Transom height vs Engine Shaft length

[Murray]

I’m looking to repower a 1974 12’ aluminum (max rated 10hp) with a gently used DF9.9 15” short shaft. My measured transom height is 161/4”. My research has indicated that a 15” short shaft will be sufficient. Am I correct in my assumption? The reason the individual is selling is the DF9.9 is too heavy and too hairy for their 12’ aluminum boat and I’m kind off wondering if I should maybe look for a smaller engine? I have no experience with smaller tiller outboards. Any thoughts would be appreciated. Thank you in advance.

[Moonlighter]

Hi Murray

There are millions of 12ft tinnies running around in Australia with 9.9hp or 15hp engines. With one or two persons on board they are very snappy performers!

Link attached is to one of our best manufacturer’s spec sheets for their standard light tinnies. You can see if thats similar to your boat. They have other models so have a look around on the website and you will likely find something similar. (But none of our tinnies are riveted, since, oh about 1970, they’ve all been welded hulls)

The 3.75 meter is roughly the 12ft model and they cite 15hp as the max.

Now bear in mind that these boats will have been designed with the weight of 4 strokes in mind, so look at the beam etc in the specs to compare to yours.

https://www.seajayboats.com.au/boats/angler/

I think the transom length on your boat should be OK.

cheers

Grant

[Murray]

Thanks for your thoughts. Talked to the gentleman about his concerns, he repowered from a mid 70’s 9.9 Merc and “it just didn’t feel right so I put the Merc back on”.

[Moonlighter]

Theres going to be a substantial weight increase for the 9.9 4 stroke vs the 70’s 2 strokes.

You would definitely notice the difference - although maybe thats one of those things where if you dont have the immediate past experience of having the 2 stroke on there to influence you, it might not feel so bad.

The beam of the boat is whats going to affect buoyancy more than anything. Be interesting to compare the Seajay beam with yours?

good luck!

[Murray]

The boat is about 1000km away but my internet research has lead me to believe my beam is about 0.3 meters narrower than the Seajay boats. The deal fell through, virtually a brand new engine with the serial number tag removed. Thanked him very much for his time and called the police. I did find a gently used Honda 9.9 instead, comparable weight to the Suzuki 9.9 so I now own a Honda. Got a feeling I’m going to need a sandbag or two in the front.