Cowl Ventilator

What are they?
CowlVentilatorFor a long time the most common ventilator was the cowl ventilator until the advent of forced ventilation using power-driven fans. Used on all types of vessels it was used to supply fresh air and remove foul. The ventilator could be rotated to face forward to gather fresh air and rotated to face to the stern to extract bad air.

The diagram to the left1 gives some basic (relative) dimensions of a Cowl Ventilator.

and more..
The Boiler Room is fitted with two cowl ventilators, of larger diameter than those in the Engine Room. They are fitted in the two forward corners of the boiler hatch and stand above the Fidley Hatch to have a good draft. The cowl hood of all these ventilators is built separate from the ventilation trunk pipe (which is round in section) and rests on top of the trunk as a guide, so the cowl can revolve. On the outside of the cowl, below the throat and just above the trunk a gear rack is fitted to engage with a pinion gear wheel. (operated from Engine or Boiler Room). The ventilators in the Boiler Room are made larger in diameter as they are also used as elevator shafts. The Boiler Room has ash buckets which are used to raise and dump ashes over the side into the dump scows while in harbor, if the steamer is a coal burner.2

The above shows how I could use a number of ventilators – 4ea of one size and 2ea larger versions.

Follow me and Curves
One of the most useful tools in Sketchup is the ‘Follow Me’ too. You can create a path .. a straight line or a series of joined curves and pull a shape through that path. You can make molding follow a wall .. the handrail of a curving staircase .. or a pipe elbow. Once you understand how it works it isn’t that hard.

Using the pipe elbow as an example – you create a curve and then use the Follow Me tool to push a ring along that curve to create the elbow. That curve by the way isn’t necessarily 90° even when you want a right angle elbow. The reason is that when you look closely at the curve it is a series of straight lines. The greater the segments the smoother the curve. If you think about it a second you can see that depending on the way the software creates this curve a curve might not end exactly at 90°. The best way I think to understand this is to think .. if you tell the software to draw a circle .. with three sides it is a triangle. If you tell it to use five segments to draw a circle then it is a pentagon. I normally use at least 60 segments for a circle – depending on the size of the object we are wanting to print in 3D .. if the segmentation is less then the resolution of the printer then there is no need to increase that segmentation.

This is I suppose useful information to anyone wishing to use Sketchup but if I try to create a reducing elbow – which is really what the cowl ventilator is at a basic level, we have a problem. Sketchup’s native Follow Me tool won’t do that .. so we need a plugin.

Enter .. Crviloft

curviloftThis plugin does various things but here the “Loft along Path” is what we are interested in. The link here is to You have to register to download (and possibly even see) the plugins but that is simple and easy enough.

The plugin on this page is described as being released in BETA without documentation. There may be a newer version out there but for the purposes of this tutorial – using it to Loft along a Path, no documentation is needed.

[Plugin Beta] Curviloft 1.3a – 27 Nov 13 (Loft & Skinning)

I swiped the photo on the left from the link. Notice that the elbow shown moves from a circle to a ‘wonky’ shape. That is how we will use the plugin .. just not so wonky .. or at least .. a different wonkiness ..

overlayThe first thing I needed to do was get some dimensions – or more accurately .. relative dimensions. From the book – Working Scale Model Merchant Ships by Tom Gorman we have the diagram showing the basic relative dimensions of what is labeled “Standard Pattern Cowl Ventilator”. I imported this in Sketchup as an Image so I could overlay it with some lines. One line following the Cowl opening and another line across the trunk even with the bottom of the first line. I added an arc from the center of both of these lines (this would be the path). I added a dimension from the center of the trunk over to the center of the cowl line and using the Protractor tool found the angle that the cowl leans out. Finally I used the Tape Measure Tool to dimension the cowl line to 1. The other dimensions change instantly relative to this ‘master’ dimension.
cvRemoving the image so we just have our dimensioning lines left. We have the mouth of the cowl set by a line at 1. The trunk dia by a line at half that at .5 and the horizontal distance between the center of the trunk and the cowl opening at .328 .. and that the cowl leans over at 97.2°

In addition to this we only need the dia of either the trunk or the cowl to get started.

Step 1 – make a copy
cv2Ok. Now in Sketchup I switch from Parallel to Perspective View – I just like working in Perspective.

I made a copy of the diagram and moved it to one side. Three reasons – 1) always good to work with a copy, 2) leaves the original to refer back to and 3) mostly -this is so you guys see what I am doing. I’m trying to show something that is dynamic using static pictures.

To copy you press “M” (Move) and then press and release the Cntrl Key. I held the ‘Left-Arrow Key’ down and move and release the copy where I want. The Left-Arrow Key will constrict movement just to the Y-Axis.

I delete everything except the two lines and the arc.

Step 2 – make a plane
cv3The larger – semi-vertical line represents the cowl dia. Note I said ‘semi-vertical’. It is angled back at 97.2°. To make it easy to draw a circle I extended lines out from the ends of the lines .. the blue lines .. and then connected the ends of those with a pair of additional lines .. the red lines .. so a plane is formed at that 97.2°.
Step 3 – draw circles
cv4Now when we draw a circle on that plane the circle-tool will align to the plane. Draw a circle so it touches both ends of the line. The radius will show as .5 inches (you can use whatever measurement system that makes you dance in joy. Me .. I use decimal inches). Before you do anything else .. including move the mouse .. increase the segments for the circle. I like 60 .. so I enter “60S” (60 sides) and hit enter.

Repeat on the bottom, drawing the .5 inch circle. The window at the bottom will read .25 inches Radius. You don’t have to worry about aligning the tool for this circle since it is on the XY plane.

At this point I am moving to page 2. Keep where we are in the back of your mind as I am going detour to determine the size of the cowl ventilators needed .. but will come back to this point to continue the build.

  1. Working Scale Model Merchant Ships by Tom Gorman []
  2. A Handbook of Practical Shipbuilding: With a Glossary of Terms – by James Douglas MacBride – 1918 []

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