Model Boat Mayhem

The Shipyard ( Dry Dock ): Builds & Questions => Yachts and Sail R&D: => Topic started by: Mansfield_Marauder on January 05, 2012, 10:24:05 AM

Title: Schooner Design
Post by: Mansfield_Marauder on January 05, 2012, 10:24:05 AM
I have designed a schooner type vessel using something called Delft Ship, the free version. Can anyone tell me should the centre of lateral resistance be forward of the centre of displacement, I know the centre of effort for the sails should be just ahead of COL but I'm not sure where it should be in relation to other things such as the centre of displacement
Title: Re: Schooner Design
Post by: andrewh on January 05, 2012, 01:27:30 PM
Mansfield_Marauder, hi

I will not go near the theory - not lease cos I don't really know it in detail

In practice - its not a thing to worry about because:  unless your design is weird creative then the CD will be around the middle of the hull which in general is a long parallel box with more-or-less pointy ends

Similarly - the CLA will be pretty much where you expect it to be (unless again you go for a distinctive, swoopy sheer)

My Brig, which I expected to convert to schooner rig if the brig was unsuccessful required extensive development effort has a codshead/mackerel tail hull and both the CD and CLA are about at 40% aft from bow

GOOD NEWS! :}  with schooner rig you can adjust the rig to suit the hull by trimming the size of the jibs and mizzen sail/spanker until she sails right!
This is even more painless if you make the first suit of sails in a free synthetic material - then you can cut them oout using card templates and a soldering iron, bend them on using copper wire (I use transformer wire) and see what she tells you in a moderate wind.

small adjustments can be made by the relative "set " of the sails  - letting them out (relative to the other sails)reduces the effect almost like reducing the area

hope this helps,
Title: Re: Schooner Design
Post by: Mansfield_Marauder on January 05, 2012, 02:51:52 PM
Don't have access to the plans at the moment but the CLE is just ahead of the mid way point and the centre of displacement is just aft. like you said it would be easy to adjust the sails to suit, might even step the masts so i can shift them about a bit
Title: Re: Schooner Design
Post by: andrewh on January 05, 2012, 05:05:54 PM

You can usually get a bit of adjustment from the rake of the masts - makes schooners look faster anyway :}

I usually expect to do a little rake adjustment and maybe make a mark 2 Jib, possibly with a longer/shorter jibboom (if you have such a thing)

Your hull sounds to be "regular" so it is a valid design option to find a good side view of JayDees Bloonose and slavishly copy the relative mast positions.  Demending on size etc it may be quite complex to move masts enough to make a big difference.

To be encouraging - what looks right, is right and will often sail right as well
Title: Re: Schooner Design
Post by: Jimmy James on January 06, 2012, 12:28:08 AM
Rule of thumb is  as good a way as any to find the C of B :ie put her in the bath and by gently pushing her side ways you can find her point of balance ,all ways set your sail area slightly aft of this other wise she won't tack ... Raking the masts aft will move the C of E aft as will having an overhanging boom (Tends to weather cock the vessel ) but be careful, to much sail aft may result in the vessel getting hung in Irons (stuck head to wind)
Hope this helps
Title: Re: Schooner Design
Post by: Mansfield_Marauder on January 06, 2012, 11:40:41 AM
Was going to decide what sails I wanted decide where the C of E is then fit the masts so the C of E falls just behind the boats CLE and then adjust this either by stepping the masts or changing the size and shape of the sails
Title: Re: Schooner Design
Post by: JayDee on January 06, 2012, 12:02:42 PM
Hello Mansfield,

It is much easier to build to a design that has been proved to work and sail well.
My schooner took 2 years to make, imagine, if it was not up to my expectations at the end of all that work !!.
Undertakers would have been required!.

The boat is an absolute delight to sail, no vices at all.
To help, here is a sail plan as guidance.

Title: Re: Schooner Design
Post by: Brooks on January 07, 2012, 07:33:21 PM
M M - I follow this procedure: 1. Calculate the geometric centers of hull, finkeel, rudder, and sails. Then compute the summation values, weighting each individual geometric center by it's area and distance from an arbitrary datum (I use the bow).  This is exactly like computing center of gravity for an airplane, if you are a pilot. These values provide the static  COE (center of effort, sails) and static CLR (center of lateral resistance, hull+finkeel+rudder).

2. The static centers are a start, but will be insufficient since what we really need are the dynamic centers, and they will not be the same as the static centers. You can see this effect (static different from dynamic) when your ship first gathers way on a beat - she starts off skidding to leeward, then, as she gathers speed, the keel seems to "bite" and she heads up. The bite comes from the keel starting to develop lift (as opposed to just drag when the ship was motionless), and the heading up comes from the fact that the center of lift is forward of the center of drag, giving a weathervane effect.

3. Treat the ship as if she were composed of 2 wings, one above and one below the waterline. For wings, the center of lift (dynamic) is located at the 1/4 chord point. Thus, the dynamic CLR will be located half way between the static CLR and the bow. This displacement of lift (our COE and CLR) from the geometric center of the wing  is why metal airplanes have their single spar forward of the center of area of the wing, btw.

Similarly, the dynamic COE will be located forward of the static COE; this is trickier to find since it must be weighted by sail effectiveness. Sails affect those around them, so all sails are not equal, even if they have equal area. As an approximation, I shift the static COE forward to the 1/4 chord point just like for the CLR, thus finding the approximate dynamic COE. If the dynamic CLR and the dynamic COE line up, then the ship will (probably) tack and wear. If the COE is aft of the CLR, the ship will weathervane, making tacks easy and wears difficult. The opposite situation makes wears easy and tacks hard.
4. I place the finkeel (all my ships have one) so that it's impact on the dynamic CLR makes the dynamic CLR line up with the approximate dynamic COE. I always plan for adjustment of the finkeel location, though. That is, I make provision so that I can shift it forward or aft as necessary to get the balance I want ie. so the ship will both tack and wear.

5. Even if you get the CLR and COE lined up, your ship may still have maneuvering difficulties, though. For example, my topsail schooner Aldebaran won't both wear and tack, no matter where I place the fin keel,  unless she is flying some square sails; tacking is easy under f&a only, but wears are very difficult. This is because I can't let out the mainsheet far enough to get the main boom perpendicular to the hull (due to the placement of the mainmast backstays). I need to be flying, at least, the foretopsail squaresail to get her to wear - I can use the drive of this sail to force the bow off wind for the wear. It works even better if I am flying  square sails on both fore and mainmast (incidentally, showing me why squaresails persisted so long at sea, namely they really increase maneuverability). So, all the calculation in the world may not be sufficient to get a ship to perform the way you want *smiles*. Moving the finkeel, (or changing the rake of the masts or size of the sails, as suggested by Andrew), will give you some more shots in the locker on the way to achieving good maneuvering performance.  Leave some wiggle room in your design :-)

Hope this helps.
Title: Re: Schooner Design
Post by: Brooks on January 12, 2012, 12:01:41 AM
The weighting procedure is actually very simple, I did not put in the steps, but should have, so...

1. pick a datum, say the bow.
2. multiply the area of a unit (eg. rudder) times the distance from the datum. These are "moments"  (area x distance). Calculate the moments for all of the elements (hull, finkeel, rudder, anything else below the waterline).
3. sum up the moments for the below water elements.
4. sum up the areas. Now divide: summation moment/summation area
5. The result of the division is the location of the static CLR  relative to the datum (or static COE if you did the procedure for the sails).

This is a great procedure for a spreadsheet. Then, you can change the size of the rudder, and quickly figure the new static CLR.

A spreadsheet is even more helpful for a multimasted squarerigger: you can see how the static COE changes with changes in the sails set, and thus get a handle on how to reduce sail (to cope with more wind) w/o changing the balance of the ship. For a schooner, it will give you an idea on how to reef your sails, how much to reduce the mainsail, how much to reduce the jibs,  and yet still end up with a ship that will tack and wear under the new suite of sails. It was while "computer sailing" my topsail schooner  Aldebaran with a spreadsheet that I discovered that one advantage of raking masts is that it makes the COE less sensitive to sail changes.
Title: Re: Schooner Design
Post by: andrewh on January 12, 2012, 08:17:59 AM
 <<It was while "computer sailing" my topsail schooner  Aldebaran with a spreadsheet that I discovered that one advantage of raking masts is that it makes the COE less sensitive to sail changes>>

AND it looks swoopy :}
Title: Re: Schooner Design
Post by: Brooks on January 12, 2012, 02:12:25 PM
Swoopy indeed. But the ship lives up to it's appearance, being quite the fastest of my models on the pond - Beware Honest Merchantman, eg. Volante :-)

Once I understood the difference, and significance, of the dynamic vs static centers, it made experimentation with finkeels much easier. I get the boat balanced with  one finkeel so that it tacks and wears. Then, if I try a different finkeel, I know that all I need to do, to achieve the same balance, is line up the dynamic center of the new finkeel with the dynamic center of the old finkeel. Lining up the static centers (the center of area) does not work. I noticed that if I replaced a wide finkeel with a narrow one (chordwise), the narrow one must be moved forward relative to the wide one. But when I calculated the dynamic centers of both, turns out that they are lined up the same :-).