What size engine?

[Scotty]

I'm getting close to buying a boat now and I have a mooring on the middle levels, so I want a boat that will cope with the currents of the Nene and Ouse.

 

i'm looking for a 45ft boat and i'm finding that engines in 45ft boats range from 25-35hp, which seems like quite a difference?

has anyone had any bad experiences of a 45ft boat with a 25hp or 28hp engine on a river?

 

I don't want to find myself stuggling to avoid a weir because i've bought a boat with a rubbish engine!

[BigRoj]

I'm getting close to buying a boat now and I have a mooring on the middle levels, so I want a boat that will cope with the currents of the Nene and Ouse.

 

i'm looking for a 45ft boat and i'm finding that engines in 45ft boats range from 25-35hp, which seems like quite a difference?

has anyone had any bad experiences of a 45ft boat with a 25hp or 28hp engine on a river?

 

I don't want to find myself stuggling to avoid a weir because i've bought a boat with a rubbish engine!

 

A friend of mine has a 45ft NB and travels the Trent with it. He has a 35hp Barras Shire and it does him proud, even on the tidal bit of the Trent against the flow.

It's probably better to have a bit too much power than not enough, especially with the way the weather is now!!!

Edited January 12, 2008 by BigRoj

[bottle]

Hi Scotty

 

Have a look here Beta marine guide, read the small print at the top of the page.

[NB Willawaw]

I'd go for a 35-38HP and a well matched prop, especially if you plan to drive a hefty domestic alternator or Travel Power..

We have just done the Nene and are now on the Great Ouse cruising South between Ely and Cambridge.

We sheltered from the worst and the Nene was far worse than the Ouse. You will need to punch the tide at Salters Lode.

 

We have a Beta 43 on our 62ft but no Travel Power, as we have a separate genset.

Edited January 12, 2008 by NB Willawaw

[Sam]

I'd go for a 35-38HP and a well matched prop, especially if you plan to drive a hefty domestic alternator or Travel Power..

We have just done the Nene and are now on the Great Ouse cruising South between Ely and Cambridge.

We sheltered from the worst and the Nene was far worse than the Ouse. You will need to punch the tide at Salters Lode.

 

We have a Beta 43 on our 62ft but no Travel Power, as we have a separate genset.

 

I agree with the man above

[redfastlad]

The shire will do you well and on the Trent it will push you on and give you the confidence to try the tidal reaches, the option of travel power as well makes this the best engine for NB,s i am biased as my 45hp has done me proud up and down the Trent twice we have come up river in flood and other than use a good drop of juice she powers us with a great deal of confidence and we always have good battery power and the engine is in my opion the best on the cut.

[stuart]

I've a 35hp Isuzu on a 45ft boat, no problem with rivers, 8mph no problem.

[Scotty]

I've seen a few 45ft boats with 28hp engines, but after reading bottle's Beta marine guide and hearing your responses i think i'll keep looking for something with a bit more poke.

 

I'd feel happier knowing there's some power in reserve.

 

 

Although at the moment the boats i like are gone before i can make an offer!

[steelaway]

I'd go for a 35-38HP and a well matched prop, especially if you plan to drive a hefty domestic alternator or Travel Power..

We have just done the Nene and are now on the Great Ouse cruising South between Ely and Cambridge.

We sheltered from the worst and the Nene was far worse than the Ouse. You will need to punch the tide at Salters Lode.

 

We have a Beta 43 on our 62ft but no Travel Power, as we have a separate genset.

 

I agree with the phrase - 'well matched prop' - I suspect the majority of boats are under propped.

The engine revs at your usual criusing speed should be at the peak torque revs of the engine.

This also where the maximum MPG is achieved.

[Billypownall]

I agree with the phrase - 'well matched prop' - I suspect the majority of boats are under propped.

The engine revs at your usual criusing speed should be at the peak torque revs of the engine.

This also where the maximum MPG is achieved.

Steelaway can I ask if you mean overpropped as I think this is actually the case. The cruising speed of most narrowboats is way under the peak torque revs of most engines. Since we fitted a a new propellor on our boat with less pitch and increased our cruising revs benefits accrued all round. Smoother quieter running and much increased charging from the two alternators.

[steelaway]

Steelaway can I ask if you mean overpropped as I think this is actually the case. The cruising speed of most narrowboats is way under the peak torque revs of most engines. Since we fitted a a new propellor on our boat with less pitch and increased our cruising revs benefits accrued all round. Smoother quieter running and much increased charging from the two alternators.

Hi There

My first boat was over propped and would not pull peak torque revs without overheating (Nanni).

As you say it became a new boat after changing to a smaller one.

It was recomended by Barrus that my new (Shire 45) 57ft boat be fitted with an 18x10. I tried a friends boat with this prop on his 57ft Barrus (Shire 45) and thought it under propped. Cruising revs 2200rpm.

I fitted a 18x12 - better but cruising revs 2000. I had my prop repitched to 18x14 dropped to 1900rpm but seriuse cavitation noise.

Now fitted with 19x12 and so much better cruises at 1600-1800rpm (4.3mph according to my GPS unit) peak torque on this engine. It will also pull 2800rpm (on the river Nene this year). Lord knows what speed that is but it felt like being on a destoyer.

Each engine has its own power characteristics, this is usually found in the engine brochure - shown as a graph. I have tried to tune my boat to this engines characterisics.

 

Alex

[Yoda]

Steelaway can I ask if you mean overpropped as I think this is actually the case. The cruising speed of most narrowboats is way under the peak torque revs of most engines. Since we fitted a a new propellor on our boat with less pitch and increased our cruising revs benefits accrued all round. Smoother quieter running and much increased charging from the two alternators.

 

That may be the case where your charging better but that's a seperate problem. What you have done is increase the fuel consumption and therefore the hourly cost of running the boat.

 

The prop must be sized by the sfc curve of the engine.

[Keeping Up]

Most canal boat engines have a fairly flat torque curve over virtually all their usable rpm range; the possible cruising speed, max torque, and best fuel economy, don't necessarily come at a well-defined engine speed. The useful range is also relatively restricted, between 3:1 and 4:1 as a rule.

 

You need to make sure that your slowest speed is slow enough to enable you to pass lines of moored boats without having to engage neutral, but at the same time make sure that your fastest speed is fast enough to cope with the currents of any currents or tides that you may meet. These two alone may be almost incompatible for some engines; you just cannot find a prop for example that will give you 8mph but still enable you to slow to 2mph, if your engine has only a 3:1 rev range.

 

Also you may find it an advantage to have the engine deliberately under-propped, to give you some power in reserve in order that you can continue to travel for some distance with a collection of rubbish on the propellor, until you reach somehwre suitable to remove it. Also you want to set it up so that the revs do not drop too low when at idle speed in gear.

 

All these, and other factors, may lead you to fit a prop which is not actually optimised for your particular engine.

[Billypownall]

That may be the case where your charging better but that's a seperate problem. What you have done is increase the fuel consumption and therefore the hourly cost of running the boat.

 

The prop must be sized by the sfc curve of the engine.

This statement about raising the fuel consumption has come up before and yet we noticed absolutely no difference at all. Surely the engine is not working as hard at the higher revs so why should the consumption rise to any degree? I do feel anyway that the engine itself would rarther have a few more revs on than being like climbing up a steep hill in top gear in a car.

[steelaway]

Most canal boat engines have a fairly flat torque curve over virtually all their usable rpm range; the possible cruising speed, max torque, and best fuel economy, don't necessarily come at a well-defined engine speed. The useful range is also relatively restricted, between 3:1 and 4:1 as a rule.

 

You need to make sure that your slowest speed is slow enough to enable you to pass lines of moored boats without having to engage neutral, but at the same time make sure that your fastest speed is fast enough to cope with the currents of any currents or tides that you may meet. These two alone may be almost incompatible for some engines; you just cannot find a prop for example that will give you 8mph but still enable you to slow to 2mph, if your engine has only a 3:1 rev range.

 

Also you may find it an advantage to have the engine deliberately under-propped, to give you some power in reserve in order that you can continue to travel for some distance with a collection of rubbish on the propellor, until you reach somehwre suitable to remove it. Also you want to set it up so that the revs do not drop too low when at idle speed in gear.

 

All these, and other factors, may lead you to fit a prop which is not actually optimised for your particular engine.

Hi There

I do realise that there is always a compromise to be made with all changes - including running a 19" prop with a 2ft swim

What are you meaning with the 3:1 and 4:1 range - is it the gearbox ratio?

I am always interested in making this 'skip' more efficient.

Alex

[Yoda]

Engines should not be run and loaded below an optimum rpm designated by their designers. Idle speed can be below that load speed. Your boat might have been over proped beforehand so things are now better. As Allen was trying to say in his submission, the prop has to be matched to the specific fuel consumption curve of the engine. This takes into account of the torque curve, and power.

[Keeping Up]

Hi There

I do realise that there is always a compromise to be made with all changes - including running a 19" prop with a 2ft swim

What are you meaning with the 3:1 and 4:1 range - is it the gearbox ratio?

I am always interested in making this 'skip' more efficient.

Alex

No, I am talking about the rpm range of the engine. For example, my engine has a tickover speed of 850 rpm and a rev limit of 2800 rpm, so it just about has a usable range of 3.3 to 1

 

Your speed through the water is proportional to the rpm; at engine tickover speed in gear I travel at just under 2mph and flat out I travel at about 6.5 mph (in open water)

 

Allan

[steelaway]

No, I am talking about the rpm range of the engine. For example, my engine has a tickover speed of 850 rpm and a rev limit of 2800 rpm, so it just about has a usable range of 3.3 to 1

 

Your speed through the water is proportional to the rpm; at engine tickover speed in gear I travel at just under 2mph and flat out I travel at about 6.5 mph (in open water)

 

Allan

 

Thanks for that.

It is certainly a different way of looking at performance.

Could you explain how are you calculating your figure of 3:1. (I do not have a figure for my tick over speed)

[Keeping Up]

Thanks for that.

It is certainly a different way of looking at performance.

Could you explain how are you calculating your figure of 3:1. (I do not have a figure for my tick over speed)

Nothing complicated, just the max revs (2800) divided by the tickover speed (850).

 

Stop reading now to absorb this carefully, before reading further as I leap into the realms of the unknown.

 

The max revs are defined by the engine maker, so that's a precise number - unless you are over-propped in which case the engine may not be able to reach that speed.

 

The minimum usable revs are usually at tickover speed, which again should be defined by the manufacturer, but some engines run rather roughly at or near tickover speed, especially if the prop gives them a significant load, so your minimum usable revs may be a couple of hundred revs above the tickover.

 

Put these together, and you begin to see the peril of over-propping. With my engine as an example, if I gave it a great big prop in an attempt to go really fast, I may find that I couldn't get more than 2000 revs maximum from the engine. Possibly I'd be able to do 8mph at these revs (although there is the risk that the engine would find it such a struggle to drive the big new prop that the revs would drop right down and I'd actually be going slower than before, rather like a car trying to go up a steep hill in top gear). If I could go at 8mph I'd be really happy until I throttled back; the engine would probably not be very smooth with the increased load at low revs, and I'd have to increase the tickover to 1000 rpm, but now that means that I'm still going at half the speed that I was before (1000 rpm compared with 2000 rpm) and my minimum speed is therefore one half of my maximum; even at tickover I'd be travelling at 4mph!

[steelaway]

Nothing complicated, just the max revs (2800) divided by the tickover speed (850).

 

Stop reading now to absorb this carefully, before reading further as I leap into the realms of the unknown.

 

The max revs are defined by the engine maker, so that's a precise number - unless you are over-propped in which case the engine may not be able to reach that speed.

 

The minimum usable revs are usually at tickover speed, which again should be defined by the manufacturer, but some engines run rather roughly at or near tickover speed, especially if the prop gives them a significant load, so your minimum usable revs may be a couple of hundred revs above the tickover.

 

Put these together, and you begin to see the peril of over-propping. With my engine as an example, if I gave it a great big prop in an attempt to go really fast, I may find that I couldn't get more than 2000 revs maximum from the engine. Possibly I'd be able to do 8mph at these revs (although there is the risk that the engine would find it such a struggle to drive the big new prop that the revs would drop right down and I'd actually be going slower than before, rather like a car trying to go up a steep hill in top gear). If I could go at 8mph I'd be really happy until I throttled back; the engine would probably not be very smooth with the increased load at low revs, and I'd have to increase the tickover to 1000 rpm, but now that means that I'm still going at half the speed that I was before (1000 rpm compared with 2000 rpm) and my minimum speed is therefore one half of my maximum; even at tickover I'd be travelling at 4mph!

 

Whoaaa!

I'm getting a head ache - but its good to able to talk to some one who has thought about it and has a knowledge.

Please be assured that I am not after maximum speed but maximum efficiency. I appreciate that a huge prop would cause serious problems - Ref my overheating troubles with my first boat being over propped

I am sure I have not over propped my engine but have achieved a situation where I feel the engine is working with a load it is happiest with. I do not suffer any bad side effects with this prop, but I will have to check my speed at tickover.

My initial observation is still that many people are running to small a prop, through bad advice or a lack of understaning.

My calculations are 3.6:1 on engine max revs (3500) - of load and 2.9:1 on engine max revs (2800) - on load. Tick over 950rpm

Thank's for your time I really enjoyed that thread.

Alex

[sbillis]

I have a 45'-0" boat with a Perkins Perama 29hp engine. I have done the tidal Thames, Severn, non tidal Thames (up and down in strong stream), Warwikshre Avon, Kennet up and down with levels well up, and never had a problem. The engine just purrs along.

 

Simon

nb Bulrush

[steelaway]

I have a 45'-0" boat with a Perkins Perama 29hp engine. I have done the tidal Thames, Severn, non tidal Thames (up and down in strong stream), Warwikshre Avon, Kennet up and down with levels well up, and never had a problem. The engine just purrs along.

 

Simon

nb Bulrush

 

Congratulations

You must have a pefect combination of engine to prop to boat - not every one has.

Have you done calculation for your boat?