For many Minisails the mast rake is set by the glassfibre moulds, but if you are undertaking major work then the following from Peter Matthews should be useful.
· Specify a line parallel to a notional waterline based on my notes above of:
o Top centre of the transom at the top rudder gudgeon.
o 150mm (6in) below the mast ring on the deck. (330-180)
· Follow the laser community and assign a top measurement point coincident with the top of the lower mast section.
· By using some (relatively) simple maths the mast rake angle from the nominal waterline can be measured.
To measure rake angle using the suggested measurements use the following procedure
1. The vertical side of the triangle is given by the distance from the mast ring to the top of the lower mast section plus 150mm, call this dimension “a”
2. Measure the distance from the back of the lower mast section when in place at the mast ring to the top rudder gudgeon. Call this dimension “b”
3. Measure the distance from the top of the lower mast section to the top rudder gudgeon, call this dimension “c”
Calculations
Using the cosine rule c2 = b2 + a2 -2ba. cosC where C is the mast angle from the nominal waterline. Thus
Cos C = (a2 + b2 – c2)/ 2ab
Calculate the bit on the right hand side and then find the inverse cos of the result using a scientific calculator.
This angle should then be compared with 90 deg. If above 90 the mast is raked forward and the difference gives the rake angle.
If the result is below 90 the mast is raked aft and again the difference is the rake angle
Here are the results of my boats Rupert’s and Ronnie’s using the dimensions they supplied.
|
Boat |
Rake |
|
Mk 1 Monaco (Peter) |
3.6 deg aft |
|
Mk 2 Monaco (Peter) |
3.74 deg aft |
|
Sprite (Ronnie) |
7.45 deg aft |
|
Sprite (Rupert) |
2.79 deg aft |
Measurements from Rupert's Sprite:
Mast height from raised wooden bit on the deck: 5560
Transom to inside edge of back of mast pot: 2730
Mast tip to transom top edge: 6235
And for Ronny's Sprite:
Mast height from raised wooden bit: 5635 mm
Transom to inside edge of back of mast pot: 2745 mm
Mast tip to transom top edge: 6100 mm