How much thrust does your engine produce?

[rslifkin]

Originally Posted by MicHughV

Off course, like anybody, my preference would be too pack as much power as I can into my boat....having too much being better than having too little...alas, usually, not always the case.

More power is good, but only up to the point where the hull can take advantage of it. There's a point where more power just isn't useful with any hull. And in many cases, it's really the max continuous output of the engine that matters, not how much power it makes at WOT. For some engines those are close together, for others it's a big difference. Optimal choice of gear reduction and prop is critical to making good use of that power as well.

[MicHughV]

It's interesting...I live in an area where there is a lot of tug traffic...when the tugs are not busy pushing or pulling something....they head home...
Despite having several 1,000 hp available, their top speed is marginal.
They do, however, put out a prodigious wake at full clip....like a sailboat, they have displacement hulls, and are speed restrained by hull length, but for a tug, moving at hull speed, is an easy matter.

It's interesting to note, that truck manufacturer's have entered the horse power wars. 700 hp now gives you bragging rights, but just how useful and practical is having 700 hp under the hood ???

[wsmurdoch]

These two article where a number of propellers were tested contain a lot of information and measured data.
https://www.yachtingmonthly.com/gear...ler-test-29807
http://www.propelspecialisten.dk/dow..._test_2008.pdf

Bill

[MicHughV]

I've seen these reports before.
It's interesting to note, that while the tests were run in a test tank facility, together with all the paraphanalia needed to test the props, the author recognizes that actual results will vary depending on actual boat used.

Regardless, a fixed 3 blade prop is like dragging a bucket. This is made clear.

[wsmurdoch]

Both reports measured bollard pull with real boats in real water tied to real docks answering the question "How much thrust does your engine produce?".

Bill

[valhalla360]

Originally Posted by Sailmonkey

I can easily go full throttle (gently).

How do you go full throttle gently?

[valhalla360]

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.

Thrust =/= horsepower. They are totally different measurements.

And too many variables to make this rule of thumb accurate. The biggest issue is the prop interacts differently with the water while in motion vs tied up at the dock.

While we never bothered to do a bollard pull test...We had a 40hp outboard and replaced it with a 25hp outboard but spec'd a low pitch work prop. Thrust was way better on the 25hp motor. Top end speed was better with 40hp. Of course, we never ran at top end, so it was no great loss. We could easily make hull speed.

[NSboatman]

Originally Posted by Nicholson58

24 inch 3 blade infinitely variable pitch Hundested transmission.



115 HP Westerbeke 6 cylinder tractor engine. 2:1 reduction gear. Max 1400 engine RPM. Cruise, engine 950 RPM



We vary the pitch according to temperature, rpm, speed, sea state. Immense assist sailing to weather with long pitch. Flattened blades give huge thrust at low speed for maneuvering in an anchorage.



Boat, 1984 Camper & Nicholson, 40 tons, 58 LOA

Oooh boy I’d love to have that system!!!

[Adelie]

Originally Posted by valhalla360

Thrust =/= horsepower. They are totally different measurements.

And too many variables to make this rule of thumb accurate. The biggest issue is the prop interacts differently with the water while in motion vs tied up at the dock.

While we never bothered to do a bollard pull test...We had a 40hp outboard and replaced it with a 25hp outboard but spec'd a low pitch work prop. Thrust was way better on the 25hp motor. Top end speed was better with 40hp. Of course, we never ran at top end, so it was no great loss. We could easily make hull speed.

Yep, thrust and horsepower are not the same thing and I’ve been very careful about not saying that they are.

They are however correlated. For displacement vessels with inboards turning conventional fixed props 25lb per HP is a good rule of thumb.

The situation with outboards is different. Most outboards are geared and propped for planing speeds. I expect that will cause significant differences for these engines compared to engines optimized for displacement speeds.

The outboards that are optimised for displacement speeds are the 2-3hp models and high-thrust models. On an old computer I have an article where they compared 2-3hp outboards including bollard pull. I recall the 25lb/HP was a decent estimate from that article but can’t say for sure. If I can find it I’ll post it or link to it. Eventually I will bollard pull the 6hp high thrust motor I have but that won’t be anytime soon.

[wsmurdoch]

The two magazine articles I mentioned above each tested a boat with a variety of propellers.

In the Yachting Monthly article, they tested a Beneteau Oceanis 323 with a Yanmar YM20 21 hp engine with a variety of different propellers and obtained a range of static thrusts in forward and reverse. Forward results were 270 Kgf to 195 Kgf (600 lbf to 430 lbf or 29 lbf/hp to 20 lbf/hp). Reverse thrusts were 190 kgf to 104 kgf (420 lbf to 11 lbf or 20 lbf/hp to 11 lbf/hp).

In the Segein Magazine article, they tested a Bravaria 34 with a Volvo D1-30 28.4 hp engine with a variety of propellers and again obtained a range of static thrusts in forward and reverse. Forward results were 1500N to 670N (340 lbf to 150 lbf or 12 lbf/hp to 5.3 lbf/hp). Reverse thrusts were 1500N to 700N (12 lbf/hp to 5.6 lbf/hp).

I'm not sure how to interpret the data except to say it is hard to have a rule of thumb. Obviously, engine speed, transmission ratio in forward and in reverse (thus shaft speed), and propeller diameter, pitch, number of blades, and manufacturer all matter.

Bill

[MicHughV]

I seem to recall a test performed at some time, where a sailboat was pulled by a powerboat. The tow line was rigged with a gauge to determine the line tension...which is directly related to the thrust an engine would have had to provide.

Unfortunately, I don't recall the author or source of this test, but a search of the internet might provide a clue.

I did find this, which might provide some guidance.

https://www.youtube.com/watch?v=jeZFu7G5qsY

[skenn_ie]

There is a very useful little spreadsheet, propking.xls, that will give you a fairly good indication of "bollard pull"

[Nicholson58]

Seriously, why does anyone care about thrust, especially at zero speed? Unless you operate a tug or towing service all that counts is motoring speed and fuel consumption.

[skenn_ie]

Originally Posted by Nicholson58

Seriously, why does anyone care about thrust, especially at zero speed? Unless you operate a tug or towing service all that counts is motoring speed and fuel consumption.

Because it can easily be calculated and is indicative of efficiency of the machinery.
However, propking will also give you an idea of max speed with the hull-type, power, gear ratio and prop that you have, then you can choose to compromise with a bias towards speed rather than bollard pull.

[jakkum]

My schooner had a slow running 150 HP engine and IT had a bollard pull of about 2000.kg, which is pretty good
No beatbox löss, no loss of other things on the engine.
A 30 HP Yacht diesel with beatbox and alternatief has dat leds bollardpull than a fifth, so less than 30 kg

You van rig something with a known w weight because it is infer wayer.
eight on the Bottom and try to lift it with a rope Acer a pulley. But take into account the löss of Wright coz IT is submerged.
Specific weight iron 7,8 fully wet 6.8
ND you can also do IT with a known weight on deck

Like a big bucket filling IT more and more.
Nice experiments