Gear ratio for houseboat

I have a 50k lbs houseboat with a 3.0 mercruiser. I’m swapping the stern drive and have the option for a 1.47 or a 2.0 gear ratio.

The 2.0 seems to be the more common gear ratio to attach to the 3.0. But considering this is a massive boat pushing 5-7mph with displacement hulls, should I gear down to the 1.47 for more torque headroom?

BTW I bought the boat with a busted drivetrain, so I don’t have any insight into how it performs with the current setup. I’m assuming that I will need to re-prop after some sea-trials.

Mercruiser/Quicksilver Alpha One, Gen II Sterndrive Assembly. 1.47:1 Ratio. (Quicksilver Branded) https://marineengines4less.com/mercruiser-quicksilver-alpha-one-gen-ii-sterndrive-assembly-1-47-1-ratio-quicksilver-branded/

Mercruiser/Quicksilver Alpha One, Gen II Sterndrive Assembly. 2.00:1 Ratio. (Quicksilver Branded) https://marineengines4less.com/mercruiser-quicksilver-alpha-one-gen-ii-sterndrive-assembly-2-00-1-ratio-quicksilver-branded/

 

1:47 if gearing up compared to a 1:2.0. The 3.0 will not have enough torque for higher gears.

 

Those numbers describe the reduction. 2.0 would be a lower gear than 1.47

 

OK, so I am getting the result of my gear ratios mixed up. You are saying, the 2.0 ratio will actually provide more torque and bottom end (versus the 1.47), correct?

 

Yes. The number describes the relationship between input and output. So 5000 rpm in gives 2500 rpm out. If it was the other way, you'd have 10000 rpm out, which is in the drag boat range. A supercavitating prop would be necessary.

 

Have you definitely given up on the possible proposal of using twin outboard engines instead?
Re-powering a houseboat - Stay with Stern drive? https://www.boatdesign.net/threads/re-powering-a-houseboat-stay-with-stern-drive.68464/

 

Yep, I decided to go with the stern drive again. Although I strongly considered the outboards. I just don’t think I can make it work without rebuilding/heavily modifying the rear deck.

 

My preference for moving weight is always more gear ratio less pitch and a larger prop. Kinda tough to do with a gear leg and set max diameter. Guessing the gear ratio and prop pitch doesn't amount to a lot of perceptible difference with that curb weight. Both will likely be awash with prop slip, I'd probably tend towards less prop speed from a deeper gear. But that's a hunch, don't think when factoring in significant figures I could mathematically defend that.

 

The fixed diameter is the killer. It's like outboard designers had no clue about basis physics.

 

They just designed for the market. As much as boats are integral to life here, there are so many more people in one county in Florida it's easy to understand why the ob market trends towards the go fast world.

Had really hoped when yamaha started shipping 425s sans lower unit they were gonna release a deeper lower for a bigger wheel, alas they didn't. Suzuki sales guy says the ratio for big ob sales in one county in Florida will be 30:1 sales to my state that has as much coast lime as the eastern seaboard.... needless to say they are chasing the light fast market.


Duo prop seems to help, ran a bowpicker when I was young with a little 4 banger and a duo prop, it towed pretty hard all things considered.

I wonder what the largest od and slowest pitch available to that gear leg. 3.0 gasser isn't pushing much torque for 50k pounds.

 

I wish mercruiser made a duo prop. I considered a Penta DP, but I didn’t want to go to Volvo due to the cost. The majority of the time this houseboat will be sitting on a mooring ball in a beautiful cove (not in a marina slip). So no need to move it.

Once the boat is back in the water, I will post some RPM/MPH numbers. I suspect whatever prop is on there now is not optimized.

 

Paticus,
As you say, you only need propulsion when you move and that's not often.
Around here, nobody powers their floathomes, in the rare event of a move, they just hire someone to tow it.
This would save you a ton of money and depreciation.
Your call obviously.

 

Ya, what a bunch of knuckle heads, and why did they have to make them so complicated?!
They probably weren't even engineers!
<kidding>

 

Drives Bravo Three® | Mercury Marine https://www.mercurymarine.com/en/us/engines/inboard-and-sterndrive/drives/bravo-three/