and you have the half section at station 5.

Now do that for all the other stations with stations 0 to 5 on one side of the CL and 6 to 10 on the other. Draw a line parallel to the baseline from the bow to the CL. You end up with this,

And now for the tricky bit. Join the ends of the section lines at the chine and at the sheer to see if you have a fair curve. In most instances, when drawing by hand, you won't on the first try and will have to alter the lines in plan and profile to make the curves fall fair. That can be exceedingly time consuming.

Once that's done you have this,

In this case the curves fell fair on the first go and there is a reason for that, we'll discuss that next time.

More tools of the trade

More tools of the trade

In the last post I said to make a line parallel to the yardstick 6” up, that was six real world inches not scale inches and the same for the center line. All other measurements were to scale 1/2”=1'. In this post all measurements are scale unless otherwise specified.

We ended the last post with the following grid,

On this grid we will construct the lines in plan using the centerline (CL) marked in red and in profile using the waterline (WL) in green. From the cartoon we know that the proposed draft is 6”, and it appears halfway between station 5 and station 4. Measure down from the waterline 1' on stations 4 & 5. Draw a line from the intersection of the WL and station 4 to the mark on station 5 and vice versa, that will establish the point of maximum draft thusly.

Now we need to construct a fair curve from the intersection of Station 10 and WL through the point of maximum draft to the intersection of station 0 and WL.

This brings into play new tools, french curves, ship curves and splines and ducks otherwise known as spline weights. The basic drafting set contains french curves but in order to get long fair curves you really need ship curves or a spline and ducks.

A spline is a narrow sectional piece of wood or plastic which can be bent through several points to construct a curve. The spline is held in place by lead ducks,

These things are hard to find and when you do find them they are expensive. When I was doing the Westlawn course I carved two male molds and took then to a local foundry and had them cast ten for me which was way cheaper than trying to buy them, Or you can try to make them yourself, there is a good article at Duckworks on how to do this. (http://www.duckworksmagazine.com/06/howto/splineweights/index.htm)

Once you have all your ducks in a row then you can use the spline to draw the curve of the bottom.

And then remove the X from between stations 4 and 5. Which brings up another tool the eraser, get a good one you'll be using it a lot and you'll also need an eraser shield to protect the parts of your drawing you don't want erased and a drafting brush to sweep all the eraser bits off the drawing so it doesn't smudge.

Once the bottom curve is drawn turn your attention to the bow and stern. From the cartoon we know that the top of the transom is 1' aft of station 10 and 2' above WL, using dividers mark out those points.

Problem! The cartoon length over all (LOA) is 17' but we only want 16'. If we whack 1' off the LOA will we still get the sheer (the curve of the gunwale in profile) that we wanted? The only way to find out is to draw it in and look at it. So, again using the dividers, mark out a point forward of station 0 3' up from WL and 2' forward of station 0. Then draw a line from the mark aft of station 10 to the intersection of station 10 and WL. Do the same at the bow.

From the cartoon we know that the sheer is at it's lowest point at station 7, 1' 9” above WL so using the spline and ducks we'll construct a curve from the bow to the stern through that point on station 7.

Now take a good look at that line from all directions, is it fair, does it look good. I think so, in fact I think it looks better than the cartoon. So 16' is good.

Using the same process of taking measurements off the cartoon and using our spline and ducks we can draw in the sheer and bottom line in plan.

So far so good but the real test of all this will come when you draw the lines in section like the center portion of the cartoon. We'll discuss that next time.