Query on engine size and bowthruster

[bigcol]

If you find you ideal boat, that has a BT, it will be sold as a positive assensory in the price.

But don't let it be a deal breaker

[Bugbugs]

I managed to get a nice bit of info from the BetaMarine web site, which might help people figure recommended engine size - I did a screen shot

[bigcol]

I managed to get a nice bit of info from the BetaMarine web site, which might help people figure recommended engine size - I did a screen shot

Bug bugs

Excelent info

Our boat 50 hp beta and a 6 hp BT is boarderLine,but still in the parameters.

 

Thanks

 

Col

Edited January 11, 2016 by bigcol

[Tony Brooks]

I find that a bit simplistic and seems to overstate the power required as well as suggesting a prop diameter than may be too large for the stern post in certain cases. Both myself and another member have stated on here that a nominal 36 bhp engine has been fine in a 54 & 60 ft boat. However a good resource for a starting point but by no means a bible.

[Slim]

Re a bowthruster

 

Two questions you need to ask yourself

 

How long do you intend to keep the boat and how old are you ( not a question to ask a lady and not one that needs answering publicly!)

 

I'm a fairly hefty male aged 69, single hand most of the time and my boat is 55'. When planning and fitting it out 20 years ago I dismissed the idea of a thruster. Now there are occasions where I regret that decision.

 

Good luck

 

 

Frank

[Slim]

Bug bugs

Excelent info

Our boat 50 hp beta and a 6 hp BT is boarderLine,but still in the parameters.

Thanks

Col

Can't help but notice that that table was published by Beta

 

Never felt that my 55' with a bmc 1.8 is underpowered, canals and rivers

 

 

Frank

[Tony Brooks]

Can't help but notice that that table was published by Beta

 

Never felt that my 55' with a bmc 1.8 is underpowered, canals and rivers

 

 

Frank

 

That is what I was trying to suggest but one has to be careful because the torque curve is probably more important than the BHP. I know by Bukh has a "better" torque curve for displacement boat use than a BMC 1.5 but it pretty marginal.

[bigcol]

That is what I was trying to suggest but one has to be careful because the torque curve is probably more important than the BHP. I know by Bukh has a "better" torque curve for displacement boat use than a BMC 1.5 but it pretty marginal.

I was if health was good join some friends on the Thames some time ago.

It was pointed out that my engine on the boat possibly could be underpowered.

After a phone call to Beta, the guy there said that being a modern engine it would have a lot more torque, compared to a older engine of poss 75/80 hp

 

I never made it to the Thames,

 

Col

Edited January 11, 2016 by bigcol

[Tony Brooks]

I was if health was good join some friends on the Thames some time ago.

It was pointed out that my engine on the boat possibly could be underpowered.

After a phone call to Beta, the guy there said that being a modern engine it would have a lot more torque, compared to a older engine of poss 75/80 hp

 

I never made it to the Thames,

 

Col

 

That all depends upon the ratio of cylinder capacity to stroke. Modern engines tend to be square or even over square. That means the bore is the same as the stroke or larger than the stroke. As the torque depends upon the pressure in the cylinder (mean effective pressure) multiplied by the length of the crank arm and square engines have a shorter stroke than under square engines I would suggest the phrase used has no validity. To be sure one would need to compare bore and stroke but in general I would treat that statement as marketing.

 

My nominal 36 bhp engine will push the boat upstream against the Thames on red boards between Godstow lock and Kings cut and we sued to fit 1.5s into 35ft wide beam Thames cruisers with no problems. In fact I met one on the GU about 5 years ago that was still operating well despite being 30 to 40 years old.

 

There is far too much reliance placed upon raw BHP and not enough on matching the gearbox reduction and prop to boat and engine - plus skipper's experience in reading water and getting the boat in the best place for progress. I would note that the lock keeper at Godstow got quiet animated when I dropped through the lock, turned the bow towards the layby and backed the stern into the weir stream to spin the boat round. I do not think she had seen a boater using the current to its best advantage.

[Dalslandia]

the egg or the hen?

torque or HP?

 

it is HP that move the boat, thrust HP from the propeller.

So as said, it depends a lot of how well mached the gear ratio and propeller is to the hull, a small or big prop can be 25% in diff of efficiency.

if the engine under rev, say 200 rpm or 10% that is probably less then 10% diff in HP from engine, and some of that is taken back from the under rev prop.

 

A higher torque engine can turn the over propped prop faster then a less torque engine.

 

others if we have one high torq and one low torq engine but with same HP, and same rpm at the propeller. (different gear ratio) they do the same job just as good.

 

And if propeller is matched to max HP rpm, and we throttle back, the engine torque curve have nothing to do with the thrust, because propeller demands less power/torque then engine can deliver, so two engines with same hp at same max rpm, but with different torque/hp mid range, will still just produce enough power to turn the prop at the wanted rpm. until we change the throttle setting.

 

And to the question of bow thruster, as an canal pilot said, if the owner invested in a bow thruster, the captain should use it.

Edited January 12, 2016 by Dalslandia

[Tony Brooks]

I am going to disagree with much of that.

 

BHP is quoted at full throttle and at the stated revs. The revs stated are usually well into the rev band where torque is falling while the increase in RPM more than makes up for the falling torque figure.

 

Now, unless my text books are all wrong Torque is the "twisting force" and it is "twisting force" that twists the prop shaft, not the amount of power being developed although the torque and BHP are related.

 

At any prop loading or speed it is the torque the engine produces at that particular throttle opening and speed that determines the ability of the engine to spin the prop, not the BHP at full load. If the engine can not produce enough torque to spin the prop at a given speed at a particular throttle opening the engine will run at and prop will turn at a speed at which the torque is sufficient. Again the BHP at full throttle has nothing to do with it. It is all dependant upon torque.

 

At any given throttle opening a long stroke engine must produce more torque than one with a shorter stroke although a larger piston area on a typical short stroke area must to a degree offset the shorter radius of the crank web.

[Dalslandia]

Good that you disagree.

 

power is what moves the boat, power is work don or produced.

 

torque is a force, we need force and movement to get the work don,

 

if torque is 100 and rpm 2000 we have the same power as if torque is 200 and rpm 1000

if we gear those example down to same propeller rpm we can have the same propeller on both, say we have 500 rpm on propeller torque is 400.

 

if we now throttle back to half rpm 250 on propeller, the needed power is about 12.5% out of what is needed at 500 propeller rpm, and torque is 100 at propeller.

 

the engine full throttle power at half rpm is probably higher then half power as you say, but even if it is half power or slightly less it is higher then the 12,5% the propeller need at that rpm. the extra power at the hand make it possible to increase rpm/speed.

 

That a long stroke engine have higher torque then a short stroke engine, all other the same, is correct.

 

If both engines/boats have the same gear ratio, they will have 2 different propellers, and the low rev high(er) torque engine will be believed to have more power, but it is the propeller that is different. and the thrust power will be different.

[Arthur Brown]

Find a boat that you LIKE! Then consider the power and the river. The Trent is a huge river with some huge currents you have to learn how to deal with them. Read good charts and learn to watch the weather, especially watch the weather upstream (into Derbyshire!) several hours/days before you wish to travel. Learn to accept that fast currents and slow boats don't mix, and learn to ride the tides to your benefit -which may mean travelling at silly o'clock and mooring while the tide runs against you.

[Loafer]

Good that you disagree.

 

power is what moves the boat, power is work don or produced.

 

torque is a force, we need force and movement to get the work don,

 

if torque is 100 and rpm 2000 we have the same power as if torque is 200 and rpm 1000...

 

Torque isn't a force - it's a product OF a force, and the radius of action around the pivot. Therefore it must be like 'work' (force times distance)

 

Power is Work Per Sec, so presumably ...oh dear. I've lost it.

 

Help!

[Dalslandia]

 

Torque isn't a force - it's a product OF a force, and the radius of action around the pivot. Therefore it must be like 'work' (force times distance)

 

Power is Work Per Sec, so presumably ...oh dear. I've lost it.

 

Help!

 

Yes you lost it, hehe easy don so.

 

you can have torque or you can have a linear force, but without movement you don't have a power. and nothing will be don.

Thrust power is a linear power, with a linear force and movement, like a horse pulling a canal boat. 1 HP 100% efficiency and non slipping (hopefully)

 

at walking speed propeller efficiency is probably 25% and you need 4 HP at engine and you use lille more then a litre of diesel per hour.

at 6 km/h = 3.73 MPH (1.66 m/s)

 

Thrust will then be 4 * 75 * 0.25 / 1,66 = 45,2 kg or 99 lb

Edited January 12, 2016 by Dalslandia

[WotEver]

Well, Horsepower = (Torque x RPMs)/5252 so the two are totally and inextricably linked.

 

Loads of hits on Google about this if you want to research it but in a nutshell you can't have one without the other and the one which actually represents 'work' is torque.

[dmr]

At any given throttle opening a long stroke engine must produce more torque than one with a shorter stroke although a larger piston area on a typical short stroke area must to a degree offset the shorter radius of the crank web.

 

I'm going to risk getting shot down in flames here, but I reckon this is a myth, though a very very popular myth.

If we take two similar engines of the same capacity, but one with a longer stroke, then at a given low speed they will produce the same power and hence the same torque. It all comes down to how much air they can suck in and this is primarily related to the capacity of the engine.

Long stroke engines are not good at high speed and I think many people have falsely extended this to mean:

short stroke = high speed low torque, long stroke= low speed high torque.

 

As you say, the bigger piston area does compensate for the shorter stroke!

 

....................Dave

[Dalslandia]

Well, Horsepower = (Torque x RPMs)/5252 so the two are totally and inextricably linked.

 

Loads of hits on Google about this if you want to research it but in a nutshell you can't have one without the other and the one which actually represents 'work' is torque.

 

if you have torque or force and no movement, linear or rotating, no work will be don. so mechanical work is force and movement

 

the 5252 is a konstant coming from the 33000 lb a horse is said to be able to lift one foot in one minute, (or 550 in one sec) because the motor/engine rotate it is divided with the radius of one foot but the circumference is 2 pi, so 33000/2pi =5252

Edited January 12, 2016 by Dalslandia

[Loafer]

A steam engine can have maximum torque at zero rpm, just as it starts pushing, so they say. So what they REALLY mean is that they have max FORCE at zero rpm?

Edited January 12, 2016 by Loafer

[dmr]

An electric motor can also produce full torque at zero speed (which is why they are widely used in locomotives whilst we are talking railways)

 

Work done is indeed force times distance

and Torque is also force times distance

but these are not the same sort of distance.

The torque arm is effectively at 90 degrees to the force so does not count, only any distance moved in the the direction as the force will do work.

Vectors and resolutions of forces and all that stuff.

In the rotating sense work = torque times angle.

(and for the engineering pedants that angle is in radians and is dimensionless, the torque=force times distance hence provide the correct dimensions of work!)

 

...............Dave

[Loafer]

An electric motor can also produce full torque at zero speed (which is why they are widely used in locomotives whilst we are talking railways)

 

Work done is indeed force times distance

and Torque is also force times distance

but these are not the same sort of distance.

The torque arm is effectively at 90 degrees to the force so does not count, only any distance moved in the the direction as the force will do work.

Vectors and resolutions of forces and all that stuff.

In the rotating sense work = torque times angle.

(and for the engineering pedants that angle is in radians and is dimensionless, the torque=force times distance hence provide the correct dimensions of work!)

 

...............Dave

 

Very good Dave. Succinct.

 

Well I believed it anyway.

[WotEver]

if you have torque or force and no movement, linear or rotating...

... you have 0hp

[Dalslandia]

An electric motor can also produce full torque at zero speed (which is why they are widely used in locomotives whilst we are talking railways)

 

Work done is indeed force times distance

and Torque is also force times distance

but these are not the same sort of distance.

The torque arm is effectively at 90 degrees to the force so does not count, only any distance moved in the the direction as the force will do work.

Vectors and resolutions of forces and all that stuff.

In the rotating sense work = torque times angle.

(and for the engineering pedants that angle is in radians and is dimensionless, the torque=force times distance hence provide the correct dimensions of work!)

 

...............Dave

 

Work don is force * distance / period of time

Torque is a force some distance out but no movement or time, it is a moment arm force, but not in a moment (no time) but at the moment Clear?

 

I like the picture of a horse pulling in a rope over a pulley down from the mine, lifting a bucket with a weight of 100 lb at the speed of 5.5 ft-sec (330 ft-min) (3.75 MPH) doing the work of one horse.

 

If we put a motor on the shaft of the pulley when the horse got old. and the diameter of the pulley is 2 ft, radius is 1 feet.

We need a torque of 100 lb one feet out to pull it up.

the speed of the rope is 5.5 feet per second, and the circumference of the pulley is 2pi * 1 = 6,283 feet

the rotating speed will then be 5,5/ 6,283 = ,87535 rotation per second or 52,52 rev per minut

 

so the formula for HP is torque in ft-lb * RPM / 5252

in this case 100 ft-lb * 52,52 / 5252 = 1 HP

 

IF we have an electric motor of one HP at 1500 rpm, the motor need a gear ratio of 28,56:1 to turn the pulley at 52.52 RPM

so the motor need to have a torque of 3,5 ft-lb (100/28.56)

the electric motor is 3,5 * 1500 / 5252 = 1 HP

The power isn't changing with gearing but the torque is.

 

The bucket should be just so big that the load of 100 lb of material fits, to small we lose material or to big the bucket weight to much

So the biggest propeller that can be fitted and engine can handle is the winner. :-)

​

[mark99]

In the practical sense, accept torque and power are linked, we need an engine which delivers max torque at the rev range and gearing you need it. So it's engine/gearing characteristics.

 

What you don't want is all the torque/hp high up in the rev band so you need to rev like crazy to get good screw.

 

Low revving give high torque is generally met by a large piston displacement engine (big Mean Effective Pressure) pushing down a long crank (lever).

 

That way you spend a lot of cruising time wallowing in the useful rev band. (And they sound better too!).

Edited January 13, 2016 by mark99

[Tony Brooks]

This topic stated by the OP asking about the power (BHP) of engines and I still do not see how the BHP that is measured at full throttle and a given revs can be much more than a rough indication of the torque available at other throttle openings.

 

If, as seems fairly common judging by the maximum achievable revs across a number of boats, a displacement boat is propped to provide maximum fuel economy then very soon after the torque curve starts to fall off at higher speeds the available torque becomes less than that required to turn the prop so the revs stay steady.

 

At that point, despite being at full throttle the bhp will be less than if the load created by the prop was lower so the engine could reach the speed at which the bhp was measured.

 

I am arguing that the published bhp of an engine does not tell you how well the engine can push the boat along under normal boating conditions. The torque curve will give a far better idea as long as the engine, prop & gearbox reduction are suitably matched.

 

If you wanted to use HP to gauge the ability of an engine at part throttle you would either have to use a table of BHP's measured across the whole rev & load range or drill holes in the cylinder head so you can measure mean effective pressure and calculate the hp.

 

Each engine design may have a similar torque curve to most others of a similar BHP but it will not be identical for a variety of reasons, not all to do with piston area MEP and crank throw although they may all have the same quoted BHP.

In the practical sense, accept torque and power are linked, we need an engine which delivers max torque at the rev range and gearing you need it. So it's engine/gearing characteristics.

 

What you don't want is all the torque/hp high up in the rev band so you need to rev like crazy to get good screw.

 

Low revving give high torque is generally met by a large piston displacement engine (big Mean Effective Pressure) pushing down a long crank (lever).

 

That way you spend a lot of cruising time wallowing in the useful rev band. (And they sound better too!).

 

Absolutely, they are linked but not identical so one has to look at the torque curve rather than the published BHP figure.