3.0 to 4.3

[sethvw]

Hello all,
I am new to boatdesign, but really like the site so far. I have a 21 ft v-hull with a Merc. 3.0 l 4 cylinder. The boat runs good, but I would like some oppinions on swapping in a 4.3 v6. I know the gas usage would go up but could use a little extra power. I have changed props, but believe the boat is a little large for the 3.0. What issues will i have with connecting outdrive?
thanks seth

[marshmat]

Both these engines generally ship with the Alpha outdrive. You may need to change the gear ratio in the drive (the 3.0 uses a gear ratio of either 2.0 or 2.4, while the 4.3 uses 1.62, 1.81 or 2.0 depending on boat weight and type), and you will almost certainly need to change the prop. Depending how the engine bay was originally built, the engine mounts might need some work.
I don't believe this swap is considered a complicated one; as far as I know it's a pretty common procedure.

[sethvw]

thanks for reply. how can I find the gear ratio? It is a rather large boat for the engine. I have found that the same (and even smaller boats) was available with 4.3. I have taken some measurement and the engine compartment looks good. How can I find information on exhaust conversion, bolting to out drive, etc?

thanks again
seth

[Zackman]

Here is a link to some of the Mecruiser drawings to give you an idea of what you may be getting yourself into. http://www.mercurymarine.com/sterndrive_engines

Good Luck