# What will the angle of a boat with an outboard motor be?

• I
• HRG
In summary, a jon boat with a flat bottom that is 4 feet wide x 12 feet long and has a weight of 373 lbs on one end will have an angle of 2.6 degrees relative to the surface of the water when the boat is at rest.
HRG
I want to calculate the angle of a wooden boat at rest in salt water, with a 55 lb outboard motor mounted on the transom + 3 gallons of gasoline (18 lbs) + 2 adults weighing about 150 lbs each. So just figure a total weight at the back end of the boat being 55 + 18 + 300 = 373 lbs.

The boat will be a jon boat that has a flat bottom. It will be 4 feet wide x 12 feet long. Assume that the part of the boat that is "in the water" is a rectangular box that is 4 feet wide x 8 feet long. The 4 feet to the bow plate is curved upward so will not be in the water. So for the calculation, just figure a 4' x 8' rectangular box with 373 lbs on one end.

For info, so you don't have to look it up, one cubic feet of sea water weighs approximately 64 lbs.

What will be the angle of the top of the boat relative to the surface of the water, when the boat is at rest in the water? (The top and bottom of the boat are parallel)

EDIT: I came up with an angle of 2.6 degrees.

Last edited:
How much does the boat weigh ?

depends on where these people and the 3 gallons of gasoline sit.

Baluncore said:
How much does the boat weigh ?
Since the boat will be perfectly level with nothing in it, please disregard the weight of the boat. I did add the weight of the gas and 2 adults in my original post. Just assume that all of that weight is concentrated about 1 foot from the back end of the boat. I just need the "approximate" angle the top of the boat will assume.

Arjan82 said:
depends on where these people and the 3 gallons of gasoline sit.
Please just assume that the 373 lbs is 1 foot from the back of the boat. I just want to know the "approximate" angle the top of the boat will be relative to the surface of the water. I'll post my answer when I recalculate it and see if others come up with approximately the same angle. Thanks.

Assume stern sinks displacing a wedge of water 4' wide and 8' long.
55 lb outboard; 64 lb seawater per cu ft;
Additional displacement will be 55 / 64 = 0.859 cu ft = volume of wedge.
Displaces 0.859 = ½ * 4' * 8' * depth
Wedge (thick end) depth = 0.859 / 16 = 0.0537 ft over 8'
Atan( 0.0537 / 8 ) = 0.38°
Now I see that you have changed the problem.

Baluncore said:
Assume stern sinks displacing a wedge of water 4' wide and 8' long.
55 lb outboard; 64 lb seawater per cu ft;
Additional displacement will be 55 / 64 = 0.859 cu ft = volume of wedge.
Displaces 0.859 = ½ * 4' * 8' * depth
Wedge (thick end) depth = 0.859 / 16 = 0.0537 ft over 8'
Atan( 0.0537 / 8 ) = 0.38°
My calculation with just the 55lb motor was .996 inches for the right triangle's short side. Ended up with 0.6 degrees tilt. I'm recalculating with 373 lbs on the back of the boat. Thanks.

HRG said:
Since the boat will be perfectly level with nothing in it, please disregard the weight of the boat. I did add the weight of the gas and 2 adults in my original post. Just assume that all of that weight is concentrated about 1 foot from the back end of the boat. I just need the "approximate" angle the top of the boat will assume.
Have you ever stood in such a boat? It can change many degrees in angle when moving around the boat. Also, the weight of the boat is important if it is significant enough compared to this 373 lbs.

Assume stern sinks displacing a wedge of water 4' wide and 8' long.
373 lb load; 64 lb seawater per cu ft;
Additional displacement will be 373 / 64 = 5.828 cu ft = volume of wedge.
Displaces 5.828 = ½ * 4' * 8' * depth
Wedge (thick end) depth = 5.828 / 16 = 0.365 ft over 8'
Atan( 0.365 / 8 ) = 2.6°

Baluncore said:
Assume stern sinks displacing a wedge of water 4' wide and 8' long.
373 lb load; 64 lb seawater per cu ft;
Additional displacement will be 373 / 64 = 5.828 cu ft = volume of wedge.
Displaces 5.828 = ½ * 4' * 8' * depth
Wedge (thick end) depth = 5.828 / 16 = 0.365 ft over 8'
Atan( 0.365 / 8 ) = 2.6°
Bingo. That is exactly the angle that I calculated using 373 lbs. 2.6 degrees.
Thanks!

HRG said:
I want to calculate the angle of a wooden boat at rest in salt water, with a 55 lb outboard motor mounted on the transom + 3 gallons of gasoline (18 lbs) + 2 adults weighing about 150 lbs each. So just figure a total weight at the back end of the boat being 55 + 18 + 300 = 373 lbs.

The boat will be a jon boat that has a flat bottom. It will be 4 feet wide x 12 feet long. Assume that the part of the boat that is "in the water" is a rectangular box that is 4 feet wide x 8 feet long. The 4 feet to the bow plate is curved upward so will not be in the water. So for the calculation, just figure a 4' x 8' rectangular box with 373 lbs on one end.

For info, so you don't have to look it up, one cubic feet of sea water weighs approximately 64 lbs.

What will be the angle of the top of the boat relative to the surface of the water, when the boat is at rest in the water? (The top and bottom of the boat are parallel)

EDIT: I came up with an angle of 2.6 degrees.
Here's my calculations:

1 cu ft = 1728 cu inches, 1 cu ft of sea water weighs 64 lbs.

x cu in/373 lbs = 1728 cu in/64 lbs.
x = 10071 cu inches.

volume of a right angle wedge = 10071 cu inches
(area of a right triangle) width of boat = 10071
(short leg of right triangle x long leg / 2) 48" = 10071
(short leg x 96" / 2) 48" = 10071
(short leg x 4608) / 2 = 10071
short leg x 4608 = 20142
short leg = 4.37 inches

Using a right triangle calculator on the web:
short leg = 4.37 inches ... long leg = 96 inches.
The angle = 2.6 degrees.

*** not as simple and elegant as Baluncore's answer but the result is the same angle, 2.6 degrees ***

Inches seem quite unnecessary. Why did you not do everything in feet and cubic feet ?

Baluncore said:
Inches seem quite unnecessary. Why did you not do everything in feet and cubic feet ?
I was wanting to know about how many inches the back of the boat might sink down. But yes, I could have used feet and converted the answer to inches at the end. Glad to see that my answer matches yours though. Thanks.

## 1. What factors affect the angle of a boat with an outboard motor?

The angle of a boat with an outboard motor is affected by several factors, including the weight and distribution of the boat, the size and power of the motor, and the speed and direction of the boat's movement. Environmental factors such as wind and waves can also impact the angle.

## 2. How is the angle of a boat with an outboard motor measured?

The angle of a boat with an outboard motor is typically measured using a device called a clinometer or inclinometer. This tool uses a weighted pendulum or a bubble level to determine the angle of the boat's hull in relation to the water's surface.

## 3. What is the ideal angle for a boat with an outboard motor?

The ideal angle for a boat with an outboard motor will vary depending on the specific boat and motor, as well as the conditions of the water. In general, a slightly bow-up angle is preferred for optimal performance and fuel efficiency.

## 4. How can the angle of a boat with an outboard motor be adjusted?

The angle of a boat with an outboard motor can be adjusted by trimming the motor. This involves tilting the motor up or down to change the angle of the propeller in the water. Trim tabs may also be used to fine-tune the angle and improve stability.

## 5. What are the consequences of having the wrong angle for a boat with an outboard motor?

Having the wrong angle for a boat with an outboard motor can result in decreased performance, increased fuel consumption, and a less comfortable ride. It can also put unnecessary strain on the motor and other components, potentially leading to mechanical issues.

• Mechanics
Replies
9
Views
1K
• Mechanical Engineering
Replies
17
Views
3K
• Mechanics
Replies
1
Views
1K
• General Engineering
Replies
16
Views
2K
• Introductory Physics Homework Help
Replies
15
Views
2K
• Mechanics
Replies
8
Views
2K
• Mechanical Engineering
Replies
19
Views
1K
• Mechanics
Replies
15
Views
1K
• Introductory Physics Homework Help
Replies
2
Views
1K
• General Engineering
Replies
2
Views
2K