How much thrust does your engine produce?

[fox9988]

While stationary, at WOT. Obviously it depends on the horsepower, transmission gearing and propeller.
I read a suggestion to (partially) test the engine by loading the engine at least at partial throttle, in gear, while tied to the dock. Any idea how much thrust your engine produces?

[S/V Illusion]

How would running it tied to a dock provide any quantitative measure or data of thrust?

[C420sailor]

Without a strain gauge or similar, I don’t know how you would quantitatively test this.

Edit: illusion beat me to it

[Sailmonkey]

If your worried about the lines or dock while in gear against the lines you need to change the lines or your dock.

The typical sailboat (under 45’ give or take) isn’t going to be able to break dock lines or cleats.

I’ve no idea how much thrust our 28horse kubota produces but I can easily go full throttle (gently) against a single 5/8” three strand spring line.

[fox9988]

Originally Posted by S/V Illusion

How would running it tied to a dock provide any quantitative measure or data of thrust?

Breaking a dock line with a known breaking strength or not breaking the same dock line would tell you something. Not that breaking a dock lines was the reasoning behind loading the engine while stationary.

It might tell you the engine cooling system is inadequate or the fuel filter needs changed, which was why it was suggested in another thread.

[fox9988]

Originally Posted by Sailmonkey

I’ve no idea how much thrust our 28horse kubota produces but I can easily go full throttle (gently) against a single 5/8” three strand spring line.

There's some info. Less than ~10,000 lbs thrust for your boat. Thanks

[S/V Illusion]

Originally Posted by fox9988

Breaking a dock line with a known breaking strength or not breaking the same dock line would tell you something. Not that breaking a dock lines was the reasoning behind loading the engine while stationary.

It might tell you the engine cooling system is inadequate or the fuel filter needs changed, which was why it was suggested in another thread.

That’s ridiculous. In what world is there only one dock line? And how does breaking/not breaking it tell you thrust?

Most folks monitor engine temp and fuel flow under actual use.

[rslifkin]

WOT while stationary is a bad idea. With no forward speed the load on the engine is higher and it likely won't reach rated RPM, which is a good indication that you're overloading it.

[fox9988]

Originally Posted by S/V Illusion

That’s ridiculous. In what world is there only one dock line? And how does breaking/not breaking it tell you thrust?

Most folks monitor engine temp and fuel flow under actual use.

No need for you to participate in a thread that you find ridiculous.

Sailmonkey gave one example, a spring line.

For example, if you break a line accurately rated at 5000 lbs, it tells you that your engine is developing at least 5000 lbs of thrust.
Sorry that the dock line scenario distracted from the original question. It was in answer to the question of how being tied to a dock could possibly provide any quantitative measure or data of thrust.

In the other thread that gave a suggestion of load testing the engine while stationary, the new owner of an older boat needed to motor it 2.5 hours to its next destination. The static load test was suggested to do a preliminary engine check before casting off the lines. That got me to wondering how much thrust my engine produced.

[fox9988]

Let me rephrase: Any idea how much thrust your boat produces while stationary?

[hugosalt]

My dinghy's trolling motor develops 55 lbs, but that's at WOT.
(Sorry, is a slow rainy day here on Long Island, NY)

[fox9988]

Originally Posted by hugosalt

My dinghy's trolling motor develops 55 lbs, but that's at WOT.
(Sorry, is a slow rainy day here on Long Island, NY)

Has it been rated in horsepower?

[Adelie]

4.5hp, 36kg WOT bollard pull.
Rule of thumb is 25lb/hp so that’s 3.0-3.5hp based on thrust.
My engine was a 1982 2-stroke so I wouldn’t be surprised if it was only producing that much power.
I’ve gotten a new 6hp high-thrust but haven’t tested the bollard pull.

I’ve got data for my electric motor too if anybody is interested.

[C420sailor]

I’m pretty sure mine develops 626.4lbs thrust, +/-16.3

[fgd3]

With some kind of strain gauge on the dock line you can measure thrust produced directly. Without a strain gauge you'll get a better thrust measurement with the boat untethered. Measure the time it takes to reach a certain speed and use the equation F=ma (force equals mass times acceleration).