outboard help

[seajay]

how can i tell what shaft length i need for my boats auxilary outboard (16 ft shetland cabin cruiser) and does anyone have a small outboard for sale
regards

[mark]

shaft length is usually determined by the distance from the point on transom/bracket where the motor is mounted, to 1" below the bottom of the transom in the water, which is roughly where the cavitation plate on the motor should be when in use. so on an engine, if you measure from the bit of the mounting bracket that will sit on the transon to the cavitation plate you get the shaft length





15 to 16" will be shortshaft

19 to 20" will be longshaft

with an auxilliary, its not quite as critical though, as with a planing hull like a shetland, the auxilliary ain't gonna get it up on the plane, its just going to run as a displacement hull,

are you going to clamp it on the transom, or on a bracket??

[seajay]

im putting it on a bracket .
can i use both

[mark]

possibly! measure the drop from the top of the bracket (in the down position if its a folding one) to the bottom of the transom, if its anything over 16" you can use any. Won't make any difference if the distance is only 16" but you put a long shaft on (20")

as an auxilliary both would work just as well, maximum displacement spped is generally 1.4 times the square root of the hull length, so square root of 16 foot is 4, tim es by 1.4 would give about 5 and a half knots max, which is about what I get from either a 4hp or a 6 hp mariner on a 17 foot boat

[seajay]

thanks a lot for all your advice it is much appreciated
regards

seajay