How much thrust does your engine produce?

[chasfgr]

Originally Posted by S/V Illusion

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

If it pulls the dock down, obviously a lot!
In practice it's totally fallacious - your prop will cavitate and your thrust would drop to 30-50% of the true figure.
I suppose the final part of the picture is the one restraining warp having a guage on it of some kind.
In practice I suspect you'll get a more accurate figure by calculation.

[thlamers]

We have an original Yamaha 9.9 high thrust on our catamaran and If I recall correctly it was factory rated at 218 pounds static thrust. I don't think Yamaha still provides this rating for the current models which use different gearing for faster prop rpm and lower prop pitch.

[Rothblum]

Originally Posted by 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.

I don't think it is true that the load on the engine will be higher at given RPM if the vessel is held stationary. The flow on the prop blades will be stalled, so less thrust and consequent torque on the shaft than if the boat were free to run. Yes, I am sort of an expert. Although this was not my specialty, I worked for 31 years for the David Taylor Model Basin, where measuring model and full scale boat propulsion performance was our bread and butter. Not sure if anyone bothered to measure stationary thrust, even though it is important in certain situations, like backing down or accelerating.

[Simi 60]

Originally Posted by 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?

Enough to break things


At idle rpm ropes and dock cleats were protesting
It was churning up the bottom with 5 ft under keel
And boats in berths 100ft behind us were being buffeted around

52 inch prop in a Kortz nozzle with about 70hp being used
At full noise and 350hp..........

[Jdege]

Originally Posted by PineyWoodsPete

Unless you have the bucks to experiment - prob. $20,000 at least for the dual rig - I'd stick with the diesel - and get a 16' sturdy pole with a hook as a bow-thruster as we did!

I have the bucks to experiment extensively with the existing diesel, to get a solid understanding of what I might expect from a electric installation, before I actually buy anything...

[PineyWoodsPete]

Originally Posted by Jdege

I have the bucks to experiment extensively with the existing diesel, to get a solid understanding of what I might expect from a electric installation, before I actually buy anything...

Why not leave the diesel in and have, say, a 3kW electric motor on the other side to use in docking situations, with a couple of 100Ah AGM batteries, allowing you to "steer" the stern in either direction with rudder prop-wash?

[Jdege]

Originally Posted by PineyWoodsPete

Why not leave the diesel in and have, say, a 3kW electric motor on the other side to use in docking situations, with a couple of 100Ah AGM batteries, allowing you to "steer" the stern in either direction with rudder prop-wash?

The prior owner told me he'd always intended to rig a mount for his dinghy motor, but never did.

At this point, I'm considering ideas. I'm a year or more away from making any decisions.

[Jdege]

Originally Posted by tdgaffney

I think that if you really want to know your thrust v RPM you need to find the answer through testing. The first part is to get your boat's "drag data." To do this you need three things: 1. A long rope. 2. A friend with a more powerful boat. 3. A scale, often called a fish scale, but a larger one perhaps good to 500lbs.

TEST Part 1: Run the test with your engine off, sails down. Your friend drags you (yes, you and your boat) through the water and maintains a constant speed as the drag is recorded. Run the test at different speeds and record the drag as shown on the scale at each speed. The test should start in a flat water condition but could be repeated in different sea states. The long rope is to allow the disturbed water from the tow boat thrust to dissipate. Record your speed and drag data. Plot the curve.

TEST Part 2: Just you, your boat and your engine ... and your speed-drag data. Match your boat speed to each of the data points, recording the RPM. All done.

If you are up to it, run the test, both parts, with your bottom in poor condition. Then run it again, after haul out and a smooth bottom. Might be interesting.


Have fun.

That's useful, thanks.

I wonder if the local TowBoatUs affiliate would be open to an odd contract?

[PineyWoodsPete]

How to calculate Propeller Demand (PD) at various RPM: This is totally different from the engine's "brake" HP or kW power curve, as all output readings at various RPM are with WOT, and reflect the counter-power needed to brake the output shaft down to the various RPM points, and PD is equal to max. power only running WOT. PD represents the total power consumed by prop thrust, prop tip vortex turbulence and other inefficiencies, transmission frictional heat generation, shaft friction, etc.

All you need to start with is your engine HP output a your WOT RPM - Specifics of tranny reduction factor, prop size, etc. make no difference . If your prop is perfectly matched, it's at the engine's rated max power, but at lesser RPM, pick it off the engine power curve in your manual.

This was a new concept to me until several months ago when I happened onto the below figure with my diesel's power curve along with vastly different PD curve, with a rounded output table. Playing with the figures, I found that there's a cubic relationship to the RPM differences: if the RPM are halved at any point, the output is decreased by (0.5) cubed = 0.125. On the table @ 2000RPM, max output of 121kW X 0.125=15.125. Or from 4000 to 3000, (0.75) cubed = .422 X 121kW=51 kW. For the extremes, going from 4000 to 1000: (.25) cubed = .0156 X 121kW = 1.89kW-rounded to 2 on the table.

So it's easy to construct your own table, and the obvious utility of it is to estimate relative consumption at various RPM. Take the old rubric of running your engine at 80% max RPM (NOT 80% power as I've read up-thread, I believe):

For my engine installation, my max RPM is 3850 of 4000, so 3850/4000 = (.96) cubed = .88 X 121kW = 107kW. 80% of that - (.8 cubed) = 0.51 X my max 107kW output = 54.6 kW PD at my"fast cruise" 3100 RPM.

My WOT fuel consumption reading is 32L/hr on the engine instrument, and about 16 L/hr indicated @ 3100 - a pretty good correlation with PD's

[MicHughV]

This has been an interesting thread.

For sure, there is no cut and dried formula available to suit everybody.

Every sailboat out there will be different from another manufacturer, have different design parameters, be used for different things, etc, which will result in different thrust requirements, and so on..and bottom line, sailboat hulls don't lend themselves to be optimum mechanically propelled vessels in the first place.

Lost in this thread, is that the prime motive power for any sailboat are the sails, or should be the sails, but I'm not so naive as to believe a good percentage of sailors don't use their sailboat like a powerboat, hence the thrust requirements.

At the end of the day, a sailboat must still contend with the weight and cost of the engine, batteries and fuel tankage, plus associated drag from props, shafts, etc. This is a significant outlay of $$$ for any sailboat, not to mention future maintenance.

The best one can shoot for is a good compromise, big, but not too big...however, on one issue there should be no compromise...the prop.....it needs to be feathering or folding, or otherwise have collapsible blades and off course on smaller boat, engine can be lifted out of the water.....otherwise why get a sailboat in the first place...

[Bowdrie]

OK, newer guy here, and I see that this thread is a few months old, but I'm always interested in such discussions.
Anyway, I've thought of an accurate way to measure the thrust, (not a static pull against a scale,) however I don't think the device needed has been invented yet?
If one could fasten a load-cell that measures compression between the prop shaft flange and the transmission output flange,, think of a "Drivesaver" with a load cell built into it.
The load cell could have slip-rings and brushes on it to transmit the electrical data.
One could obtain a graph of actual thrust at any RPM/boat speed.
Oh well,, I needed to think of something besides the sanding and varnishing.

[tdgaffney]

In posting #74 I outlined a testing procedure that would give you real world data (The data shall set you free!). You might take a look.