Depends on the transom height of your dinghy. If the shaft is too short for the transom you can overheat the motor from not drawing enough water. I would suggest taking it to a dealer ie Allboat business (site sponsor) and get the experts to have a look.

Ahh ok thanks bud, I'm looking to get a dinghy in a month or two and we got a spare 10hp, how do I check if the transom will fit a long leg when I'm looking at dinghies? Cheers

Measure from top of the transom to the keel.
15in = short leg
20 in = long leg.
Long leg on a short transom you will be losing power due to the extra drag. Short leg on long transom you won't plane due to cavitation as prop lifts out of the water

Ok thanks dodgy, do you know if you can get dinghies with a transom big enough for long shafts?

The skeg plate that's the flat plate above prop needs to be level with bottom of boat

Measure from the top of your transom to the bottom of your keel. If it is 15" to 17" you'll need a "short shaft" outboard. If it is from 20" to 22" you'll need a "long shaft" outboard. Tohatsu/Nissan outboard shafts are measured from the bottom of the engine mounting clamp bracket to the cavitation plate (large fin above propeller) on the lower unit - see diagram below. Generally most boats need either a short or long shaft although some boats require a 25" "ultra-long shaft" outboard. Below is a diagram on how to measure your transom for proper shaft length. NOTE: ANY OTHER MEASUREMENT OTHER THAN THE ONE DISPLAYED BELOW WILL BE USELESS INFORMATION.
 

For normal operation your engine's cavitation plate should be even with the bottom of the hull. A gap of 1-2" in either direction is acceptable and considered normal. If you are still unsure as to which shaft length your boat needs contact your hull's manufacturer. Your outboard owner's manual will also contain more information on shaft length and proper installation

I found this when i was lookin for a donk for mine