With a IOM which has set measurement for mast rake , is there a lot of difference between on the exact measurement recommended  as opposed to lets say

20mm longer /shorter . Aware that more rake aft produces weather helm .

My simplistic view is that if you have too much rake, you get too much weather helm, and to make the boat sailable, you depower the main (more twist, sheet eased) - so you are making the boat slower by reducing the power and pointing.

From reading many tuning guides, they generally say that for the A rig, the mast should be between vertical to 3 degrees rake. However if your boat builder gives a specific number - use it.

John

Edited December 7, 20223 yr by John Ball
added phrase

6 hours ago, Simon Evans said:

20mm longer /shorter . Aware that more rake aft produces weather helm .

20 mm is huge. On a rig 1, If measured at forestay, 6 mm = 1 degree...you don't want to change more than 3 degrees...

There are two elements to mast rake, firstly getting it correct makes the boat sail at its optimum, but probably as importantly having dimensions that you can go back to each time you sail gives you a repeatable process. This becomes even more beneficial with lower rigs that are not used regularly.

Find the rake that you find the best for each rig (some designers have done this for you) and once happy write this down inside your rig box or on the sail (in fine tipped marker) and use this every time you rig the boat. This should be repeatable to 2-3 mm each time for each rig.

The BG sails and design website has a good section called top tips with good advice. BG Sails & Design (bgsailsanddesign.com)

Thanks for replies ,much appreciated . Last question daft question , rake measurement taken with back stay set to right  position 

Hi Simon, mast rake can only be measured in full racing trim so you would need to have tension on your back stay for sure.

Thanks Graham , I thought so .