Numbers

BY THE NUMBERS

I ran the numbers on these two versions, here they are,

Version	RMSQ&D	RMS
Displacement	304.89 lbs	266 lbs
Block Coefficient	0.18	0.16
Prismatic Coefficient	0.48	0.53
Area of the waterplane	25.55 sqft	27.78 sqft
Centre of flotation (aft or forward of Station 5)	.4381 ft aft	.4772 ft Aft
Lbs per inch immersion	152.29 lbs	137.49 lbs

So what does that mean exactly?

Well Q&D can carry more weight at the designed waterline by about 40 lbs and it takes more weight to sink it any further into the water.

Skene's sets out that a prismatic coefficient between .49 and .55 is best for sailing vessels, any more than .55 and you have a tub, any less than .49 the vessels is so fine it drags a huge quarter wave. So Q&D is on the fine side and RMS is within the parameters set out by Skene's. However the block coefficient tells us another story, by that coefficient RMS is the finer vessel.

Skene's also sets out that the center of flotation should be between 54 – 59% of the LWL aft of station 0 or in our case, between .48 ft aft of station 5 to 1 ft aft. So Q&D is a bit too far forward and RMS is just about right.

The LWL is 12ft for both these vessels and the beam at the WL is 5ft 4in which is a beam to length ratio of 2.5 which is a little beamy, anything from 3 to 5 is better, but the beamyness in such a small vessel adds to the initial stability.

Next time we'll look at stability.

Beyond the lines

The Drawings

The lines drawing that we have just completed is just the start of the process. To make a complete set of drawings several more are required. The list can vary depending upon the design. The following are the basics for small boats.

Lines drawing

Table of offsets

Construction plan

General arrangement plan

Out board and inboard profile

Deck plan

Machinery

We'll move on to the construction plan and come back later to the table of offsets.

Before starting the construction plan a few things need to be decided. First off, are the lines drawn to the inside or outside of the planking? What material will you recommend for construction? And what construction method?

Inside or Outside

This question is not very important in a small boat but in a larger vessel it makes a great deal of difference in the displacement. If the lines are drawn to the inside then the actual hull will be wider and longer increasing the displacement. If the lines are to the outside then care must be taken in drawing the frames because where the planking has a bend in it so that it meets the frame at an angle the width of the planking is different,

it has the same thickness but the width is at an angle. This makes very little difference on a boat that is planked with 3/8 ply but a lot on a vessel where the planking runs to two layers of 2”. I draw all my lines to the inside.

Material

Will the planking be plywood or structural lumber? In a boat such as this one, which will  be utilitarian, I would use plywood and epoxy cloth over the outside, we're designing a work boat not building a piano.

Construction

Plank on frame, plywood on frame, stitch and glue, these are the choices for a hull of this size. Plank on frame is traditional but may require skills not possessed by your client, in this case you. Stitch and glue uses modern methods and materials but it requires you, the designer, to lay out the panels, not always easy to do. There is a program, Hulls, which will do it for you but it doesn't work well with Windows 7 for some reason. So for this project we will select ply on frame.

The last thing you need to decide before doing the drawing is scantlings, the sizes of the materials you have designed the boat to be built from.

There are many scantling rules to choose from. Since I did the Westlawn course I use their rules. I do like Goerge Buehlers rules which are very simple, build it big, build it strong.

So we'll end here and next time start the drawing.

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/