How much thrust does your engine produce?

[Sorcery61]

Kort nozzle

[Panope]

40 horsepower Yanmar.

Three blade, fixed, 18 inch diameter X 10.5 inch pitch - prop.

910 pounds bollard pull in forward gear @ 3300 RPM and Zero Boat Speed (Engine reaches 3800 when boat is running free)

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

[Lepke]

Some info on Bollard Pull Test, what tug do.
https://workshopinsider.com/bollard-...est-procedure/

[MicHughV]

yep, it's not an exact science.....too many variables....at the end of the day, your average 40' sailboat diesel engine likely can produce somewhere around an optimum of 500 lbs of thrust....give or take 50 lbs...it's a good working number to play around with....

[Adelie]

Originally Posted by MicHughV

yep, it's not an exact science.....too many variables....at the end of the day, your average 40' sailboat diesel engine likely can produce somewhere around an optimum of 500 lbs of thrust....give or take 50 lbs...it's a good working number to play around with....

Where did you get the 500lb value?

[thinwater]

Originally Posted by Adelie

Where did you get the 500lb value?

The posts in this thread are 22-26 pounds/hp, I assume forward. Not all were WOT, so perhaps a bit more in some cases. Reverse would be less.



I've measured 23 forward and 15 in reverse with a 9.9 hp Yamaha high thrust sail drive outboard.



So do your own math. 22-30 pounds/hp at WOT in forward, depending on the prop. Somewhat less in reverse, depending on prop asymmetry. Outboards are less to a lot less because of ventilation cause by exhausting through the hub.


How that relates to rode tension is complicated, depending on boat size-shape-weight, rode, and exposure to waves. Typically, it will be similar to 35-40 knot winds, but less than a strong squall.

[donradcliffe]

I agree with 1 ton per 100 hp as an order of magnitude estimate, and it seems to agree pretty well with test results between 10 and 10,000 hp.

[fox9988]

Interesting replies, thanks.

[MicHughV]

Adelie,

Some years back, I took it upon myself to find out. This is how I came across the "Propellor handbook". I am a marine engineer by profession, now retired, but I needed some facts and figures to assist and/or guide me in my search.

My boat at that time, was just shy of 40', with a displacement of around 20,000 lbs and a LWL of around 30'. The boat had a fin keel and skeg-hung rudder.
As I recall, it took 5-6 pages of calculations to arrive at a number.

The reason for my interest was $$$$....or lack of $$$$ as the case might be.

I had sailed for a number of years without an inboard diesel at all.
When the writing finally appeared on the wall for me, I went diesel engine shopping.

The important goal being to try and find the most bang for my buck.

It takes relatively little horsepower or thrust if you will, to get a sailboat moving at a moderate pace, but the horsepower/thrust requirement escalades rather quickly to enable the same boat to approach hull speed or punch thru' waves.

And too, one can't run a diesel at WOT and expect it to last very long, so the "optimum" thrust should be at "cruising" rpm.
As seen from the post # 32, about 430 lbs of thrust was recorded at 2,250 rpm...for a 3 blade prop. This jives, more or less, with what I calculated, though my calcs at the time were based on a 2 blade prop and a moving boat.

Hull speed for my boat, using the traditional square root of LWL x 1.3 was about 7 knots.
Long story short, my calcs appeared to show I needed an engine capable of providing around 450 lbs of thrust at cruising rpm. In my mind, cruising rpm is around 2,200 rpm. Short bursts at higher rpm are tolerable if kept within reason.

Matching this requirement to engine horsepower requires more calcs. Horsepower being an elusive number, engine torque is a better guide, but so too is transmission ratio's, prop selection, prop speed, as well as tip clearance, hull design and other variables.

Nonetheless, I shopped around and researched several diesel engine manufacturers. Besides the horsepower and torque requirements, there was also the issue of weight and size, gearbox transmission, fuel consumption, etc, as the engine would have to be retro-fitted into a tiny compartment.

"The propellor handbook" is an excellent source of information. Within the book, are examples of how to determine the thrust required for most any given boat. The book is a bit of a technical read, but relatively easy to follow and understand.

Trust that this response addresses your question. It's a complicated science, with many variables, and I'm no expert on the matter, but the 500 lb thrust number, give or take, appears to be a good " working" number for your average 40' boat.

As noted above, about 1 ton per 100 hp equals about 1,000 lbs for 50 hp....assuming this to be at WOT.
A 40 hp engine obviously would be less. the post # 32, appears to indicate 910 lb thrust at 3,300 rpm for that particular boat, prop and engine, so we are in the ball park.

It's a very complicated science, with many variables. At the end of the day, most boaters would not want to go thru' this exercise, either by tabulation or in the water testing, so generally accepted "rule of thumb" approaches seems to satisfy most people.

[waterman46]

Oh man, this one is fun! F=MA and V=AT. Isaac Newton, doncha know! If you know the mass of your boat and have a stopwatch, go in reverse at exactly one knot. Then put the engine in forward gear, rev it up and start your stopwatch. When you get to 1 knot forward speed, note the stopwatch time. The "velocity" in the formula is really the change in velocity of an object. In this case, that change is 2 knots because we are going from -1knot to +1knot. Mass is your boat weight in kg. From the above,

F(thrust in newtons)=Mass(kg)*V(m/sec)/T(sec).

The conversion to your favorite units is left as an exercise for the student.

Why did I start at reverse, go to zero and then forward the same amount? That's because the boat's resistance to the water will be a factor to consider. Including an approximately equal resistance both helping to slow the boat (while in reverse) and hindering the boat (while in forward) should nearly cancel out the water resistance effects.

Of course, this thrust measurement is pretty useless except for maneuvering, because most of the time you are more interested in the thrust at cruising speed or full throttle speed. In that case you get more thrust if the propeller and transmission are correctly optimized.

[MicHughV]

the "Propellor Handbook" goes to great lengths to explain the difference in hull form...or "wetted surface".....a 40' long barge that is 20' wide with a draft of 5' is nowhere near the same as a 40' sailboat with a 12' beam and a fin keel...that they share the same LOA is really meaningless.

The displacement of the barge is about 5 times that of the sailboat. On the opposite end, a 40' tug, fitted with twin 2,000 hp diesels falls into a category all of it's own. A 40' round-the-world racer with daggerboards and canting keel also does not fit most sailboat criteria.

For your basic sailor, presuming that to be us, the posters on this board, knowing the " wetted surface" of your vessel is a key component...to know this requires knowing the hull lines. My first boat was a hard chine steel vessel, with steel frames at set intervals, that I built myself. Knowing this, I could replicate a "scale" model of my boats hull on some graph paper, which allowed me, with some accuracy, to determine the "wetted surface" of the hull.

To determine a modern round hull " wetted surface" would require having the boat plans in hand and AutoCad, two tools most sailors do not have, so some " assumptions" have to be made to get to a reasonably accurate number.

Nautical architects have a variety of computer programs these days, that can tweak this, that or the other in other to get the result that they want or are shooting for.

Getting back to the original question, how much thrust can your engine produce requires a rather in-depth knowledge of many and varied issues.

[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

[rslifkin]

On my boat, I consider the available thrust as "more than enough to break stuff". 2 engines, 340hp each, driving 22x25 props through a 2.57:1 reduction. Idle reverse on both is enough to stretch the anchor rode to about what I see in 25 - 30 kts of wind, so I consider 900 - 1000 to be the maximum power for setting an anchor (and I'm off the bow before they go above idle, as any kind of failure under the additional tension would carry a significant risk of injury).

Idle thrust while moving freely is enough to push the boat along at just over 4 kts. And there's enough blade area to make good thrust that the last time I towed someone, idle was still giving about 3.5 kts with a 25 footer side tied to me.

As far as testing while tied to a dock, I'll put an engine in gear, but not above idle and generally only 1 engine at a time.

[Adelie]

Originally Posted by MicHughV

Adelie,

Some years back, I took it upon myself to find out. This is how I came across the "Propellor handbook". I am a marine engineer by profession, now retired, but I needed some facts and figures to assist and/or guide me in my search.

My boat at that time, was just shy of 40', with a displacement of around 20,000 lbs and a LWL of around 30'. The boat had a fin keel and skeg-hung rudder.
As I recall, it took 5-6 pages of calculations to arrive at a number.

The reason for my interest was $$$$....or lack of $$$$ as the case might be.

I had sailed for a number of years without an inboard diesel at all.
When the writing finally appeared on the wall for me, I went diesel engine shopping.

The important goal being to try and find the most bang for my buck.

It takes relatively little horsepower or thrust if you will, to get a sailboat moving at a moderate pace, but the horsepower/thrust requirement escalades rather quickly to enable the same boat to approach hull speed or punch thru' waves.

And too, one can't run a diesel at WOT and expect it to last very long, so the "optimum" thrust should be at "cruising" rpm.
As seen from the post # 32, about 430 lbs of thrust was recorded at 2,250 rpm...for a 3 blade prop. This jives, more or less, with what I calculated, though my calcs at the time were based on a 2 blade prop and a moving boat.

Hull speed for my boat, using the traditional square root of LWL x 1.3 was about 7 knots.
Long story short, my calcs appeared to show I needed an engine capable of providing around 450 lbs of thrust at cruising rpm. In my mind, cruising rpm is around 2,200 rpm. Short bursts at higher rpm are tolerable if kept within reason.

Matching this requirement to engine horsepower requires more calcs. Horsepower being an elusive number, engine torque is a better guide, but so too is transmission ratio's, prop selection, prop speed, as well as tip clearance, hull design and other variables.

Nonetheless, I shopped around and researched several diesel engine manufacturers. Besides the horsepower and torque requirements, there was also the issue of weight and size, gearbox transmission, fuel consumption, etc, as the engine would have to be retro-fitted into a tiny compartment.

"The propellor handbook" is an excellent source of information. Within the book, are examples of how to determine the thrust required for most any given boat. The book is a bit of a technical read, but relatively easy to follow and understand.

Trust that this response addresses your question. It's a complicated science, with many variables, and I'm no expert on the matter, but the 500 lb thrust number, give or take, appears to be a good " working" number for your average 40' boat.

As noted above, about 1 ton per 100 hp equals about 1,000 lbs for 50 hp....assuming this to be at WOT.
A 40 hp engine obviously would be less. the post # 32, appears to indicate 910 lb thrust at 3,300 rpm for that particular boat, prop and engine, so we are in the ball park.

It's a very complicated science, with many variables. At the end of the day, most boaters would not want to go thru' this exercise, either by tabulation or in the water testing, so generally accepted "rule of thumb" approaches seems to satisfy most people.

Ok so your previous post said the engine you would normally see in a 40’ boat could provide 500lb thrust when what you meant was you would need 500lb to get to hull speed in a 40’ boat
The normal engine in a 40’ boat is 40-50hp which should be able to provide about 1000-1260lb thrust.

[MicHughV]

Hull speed for a sailboat is also an elusive target.

Hull speed generally being defined when the boat moving thru' the water generates a bow wave and stern wave, with the peak or crest of each wave at the outer ends of the boat..
To exceed hull speed, would require a boat to push past the bow wave, sometimes occasioned by surfing down a wave...for a very brief moment.

My experience has been that most sailboats do not travel under power at hull speed. Rather, the diesel is run at an optimum rpm, which generally speaking, provides a "cruising" speed shy of hull speed.
Add some adverse wave action, and the "cruising" speed of a sailboat is severely impacted.

Sailboats, by their very nature, do not make for optimum powered cruising designs.
Complicating the issue, is that LWL generally dictates your boat speed, regardless of all else, as sailboats have displacement hulls vs. planing hulls.

Finally, for me, anyway, I have a sailboat, not a powerboat, and to this end, have a maxprop, as prop drag under sail is a pain in the rear end.

I don't think that there is an "ideal" situation for powering a typical sailboat. At the end of the day, it's a compromise.

When I first started to look into the matter..marine diesel engines were notably large, heavy and cumbersome with, generally speaking, low rpm.. The modern day marine diesel is a far cry from the beasts of bygone years. Add to the that, turbo-charging...

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.