Thrust measurements

[Fastjeff57]

Dumb question: Anyone know how to calculate the thrust in pounds of force developed by an outboard motor? (Electric OBs are rated in pounds of force, but not in hp.) I'm looking at designing a tilt system and need to know what kind of force it has to resist.

Thanks,

Jeff

[Mark75H]

As I understand it, you want to calculate how much thrust the motor is applying to the water so you can know how much force is being applied to the assembly you want to build.

If you learn how much force you are making at the prop ... you will still have to make a correction at the bracket location ... half way between the prop and the tilt pivot will have twice the force being applied to the transom.

Subtracting "slip" as has been suggested to you on another forum is incorrect. Slip is force that the prop applies but is "consumed" by the boat as friction against the water - its still part of the push against the transom

[david bryan]

other factors need to be addressed i would not expect you would have as much force on the transom on 100 pound boat as you would on 1000 pound boat unless you where tied to the dock. But look a the Mercury kick outs they have done lots of research

[Fastjeff57]

Sam:

Thanks for the come back. Calculating the force on the transom is hte easy part! What I need is the force produced by the prop. I double checked Min Kota's e-motor site for kWs as well as thrust, but no luck.

Gotta be a way to do this!

Jeff

PS to David: There's no difference in thrust on a 100 or a 1000 lb boat if the motor is at full song. One just goes a lot faster.

[MN1]

Does this help?
Mark N
http://sites.mercurymarine.com/porta..._schema=PORTAL

[Seagull 170]

As I understood it, slip is the difference between the theoretical speed & the actual speed

i.e. if you have a 12" pitch prop running at 5000 revs, your theoretical speed would be 12" x 5000 x 60 = inches per hour which when turned in miles per hour = 56.8 miles per hour

But the speed you record at 5000 revs in your boat using a 12" pitch prop, on your GPS is 45mph, your slip is 20.7%, which is caused by a combination of hull drag & prop inefficiency.

& I think you will find that if your prop is exerting 100 lbs thrust into the water it will put the same thrust 100 lbs onto the transom. As the motor is not acting as a lever but as a fixed arm, otherwise we would all be using the longest leg motors you could find!

Your answer is only right Sam, if the pivot point is attached to a fixed point, not on the boat.

Perhaps Ron would be the best qualified to pass comment on this!

[david bryan]

Sam:

Thanks for the come back. Calculating the force on the transom is hte easy part! What I need is the force produced by the prop. I double checked Min Kota's e-motor site for kWs as well as thrust, but no luck.

Gotta be a way to do this!

Jeff

PS to David: There's no difference in thrust on a 100 or a 1000 lb boat if the motor is at full song. One just goes a lot faster.

funny transoms are torn out not pushed in

[Fastjeff57]

Mark nailed it!

Prop thrust = 375 x Hp (output)/ boat speed.

Sample calculation: 47 lbs of thrust for a 50 hp motor pushing a boat 40 mph. (Not much force.)

And I was wrong about the force on the transom of a 100 lb vs. a 1,000 lb boat, even at full throttle, for when boat speed goes down, force goes up.

Thanks all,

Jeff

[JohnsonM50]

Mark nailed it!

Prop thrust = 375 x Hp (output)/ boat speed.

Sample calculation: 47 lbs of thrust for a 50 hp motor pushing a boat 40 mph. (Not much force.)

And I was wrong about the force on the transom of a 100 lb vs. a 1,000 lb boat, even at full throttle, for when boat speed goes down, force goes up.

Thanks all,

Jeff

Interesting Q & A's Ive got ideas but am not so sure as to write then in stone. Consider this tho, It makes sense that it takes more thrust to get going than to hold top speed [ever push a car? lols] The bracket also has to be able to withstand max straight line force + pounding & turning. Even the smoothest boat is just absorbing the hammering well. It will need to withstand side force to turn & when halfway between the propshaft & tilt pivot point there wont be more thrust on the boat but there will be double the squeeze on the bracket. I think theres more question about the question than anyone being wrong.

[Fastjeff57]

No wonder I was such a bad engineer! I dropped a decimal place in my calculations: It should be 460 plus pounds of force from a 50 hp motor going 40 mph.

Sorry!

Jeff