How Long the Shaft of an Auxiliary Motor Should I Get?
 

I am planning to get a 9.9hp outboard motor as an auxiliary motor. But
I don't know how long the shaft should I get. Should this be 15", 20",
or 25"? I have a feeling that the shaft length is depending on the
height of the transom. The transom height is 21" as measured from the
top edge of the transom (where the main motor is mounted on) to the
bottom of the transom as shown in this picture of my transom:
http://www.geocities.com/jaykchan/Ma...o_Transom.html

Just in case the transom height is not the right info for determining
the shaft length, I also provide the following info. Hopefully,
between the picture and the following info, we will be able to
determine the right shaft length:
Main Outboard Shaft Length: 20"
Boat Draft with Main Outboard is Up: 26"
Boat Draft with Main Outboard is Down: 12"

To complicate thing a bit, I likely need to mount the auxiliary motor
on an motor bracket, such as this "Offshore Auxiliary Outboard
Brackets" from Garelick:
http://www.westmarine.com/webapp/wcs...allpartial/0/0
This is supposed to lift the motor vertically by 15 1/2''.

Or this "Lift & Lower OB Bracket (for less than 20hp motor)" from
Garelick:
http://www.westmarine.com/webapp/wcs...allpartial/0/0
This is supposed to lift the motor vertically by 11-1/4".

I want to use a motor bracket. The reason is that there is a 6" tall
object on the spot where I am supposed to mount the auxiliary motor
(please see the picture in the link above). This means I cannot simply
mount the auxiliary motor over the edge of the transom.

Will the use of motor bracket affects the shaft length that I should
get?

Thanks in advance for any help.

Jay Chan