How much thrust does your engine produce?

[CaptCorn]

A propeller with zero speed of advance provides about 15 to 20 pounds of thrust per engine horsepower. It depends upon the propeller design and the speed of the water entering the propeller. So for example a 30 hp engine at full RPM would pull on a fixed dock between 450 and 600#. The engine may be lugged down because the torque absorbed by the propeller at zero speed exceeds the torque available by the engine if tied to a fixed dock and may not get to rated RPM. In that case the thrust would be reduced.

[merrydolphin]

Don’t know the numbers but from a practical application point of view:
It produces enough to motor Straight into a near gale and maintain 5+ kts!

Very few single engine sailboats under 50 ft will do this. The reason is a so often forgotten part of the drive train, the reduction gear (and respectively the prop).
Not HP. Yes that’s part of it but only. Part. I had a typical 45’ aux sailboat that had 80 hp and a more typical 2.1: 1 reduction and 18” 3 blade fixed prop. In the afore mentioned conditions I would be lucky to not be going backwards, straight into 30+ Kts and 3+M seas. next boat 46’ with 85 hp would do over 5kts all day in these conditions. It motored as good as a trawler I had.
The difference. It had a 3:1 reduction gear swinging a 27” fixed 3 blade prop. A huge difference. Most aux. sailboats suffer from way too much slippage under heavy going. Yet this same 46’ boat could knock off near 200 mile days under sail. If I were to fit a quality feathering prop, even better. But the big prop really didn’t effect me nearly as much as you would think.

Don’t know why more serious boats marketed for Cruising don’’t address this issue.

[kenrobmac]

A very crude estimate of thrust would be about 1 ton of thrust per 100 hp.

This would assume a decent 3 or 4 blade prop and a reasonable reduction ratio.
If you have a 2 blade yacht prop will l not achieve this sort of number.

It is unlikely you will be able to achieve WOT as the prop will cavitate/vertilate long before WOT

I would expect you would load up the engine to cavitation and then the thrust will flatline.

I hope this is helpful

[valhalla360]

Originally Posted by skenn_ie

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.

Actually, it doesn't give a good indication. Generally, you won't be able to get up to the same maximum RPM and resistance the motor sees will be different at all RPM levels.

On a boat in the water, taking it out and running at various speeds and conditions gives you an actual indication of efficiency and capability.

[wsmurdoch]

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.

I bought a 0-2000 lb pull scale. I have used it to measure the reverse bollard pull of my sailboat. I have also used it to measure the load on the anchor in different wind strengths. I have also seen the dramatic effect of waves on the anchor load. I now know that the maximum load that my engine operating in reverse can put on the anchor is about equal to a wind load of 25 kts with no wave action. I know that having backed down on my anchor at 3500 rpm I need to start paying attention when the wind is greater than 25 kts or when the boat is pitching because my anchor set is untested at the expected higher loads.

I have also loaned it to others and gotten a few beers for my effort.

[henryv]

Yamaha 9.9 high thrust models are indicated to produce a thrust of 250 lbs.

[Cheechako]

You can run your boat at the dock all you want, even high throttle with good cleats and lines. PS: you will create turbulence to the boats around you though. No measurement of thrust from this exercise though.

[betwys1]

Checking all the thrust/HP given in this thread: 20,27,10,12,22,17,22,25
gives a mean estimate of 19.4 +- 5.6 lb/HP

[Martkimwat]

It's interesting that most dinghy outboard tests, whether IC or electric use a static dock-pull with a strain gauge to compare each engine/motor with each other, and also most inexpensive electric motors(trolling motors) are rated by their static thrust, so, for example, my Bison electric motor is rated at 69Lbs thrust which is supposedly equivalent to a 2.5Hp IC engine. it certainly feels nearly as good as my Mariner 3.3Hp. I don't see why a similar setup could not be used as a comparison for an inboard engine. Obviously, a larger strain gauge would be needed and I agree you probably won't achieve max rpm but the question asked was 'how much thrust does your engine produce' and static thrust is the only thing that would make sense as it would be a constant, as in any other condition, it will vary according to speed(greater speed, lower thrust till equilibrium is reached), also for any particular boat and engine, the propellor fitted would make a huge difference as well.

[Jdege]

I've been thinking about exactly this problem.

I just bought a boat that was designed to have two props - each driven by a 5HP gasoline engine.

A prior owner removed one prop and on the other installed a 27HP diesel (and added two 25 gallon tanks.)

It's my sense that:

• 27HP is far more engine than this boat needs, and
• 50 gallons is far more fuel storage than I'm every going to need, and
• steering the boat in and out of slips, with only one prop, is very difficult.

With respect to the last, this is a very shallow-draft boat, with a long and shallow keel, and the props alongside the keel, blocking any kind of prop walk.

So we I get the boat ready to splash I've been wondering about replacing the diesel with a pair of electric pod drives.

The big question I need to answer, before I make any decision, is how powerful would these need to be? 1kW? 3kW? 6kW? 9kW?

What I was thinking was to record just how fast the boat moves, at various RPMs, and then to tie to a dock or bollard with crane scale and measure just how much thrust is being generated at each RPM.

With that I could look at the power curves of the various pod drives and get a sense of how fast they would push the boat, and for how long.

And from that I could judge which drive to use, or whether to abandon the whole idea and stick with diesel.

Thoughts?

[valhalla360]

Originally Posted by Martkimwat

It's interesting that most dinghy outboard tests, whether IC or electric use a static dock-pull with a strain gauge to compare each engine/motor with each other, and also most inexpensive electric motors(trolling motors) are rated by their static thrust, so, for example, my Bison electric motor is rated at 69Lbs thrust which is supposedly equivalent to a 2.5Hp IC engine. it certainly feels nearly as good as my Mariner 3.3Hp. I don't see why a similar setup could not be used as a comparison for an inboard engine. Obviously, a larger strain gauge would be needed and I agree you probably won't achieve max rpm but the question asked was 'how much thrust does your engine produce' and static thrust is the only thing that would make sense as it would be a constant, as in any other condition, it will vary according to speed(greater speed, lower thrust till equilibrium is reached), also for any particular boat and engine, the propellor fitted would make a huge difference as well.

For a trolling motor, thrust makes more sense as a useful measure. Top speed isn't usually a major consideration (which is determined by HP up to hull speed for displacement operation).

If you are out on your bass boat and it's blowing 20kts, you want to know if you can hold position. Available thrust is what determines if you can hold position.

[PineyWoodsPete]

Originally Posted by Jdege

I've been thinking about exactly this problem.

I just bought a boat that was designed to have two props - each driven by a 5HP gasoline engine.

A prior owner removed one prop and on the other installed a 27HP diesel (and added two 25 gallon tanks.)

It's my sense that:

• 27HP is far more engine than this boat needs, and
• 50 gallons is far more fuel storage than I'm every going to need, and
• steering the boat in and out of slips, with only one prop, is very difficult.

With respect to the last, this is a very shallow-draft boat, with a long and shallow keel, and the props alongside the keel, blocking any kind of prop walk.

So we I get the boat ready to splash I've been wondering about replacing the diesel with a pair of electric pod drives.

The big question I need to answer, before I make any decision, is how powerful would these need to be? 1kW? 3kW? 6kW? 9kW?

What I was thinking was to record just how fast the boat moves, at various RPMs, and then to tie to a dock or bollard with crane scale and measure just how much thrust is being generated at each RPM.

With that I could look at the power curves of the various pod drives and get a sense of how fast they would push the boat, and for how long.

And from that I could judge which drive to use, or whether to abandon the whole idea and stick with diesel.

Thoughts?

Having co-owned a Meadowlark 37 w/ 25HP Westerbeke-30 diesel years ago, with her 36' waterline and hull speed ~8kn, the diesel would drive her to near 7.5. But we cruised at about 6kn through the lovely FL Gulf coast ICW where little wind except on a couple of large bays or outside, using less than a gal./hr on a 40 gal. tank with a 200+nm range.

My 33' hybrid M/V has a 7/5kW 48V motor/gen in-line with the 165HP diesel which can be separated by a clutch. But at WOT, the electric drives her at only 5kn, but only briefly as voltage from the 11.5kWh lithium bank drops, then ~ 4kn., with a range of prob. 15nm under ideal conditions, Doesn't take much wind/seas to make it fairly useless. The Meadowlark weighs ~16,000# vs. my 11,000 loaded, and surely has more windage with it's ketch rig.

While two 6kW motors might be adequate for most conditions, to get even a 10nm range against mod. wind/sea would take a huge battery capacity - and then you have to recharge..either generator or plug-in.

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!

[Adelie]

Originally Posted by Jdege

I've been thinking about exactly this problem.



I just bought a boat that was designed to have two props - each driven by a 5HP gasoline engine.



A prior owner removed one prop and on the other installed a 27HP diesel (and added two 25 gallon tanks.)



It's my sense that:

• 27HP is far more engine than this boat needs, and
  
• 50 gallons is far more fuel storage than I'm every going to need, and
  
• steering the boat in and out of slips, with only one prop, is very difficult.
  

With respect to the last, this is a very shallow-draft boat, with a long and shallow keel, and the props alongside the keel, blocking any kind of prop walk.



So we I get the boat ready to splash I've been wondering about replacing the diesel with a pair of electric pod drives.



The big question I need to answer, before I make any decision, is how powerful would these need to be? 1kW? 3kW? 6kW? 9kW?



What I was thinking was to record just how fast the boat moves, at various RPMs, and then to tie to a dock or bollard with crane scale and measure just how much thrust is being generated at each RPM.



With that I could look at the power curves of the various pod drives and get a sense of how fast they would push the boat, and for how long.



And from that I could judge which drive to use, or whether to abandon the whole idea and stick with diesel.



Thoughts?

I would be very hesitant to get a pod drive. Over time water gets past the seals into the motor. Then it’s just a question of whether corrosion or a short kills the motor.
Electric trolling motors don’t spend that much time in the water. And the daily hot cold cycle will push small amounts of water vapor back out past the seals.
Pods on large vessels have pumps to push seepage up their stems into the main hull and the overboard.

[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.

[Adelie]

Originally Posted by PineyWoodsPete

Having co-owned a Meadowlark 37 w/ 25HP Westerbeke-30 diesel years ago, with her 36' waterline and hull speed ~8kn, the diesel would drive her to near 7.5. But we cruised at about 6kn through the lovely FL Gulf coast ICW where little wind except on a couple of large bays or outside, using less than a gal./hr on a 40 gal. tank with a 200+nm range.

My 33' hybrid M/V has a 7/5kW 48V motor/gen in-line with the 165HP diesel which can be separated by a clutch. But at WOT, the electric drives her at only 5kn, but only briefly as voltage from the 11.5kWh lithium bank drops, then ~ 4kn., with a range of prob. 15nm under ideal conditions, Doesn't take much wind/seas to make it fairly useless. The Meadowlark weighs ~16,000# vs. my 11,000 loaded, and surely has more windage with it's ketch rig.

While two 6kW motors might be adequate for most conditions, to get even a 10nm range against mod. wind/sea would take a huge battery capacity - and then you have to recharge..either generator or plug-in.

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!

If the OP needs to go to weather ….. raise the sails.

Motoring with a failed mast is possible but unlikely.

Motoring into seas with no wind is more likely but also comparatively rare.

Needing to meet a schedule is fairly likely and probably the best reason to need a diesel. If I was cruising I would not be keeping a schedule and if I was I would not rush to meet it.