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[drover]

I am very much a beginner to building and sailing model yachts. I have been looking at some designs and note that several use a mast step with multiple choice for the position of the mast. I am wondering how in these cases what guidelines you follow to find the best position for the mast. I assume it mainly by trial and error but how do you know when you've found the optimum position? Just a guess but is it something like finding a position where the boat will hold a certain course without having to make adjustments with the controller.


Thanks for any explanation

[rmaddock]

"Normally", a mast will be stepped so that the center of effort of the sails is slightly forward of the hulls center of balance.

You can find that center by trial and error by putting the hull in water and pushing it sideways with a finger.  There will be a point along the hull where pushing will make it go perfectly sideways, rather than it's twisting.

Putting the CofE slightly forward of this gives the model what is called "weather helm".  This means that a gust will actually push the boat into a turn into the wind....which is a safety feature.

If you have differing suits of sails for a boat (for different wind conditions) then the CofE moves and so you need to move the mast too.

Of course, that's the theory from full size boats and there are those on here that will disagree 

Cheers,

Robert.

[dreadnought72]

^ Robert's use of the word "forward" needs to be replaced with "backward" for weather helm. 

Incidentally, drover, the hull's centre of lateral resistance can change backwards or forwards depending on the heel of the hull.

So trial and error is your best bet.

Andy

[tigertiger]

The adjustment is there if to allow for different effects from different sets of sails.
If you only use one set of sails you only need to do this once.



Look on line for advice about what lee helm and weather helm are.
Sail you boat if it tends to naturally steer into the wind (if left to its own devices) it has weather helm. If it steers away it has lee helm. Then adjust the mast footing position accordingly.


This is really just the course/crude tuning. Fine tuning can then be done, and that can make a big difference to how she sails.

[drover]

Thanks gentlemen for your advice. Once I've actually built a yacht looks like an interesting bit of trial and error.

[rmaddock]

^ Robert's use of the word "forward" needs to be replaced with "backward" for weather helm. 

Yes, Andy, I realised that in the middle of the night 

[Netleyned]

A good position for a mast is 'upright'. 

Ned

[tigertiger]

A good position for a mast is 'upright'. 

Ned

Another good position is on the boat, and not in the shed back home 

[mrpenguin]

Another good position is on the boat, and not in the shed back home 

Doesn't matter where the mast is if the transmitter is still at home 

[roycv]

Hello Drover.  The first reply and mod to it are good.

I have set up sail position on several scratch built model yachts and have not had to change them after first sail. 
It is the sail position which is the first consideration.  Work out the centre of effort for the combined (assumed 2 sails) sails and this point should be 4% behind the centre of lateral resistance of the WATERLINE length of the hull.

The mast position then is where it is so to speak.  If you step the mast on the deck it can be adjusted easily.  If the mast goes down to the keel then you have to be more precise.

I altered a Panache yacht (30 inches loa) to have 2 masts (3 sails) so had to re-calculate everything.  The arithmetic works and she sailed nicely balanced straight away.

Yes, the clr does change at different speeds but makes little difference for small models, though my experience is only up to 45 inches loa.
I have a small J class (45 inches) and have allowed the flying jib to run on a loop which is adjustable but not under RC and this has been no problem with the balance of the sails.

To help with the "finger" method of finding the clr, I stick some home type masking tape on the side of the yacht hull (weighted of course to the final ballasting waterline) this means you can mark the hull with a pen without damaging the paint finish.
If all works out you should be able to set the sails at 30 degrees or so and she should sail straight slightly off the wind without having to use the rudder.
good luck Roy