Propellors

[Dalslandia]

I understand what Tony is saying, if just as an example, we have two identical boats, and use the max practical diameter possible to install, we have one with a 30 hp engine and one with a 50 hp engine, the gearing and pitch used so both engines can produce its max power, and lets say that the boat goes so fast that the 30 hp give a good speed so the relative big propeller give 50% efficiency.

 

Okay the 50 HP boat will have 50 hp on the same disk area, so higher prop load, and efficiency a bit lower, say 40% just to pic a number, but probably likely.

 

now we see the 30 hp boat have 15 HP that produce thrust, THP = Thrust HP

 

the 50 HP boat have 20 THP and it will go, cube root 20/15= 1,1 times faster, yes 10% faster.

 

BUT if the 50 HP boat had a smaller propeller the efficiency might be only 30% and the Thrust HP is 15 THP, and it will be just as fast as the 30 HP boat, but use 50/30= 67% more fuel for same work.

 

These numbers is taken straight out of the thin air, but show the relations in real world.

Edited July 31, 2015 by Dalslandia

[Tony Dunkley]

 

Up to 43-44 HP. The engine will only produce enough power to maintain the speed demand

 

Richard

 

I think what you really mean is load demand, but in any case the engine can't get above 2200 rpm (underway and in gear), so it's on full load at those revs anyway.

Edited July 31, 2015 by Tony Dunkley

[RLWP]

 

I think what you really mean is load demand, but in any case the engine can't get above 2200 rpm, so it's on full load at those revs anyway.

 

I guess I'll have to re-read the thread. I missed the bit about being load limited to 2200

 

Richard

[Dalslandia]

And that is also something to read, think and understand about, the difference between published power curve vs RPM, and propeller demand curve.

 

if as in this case the engine do 2200 at full throttle, the power is 43ish

 

if it can rev to its rated max power rpm it will be 55 at 3000 as in this case, and this engine is probably rated with all necessarily items.

 

if we gear and pitch it so it can reach 3000 rpm and 55 HP, and then throttle down to 2200, we can forget what the graph say, because it is a full throttle curve, full throttle at all RPM, now when throttled back to 2200 the propeller demand curve say ~21-22 hp

[peterboat]

Got what you say but both Mike and I would have to change gearboxes and propellers to achieve better power at the propeller. Also I could only accommodate a 21 inch prop at the most, the ideal is the biggest prop you can fit turning slow but the engine operating at its maximum torque. What needs to happen is boat makers need to go to school to make better boats with the right propulsion equipment. Never going to happen

 

Peter

[Dalslandia]

Got what you say but both Mike and I would have to change gearboxes and propellers to achieve better power at the propeller. Also I could only accommodate a 21 inch prop at the most, the ideal is the biggest prop you can fit turning slow but the engine operating at its maximum torque. What needs to happen is boat makers need to go to school to make better boats with the right propulsion equipment. Never going to happen

 

Peter

 

Absolutely correct Peter. can't be said better. and some distance between prop, skeg and hull is good, like 15-17% of the diameter up to hull,

[Neil2]

I'm just checking my understanding on a couple of things here.

 

Given that an engine like the Isuzu isn't going to be designed for operating flat out, for practical purposes the 2200 rpm is probably going to be it's "maximum" rpm.

 

So changing the gearbox ratio isn't going to allow the engine to produce any more power.

 

But it will generate more torque so you could have a much bigger prop.

 

But even so, the engine is not producing any more power so the boat can't go any faster - yes?

 

Unless the efficiency of the wider prop makes a big difference?

Edited August 1, 2015 by Neil2

[peterboat]

I'm just checking my understanding on a couple of things here.

 

Given that an engine like the Isuzu isn't going to be designed for operating flat out, for practical purposes the 2200 rpm is probably going to be it's "maximum" rpm.

 

So changing the gearbox ratio isn't going to allow the engine to produce any more power.

 

But it will generate more torque so you could have a much bigger prop.

 

But even so, the engine is not producing any more power so the boat can't go any faster - yes?

 

Unless the efficiency of the wider prop makes a big difference?

Yes if thats the way they explain it and I understand it

 

Peter

[nebulae]

Our boat has a shallow draft,for a steel boat,of 20"". The prop is a less than perfect 17"". Although this gives an adequate performance on narrow canals,it is one of the factors limiting performance in open water. With previous boats,we have had the weedhatch up freqentlly. With this boat,we have not had the hatch up for over a thousand lock miles. I guess all these deep drafted boats with big props are collecting all the junk,while we slide over it.?

[nebulae]

The 42 hp Isuzu engine in our boat is desgned to run flat out at 3,000rpm because it is a general purpose industrial engine,intended for pumps,generators etc,where it would have to run at 3,000rpm to deliver the machine output. H.P.I. the marinisers,restricted the r.p.m.and fitted a 2/1 gearbox,to reduce prop speed. I dont think I could live with the noise at max revs .I do not have access to the power curve for this engine,but I believe that the 42hp quoted by Isuzu is delivered at 3,000rpm. I presume that the actual hp is less than the quoted hp.(Maybe as little as 25/30hp.

[Dalslandia]

All boats beeing different, and owner demand is different, and all is differently good at different situations.

 

TOP speed don't change much with power, or thrust power as seen in my previus example, but efficiency at slow, or cruise speed change a lot, it is mainly fuel economy, but haveing a big prop and dragging the keel in the mud doesent improve effeciency. So as the swedish saying Lagom is best.

[Dalslandia]

The 42 hp Isuzu engine in our boat is desgned to run flat out at 3,000rpm because it is a general purpose industrial engine,intended for pumps,generators etc,where it would have to run at 3,000rpm to deliver the machine output. H.P.I. the marinisers,restricted the r.p.m.and fitted a 2/1 gearbox,to reduce prop speed. I dont think I could live with the noise at max revs .I do not have access to the power curve for this engine,but I believe that the 42hp quoted by Isuzu is delivered at 3,000rpm. I presume that the actual hp is less than the quoted hp.(Maybe as little as 25/30hp.

You can probably use the 55 HP curve with a factor, 0,76

The isuzu beeing asian i am sure they use iso standard and the output is close to reality

[nebulae]

Interesting, So at 2,000rpm our Isuzu is producing 31.92hp.This would be flywheel hp. Allow gearbox loss on a 2/1 gearbox of(say)5%,I am left with 30.2hp.With a propellor efficiency of (say) 50% ,I am left with 15.1hp,to drive the boat. I can get 5.9 knots in deep still water,which is close to the hulls theoretical max displacement speed .As discussed on an earlier post,modern narrow boats have a very inefficient hull form ,so not a bad perfomance.

[Neil2]

You can probably use the 55 HP curve with a factor, 0,76

The isuzu beeing asian i am sure they use iso standard and the output is close to reality

 

Are you sure about that Dalslandia?

 

Most modern engines seem to refer to ISO 8665 but as I understand it that just specifies the conditions under which the engine is to be tested. ie ISO 8665 still allows the engine power to be quoted as either at the flywheel or at the prop with all ancillaries. I reckon if the manufacturer doesn't make it clear how the power output figures are to be interpreted you can assume it's a gross figure.

 

Edit - I've just realised I may have misinterpreted your statement and maybe you meant the 55hp can be taken as a true gross figure with a factor of 0.76 applied to give the likely output at the prop?

Edited August 1, 2015 by Neil2

[magnetman]

All boats beeing different, and owner demand is different, and all is differently good at different situations.

 

TOP speed don't change much with power, or thrust power as seen in my previus example, but efficiency at slow, or cruise speed change a lot, it is mainly fuel economy, but haveing a big prop and dragging the keel in the mud doesent improve effeciency. So as the swedish saying Lagom is best.

What is Lagom?

 

I am enjoying your technical input to this thread.

One thing I have always liked is when you put a boat in gear and it noticeably moves forward immediately and the stern drops a bit. Its not that common with canal boats in my experience.

I mean without increasing speed above tickover.

 

I have owned and driven several narrow boats and the only one which did this was an old motorised horsedrawn bantock narrowboat which had a large Petter air cooled engine (pj4) and a big propeller (I don't know exactly what size it was but it was big)

My barge does this as well and I can spend all day just above tickover so its very fuel efficient and a good turn of speed.

 

I am not a person with technical knowledge about propellers but I think this may be partly attributable to pitch.

 

So is an increase in pitch more or less effective than an increase in diameter in terms of engine efficiency?

 

I suspect also that a larger engine may actually be more efficient because it is better matched to the most effective propeller size for a particular boat.

 

I'm sure its been done before on the forums but no two threads are ever exactly the same

Edited August 1, 2015 by magnetman

[Dalslandia]

 

Are you sure about that Dalslandia?

 

Most modern engines seem to refer to ISO 8665 but as I understand it that just specifies the conditions under which the engine is to be tested. ie ISO 8665 still allows the engine power to be quoted as either at the flywheel or at the prop with all ancillaries. I reckon if the manufacturer doesn't make it clear how the power output figures are to be interpreted you can assume it's a gross figure.

 

Edit - I've just realised I may have misinterpreted your statement and maybe you meant the 55hp can be taken as a true gross figure with a factor of 0.76 applied to give the likely output at the prop?

I meant to have rough idea what the 42 HP Isuzu power curve look like, the 55 hp can probably be used with a fuss factor

 

I know there are many standards, the now old DIN was good.

[Dalslandia]

What is Lagom?

 

I am enjoying your technical input to this thread.

One thing I have always liked is when you put a boat in gear and it noticeably moves forward immediately and the stern drops a bit. Its not that common with canal boats in my experience.

I mean without increasing speed above tickover.

 

I have owned and driven several narrow boats and the only one which did this had a large Petter air cooled engine (pj4) and a big propeller I don't know what size it is.

My barge does this as well and I can spend all day just above tickover so its very fuel efficient and a good turn of speed.

 

I am not a person with technical knowledge about propellers but I think this may be partly attributable to pitch.

 

So is an increase in pitch more or less effective than an increase in diameter in terms of engine efficiency?

 

Thank you magnetman

 

Lagom is one single word for when it isn't to much or to little, just about right, but don't have to be full or perfect

 

Saw a Piper Dutch barge named Lagom.

 

one aspect to a efficient big slow propeller, with lot of pitch is prop walk. but it might be a can of worms.

With Dalslandia I can go dead slow into the looks in gear, that help with steering, between locks in narrower canals I increase rpm one or two hundred rpm.

Idle is 650-700 and a 3:1 SCG gearbox, 26" pitch.

 

I was skipper on a other boat a few times in this same canal, a few inch bigger boat, flat bottom, big propeller and a hefty prop walk, it was not half as good as Dalslandia to take into the canals and locks, and did go to fast in gear, and steering with a joystick was no joy either in the canal, on the lakes it is ok though, only time it did go slow was when halfway into the lock, with a boat as a cork on the gate, the water inside the lock had problem passing the boat...

Edited August 2, 2015 by Dalslandia

[magnetman]

Yes it probably is a can of worms as you say.

 

I just like the feeling something is happening when going into gear rather than the propeller just spinning round without doing anuthing. It implies efficiency but as you say prop walk or paddlewheel effect can be a bit irritating if its severe. And it was severe enough on the Bantock to mean it just wouldn't turn properly to the left going forwards (port) I did once end up in the bushes because of this

Edited August 1, 2015 by magnetman

[Dalslandia]

Yes it probably is a can of worms as you say.

 

I just like the feeling something is happening when going into gear rather than the propeller just spinning round without doing anuthing. It implies efficiency but as you say prop walk or paddlewheel effect can be a bit irritating if its severe. And it was severe enough on the Bantock to mean it just wouldn't turn properly to the left going forwards (port) I did once end up in the bushes because of this

it is a questions on tonnage too, a 2 ton boat doing 25 kts+ with 300 HP, or as Dalslandia, 163 hp and 70+ ton, when going into gear it hardly is noted in a half second or more. different from friends boat with 300 HP and Duo-prop that do 5 kts idle in gear

[magnetman]

I think for this thread we are talking about heavy displacement canal boats rather than sports cruisers

 

Some of the canal boats I have driven take a lot more than half a second to begin going forwards. I may be wrong but to me the fact a propeller 'bites' the water at slow engine speed (Tickover about 900rpm on an engine rated to 2700rpm) implies efficiency, provided that it is also able to increase output up to its rated maximum rpm.

 

Maybe it just implies that the engine is too big?

 

(Edit to change Rickover to tickover as my phone seems to think I meant Rickover )

Edited August 1, 2015 by magnetman

[nebulae]

On a point of clarity .I dont know if this applies to other Isuzu or other Japanese engines,but our Isuzu 42hp is rated by the ENGINE manufacturers at 42hp at 3,000rpm. Although it was advertised as a 42hp by H.P.I. the marinisers,as the engine was de-rated to 2,000rpm by them,it was not actualy a 42hp in marinised form .If you are working on the advertised hp rather than a dynamometer reading on the gearbox coupling,your calculations are going to be way out.

[Dalslandia]

On a point of clarity .I dont know if this applies to other Isuzu or other Japanese engines,but our Isuzu 42hp is rated by the ENGINE manufacturers at 42hp at 3,000rpm. Although it was advertised as a 42hp by H.P.I. the marinisers,as the engine was de-rated to 2,000rpm by them,it was not actualy a 42hp in marinised form .If you are working on the advertised hp rather than a dynamometer reading on the gearbox coupling,your calculations are going to be way out.

right, if torque was constant (it isn't) you would have 42x (2000/3000) = 28, maybe little more, guessing that max torque is in that region. minus the gearbox drag.

 

The 55 HP Isuzu curve x 0,76 factor give 30,4 hp at 2000. minus 3% and you have 29,5 at the shaft, 28,6 at the propeller ...

 

So as you say what it say on the leaflet or colour full delivery box doesn't always agree with the reality.

 

For those that don't have a firm grasp about HP Horse Power, or kW kilo Watt, for that matter, Power is work don, using the force and repeat the work often, can have one small shovel, shovel fast, or one big shovel, shovel slow, if it does the job in same time, the power created/used was the same.

 

An engine, or motor, turn and can make that with a torque (force) if the torque is constant and the rpm is increased more power - more work don.

same if torque is increased and rpm constant.

 

An imperial hp is said to lift 33000 lb one feet in one min, or 550 lb in one sec.

An metric HP is said to lift 75 kg one meter in one sec.

 

Torque is measured one feet or one meter out from shaft centre, but power is movement with a force, engines isn't often linear but rotate, so distance around is r * 2 * phi, and radius is 1.

 

The engine rotate and torque is in foot-lb so 33000/ (2 * phi) = constant 5252

 

if engine have torque 100 ft-lb and rotate with 1000 rpm, the power is 100*1000 / 5252 = 19 Imperial HP

 

Metric torque is often in Newton meter. or sometimes kg-m

 

75*9,81*60=44145 / (2*phi) = 7026 (or 716 if torque is in kg-m)

 

so, 100 Nm at 1000 rpm / 7026 = 14 PS or metric HP

Edited August 2, 2015 by Dalslandia

[peterboat]

Hi all

 

Well the repitch on the prop has helped on the Trent it produces more power when needed which was the issue before. I also got rubbish caught in the prop which caused me problems but the engine could still rev to 2200 so just managed to get out of serious trouble until a couple of reverse bursts cleared the prop and full steerage was returned.

So for me the exercise was worthwhile the boat is easier to reverse and moor up, and when fighting the current the engine can run at full power so it pushes the tide which it didnt before

 

Peter

[magnetman]

That's good news - thanks for the update

[blackrose]

In the light of this latest post from you, I no longer think that you don't understand the basic principles, . . . . I'm absolutely certain that you don't.

The feeling is entirely mutual I assure you.

 

Your figures are a load of tosh and there are other contributors to this thread who seem to understand the principals far better than you.

Edited September 2, 2015 by blackrose