Given the size of your tug at 3 foot, go with geared motors from MFA 2.5:1 for torque with a decent amount of power to get out of trouble or 6:1 for full on torque, at £20 a pop safe to say it won't kill your wallet like others mentioned above!
2.5:1 geared here
http://www.cornwallmodelboats.co.uk/acatalog/MFA_919D251.html6:1 here
http://www.cornwallmodelboats.co.uk/acatalog/MFA_919D61.html They will only require a 25A esc at max but easily run them from twin 20A with a mixer function if its fixed korts, if there steerable forget the mixer and have independent control (which i promise you won't regret!) not everyone's got deep pockets for the t12 at £40 a pop i don't think so when 2 are normally required for the average tug!
i have no clue why people are suggesting using 70mm props on a 3 foot model? seriously sort it out with the right info!
heard of models falling into holes? fitting 70mm's you'll soon find out!
you should be looking to use around the 40-50mm korts with the appropriate prop to fit inside so 38mm for 40mm korts and 45mm for a 50mm korts ( remember to get lh/rh 4 blade brass props )
just note i have mentioned korts all through since you have not stated what era the tugs based on im just assuming its a plank on frame constructed wooden tug.