How much thrust does your engine produce?

[kenbo]

I just ran your boat, a Catalina 30 with a Universal 5411 engine with 2:1 reduction through a prop program I have. Comes up with 294 lbs. of bollard thrust. If you have a different engine/reduction let me know and I'll rerun the numbers.

That's at WOT, full 11 hp.

[fox9988]

Originally Posted by 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.

Much lower than I'd have guessed, thank you.

[fox9988]

Originally Posted by kenbo

I just ran your boat, a Catalina 30 with a Universal 5411 engine with 2:1 reduction through a prop program I have. Comes up with 294 lbs. of bollard thrust. If you have a different engine/reduction let me know and I'll rerun the numbers.

That's at WOT, full 11 hp.

That's what I have. Thanks.

[HankOnthewater]

Even there is ambiguity up-thread, there is an easy way of calculating the thrust of a boat's engine.... I htink.
When building a sea rescue boat the naval architect asked me about the HP of the engines, in relation how strong he has to make the towing post. He said that 800 HP would give about 10x that 800 in pounds, ie 8000 lbs.
Of course the actual figure is depended on losses in gearbox, shaft bearings and most of all the ability of the propeller to translate that power into thrust.

If that is true, than a 50 HP engine that many of us have, provides about 500 lbs in thrust, not enough to break any ropes.

What does break ropes (and rips cleats of pontoons and boats), is shock loading. Only when one part has some velocity, comes displacement of the vessel into play.

BTW if anyone want to borrow my loadcell, you're welcome, but I have not measured the bollard pull of my boat's engine.

On edit: is bollard pull and thrust the same thing?

[BillKny]

Originally Posted by 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).

That method will not even come close to working.

Friction, and pushing water out of the way are much bigger issues that inertia in getting a boat moving.

"Thrust" is kind of a useless term here, what the OP is actually asking about is called "bollard pull" and is different than what happens as the boat is moving. Given the prop geometry (diameter, blade area, pitch) and the rotational speed it can be calculated pretty accurately.

[thinwater]

FWIW, if you are worried about breaking a line with static thrust, your tie-up is too lame to handle any sort of storm. Just sayin' (and I have measured these things).


It varies a lot with the prop and engine, but 25-35 pounds/HP is ballpark. Less in reverse.


Obviously, a powerboat can deliver dangerous amounts of thrust.


Why would you want to know bollard pull, vs. thrust underway? Setting an anchor is the practical reason IMO. Compare that with the rode tension in a known amount of wind, and you have a ballpark idea about how much of a storm you are setting equivalent to. For an average sailboat, bollard pull in reverse is about equal to 35-45 knots wind speed at anchor. Multihulls are often less, since they have less motor and more windage.


As for testing the engine, IMO testing the engine in the slip after significant work is smart. Does not have to be WOT, but should be for long enough to shake out early problems. 10-20 minutes? Then taker her out.

[bjp447]

I am So confused!!!

[MicHughV]

The larger question is....how much thrust is required to push a sailboat at hull speed.....the answer is complicated.

Many factors need to be taken into account....displacement....LWL....hull /keel type.....depth of prop...type of prop.....transmission gear ratio...etc..etc...
Pushing a sailboat thru' calm water is one thing, but pushing a boat thru' a chop or wave is quite something else..
The " Propellor Handbook" authored by "Dave Gerr" is a good read. In it, you will find charts, graphs, explanations, work sheet, etc, etc, that can provide you an answer for your particular boat.

To the OP's question....most diesel/ prop combo's for sailboats are designed so that the engine can reach it's maximum operating rpm with a given prop for a given boat. Too little rpm = engine is overloaded, prop or pitch too much....too much rpm = prop or pitch is too small.

While tied to a dock, your engine is unlikely to reach it's maximum operating rpm. While a " bollard pull" can give you some indication of thrust, it is not a true answer, as the boat is not moving.
Don't confuse a 2,000 hp tug diesel engine and it's related "bollard pull" with your basic 50 hp sailboat diesel. These engines are designed for different purposes.

Furthermore, diesel engines have different "duty" ratings...you cannot expect to run a sailboat diesel engine at WOT continuously and expect it to last very long.

Get the book.....

[NSboatman]

Done this. Was curious, so used a load cell and tied off stern-to to our mooring (wanted to test it anyway); and went for it. I have a 42000 lb steel boat with a weeny old Perkins 4-154 (65hp). Slowly bringing her up to cruising rpm (2000) I got about 675lbs of thrust. At this rpm we’d be doing 6.75 knots in flat water.

Going further to 2600rpm (not quite wot) added only another ~100 lbs.

Was running an old fixed 3 blade then, 17x 14, thru a 2.7:1 reduction.

What was really cool was how much this changed if I got a bit of way on first. Momentum is a real thing, specially when you have 20 tons moving…

[MicHughV]

Sounds about right. I recall doing some calcs many years ago, 20,000 lb displacement........30' LWL boat, and I came up with around 450 lbs of thrust required to get to hull speed...

[Adelie]

Originally Posted by 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).

F= ma doesn’t work well here because there is drag and drag in water is not a linear function.

[Nicholson58]

Static pull at zero speed means the prop has turbulent flow. It will not develop full thrust until proper hydrodynamics is established. Further, stalled, the engine will not make sufficient rpm to place the system at rated HP. The prop is pitched to operate at around the cruising speed with the engine in the peak power part of the HP-torque-rpm curve. (Maximum efficiency). If you need to know stalled system thrust use a load cell or scale in the dock line. This will only really tell you the maximum load you can apply to set the anchor.

A solid example: my dinghy with a 9” pitch cannot plane with two people. Motor never reaches high rpm. Change to 8” pitch and we plane with two people and groceries. High load, stalled motor and prop = low thrust.

[Adelie]

Originally Posted by BillKny

That method will not even come close to working.



Friction, and pushing water out of the way are much bigger issues that inertia in getting a boat moving.



"Thrust" is kind of a useless term here, what the OP is actually asking about is called "bollard pull" and is different than what happens as the boat is moving. Given the prop geometry (diameter, blade area, pitch) and the rotational speed it can be calculated pretty accurately.

Thrust underway and thrust at zero speed (bollard pull) are different, but they are related and bollard pull is MUCH easier to measure.

[Adelie]

Originally Posted by MicHughV

The larger question is....how much thrust is required to push a sailboat at hull speed.....the answer is complicated.



Many factors need to be taken into account....displacement....LWL....hull /keel type.....depth of prop...type of prop.....transmission gear ratio...etc..etc...

Pushing a sailboat thru' calm water is one thing, but pushing a boat thru' a chop or wave is quite something else..

The " Propellor Handbook" authored by "Dave Gerr" is a good read. In it, you will find charts, graphs, explanations, work sheet, etc, etc, that can provide you an answer for your particular boat.



To the OP's question....most diesel/ prop combo's for sailboats are designed so that the engine can reach it's maximum operating rpm with a given prop for a given boat. Too little rpm = engine is overloaded, prop or pitch too much....too much rpm = prop or pitch is too small.



While tied to a dock, your engine is unlikely to reach it's maximum operating rpm. While a " bollard pull" can give you some indication of thrust, it is not a true answer, as the boat is not moving.

Don't confuse a 2,000 hp tug diesel engine and it's related "bollard pull" with your basic 50 hp sailboat diesel. These engines are designed for different purposes.



Furthermore, diesel engines have different "duty" ratings...you cannot expect to run a sailboat diesel engine at WOT continuously and expect it to last very long.



Get the book.....

How much thrust is required to reach hull speed is not a difficult question, it’s a fairly simple question that is somewhat complicated to test. Prop depth and angle have nothing to do with it, nor transmission ratio and transmission losses.

Default assumption is flat water no current.
If you want to know thrust levels at various seastates and headwinds it gets somewhat more complicated.

If you want to calculate engine horsepower required to reach hull speed then all the info about props, transmissions and engine specifics become important and it is very complicated. And then you need to start differentiating all the different flavors of HP so everyone is clear what you are talking about.

Don’t conflate thrust and HP.

[Sorcery61]

As a general rule of thumb you can expect to achieve about 1 ton of bollard pull for each 100 hp of your vessels engines
This does assume that engine, gearbox, and propeller are as close to optimal as possible
A 15 to 18% increase is possible if running propellers in Joey nozzle ( again making assumption that all is a proper engineered fit)
Running WOT while moored to a dock is likely to be a poor test and may cause damage to the Dick , your vessel mooring points, or other vessels in area
Extreme caution and a well planned / executed test is warranted
Watch your engine cylinder exhaust temps when performing any max performance tests as well