Boat speed vs engine speed

[FidoDido]

Last time I went out, I decided to use a speed app on my phone to see what speed I'm doing at various engine speeds.. I got a surprise:

 

slow tickover: 2.7 mph (800 rpm)

"fast" tickover: 3 mph (i.e. just off tick over - maybe 1000-1200 rpm)

Middle range, where I normally cruise 3.4 mph

 

Gunning it, such that there was a breaking wake. 4 mph

 

I was genuinely surprised that tick over or just above, where there is no load on the engine particularly, made the boat go so quick.. and then how much engine revving doesn't make it go much quicker! Made me think, how much fuel am I wasting pushing the engine harder for an extra half a mile an hour? I think I'll potter in the just above tick over range, and see if my fuel usage is much reduced.

 

Is this common?

 

42 Hp engine. unsure of prop size. 70ft narrowboat, about 2 ft 6 draft.

 

Of course, I've no idea of the accuracy of the phone app!

[Paul C]

What depth of water, what draft of boat (I know you've mentioned it but I've listed it for completeness for others), and how far from the bank were you? It makes a big difference.

Edited June 7, 2014 by Paul C

[Alan de Enfield]

 

 

Is this common?

 

 

Different rev range - but yes the principle is common.

 

You get to the point (with a displacment boat) that all you do is pile water up in front of you, getting progressively harder and harder to push against - you could try 10,000 revs and a 100hp engine but it wont go any faster.

[by'eck]

Generally speaking the stern digs in with greater engine power and causes boats stern to drag along the bottom of the canal. Prop becomes less efficient (cavitation) if the boat a cannot accelerate to a speed allowing the prop to bite on clean water as well, not to mention churning up mud.

 

There comes a point where reducing engine power increases your speed.

[bottle]

All that Paul C is correct

 

but

 

generally your findings are true that is why I travel at about 1250 rpm (similar engine, 10 ft shorter, same draught)

 

Similar engine PDF data sheet, showing fuel consumption at different rpm's

 

http://www.betamarine.co.uk/downloads/technical_data_sheets/inland_engine_data/B43_Tech_Photo_Sheets/B43-IDS-0311.pdf

 

edit; to add link

Edited June 7, 2014 by bottle

[jelunga]

Last time I went out, I decided to use a speed app on my phone to see what speed I'm doing at various engine speeds.. I got a surprise:

 

slow tickover: 2.7 mph (800 rpm)

"fast" tickover: 3 mph (i.e. just off tick over - maybe 1000-1200 rpm)

Middle range, where I normally cruise 3.4 mph

 

Gunning it, such that there was a breaking wake. 4 mph

 

I was genuinely surprised that tick over or just above, where there is no load on the engine particularly, made the boat go so quick.. and then how much engine revving doesn't make it go much quicker! Made me think, how much fuel am I wasting pushing the engine harder for an extra half a mile an hour? I think I'll potter in the just above tick over range, and see if my fuel usage is much reduced.

 

Is this common?

 

42 Hp engine. unsure of prop size. 70ft narrowboat, about 2 ft 6 draft.

 

Of course, I've no idea of the accuracy of the phone app!

I think it is called hull design speed or similar

A little power gets you to this speed but to go faster requires a disproportionate amount of power. .

Our is similar.

[OldGoat]

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

[Alan de Enfield]

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

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

 

Did he design boats ?

[John V]

I think it is called hull design speed or similar

A little power gets you to this speed but to go faster requires a disproportionate amount of power. .

Our is similar.

 

 

as an example 28' boat 25' waterline displacement 4 tons to increase from 8 knots to 12 knots (33.3% increase) requires a 275% increase in engine power......38shp @8kts 105 shp @ 12kts

 

 

edit to add this is in free water no bottom drag or width/depth restrictions

Edited June 7, 2014 by John V

[MtB]

Yes your experiment seems consisten with my own experience. Try in in deep water on, say The Thames and you'll get more intuitive results.

 

This also explains why boats passing me at tickover often appear, from inside my boat, to be going past almost as other boats who barely bother, if at all to 'slow down'. Reducing engine speed has only a marginal effect on boat speed in shallow water.

 

MtB

[RLWP]

My sign that I'm pushing too hard is the speed of the water passing the boat relative to the speed of the towpath (yes, I know)

 

If the towpath is passing slowly and the water is passing quickly, back off the power

 

It's a good way of keeping crap off the prop too, you don't suck it up off the bottom of the cut

 

Richard

[GUMPY]

Some comparative figures from my GPS boat is 60ft 2ft5 draft 50hp beta 19x? Prop

Tickover 850rpm canal 2.7 deep water 3.0mph

1200rpm canal 3.5 deep water 4.5mph

1500rpm canal not worth going there deep water 5.5mph

1900rpm. Ditto. deep water 7.5mph

In 2 years I have only once had it flat out and that was when there was a bit of fresh on the Ouse and GPS read 11.5mph.

[Robbo]

 

Is this common?!

 

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

[FidoDido]

What depth of water, what draft of boat (I know you've mentioned it but I've listed it for completeness for others), and how far from the bank were you? It makes a big difference.

 

It was on a very shallow bit of the Trent + Mersey.. Indeed I can hear cavitation at about 1400 rpm. In deep water, its much better - boat feels very "free". Was doing 4 mph up the Trent from Beeston to Cranfleet with a reasonable flow on the river a weekend or two ago - at about 1700-1800 rpm, so had plenty left. No point in pushing it though. That "felt" like a comfortable speed.

 

In Harecastle Tunnel, I can only go at tick over, or the amount of water pulled past the boat makes it very difficult to steer. It may well be over propped.. On a shallow canal, I can often only go at tick over. Then, now I know tick over is around 3 mph, thats perfectly fine. I'll just poodle along at minimal engine revs, and enjoy the scenery

[Keeping Up]

I've often wondered what is the best speed to go through a narrow tunnel (I'd class Harecastlle as narrow, compared with say Blisworth where you can go as fast as you like), especially if there is a flow of water in the opposite direction such as on the Llangollen. At tickover speed you hardly move, but high revs make the water flow past you extremely fast.

[jelunga]

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 ?

No. I thought he did motorcycles.

[Guest]

Being a nerd, I have measured my boats speed between bridges. It usually comes out at around 2mph at just above tickover (engine stops rattling and alternator becomes efficient at this point about 1000rpm) At 1300 rpm it comes out at around 3.5mph. I have never measured it at higher revs, but don't believe it would make above 4mph on an average canal. It just makes more noise and chucks more water around.

 

Boat is 57NB with Vetus 415 33hp about 2ft draught and slightly over-propped.

[Guest]

Yes your experiment seems consisten with my own experience. Try in in deep water on, say The Thames and you'll get more intuitive results.

 

This also explains why boats passing me at tickover often appear, from inside my boat, to be going past almost as other boats who barely bother, if at all to 'slow down'. Reducing engine speed has only a marginal effect on boat speed in shallow water.

 

MtB

Pity more people don't understand that point. It is a psychological thing that if they hear the revs drop, well you must have slowed down. I have read about people with those constant speed engines getting moaned at for not slowing down, when in fact they have reduced the prop. speed.

[Naughty Cal]

Tickover 700rpm = 3 knots

Canal cruising revs 900-1000rpm = 4 knots

River cruising revs 1200-1500rpm = 6 knots

Fast cruise revs 3000rpm = 25 knots

Flat out 3800rpm = 30 knots and no fuel!

[Guest]

Tickover 700rpm = 3 knots

Canal cruising revs 900-1000rpm = 4 knots

River cruising revs 1200-1500rpm = 6 knots

Fast cruise revs 3000rpm = 25 knots

Flat out 3800rpm = 30 knots and no fuel!

I would suspect at high speed you notice an increased fuel consumption when the tank is full? ie using fuel to carry fuel?

[Alan de Enfield]

................. using fuel to carry fuel?

 

Is there an alternative ?

[Naughty Cal]

I would suspect at high speed you notice an increased fuel consumption when the tank is full? ie using fuel to carry fuel?

There is a noticeable drop in speed when the tank is full but we don't really have a choice but to carry fuel!

 

Towards the end of a five hour run you get a noticeable increase in speed for the same revs. It's time to have a pit stop then though and refuel!

[Guest]

 

Is there an alternative ?

None whatsoever (except for a motor-sailer.) Just interesting (to my small mind) that the MPG of some vehicles increases noticeably as the fuel load decreases. That is of course what averages are about.

Edited June 7, 2014 by Guest

[RLWP]

Flat out 3800rpm = 30 knots and no fuel!

 

That reminds me of this:

 

 

Apparently, when the engines are lit, a Saturn 5 rocket is too heavy to fly. It is the fuel being used up that gives the gentle launch

 

And it uses up fuel at an enormous rate

 

Richard

[Guest]

There is a noticeable drop in speed when the tank is full but we don't really have a choice but to carry fuel!

 

Towards the end of a five hour run you get a noticeable increase in speed for the same revs. It's time to have a pit stop then though and refuel!

Yes fuel comes in useful in a power boat

I remember a friend had a similar style of boat to yours at Hoylake, and the effect of a full tank was very noticeable indeed. Great boat it was, went like the clappers.