Virtual Design - part 5

First lines

Let's start the design. First some 3D sketches.
Here is a scan of the "design spiral" extracted from the excellent Cyrus Hamlin book "Preliminary Design of boats and ships".



Depending on the boat, some elements may be less important or even disappear and the sequence may be slightly different.
We will not try to solve every detail during the first turn, just get closer.
We will have to make changes a few times but when nothing changes anymore during a full turn, the design is mature.
(That's usually when I completely change my mind about something major and drop the project . . . just joking.)

Let's go through some sketching and see if we can make a first full turn.
We could consider that we already made full turn without even drawing a line:

  • the general concept is clear
  • the hull form is almost clear, we may hesitate a little bit about balance, wide sterns etc. but not much
  • for arrangements we will try to have a nice double in the stern but need some real sea bunks in the middle or is there another solution?
    I owned a sailboat of my design that had a double berth in the stern, under the cockpit and it was my preferred sea berth. It was rigged with a removable leeboard in the middle. It was altogether narrow enough to sleep well in bad weather and with the leeboard, became a queen size bed on anchor.
    This could allow us to have shorter seats in the saloon and a second large double in the peak. Whichever way we go, it can work.
  • Powering: priority to outboard version but an optional inboard should be possible
  • Construction: our composite method
  • Spars and rigging: sloop, 7/8, optional spi pole
  • Specs: see the previous table
  • Economics: inexpensive but not cheap

Back to the start and hull form, let's sketch it.

A good starting point could be a 30' design on which I worked years ago.

This is a proven design and we know his pros and cons very well.
We can improve: add a skirt because the dynamic waterline is too short and move the sail plan, it is too much forward.
This boat was designed before furling jibs became well accepted in offshore sailing and sail reduction was done mostly on the main. We'll move the whole thing back with the keel: that is why we show the sail plan, keel and their centers.

Scaled down to 27', with lines updated just a little bit, a small skirt added, and a new sail plan, here is what the hull looks like:

Too much rocker but note how the sail plan and appendages moved. At this point, the lead is 10%.

You may wonder why we worry about the sail plan so early in the design. It's because we want to know where the mast bulkhead goes for the inside arrangement. We sketched a cabin profile and deck: we have 6' under the roof in a boat that is 26.5' long but standing in the bilges with no allowance for roof beams.
Also, see how we will cut the skirt along the backstay line.


Same hull, quick render

and from the stern.

Let's run some quick hydrostatics calculations to see how she will float.

I expect the worse because of the rocker but we have to start somewhere. Here is the raw output of the hydrostatics:


Hydrostatic output file RhinoStatics v1.0 Copyright, 2002, BaseLine Technology

Inputs:
Displacement Draft 0
Trim LCG Depth Linear units, Model mm. Linear units, Output mm.
Weight units, Input Kg. Weight units, Output Kg.

Outputs
Length, Overall..............26.49
Waterplane Area...11304732.00 Sq mm
Beam.......................2957.01
Kg/Cm Immersion...........115.89 Kg
Depth, Midhsip.................N/A
LCB......................3791.91 mm
Length, Waterline.......7173.83 mm
VCB......................-152.26 mm
Beam, Waterline.........2288.60 mm
LCF......................4060.07 mm
Baseline Draft ............0.00 mm
Mom to trim 1 cm....95419.06 mm.-kg
Fairbody Draft ..........403.97 mm
Trim, Degrees..................0.00
Displacement...........2,773.31 Kg
Trim, ......................0.00 mm
Volume.................95.54 Cu mm
BMt.........................4.12 mm
Disp/0.01L^3................209.36

Ratios
Underbody Hull Length / Breadth............3.1346
Length / Beam................2.7301
Breadth / Draft.............5.6652
Beam / Depth....................N/C
Length / Draft.............17.7581
Length / Depth..................N/C

Coefficients
Block Coefficient ..........0.4079
Disp/0.01L^3.................209.36
Midship Coefficient.........0.7168
Prismatic Coefficient.......0.5690
Waterplane Coefficient.....0.6886


Don't worry about the spelling mistakes and the mix of units: that program is accurate.
For subsequent calculations, we will not show the whole file, only what is relevant.
Soon, we will switch to US units but metric is so easy: one liter is one kg and 1 cubic meter is 1,000 kg: no factors, less errors.

It's a little on the heavy side compared to week-end boats but if we want real cruising capability with a decent payload, we can not reduce it too much.

A displacement goal of 5,500 lbs with an empty boat weight around 4000 lbs, 2,000 lbs ballast is fine for our boat definition.
(We switch units all the time . . . get used to it)


The curve of areas look good. If this was the RM800, you would see the apex way to the right.



I would like to change the lines a little bit, reduce displacement, see if we can't get the accomodations we want in a smaller hull and maybe go for a larger main sail, smaller foretriangle but optional assymetrical spi? Maybe we can get what we want in less than 27', 25 feet maybe?

There is still a lot to decide. This was just a quick test to see where we stand with that length and hull shape.

Next step: a new hull.

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