Re: transmission?
eric,
Let me try and summarize this for you. You asked about a tranny and it appears you wanted to bring RPM down at high speeds. If you are searching for power which could mean you want better performance out of the hole (acceleration) then a tranny would be used to get more RPM early. The same is accomplished with a lower pitch prop. Say 17" or 19" etc. A 24" is usually considered a high speed prop and is not typical in less than 300 bhp applications.
However . . . this brings us to gear ratio. Although I do not think this is your situation because it is simply not available, a high numeric ratio like 3:1 could explain the 24". Lower gearing (high numeric ratio) could allow a small engine to spin more pitch. It is not right, but it might work. I highly doubt this is what's going on
Sooooo . . . this leads me to believe that you have too much pitch which should make her very slow to get up and go, and would also make her slow at top speed. I am also guessing this is why you thought about a transmission. Due to your automotive background you maybe thought "if I can downshift, I could get her going better, and then up shift to run faster at higher speeds." But this is where I believe you are going wrong . . .
With a boat, you just need to think of a heavily loaded vehicle going up hill with a single gear. What you want is gearing that will allow you start the vehicle rolling and then get to max speed (up the hill). Since your gear ratio is fixed (inside the Alpha although there are different ratios typically for 4, 6 and 8 cyl) the only way to fine tune is with prop pitch. A 24" would be like using tall tires on a four banger Toyota pickup to pull a heavy load uphill. Bad choice.
The only way we can know this is to get your top speed at Wide Open Throttle (WOT). Test this with a typical load and trimmed properly for max speed. Your Tach has to be dead nuts accurate and speed must be dead nuts accurate (GPS). We need the gear ratio to confirm all of this, but if the other three are right (speed, RPM and pitch) we can make some reasonable recommendations.
What will never happen is making the 3.7 more powerful. If she is running properly, there is not anything reasonable you can do. Funny enough we say similar things about V8s even although there are some major exceptions. Generally speaking Marine engines are limited in their ability to make more power due to the fact that they have water near exhaust ports. Radical cams cause water to get sucked in, so it is generally recommended to put the street mod thinking away. Unless you can get more air to her (blower) you are limited.
Annnnnnndddddddd, despite the fact that I hate to be the bearer of bad news, the 470 (3.7) is just not considered a good engine to throw money at. You can wish it was all day long, and a good running one with proper maintenance may lead to many, many happy days on the water. But it is generally accepted, even by the people who made it, that you should not spend money on them once they go bad.
So get us the data, and we'll help you dial her in. But people here will never recommend that you try and make what is at best a 150 bhp (flywheel) setup anything more than that. Just not going to happen.
