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[tonyH]

Over the last couple of months I've been using up scrap stuff for this. I'm only going to use the fore and aft sails and the square rig will be tightly furled. I intend to have a weighted sailing keel to replace the centre keel so is it trial and error to balance the sizes of the fore/aft sails or is there a rule of thumb method to start from? Any guidance would be much appreciated.
Thanks
Tony

[warspite]

My sovereign (Airfix 1:180), it was more about the weight above the waterline whether she healed over or not, it may mean a deeper dagger board with bulb than is scale correct i.e. the top half of the dagger board to the underside of the hull is lighter than the bulb with the weight in the whole of the bulb to give more 'righting moment', just my tuppence worth.

[tonyH]

All 3 keels on the real ship lowered down from the hull. HMS Trial was a test bed for the idea so she'd be good for shallow estuary waters. She's also quite beamy in the Dutch yacht way. For sailing I'm going to replace the centre drop down with a dagger board and lead bomb to suit, once I know the all up weight. What I'm thinking of is the comparative sail areas fore/aft of the dagger board and the best way of balancing the forces in the assumption that the dagger board is, effectively, a fulcrum. I'll probably cut the first sails from plastic bags and chop away but it would be useful to have at least sort of "cunning plan"

[roycv]

Hello the arithmetic is simple.  My first look at the proposed sailplan is that it is a tad too much. 
You have to work with some "knowns"  For instance hull length and displacement including keel weight.

Starting from there get the keel weight to 40 / 50% of the total displacement.  Work out the practical depth the keel will be.  Rule of thumb now as a starter.

Weight of keel in say pounds multiplied by the depth of keel from waterline in inches.  Call the answer in inch pounds.
What is the wind like where you sail?  A 20mph wind has a force of 1 pound per square foot of sail, this we can use as a guideline.  The point at which the wind affects the boat is the centre of area of the 3 sails.  If you can't work this out then to be honest don't bother to read on!  Not meaning to be offensive but it is pre-GCSE level sums.

The hull will react to being pushed sideways, you said only fore and aft sails, to find out how the hull will react, load it down to the waterline in the water and using a finger push the boat sideways, it will slide off to left ot right but moving the finger and trying a few times you will find a sweet spot where the boat travels sideways only, mark this point.  I use sticky masking tape and a pencil.

The previously calculated centre of areas (?) (NB NOT the mast) for the sails should be placed 4% of the WATERLINE length of the boat forward of this point.
Measure the sail area centre height above the waterline in inches divide this into the the numbers you have for the keel (weight and distance to waterline) and this will tell you how much sail area in square feet you can use.

You can have a larger sail area but it will make the sailboat more tender.  Likewise with these numbers you can increase and decrease the distances and weights to manipulate where you want to be.
It really is easy and the results are guaranted!
I have used this calculation in the last 4 yachts I have built or restored and it is totally effective.  The others were already where the designer wanted them.

You do have some tuning you can do with a few degrees of mast slope or adjusting sail trim.
I do hope this helps, some get by with guesswork but have to have the rudder hard over to stay on course.
regards
Roy

[tonyH]

some get by with guesswork but have to have the rudder hard over to stay on course.

Cheers Roy, it's exactly what was needed! It'll be good to re-open the trig and geometry side of the brain again.
Tony

[roycv]

Hi Tony glad to have been of help.  I do the initial C of E calculation by construction on gragh paper.  Then with a rough idea of what to do I cut out sails full size and work on the kitchen table moving sails and mast with reference to the hull size to get it looking right.
Using my methods I put a gaff rig on the Bella (Bermuda rig) yacht and it was doing the initial full size sail check that I realised I needed a bowsprit as the mast would not move any further aft.  The additional fore sail, which is partially adjustable, has been a bonus as this improves the airflow across the sails and speeds her up.  She sails very well, hands off on a reach and spins on a sixpence.  All on the basic hull and rudder.
If you hit any queries let me know.
Good luck
Roy

[tonyH]

Thanks Roy, I'll keep you posted!
Tony

[tonyH]

Hi Roy,
Finally got Trial on the water and there's enough capacity to have a decent lump of lead underneath so I can now get on with the keel, masting and rigging and see how your system works