Boat transmission. What am I missing?

[Chocaholic]

Perhaps the stupid question of the day but…I’ve owned probably a dozen boats over the years, and a few jet skis. I’ve always wondered why only a single speed transmission? Top speed of the boat is purely a function of rpm and prop pitch. Our current boat has a 300hp V8 engine that must run at 5500+ rpms’s to maintain 45 mph. With one gear change, it seems those rpm’s could drop while maintaining speed, using far less fuel and perhaps allow smaller, more fuel efficient engines in boats.

With an automobile, you can shift gears to run the engine at lower rpm while increasing speed of wheel rotation. Even if you only had one gear change, it seems you could cruise along at reasonable rpm while maintaining 40+ mph on the water.

The technology certainly exists. So, what am I missing?

[onewhippedpuppy]

I assume complexity and cost? The new Mercruiser 600 HP outboards do have a second gear that’s like an overdrive and improves efficiency. They also have a fixed upper and the lower drive portion rotates to steer, which is pretty cool.

[ryans65]

The propeller in the water is acting as a torque converter and the final drive in one. Unlike a car where road speed is directly proportional to the speed at the final drive unit a propeller is constantly variable almost like a CVT. If you want the engine RPM down at max speed you would change propellers like you would change your final drive ratio in a car. I'm sure your boats manufacturer matched the prop size and pitch relative to your engine and boat size/ hull style. Its probably a good compromise for whatever your boat was meant to do.

[onewhippedpuppy]

Here’s a quick video of the 600 HP V12 Mercury outboard. Pretty freaking cool. Not uncommon for go-fast center consoles to have four or five of them.

https://youtu.be/nN42mq5eSUc?si=gOQbBzEqCUm6xHpK

[unclebilly]

Change your prop if you think your hull can go faster. More diameter and more pitch.

[onewhippedpuppy]

Changing your props is like changing the gear ratio in your differential. Higher pitch gives you better hole shot and acceleration, but less top end. Conversely less pitch will give you slower acceleration, slower time to plane, and higher top speed. On one of my prior boats it struggled to get on plane quickly when loaded and pulling kids on a tube, I went from a 3 blade prop to a 4 blade with higher pitch and it made a BIG difference. But I lost 5 MPH in top speed.

[Dantilla]

The engine/prop combination is a compromise, but optimized towards top-end speed.
I'm guessing it would take gobs more horsepower to go much faster than 45 mph.

A higher gear would bog the engine down out of the power band.
The purpose of another gear would be a lower first gear to get up on a plane quicker.

[Chocaholic]

Well, (smartass comment) changing the prop is hard to do while cruising along at 5500 rpm. I’m aware that prop pitch will affect rpm at top speed, but it also affects low speed torque (like when pulling up a skier). Ours has a Bravo 3 (2 props spinning in opposite directions) in an effort to optimize torque and top speed. Seems with a simple and small two speed tranny, you could have the best of both. With a modest sized ski boat costing $150k (Mastercraft, Malibu, Cobalt), cost probably isn’t the issue. Many boats have power steering and even power wakeboard towers (Regal), plenty of complexity.

Cruising along at 45mph, it would be nice to have overdrive to drop rpm’s from 5500 to say 4000.

[ryans65]

I'm not being sarcastic or negative or anything when I ask this but what makes you think your boats engine may be more efficient at 4k rather than 5500? Less RPMS dont always equate less fuel consumption, less stress etc.... If that were true we would all have cars with engines that are basically operating at idle speed on the highway. Your engine may be pretty happy all day long at 5500. Just a thought.

Originally Posted by Chocaholic

Cruising along at 45mph, it would be nice to have overdrive to drop rpm’s from 5500 to say 4000.

[ryans65]

This is another thing to consider and that is extra gears hardly ever equate to an increase in top speed, its quite the oppposite. Adding extra gears on top almost always equate to shorter ratios down low so the engine can be kept in its optimum power band at any particular speed.

Originally Posted by Dantilla

The engine/prop combination is a compromise, but optimized towards top-end speed.
I'm guessing it would take gobs more horsepower to go much faster than 45 mph.

A higher gear would bog the engine down out of the power band.
The purpose of another gear would be a lower first gear to get up on a plane quicker.

[onewhippedpuppy]

There are a lot more considerations with a prop, in a way it’s like a rear differential combined with your rear tires. Props have a sweet spot for efficiency too, and if you spin it too fast you start getting slippage or cavitation. You want it to bite but not slip, that’s a balancing act. I did a lot of research before I changed the prop on a boat years ago, but I’m not expert. You can science project the hell out of this topic if you want.

Or if you want something new to look at, research Sharrow props. I believe they just came out with counter-rotating props for Bravo drives.

[Dantilla]

Too bad props are so expensive, especially when you need two.
Otherwise, an experiment. If your current props are what- 19" pitch?
Try 23" props (with the boat as light as possible- No passengers) and see if the boat will maintain 45 mph, or just lug the engine.

I do recall long ago some flatbottoms with the big V8s and over-the-transom exhaust using Powerglide 2-speed transmissions. Once up on a plane you could hear the rpms drop as it shifts into 2nd.
Outside of the heavy engine, these boats were featherweights.

[juanbenae]

Originally Posted by Chocaholic

Well, (smartass comment)

When did you get so sensitive? You are the king, well maybe the queen, a prince at a minimum of the smart-ass remark in that other forum.

It would seem with the craze today being the wake board requiring low end torque to push a ton of water the top end speed is less desired possibly? Unless you are some redneck in a sparkly bass boat that has to go 65mph to get to your choice spot before the other redneck seems top speed is something less important.

The other thing is that more gears means more weight, more room required to place a larger by nature trans. If you wanna go faster buy a boat with that being it's thing.

[Dantilla]

I used to swap props back and forth often on a boat I used to own.
The Go places quickly prop, and the Yank skiers up out of the hole prop.

[id10t]

Originally Posted by Dantilla

The engine/prop combination is a compromise, but optimized towards top-end speed.
I'm guessing it would take gobs more horsepower to go much faster than 45 mph.

Might not even be possible. Things moving thru a medium like air or water have max speeds. For a displacement boat, this is called hull speed (based on waterline lenght). For a planing boat, at some point the design of the boat will plane it out of the water too much and then things get very exiting for a short while.

[onewhippedpuppy]

Originally Posted by Dantilla

I used to swap props back and forth often on a boat I used to own.
The Go places quickly prop, and the Yank skiers up out of the hole prop.

That’s great when you have a single engine and single prop. The dual counter-rotating props are something like $1500 for a set, and if you have twins that’s double the fun.

[BK911]

Originally Posted by Chocaholic

Well, (smartass comment) changing the prop is hard to do while cruising along at 5500 rpm.....

Ha!
Well done my man. Well done.

[look 171]

Electric motor, man. Problem solved and faster out of the hole too. OK, OK, flame suite on.

[onewhippedpuppy]

Originally Posted by look 171

Electric motor, man. Problem solved and faster out of the hole too. OK, OK, flame suite on.

High voltage and water, it’s a match made in heaven!

In all seriousness I could see EV boats catching on, you get the security of a motor with the solitude of a sailboat. But the range issues are even worse than cars, boats get horrible MPG. Mine cruises at about 0.8 MPG at 35 MPH. Most of the EV boats on the market give you maybe an hour of run time, at least the ones I have seen in articles. Then you have the logistics of charging high voltage on a dock in close proximity to water. A lot of lake drownings occur near docks due to stray voltage.

[id10t]

Originally Posted by onewhippedpuppy

High voltage and water, it’s a match made in heaven!

In all seriousness I could see EV boats catching on, you get the security of a motor with the solitude of a sailboat. But the range issues are even worse than cars, boats get horrible MPG. Mine cruises at about 0.8 MPG at 35 MPH. Most of the EV boats on the market give you maybe an hour of run time, at least the ones I have seen in articles. Then you have the logistics of charging high voltage on a dock in close proximity to water. A lot of lake drownings occur near docks due to stray voltage.

Probably not what y'all are thinking but I get 2.5-3 hrs run time on my trolling motor powered canoe with a single 100ah battery. Can go faster upwind and upcurrent than I can paddle on flat and still water.