O'Day 240 Outboard Motor Shaft Length

[BillFred]

Go great deal on O'DAy 240, part of why was there is no motor, planning to buy a 5 HP Honda, should I get the short (15") shaft or the long (20" shaft?

 

[jssailem]

Welcome Bill

Congrats on your new to you boat.
You might want to wait til you get the boat home. You want the shaft length to sit in the water. That is determined by where the mount on your boat is set. You may get a boat with an adjustable mount. This may allow you to save a couple of boat bucks on the standard shaft instead of the long shaft.

Good luck.

 

[BillFred]

Boat is "home" working on cleaning her up and provisioning for 2025 season. Mount is fixed.

 

[kappykaplan]

Mount is fixed.

You'll probably want a long shaft then.
Welcome to the forum!

 

[jssailem]

Measure from the mount to the bottom of the boat. Then, compare it to the outboard at a local shop. Ideally, you want the prop to sit in the water. at least 10-12 inches. You do not want the prop to pop out of the water for small waves.

 

[BillFred]

That's what I am thinking, thanks! A couple of extra bucks, but a friend has told me a boat is a hole in the water you pour money into. Oh well...

 

[jssailem]

Here are some suggestions from a prop shop on the web.

The Ideal Prop Depth
The general rule is that the center of the propeller should sit about 3-5 inches below the bottom of the boat's hull. This allows for optimal water flow while keeping the propeller deep enough to avoid cavitation. The exact placement can vary based on boat type, engine design, and boating conditions.

Key Guidelines for Setting Prop Depth:

1. Hull Clearance: Ensure there's sufficient space between the propeller and the boat's hull to avoid turbulence.
2. Waterline Proximity: The prop should operate in clean water, typically below the wake turbulence created by the boat.
3. Shaft Angle: Maintain a level or slightly downward shaft angle to maximize thrust.

Factors Affecting Prop Depth

1. Boat Type: Deep-V hulls may require a slightly deeper prop placement than flat-bottomed boats.
2. Engine Size: Larger engines generally have deeper prop setups to harness their power effectively.
3. Water Conditions: Shallow or debris-filled waters might require adjustments to avoid damage.
4. Trim Settings: Proper trimming balances the boat and helps set the prop depth correctly.

 

[jssailem]

If you use the 3-5 inch value below the hull and the depth of the hull for a typical boat, you will be about 10-12" beneath the water surface.

I suspect Kappy is correct regarding the 20" shaft. On the other hand, if you can get an adjustable mount, more options are available.

The challenge is that some boat builders put a rocker shape on their hulls. This sets the transom high out of the water. You want to examine your boat before you buy the outboard.

 

[Project_Mayhem]

The longer the better. My O'Day 25 had issues with keeping the prop in the water. I'd recommend an 8+ horsepower engine, especially if buying used. It helps with emergency stops which will inevitably occur around the docks and marina. I'm happy to offer some tips on how to handle this boat at lower speeds. It's very temperamental. Depending on your mount, you may run into issues with the motor's tiller hitting the transom when tilting it up

 

[BillFred]

Thanks mayhem, going for longer shaft!