Boat speed vs engine speed

[blackrose]

Yes, all boats do it (unless the boat can plane), your boat maybe over propped which will highlight this "issue" earlier.

 

I think over-propping is a big part of the reason why the effect of diminishing returns (engine revs/speed), is so exaggerated on many canal boats. Since I had the pitch reduced on my prop I get a lot more speed with greater engine revs. The correlation between the two can never be linear on any vehicle of course - I think it's the same with a car to a lesser extent due to drag and air resistance, so there will always be diminishing returns.

Edited June 7, 2014 by blackrose

[Guest]

 

I think over-propping is a big part of the reason why the effect of diminishing returns (engine revs/speed), is so exaggerated on many canal boats. Since I had the pitch reduced on my prop I get a lot more speed with greater engine revs. The correlation between the two can never be linear on any vehicle of course - I think it's the same with a car to a lesser extent due to drag and air resistance, so there will always be diminishing returns.

Interesting. I wonder why so many canal boats appear to be over-propped? I have read of many that are, but few that are under-propped.

[blackrose]

Interesting. I wonder why so many canal boats appear to be over-propped? I have read of many that are, but few that are under-propped.

 

I think in part in comes down to this idea that maximum hp is achieved at maximum torque, which is not always the case especially with modern, higher revving engines which unlike the vintage thumpers, have relatively shallow torque curves. For example, my engine achieves maximum torque at about 1900rpm, but power output keeps going up until it reaches maximum revs at 3000rpm. It's never going to reach 3000rpm in gear of course, but before the prop was repitched it only revved to 1950rpm and after repitching it achieves 2250rpm and that's resulted it a lot more power. Being too overpropped is like driving a car up a steep hill in 4th gear. It overloads the engine.

 

Power & torque curves for Isuzu 55:

 

But that still doesn't really explain why canal boats are routinely over-propped. I suppose you wouldn't want an under-propped boat, so most boats will be over-propped to some degree which is usually fine, but in the case of canal boats they sometimes go too far over and you end up losing power.

Edited June 7, 2014 by blackrose

[John V]

img459 by mudlarker2, on Flickr

 

 

 

an example of an under propped changed to a correctly propped

[Keeping Up]

Mine is deliberately slightly under-propped because not only does that mean I can travel more slowly if I want to, also there is power in reserve to continue turning the prop when it has a plastic bag or other rubbish wrapped around it. I can still manage just over 7mph on open water which I reckon is plenty.

[blackrose]

Mine is deliberately slightly under-propped because not only does that mean I can travel more slowly if I want to, also there is power in reserve to continue turning the prop when it has a plastic bag or other rubbish wrapped around it. I can still manage just over 7mph on open water which I reckon is plenty.

 

Does that affect things when you want to stop quickly? If a prop is too flat (under-pitched) it can also slip and lack bite when you want to pull away.

[Robbo]

Interesting. I wonder why so many canal boats appear to be over-propped? I have read of many that are, but few that are under-propped.

 

It lowers engine revs at canal cruising speed so the engine is quieter and can improve response. You lose top end speed which isn't important on canals. Doesn't make a difference to the amount of fuel used as your still pushing the same weight boat.

[Keeping Up]

Does that affect things when you want to stop quickly? If a prop is too flat (under-pitched) it can also slip and lack bite when you want to pull away.

I certainly haven't found that, it certainly stops well enough and the cloud of exhaust smoke if I do an emergency stop suggests that the engine is working hard then. It's only slightly under propped, maybe by 10%, and I suppose it also depends on what is meant by under propped. It's under for the engine, in that it can easily reach Max revs while still not quite using full power, but I wouldn't say it was under for the boat.

 

Its a fairly coarse pitch because I use a 3:1 box. That seems to give it plenty of bite, even when towing.

[Guest]

It lowers engine revs at canal cruising speed so the engine is quieter and can improve response. You lose top end speed which isn't important on canals. Doesn't make a difference to the amount of fuel used as your still pushing the same weight boat.

Makes sense I suppose. As we only use a fraction of the horsepower, I expect it won't make a difference to fuel consumption. The only down side I can see is that the alternator might be running a bit less than its optimum speed. That of course could be cured with different pulley ratios. As Blackrose points out, you can get away with slight over-propping.

[ditchcrawler]

I think I'll potter in the just above tick over range, and see if my fuel usage is much reduced.

 

 

If you do, please pull over when someone catches you up. 1 MPH can be the difference between Watford and Buckby top if you have been boating all day.

It lowers engine revs at canal cruising speed so the engine is quieter and can improve response. You lose top end speed which isn't important on canals. Doesn't make a difference to the amount of fuel used as your still pushing the same weight boat.

I don't think you are quite right there, I can pull my boat at a very slow walking pace, I can't run with it so it must require more energy.

[Robbo]

If you do, please pull over when someone catches you up. 1 MPH can be the difference between Watford and Buckby top if you have been boating all day.

I don't think you are quite right there, I can pull my boat at a very slow walking pace, I can't run with it so it must require more energy.

Of course the quicker the boat goes the more fuel you will use, but that has nothing to do with engine speed! It's the load on the engine that uses fuel not the amount of revs it's doing.

[FidoDido]

If you do, please pull over when someone catches you up. 1 MPH can be the difference between Watford and Buckby top if you have been boating all day.

 

I'm a courteous boater, and indeed always let people past, either at a lock or if one is not coming up, then pull over. I've been in a hurry too, and know how frustrating it can be.

 

That said, I'm often caught behind boats whose speed is signifincatly slower than I can achieve at tick over, so have to coast etc a lot.. This is also very frustrating.

[Tam & Di]

If you are interested this explains why an increase in power does not always result in an increase in speed.

 

http://www.boats.com/boat-content/2010/03/crunching-numbers-hull-speed-boat-length/

 

It is sailing boaty but is fairly relevant

 

Did he design boats ?

 

Hmmm.

 

By that formula a 72' narrowboat has a hull speed of 13.8 mp.h., which he says it can maintain effortlessly but that an enormous increase in power is required to exceed it!

 

Really?

 

 

Tam

[John V]

I have been unable to get to any more than the opening headline of the article but working backward I would guess they are working at V sq root L X 1.34 or 1.35 which is fine for a displacement or semi displacement conventional hull. Narrowboats have more of the hydrodynamic qualities of a brick compared with sea going craft so I would expect them to be nearer to a factor of 1.1 this would give a hull speed in deep water of nearer 9.4 kts instead of the 11.7 you quoted.

 

with the same engine and power of the first example but with the hull reduced in length to 56 foot the speed would reduce to 8.2 kts

and for a 40 footer 7 kts all of which sounds about right.....I don't think anyone will ever have worked out the "K" factor for narrowboats so I don't think I will bother trying to find my textbooks.

[nebulae]

Many years ago,Practical Boat Owner produced a very good guide to optimum propellers.At the end of the article was a "Perfect Prop Chart"The article was written in a way that was easy to understand by the layman,by Nigel Warren .I think there may be copyright issues so I cant reproduce it .I also have a graph produced by the Manchester Ship Canal Mech and Elec Engineering Dept. This shows,for instance,that ,a 50ft boat can achieve 4 knots in still water with 7hp.

[FadeToScarlet]

 

Hmmm.

 

By that formula a 72' narrowboat has a hull speed of 13.8 mp.h., which he says it can maintain effortlessly but that an enormous increase in power is required to exceed it!

 

Really?

 

 

Tam

 

It probably could do, if it were built to very fine lines indeed, e.g. 25' long, properly curved swims, rounded hull (not flat bottomed), very fine bow and sleek exit, etc.

 

A bit like Duchess-Countess.

 

Thinking about it, the old packet fly boats were apparently much narrower than normal, 5' beam sometimes, to allow them greater speed.

 

Hmmm.

 

By that formula a 72' narrowboat has a hull speed of 13.8 mp.h., which he says it can maintain effortlessly but that an enormous increase in power is required to exceed it!

 

Really?

 

 

Tam

 

It probably could do, if it were built to very fine lines indeed, e.g. 25' long, properly curved swims, rounded hull (not flat bottomed), very fine bow and sleek exit, etc.

 

A bit like Duchess-Countess.

 

Thinking about it, the old packet fly boats were apparently much narrower than normal, 5' beam sometimes, to allow them greater speed.

[John V]

I seem to remember reading somewhere that the packet boats used a burst of speed to get on top of the wave in the canal and then could back off and hold a high speed with less effort

Edited June 10, 2014 by John V

[Naughty Cal]

I seem to remember reading somewhere that the packet boats used a burst of speed to get on top of the wave in the canal and then could back off and hold a high speed with less effort

That's much how a planing hull works. You need huge power to get the boat to climb out of the water but once it is out and planing you can back right off the throttle, stay on the plane and maintain the speed.

[PhilR]

I seem to remember reading somewhere that the packet boats used a burst of speed to get on top of the wave in the canal and then could back off and hold a high speed with less effort

 

 

That's much how a planing hull works. You need huge power to get the boat to climb out of the water but once it is out and planing you can back right off the throttle, stay on the plane and maintain the speed.

 

IME the comparison might appear similar, but in the case of the old packet boats they were still displacement boats. They would go fast enough to push a wave (wall of water) in front of them and then attempt to catch it up and ride it like a surfer. That is only possible in a restricted cross section like a narrow canal. I'm sure it did wonders for soft sided banks although it might not normally be a breaking wave!

 

For a boat like Naughty Cal, when at planing speed most of the hull will be out of the water. But that does not require a restricted cross section of water to achieve.

[Keeping Up]

A soliton perhaps?

[John V]

A soliton perhaps?

 

I wonder if the effect that was being described was that, certainly interesting. I just wish I could remember where I read about packet boat speeds, I'll have to go and dig through my bookshelves.

That's much how a planing hull works. You need huge power to get the boat to climb out of the water but once it is out and planing you can back right off the throttle, stay on the plane and maintain the speed.

 

I realize that, as my "going fishing" boat "Pippin" is 16' planing hull with a 40hp 2 stroke on the back but the article I read was something different

[Keeping Up]

 

I wonder if the effect that was being described was that, certainly interesting. I just wish I could remember where I read about packet boat speeds, I'll have to go and dig through my bookshelves.

 

Quoting from a post that I made a year ago:

 

"A few years ago I experienced something which was a little bit similar to a soliton.

 

Coming back down from the head of the Ashby, my attention was wandering as I left a relatively deep wide section to enter a narrow, shallow wooded cutting. Normally it is impossible to travel much above tickover speed through that cutting without making a wash, but on this occasion I was travelling quite fast from the open section beforehand and I simply forgot to slow down.

 

As we entered the narrow, shallow section the boat naturally started to slow down, but then my stern wave caught me up. At this point, just as I realised where I was and reached for the throttle to slow down, the stern wave lifted the back of my boat and it seemed as if I started "surf-riding" down the front of the wave (which was not breaking, I hasten to add). I soon discovered that if I kept the throttle open, this seemed to feed the wave so that it didn't die down; indeed I found I could open the throttle further and further to maintain a very large, smooth, non-breaking wave behind me - upon which the stern of the boat rose higher and higher. I estimate that I travelled about a quarter of a mile at a speed that was probably a "little bit" above the BW speed limit before my attention wavered for a few seconds and I strayed a couple of feet off the centre-line of the canal. At that point the symmetry was lost, the wave developed a breaking crest at the bank and I had to slow to my usual tickover speed.

 

I have often wondered about this phenomenon, and have tried unsuccessfully a couple of times to re-create it."

Edited June 11, 2014 by Keeping Up

[Machpoint005]

It's all the fault of an Italian chappie - name of Bernoulli.

Swiss

[Beaker]

Cheese

 

ETA Sorry, wrong thread.

Edited June 11, 2014 by Beaker

[Jamesandlinda]

Two factors. One is limited size of canal & as water won't compress (much), going faster in small waterways just doesn't work. The other is the blunt angle of the swim. Old trad 72ft narrow boats had a 17ft swim. Make it shorter & you get more cabin space, but at the expense of limiting the amount of water getting to the prop.