The Thin Green Line

The Thin Green Line

In my last post I said that I would explain the use of the multiple waterlines. Firstly, why are they green? Because I like them green, as soon as I see a green line I know it's a waterline.

Why are there so many? Because we will use them to develop the 3D shape of our boat along with other reference lines.

So far we have our midsection, bow transom and stern transom. The thing that joins these entities is the water line, By transferring the location of the LWL on these three sections we can start to develop the shape.

First we draw in several waterlines. I will explain further later.

Next we locate the point of contact in plan of the LWL and the three sections thusly, note the red circles,

and join the points with a fair curve, very easy on the computer.

Then we do the very same thing for each of the other waterlines like this,

Until we have this,

You can deduce from what we have just done the reason for the many waterlines.

Now we can go back and construct the other sections using the waterlines we have just created.

Onward

Onward

The question in the last post was where do we go from here. Well we go back to here and using the grid we constructed in the last post showing the profile and plan we add our new midsection. We can use the pram bow from RMS and we can take RMS's transom and round it to give us this.

You'll notice that I'm using the technique we discussed here to transfer the basic dimensions.

So now what?

Well we can transfer the sheer to the sections by using the same technique giving us this,

If we connect up all the intersections and the end of the midsection and bow transom correctly we have the sheer from bow to stern on the sections drawing like this.

You'll notice a couple of oddities, for one we don't have a section to the right of the centreline and there are extra green lines.

Lets deal with the green lines first, they are water lines, the LWL is the lowest one. I will explain their use in the forthcoming posts.

The missing section is section 6 and it's not there because we have no idea of the shape but we'll use what we do have to determine that shape as we develop the new round bottom boat.

Round and round we go

Redesign

As we saw RMS was designed as a flat bottom boat, but would it be better as a round bottom boat?

We'll have a go at a redesign based on RMS. Here's RMS midsection as originally designed,

That midsection has a submerged area of .87 sqft. With a prismatic coefficient of .56 and a length on the waterline of 12.24 ft. that's a projected displacement of 373 lbs.

A more rounded shape with the same over all dimensions looks like this,

If we overlay the new midsection onto the old midsection it is obvious that we would lose a significant amount of displacement.

The new midsection area is .76 sqft. Using the same prismatic coefficient and the same WL we get a projected displacement of 325lbs a loss of approximately 50 lbs. If we can live with that then lets proceed.

But how to we proceed from here?

Construction Redux Anew

Construction Redux Anew

Way back here and here we talked about the construction plan and I suggested that you build a few boats to find out how they go together. When you are considering a design you should have some idea of the construction method you intend.

Let's look briefly at RMS, which is, as designed, a hard chine boat which lends itself to plywood and carvel construction.

 
You could do the sides in clinker (lapstrake) but that's a complicated joint at the chine. However there is a little twist. If we look at the lines plan,

You'll notice that the bottom of the boat is not altogether flat, and there is a little triangular section in way of the keel. 

There are several ways ways to do that and at least one of them should be shown in your construction plan. I'll show you one,

This diagram shows ply planking 1/4” for the sides and 3/8ths for the bottom, the chine and keel joint should be epoxied with fiberglas tape.

But should this boat be round bottom? Would that change the design significantly?

Here's the redesign,

It looks much the same as the picture at the top of the article but it is a very different boat. And we'll explore those differences anon.

Shrouds and stays

Shrouds and Stays – Yes/No

Here is the new interior arrangement in plan,

and profile

The bury, the distance from the deck mast hole to the mast step, is 20” and the foredeck is 32” wide in way of the mast. The mast is well supported by the foredeck and the mast support beam that I plan not to have a forestay or shrouds (stays go fore and aft, shrouds athwartships) which simplifies rigging.

In place of the rowing bench in the original I have gone with a fixed thwart and with the removal of the engine well we have enough room for sternsheets.

There is enough room between the sternsheets and the rowing thwart to sit in the bottom of the boat when sailing.

The pad eyes for the sheets are shown in the profile and plan but the location of the oarlock is not as that location depends very much on the rower, although generally speaking 12 to 14 inches forward of the thwart is usually right.

The standing lug employs two sheets which are led to the quarters, this allows for sail adjustment absent a boom. The biggest problem with this sail is the twist that develops at the head. That can be corrected with a yard vang, a line from the end of the yard to the quarters to adjust for twist, but that is overly complicated for this size of boat.

So there you have it, Row, Motor, Sail (RMS).

Next time we'll talk briefly about construction and would this be better as a round bottom boat.

Changes yet again

The Look Astern and Other Changes

The stern on the original design was also quite complicated. It had a motor well for the outboard which could be sealed off when the motor was not fitted by a plate which was held in place by a bungee cord attached top a removable curved piece that fit into two slots in the side of the well. The space on either side of the well was for storage and flotation.

So in the new version the transom is flat across and the well is eliminated. And an electric trolling motor is fitted through a PVC tube right aft of the dead wood. The original also had a leeboard which in the new version I've traded for a centreboard.

Because all the spars had to fit into the hull for rowing they were quite short which constricted the sail area. It was about 49 sqft, a boomless standing lug sail, with sheets led to each quarter. There were no permanent stays or shrouds.

This is the new design with the old rig.

The previous design was more row, motor, sail. In the new design I wanted more emphasis on the sail and less on the motor so I increased the sail area to 80 sqft keeping the the same foot and yard as in the old sail which means that the mast is now some 17 ft tall which might require stays and shrouds. So here we have the new rig. You'll notice that the centreboard is located in exactly the same place as with the old rig. But, because of the shifted centre of effort of the sail, it is deeper.

The sheets also had to be beefed up a bit given the increase in sail area.

We'll look at the interior arrangement next time and that will determine how we can lay out the shrouds and stays or even if we need to use them.

RMS Some More

What's the diff?

In the hull proper there isn't much difference. In the original version the bow transom was curved at the top and also curved athwartships. Working out the build for this little quirk was difficult and it may be beyond the experience of the typical backyard builder so I made the new transom flat.

In the original design the keel was a flat plate with a the keel  attached to it. This came out of a design I was toying with much earlier. Here is a picture of a model of that design. I have long since lost the actual drawings during several moves but the model remains. You can see in the picture of the bottom the space for the keel piece which is a tapered, curved and beveled 2x12. Again probably not the best design for a backyard builder. It took me quite a while to get the model right.

I am a great believer in models. When I did the design for the WoodenBoat competition I made a half model to make sure the design would come together off the paper.

In the next  picture you can see the keel piece.

The change to the keel, which adds a box to the keel was to get the flow right to the new motor which is electric versus gasoline and we'll delve further into that as we develop the design. I don't think that the box keel will be that difficult to build as long as the build method is ply or plank on frame versus stitch and glue.

My instructor at Westlawn commented that this would be a wet boat because the sides are almost up and down at the bow, he obviously hadn't spent any time looking at Phil Bolgers designs for small boats.