Query on engine size and bowthruster

[mark99]

I've ridden quite a few very high horsepower motorcycles and have, in the garage, a couple of big displacement "V" "twins" with significantly less HP.

 

However it's my V twins that pull my arms off at low revs and kicks into dust the high horsepower bikes around the "lanes" as the power and esp torque is delivered very low down the rev range.

 

Get to 60/70 mph and I'm left in the dust as the revs go up and the multicylinder's bikes spin up and go into warp speed!

 

Low down power/torque is king in my book.

Edited January 13, 2016 by mark99

[rgreg]

Just to add to the bowthruster debate. Mine broke five years ago and I have not got round to having it repaired. The only time that I occasionally miss it is when reversing into the marina berth in a cross-wind.

[Dalslandia]

Tony, I do not understand what you don't understand, you seems to have it right.

 

it is just that HP is connected to torque and rpm, we can not mesure HP but we can mesure torque and calculate HP

 

BHP is a somewhat bad statment, it can mean British HP, Brutto HP or brake HP, and in imperial world IS Brutto HP without the normal losses from sea water pumps, exhaust/silencer airfilter, and alternator (light load) look at Beta's HP curves, they have both drawn in the diagram, a red and a green curve.

 

The full throttle curve (full throttle at all RPM) is often a sort of a convex curve, that start to fall of at max HP RPM, or sometimes increase till a red line of the engine.

the torque curve often fall of earlier as you say.

 

If we now prop the boat/engine so max Red line is just reached with throttle at the forward stop, the engine develope its max power.

All RPM below max will have partial throttle and power. with spare power at hand for acceleration.

 

But as soon as we throttle back we can not see the power created by look at the RPM and engine curve in the book. Att all rpm below max, the power fall of with the demand of the propeller by the rpm^3 or close to that, in a concave curve, where the concave and convex curve cross each other is max rpm we can get.

 

As I wrote earlier at 50% rpm the power needed to turn the propeller is 12.5% and at 75% rpm we have 42% power, at 85% rpm we have 61% power, and at 95% rpm we get ~86% power

This goes for boat speed as well, as long as we are below max hull speed. some use the ^2.8 factor

 

At 50% rpm the power is 12.5% but torque 25%

At 75% rpm the power is 42% but torque 56%

At 85% rpm the power is 61% but torque 71%

At 95% rpm the power is 86% but torque 90%

Full throttle power at 75% RPM (if loaded to reach max 75% rpm) is probably higher then 75% due to the torque curve is probably close to its max here (different on different engines)

 

But with a propeller connected to an engine or motor, the engine never develop more power then the propeller need for that RPM, except for accelerating.

Edited January 13, 2016 by Dalslandia

[dmr]

 

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?

 

On this occasion you are not quite right, and that is a rare thing.

 

Work done is just force times distance.

force times distance / period of time is Power.

Power is the rate of doing work!

 

Force and distance are both vectors, so direction matters, so only the distance moved in the direction of the force will do work.

Torque is force times distance and that distance does not change (not usually) but more importantly that distance is not in the direction of the force.

 

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

Po

[Dalslandia]

I've ridden quite a few very high horsepower motorcycles and have, in the garage, a couple of big displacement "V" "twins" with significantly less HP.

 

However it's my V twins that pull my arms off at low revs and kicks into dust the high horsepower bikes around the "lanes" as the power and esp torque is delivered very low down the rev range.

 

Get to 60/70 mph and I'm left in the dust as the revs go up and the multicylinder's bikes spin up and go into warp speed!

 

Low down power/torque is king in my book.

 

on the road, high torque at low rpm means high(er) power at low rpm and the opposite in low torque machines, as long as the tyres don't slip you can use all power (torque*RPM) at that RPM, if it slips the rpm goes up quicker then the speed, just as with a propeller ...

 

On this occasion you are not quite right, and that is a rare thing.

 

Work done is just force times distance.

force times distance / period of time is Power.

Power is the rate of doing work!

 

Force and distance are both vectors, so direction matters, so only the distance moved in the direction of the force will do work.

Torque is force times distance and that distance does not change (not usually) but more importantly that distance is not in the direction of the force.

 

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

Po

 

You are right there, work can have been don long ago. and it don't say how long time it took. (I hade that thought in my mind but thought no one would pick it up)

A green one.

Edited January 13, 2016 by Dalslandia

[dmr]

I've ridden quite a few very high horsepower motorcycles and have, in the garage, a couple of big displacement "V" "twins" with significantly less HP.

 

However it's my V twins that pull my arms off at low revs and kicks into dust the high horsepower bikes around the "lanes" as the power and esp torque is delivered very low down the rev range.

 

Get to 60/70 mph and I'm left in the dust as the revs go up and the multicylinder's bikes spin up and go into warp speed!

 

Low down power/torque is king in my book.

 

That's why a high capacity engine is always so much nicer than a little turbocharged one.

 

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

[Dalslandia]

About published BHP,

 

As I am a propeller designer and propeller software developer for airplanes, it is important to know or have a good guess what the REAL installed HP is when calculating both speed and propeller. sales talk is no help here. and the same goes for speed.

Engines from England, USA and Australia is sold by the BHP, But not all.

Engines from Germany, Sweden, Asia, often publish more truth full HP numbers.

 

When designing a propeller for a BHP published engine, usually 80-85% of the published BHP is used, and that lead to a good matched propeller.

This seems to be a good approach for boat propellers to a "BHP" engine as well.

 

The installed power is affected by items like airfilter, exhaust pipe (bends) silencer, number and size of alternators, pumps, hydraulic and water. and also of air temp into the engine, and temp of the diesel fuel reaching the engine.

 

Re-reading Tonys post,

having a high power high rev engine where the power isn't used, with a not thought much about propeller size and gear ratio, compered to a low power low rev, high torque engine and low rev big propeller, the latter wins, not because of high torque but because of a better propeller suited for the low speed cruise, and the sound of that engine wins too.

 

A good or a bad propeller size can differ as much as 23-30% in efficiency, so having a 30% stronger engine don't help if the propeller is a miss match. Why does those old NB "win" over the new one with modern engines? I say prop size and shape of the hull.

but we have the boat we have, so doing the best out of it, is to have the biggest propeller that is practical, and practical is minimum 2" clearance top and bottom, or 10-15% of the diameter, witch ever is bigger. (10% of an 18" is 1,8")

Then pic a gear ratio that give that diameter and that let the engine rev to its max rated power

Then the pitch should be picked from speed and prop rpm.

 

The propeller is losing efficiency at slow speed and as we have seen don't need much power at low rpm, If we now have a big engine and don't need all that power on a river or lake, it can be a good idea to over prop it slightly, and efficiency is improved at low speed especially in shallow water, it also load the engine little more at low rpm making the engine more efficient. but it should not smoke to much at WOT

[Chewbacka]

About published BHP,

 

As I am a propeller designer and propeller software developer for airplanes, it is important to know or have a good guess what the REAL installed HP is when calculating both speed and propeller. sales talk is no help here. and the same goes for speed.

Engines from England, USA and Australia is sold by the BHP, But not all.

Engines from Germany, Sweden, Asia, often publish more truth full HP numbers.

 

When designing a propeller for a BHP published engine, usually 80-85% of the published BHP is used, and that lead to a good matched propeller.

This seems to be a good approach for boat propellers to a "BHP" engine as well.

 

The installed power is affected by items like airfilter, exhaust pipe (bends) silencer, number and size of alternators, pumps, hydraulic and water. and also of air temp into the engine, and temp of the diesel fuel reaching the engine.

 

Re-reading Tonys post,

having a high power high rev engine where the power isn't used, with a not thought much about propeller size and gear ratio, compered to a low power low rev, high torque engine and low rev big propeller, the latter wins, not because of high torque but because of a better propeller suited for the low speed cruise, and the sound of that engine wins too.

 

A good or a bad propeller size can differ as much as 23-30% in efficiency, so having a 30% stronger engine don't help if the propeller is a miss match. Why does those old NB "win" over the new one with modern engines? I say prop size and shape of the hull.

but we have the boat we have, so doing the best out of it, is to have the biggest propeller that is practical, and practical is minimum 2" clearance top and bottom, or 10-15% of the diameter, witch ever is bigger. (10% of an 18" is 1,8")

Then pic a gear ratio that give that diameter and that let the engine rev to its max rated power

Then the pitch should be picked from speed and prop rpm.

 

The propeller is losing efficiency at slow speed and as we have seen don't need much power at low rpm, If we now have a big engine and don't need all that power on a river or lake, it can be a good idea to over prop it slightly, and efficiency is improved at low speed especially in shallow water, it also load the engine little more at low rpm making the engine more efficient. but it should not smoke to much at WOT

Thanks for that - I do like learning new stuff. Have a greenie

[Dalslandia]

Beta 150, with 147 BHP but also the ISO 8665 curve in this case 117 HP ~80%

http://betamarine.co.uk/wp-content/uploads/downloads/technical_data_sheets/inland_data/B150-IDS-0311.pdf

 

http://betamarine.co.uk/wp-content/uploads/downloads/sales_brochures/1720-50150-ISB-200512.pdf

But same engine here show ISO 8665 130 HP ~88 %​

 

in the graph, we see torque

also the two HP curves

and fuel used with propeller load, fuel used per HP is almost constant, but rise little at low rpm.

if there was an Propeller demand curve it would have looked much like this fuel curve but would meet the HP curve at max power.

​

Edited January 13, 2016 by Dalslandia

[steve7a3]

I have a bow thruster and, when I first started out, I found it a great help however as I've gained experience and knowledge from other boaters have found it far more of a convenience than a necessity. Mind you, the on time it was invaluable was when I became hard aground on a sandbank just inside Tattenhall Marina (Warning: there is a sandbank just inside Tattenhall Marina. It's easy to find if you turn hard left as you come under the bridge! ). The bow thruster enable me to "twist" the boat around and unstick the stern. Without it I could well still be sitting there! If you can find the boat you want with one then get it but don't consider it a necessity in most situations.

[Loafer]

I have a bow thruster and, when I first started out, I found it a great help however as I've gained experience and knowledge from other boaters have found it far more of a convenience than a necessity. Mind you, the on time it was invaluable was when I became hard aground on a sandbank just inside Tattenhall Marina (Warning: there is a sandbank just inside Tattenhall Marina. It's easy to find if you turn hard left as you come under the bridge! ). The bow thruster enable me to "twist" the boat around and unstick the stern. Without it I could well still be sitting there! If you can find the boat you want with one then get it but don't consider it a necessity in most situations.

 

Isn't that what your pole is for?

[steve7a3]

 

Isn't that what your pole is for?

Oh, believe me, we tried that! Tried pushing, tried reverse, was tempted to try paddling in the water to push. Eventually remembered the bow thruster and thought "Well, why not!?" and it worked.

[Loafer]

Oh, believe me, we tried that! Tried pushing, tried reverse, was tempted to try paddling in the water to push. Eventually remembered the bow thruster and thought "Well, why not!?" and it worked.

 

Ah ok! I'm impressed then. I would have thought a bloke with a pole would out-push a bowthruster!

[steve7a3]

 

Ah ok! I'm impressed then. I would have thought a bloke with a pole would out-push a bowthruster!

Not this little bloke!

[Peter X]

As much of my boating experience before I first got on a narrow boat was punting, I have a tendency to reach for the pole more readily than most. A narrow boat being far heavier than a punt, it responds far more slowly, but a pole is still very effective especially when pushing against a bank, to help when winding or with other tight turns. It's less useful when pushing against the bottom, which more often than not on a canal is mud rather than gravel, so will tend to suck you back when removing the pole. I wouldn't expect it to be very much help if pushing against silt, which is basically fine sand and will just give way.

 

When Foxy and I ran the bow aground in mid-channel on the Rochdale last month on silt deposited by the flood, I think the thing that really helped get us free was pulling first from one side then the other on the stern line, thus using the movement of the stern to make the bow enlarge its hole in the silt and loosen the grip of the silt, enough for the engine to be able to reverse us off. It helped that we were in a lock entrance with someone at each side to do the pulling.

[Dalslandia]

Someone wrote this in 2013. I have not changed any thing in the text.

 

<The topic about power versus torque always come up, does*t matter if talking cars, airplanes or boats, boats and airplane have several things in common, propellers, rudder, and drag, it works the same way. (Maybe not the rudder)

 

First torque without movement is not power, and power is simply said work don.

To do the work, it can be made in small bits, large bits, fast or slow.

 

The bigger bits and the faster, the more power, more work don in a specified time.

 

An efficient propeller will need less power to do the same job. I have designed airplane propellers that have been 25-26% more efficient then the propeller it have replaced with same power from engine, it is not sales talk or bragging, it is plane! data from user. No one is more surprised then me.

 

I am just telling that because it show that the propeller is very important, it is like going from 40 to 50 HP, it does not change speed much but take off and climb, and not least fuel consumption at cruise. 25% more power increase speed 7.7%

 

Let’s say we have an old boat with a big propeller, a single cylinder, long stroke, direct drive engine, say it cruise at 500 rpm and at 10 hp just to have a number.

 

We don't like the oil it spit out from the exhaust and leak under the engine, so we want a modern engine, we find a 25 hp engine from Taiwan, 25 hp at 4000 rpm,

 

we want to keep the old propeller, that fits the boat so well, say the engine make 15 hp at 2000 rpm, we want the propeller to do 500 rpm when engine make 2000, so it have to be geared down 4 times.

 

Just as before the propeller need 10 hp at 500 rpm, the gearbox take some 3% or more, so engine will produce 10,3 hp at 2000,

 

The torque and rpm at the propeller is the same, so is the power at the propeller.

The torque at the engine is <4 times less, but power is the same (plus the power the gear box drag down the engine with)

That little rice boiler can produce 15 hp at 2000, on the paper, so we can throttle back some, if it is a modern engine tested with all needed items like water pumps and alternators (low load) we still have some power to increase speed with, but when charging the batteries there is not enough power. But there is a 35 hp engine also. But the 45 HP engine is just £500 extra and come with a bigger alternator.

 

So we buy the bigger engine, it is also 33% more powerfull then what the guy moored next to us have.

 

It is producing 45 hp at 3000 rpm, with a 3:1 gearbox it will do 1500 with 500 on the prop

A 2:1 and we get 1000 rpm on engine and 500 on prop.

 

We still want to keep that old prop, just for nostalgic reasons, not for saving some money because the big engine was expensive.

 

With hot air in the engine room, good silencer, and some load on the bigger alternator, we might have 80% of the original 45 it say on the box, so we have maybe 35 HP to deliver to the propeller shaft.

 

Say it still need 10 hp at a 5 mph, why wouldn’t it. With 35 hp we can go 35/10= cube root out of 3,5 = 1,5 times faster ~7,5 mph

 

To do that we need about 1,5 times more RPM at the prop 500*1,5=750 rpm with the same diameter and pitch, But the engine need to turn 3000 rpm to produce the 35 net HP, so we need a gearbox ratio of 3000/750 = 4 if we want all power from engine, so it will still rev 2000 at 5 mph. and produce 10.3 HP because thats what propeller and gearbox need at 500 rpm

Or we give up those potential HIGH speed runs, with a different gearing. Or cave in and buy a smaller propeller, but it will need a new prop shaft too … what will wife and mother say?

 

So my guess is they say: it is good as it is no need to change anything.

 

Jan>

​

[Loafer]

Someone wrote this in 2013. I have not changed any thing in the text.

 

<The topic about power versus torque always come up, does*t matter if talking cars, airplanes or boats, boats and airplane have several things in common, propellers, rudder, and drag, it works the same way. (Maybe not the rudder)

 

First torque without movement is not power, and power is simply said work don.

To do...

 

......eller and gearbox need at 500 rpm

Or we give up those potential HIGH speed runs, with a different gearing. Or cave in and buy a smaller propeller, but it will need a new prop shaft too … what will wife and mother say?

 

So my guess is they say: it is good as it is no need to change anything.

 

Jan>

​

 

I think I got most of that. Most enlightening!

 

You should have an honorary CWDF degree for that! Greenie from me!

Edited January 14, 2016 by Loafer

[blackrose]

Hello, my first post, so please be gentle

I am a female single hander (24' plastic boat for 5 years) looking to buy a 38'-48' liveaboard narrowboat. I have a couple of queries if anyone could kindly give me some advice please.

 

I feel I'm going to need the security of bow thrusters to help me get front end in/give some assistance in turning. How important/useful are they to single handers, as I am severely limiting my choice of boats by having this on my wishlist. I am sure I have read somewhere that they can't be retro-fitted, is that right?

 

I have a BT on my 57 x 12ft, 32 tonne widebeam as I'm also single-handed, but for a 48ft narrowboat I really don't think you'll need one.

Edited January 16, 2016 by blackrose

[Loafer]

There is a good deal of satisfaction to be gained from having no bow thruster and learning to do everything the so-called 'proper way'.

 

It takes about a week!

[dmr]

There is a good deal of satisfaction to be gained from having no bow thruster and learning to do everything the so-called 'proper way'.

 

It takes about a week!

 

Half agree, its a huge satisfaction, but I think it takes half a lifetime!

 

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

[blackrose]

There is a good deal of satisfaction to be gained from having no bow thruster and learning to do everything the so-called 'proper way'.

 

It takes about a week!

There's quite a good deal of satisfaction to be gained from having a bow thruster and doing it the so-called "cheats way" too. (Especially if there are some traditionalists around to annoy!)

 

It takes about two seconds!

 

It really depends on the boat. On a big boat like mine a BT is very handy indeed when single handed. But having a bow thruster doesn't mean you can't have the option of learning how to do it without one. Just leave the BT switched off with the batteries isolated.

Edited January 17, 2016 by blackrose

[Loafer]

 

It really depends on the boat. On a big boat like mine a BT is very handy indeed when single handed. But having a bow thruster doesn't mean you can't have the option of learning how to do it without one. Just leave the BT switched off with the batteries isolated.

 

Now then, blackrose. Do YOU actually do that? If you do, then I have 50% more respect for you than I did, errrr, before!

[Dalslandia]

In the 30+ years we had the BT it have been 3-4 times it have not worked, 2 times it was the a fault in the BT, one the fuse went, 30 years later the help contactor gave up in the port direction, the other two times the gen set been inoperative. there is a 4-5 places where it is not possible to turn with out a BT or a spring, especially if windy, also the locks is wider on the inside then in the gate, so is save paint using it little when leaving the lock.

those times it is not operative, the left hand automaticly seeks the joystick and soon i get reminded it does not work. Dalslandia have a lot of airdraft, I have to ad.