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/