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.

Construction Plan

Construction plan

The Westlawn scantling plan is a series of calculations based upon the constant C which is derived from the length overall (LOA) and the extreme beam (EB) using the equation (sqrtLOA +EB)/16. That constant is then applied in other equations to determine the scantlings. Suffice it to say that what you get is this,

PART                                                                      SIDED                                        MOLDED
PLANK KEEL                                                      0-1-2-0                                           0-2-4-0
UPRIGHT KEEL                                                  0-2-4-0                                           0-3-1-0
STEM                                                                    0-2-1-1                                           0-3-6-0
HOG                                                                      0-0-6-1                              KEEL PLUS 2 TO 2.5 IN.
TRANSOM                                                           0-0-7-1
BENT FRAMES                                                   0-0-7-1                                            0-1-2-0
BENT FRAME SPACING                                   0-7-4-0                                            0-0-0-0
FLOOR TIMBERS                                               0-1-2-0                                            0-3-6-0
SAWN FRAMES (SIDE) HEAD                         0-0-7-1                                            0-1-7-0
HEEL                                                                    0-0-7-1                                            0-4-3-0
SAWN FRAMES (BOTTOM)                             0-0-7-1                                            0-1-7-0
SAWN FRAME SPACING                                  1-8-0-0
DECK BEAMS (UNDER WALKING AREA)    0-0-6-1                                           0-1-2-0
SINGLE PLANKING                                          0-0-5-0
DOUBLE PLANKING INNER                           0-0-2-0
DOUBLE PLANKING OUTER                          0-0-3-0
CHINE                                                                  0-0-7-1                                            0-1-4-1
SEAM BATTENS                                                 0-0-3-1                                           0-0-7-1 OR MORE
CANVAS COVERED DECK                               0-0-5-0                                           0-0-0-0
CABIN ROOF BEAMS                                        0-0-5-0                                           0-0-7-1
CABIN ROOF (NOT WALKED ON)                  0-0-3-1                                           0-1-7-0
STRIP DECK                                                        0-0-6-0                                           0-0-7-1
CABIN SIDES                                                       0-0-6-1
CLAMPS AND BILGE STRINGERS                  0-0-6-1                                           0-1-7-0
ENGINE BEDS                                                     0-2-4-0

which reads in feet-inches-eigths-sixteenths. The reason there is no molded for the engine beds is that that measurement depends on the motor.

Hang on, I can hear you say, what does sided and molded mean? Sided is width and molded is depth. Except for the hog where the opposite is true.

We now have to apply that to our design.

You'll notice that the suggested spacing here is 20 inches whereas our station spacing is 15 inches plus a bit. I like to start the spacing from station 5 so we copy our lines drawing and erase the station lines except for station 5 and draw in the frame spacing remembering that the station line at 5 is the centre of the frame at 5.

The sawn frames, which is what we're using, are sided 7/8 plus 1/16 near as all get out to an inch so we'll heft up to an inch.

You will recall that I said from here on in I would be doing all the drawing in the CAD program and here's how you do it.

At the intersection of the waterline and station 5 draw two circles, one a half inch radius and the other a 20 inch radius. The first gives you your frame siding and the second the frame spacing. Draw two parallel lines through the intersections of the 1 inch circle and the waterline, then copy those two lines to the intersection of the 20 inch radius circle and the waterline and tell the program you want 4 copies, do that in either direction and you end up with this.

You'll want to tidy the drawing up and move the frame at the bow back about 4 inches and the frame at the stern forward about the same amount to get this.

There you have your frames all laid out.

I do the stem next and then the transom and then work aft from the bow as this is how I build an actual boat.

Our scantlings show the stem as sided 2 1/8 th and molded 3 6/8 ths. This is can be easily constructed out of three pieces of 1x4 glued together with a filler piece glued to the after edge. Using this form of construction allows for a constructed knee integral with the stem. For example,

However, having said that, you can draw what you want but the builder will build it their way unless they're an absolute novice. Be prepared for questions if they are.

The transom is 7/8 ths and a sixteenth, again close to an inch, up is better than down so 1” ply for the transom. And again the built up knee for the transom. 

 

Then the bottom frames go in, sided 1”, molded 2”. From frame 5 forward, forward of the side frame and from frame 5 aft, aft of the side frame, like this,

You'll notice that I show the bottom frame at the final aft frame let through the transom knee and the one forward above the stem knee, this is the way I build it, others might do it differently, that's up to them.

Next time we'll talk about chines, gunwales and stringers.

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.