Why don't boats have multi-speed transmissions?
 

I can make some assumptions, but short of a V drive using an auto trans, why don't boats use multi speed transmissions?

I have seen videos and threads on high performance boats with 2 or 3 speed transmissions. They seem to be for slowing the high pitch props down around docks and idling speeds, and keeping a "peaky" race engine in the power band (I'm certainly no expert, just what I've seen and read).

For most boats, prop slip will allow a boat to come on plane and cruise at lower speeds, negating the need for a multiple speed transmission. If the boat will turn the correct wot rpms and come on plane reasonably, then why would you need a transmission?

I've kinda figured that you tune a engine and prop combo for top speed, and prop slip acts like a torque converter to a degree. I was curious if there's been much experimentation with it. I guess in a high enough performance setup you'd need a gear reduction for slower maneuvering.

My non boat owning friends always ask me this question but I never had a solid explanation.

Several years ago it was all the rage, racers used them to gear down for cornering and acceleration out of the corners, but I think they proved too "complicated" to use properly and just another complex item to fail under extreme race conditions. Some of the current racers may weigh in on this, but I think they are used anymore, and some governing bodies may have outlawed them.

Have one...21 Eliminator Daytona, factory B/B turbo 400, pretty cool to drive, manual shift, really dont use low gear..but is a V-drive

http://s227.photobucket.com/user/ezs...dscsr.jpg.html

One of my favorite lake boats ever. Those tunnel hulls are gorgeous and everybody knows what's up when you hit that key. ;) It's been forever since I've been in a V drive. Can you actually feel it shift?

I never thought about it much but any final gearing is determined by the power available and torque needed and a transmission just reduces loads until you get to that point. On a heavy boat the prop just slips up to that point like a viscous coupler.

A car can upshift and cruise at 80 mph at under 3,000 rpm. A boat has a much greater amount of drag and needs to be near it's peak output to maintain a high speed. Shift it up and it will just keep slowing down. Usually there is enough prop slip to get the boat on plane and moving.

Back in the 80's Mercury tried it. They were designed for the heaver boats. They were junk and didn't ever work the way they were designed. B & M did a drop in replacement for the Mercury for a while. ZF did some 2 and 3 speeds for racing APBA outlawed them after about 2 years of guys racing with them. Usually only the unlimited budget teams had them. The ones from the other side of the globe. When Mercury racing came up with the #6 and guys aerated them to get slippage to get on plane that took of the problem to get a big heavy race boat on plane.

One of my favorite videos

https://www.youtube.com/watch?v=MhG6JZpDAXQ

I believe JD Onken (Don's son) ran heavily modified powerglides in his 32 Skater. Not sure who has the boat now but I've seen it in the Skaterfest pics recently.