It's all about profiling

Outboard profile

The out board profile is what anyone looking at your vessel from the side will see. One of the perils of drawing outboard profiles by hand is the perspective effect caused by you looking at the profile with its top edge away from you. What appears to be a neat boat on the drawing board suddenly doesn't look all that swell when pasted up on the wall. The advantage of computer drafting is that you are already looking at the boat as it would appear on the wall.

When you are drawing the outboard profile you should include everything that you, as the designer, planned to have fitted on the boat, the builder/owner may have other ideas. So be it.

So we start with the outline of the hull as we drew it in the construction drawing. And remove any extraneous lines.

So we need to add a house. If you recall the original design called for a place to get out of the wet and an open cockpit. The design was also for up to three adults so two can be out of the wet and the other can steer. So the house should be at least half of the waterline length. And a way for those inside to see outside, a port perhaps and or a hatch. Now a comfortable seat needs to be about 15 or 16 inches high and with the same depth and minimum head room is 38 inches so the cuddy top should be some 4'6“ above the midship frame. A hatch needs to be a minimum of 2 feet square. The port can be any size or shape but should fit with the overall design. I always like a small foredeck, not that you would want to stand on it, but it gives a place for a substantial samson post forward and a small storage locker below.

So all that would look like this.

The cuddy would be open at the back for air in the cuddy and also to provide air for the engine which is air cooled. We could also decide upon a rudder at this stage.

The rudder could be transom hung or be under the boat on a rudder shaft. Because this is a small slow boat I think that a transom hung rudder would be better and fits with the type of boat.

OK you say but what about the propeller and engine. Well in this profile you can't see the engine but you're right about the propeller. The problem is you can't show the propeller until you decide the angle of the shaft and to do that you need to decide where the engine is going to go. Which brings us to the inside profile.

I lied when I said in the last post that we would do the deck plan, we really need to establish where the engine will go and what the shaft angle will be because then we can finish the outboard profile and the construction drawings for the keel.

Inboard Profile

We start with the almost completed construction profile, see why this is easier on a computer.

Then we add the cuddy that we drew on the outboard profile.The port, samson post, fore deck and some seating at the transom and in the cuddy.

Now for the engine. We have the specifications for our engine. It is 15.4 inches long, 12.6 inches wide and 13.6 inches high, the shaft height above base is 4.17 inches to the centre of the shaft. The propeller that we have is 8 inches in diameter so once we draw that in with the shaft exiting from the back edge of the keel and the propeller tip 2 inches away from the bottom of the boat, to stop thumping, we can establish a shaft angle of 5 degrees which will not interfere with the lubrication of the engine.

There is a little bit of a problem, the prop spins just a little below the keel which, if the boat takes ground, will cause damage to the prop so we'll put a skeg on the keel to protect the prop.

Now that we have established the shaft angle we can extend that back into the boat and determine where the engine will sit and draw in the engine bearers. But we have a problem, the optimal RPM for the prop is 600 RPM the engine runs optimally at 3600 RPM so we need reduction gearing. There are two ways to accomplish this, an actual reduction gear set up or v-pulleys. Given that this project is driven by cost, V-pulleys it is. A 2” steel pulley on the engine output shaft and an 8” pulley on the propellor shaft.

All of which means that the engine must be several inches above the shaft centreline probably, about a foot. There are calculators for this at http://www.gizmology.net/pulleysbelts.htm. For the purposes of this exercise we're going to place the two shafts 12” apart. Given that distance we get this,

The square box is the engine, the area under it is the engine mount, the beds should cover at least three frames to reduce vibration and strain on the hull. Covering the shaft coming into the boat is the shaft log. The concentric circles on the engine are the exhaust. We can extend the exhaust to show on the outboard profile and draw it in.

And there is the almost complete outboard profile and the almost complete inboard profile.

Outboard Profile

Inboard profile

Next time we'll finish both these drawings and move on.