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Engine size for a 55 - 60 ft NB


Bob692

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We are about to buy our first second hand narrow boat after many years of hiring. We are after a 55 - 60 ft for extended cruising (almost continuous) and would like to do rivers as well as canals, ie Thames, and Ribble link to the Lancaster etc. Is a 38HP with PRM 120 or equivalent man enough for the job or do we need a 42 HP with PRM 150.

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38HP should be enough, even on rivers but I would suggest that a hydraulic gearbox, 150 and up rather than the mechanical 120 would be a better long term bet.

Bigger is always better though.

Remember that the old BMC 1.5D was at its best only 30HP and there are thousands of those still in use.

Edited by Boater Sam
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Bigger might be better if it was built like that but if a new bigger engine has been installed as a replacement its possible that the cooling will be marginal and you'll only find out when you take it on the rivers. 

 

I think 38HP will be enough but as boater Sam says a 150 is the way to go. The 120s seem to be more fragile. 

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I agree with those who say make sure you have the hydraulically shifted PRM 150 box.  The mechanically shifted PRM 120 seems to be significantly inferior.

 

42HP doesn't sound very different to 38HP to me, but then it is not all about absolute HP, and maybe the differences in the numbers for torque are bigger than that small increase in HP would seem to imply.

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I would add that power is not the be all and end all of it. The skipper's ability to read the water, use flows to their advantage and most of all know when its time to tie up and wait out the flow is perhaps more important.

 

I have never felt my 54ft boat with a DV36 - probably around 28 to 15 usable bhp is in any way underpowered and that includes turning across the tine at West Stockwith on the Trent and stemming the tide to allow cruisers to clear the lock.

  • Greenie 1
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Note to self - prepare to be shot at....

 

My boat's a 60 footer on the Thames. I fitted a 50HP Beta with a PRM 260......

 A 42HP would probably have done 'well enough' but  Beta's pricing worked better with the larger base engine and I wanted the Sea-Power generator pack  for a plan that never came to fruition.

The PRM is a solid chunky bit of kit and it's taken a lot of hard work, whereas the 150 - I think - is a bit of a toy. In those days the 120 / 150 was only fitted to much smaller engines.

 

You don't need ooddles of power on the Thames per se but it doesn't half help if you're caught out on 'Reds' or want to make a bit of progress when the River's on Yellows.

 

Others will doubtless disagree....

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When looking at a boat its worth comparing the size of engine to the size of the skin tank. A rule of thumb is 1 sq ft for each 4HP of power so a 40HP engine will require 10sq ft of skin tank. Many are too small and this means that at max power you run the risk of over heating.  The one on our boat is really too small and a some point I will get a second one fitted.

On the subject of gear boxes we have a PRM 260 married to a BMC 1.8 - its a good combination.

 

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4 hours ago, Bob692 said:

We are about to buy our first second hand narrow boat after many years of hiring. We are after a 55 - 60 ft for extended cruising (almost continuous) and would like to do rivers as well as canals, ie Thames, and Ribble link to the Lancaster etc. Is a 38HP with PRM 120 or equivalent man enough for the job or do we need a 42 HP with PRM 150.

Yes is the short answer. I had a new build a few years ago with a beta 38 and a prm 120 on a 57 footer. I did the tidal rent on a massive spring tide and rounded up and pushed it to get in at West Stockwith. HOWEVER whilst the unit did the job I reckon I was being a tight arse when I specified the 120 as it was rather clunky and didnt inspire me as to its longevity. It did the job and I sold the boat aged just over a year but if I had done another I would have fitted a better gearbox.

3 hours ago, Tony Brooks said:

Mien did just fine going up stream to Kings Lock and Dukes cut on reds a few years ago and as I said nominally only 36bhp but probably developing far less.

Yours though Tony is a " proper " engine. I had a dv 36 in one of my boats, they are first class bits of kit.

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Our boat was 58ft with a Beta 38  PRM 120 all working fine after 6000hrs. Between them, the two owners had accomplished, 2 return Ribble links. At least 4x on the Tidal Trent to Keadby, 2 return trips Selby to Naburn and  one trip  Selby to Goole. Tidal Thames to and from Limehouse 5x and River Severn to Glouchester 2 returns.  No problems.

Still deep water showed 6mph at 2300rpm heading upstream on the River Weaver. Above this was noisy and uncomfortable as the prop intermittantly vented. 

Ribble Link comfortable at 2100rpm for the two hours.

A Beta 42 with the prm 150 would give more to spare, perhaps enough get down to Hull on the Trent, but do not turn a good boat down because it has "only s Beta 38/35 with the PRM 120 box"

 

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8 hours ago, OldGoat said:

Note to self - prepare to be shot at....

 

My boat's a 60 footer on the Thames. I fitted a 50HP Beta with a PRM 260......

 A 42HP would probably have done 'well enough' but  Beta's pricing worked better with the larger base engine and I wanted the Sea-Power generator pack  for a plan that never came to fruition.

The PRM is a solid chunky bit of kit and it's taken a lot of hard work, whereas the 150 - I think - is a bit of a toy. In those days the 120 / 150 was only fitted to much smaller engines.

 

You don't need ooddles of power on the Thames per se but it doesn't half help if you're caught out on 'Reds' or want to make a bit of progress when the River's on Yellows.

 

Others will doubtless disagree....

 

Yes I'll disagree. 

 

I've got a 55hp Isuzu with a PRM 150 pushing a 30 tonne 55 x 12ft widebeam. In theory it should be underpowered but I've had the prop repitched and it's well matched and I have loads of power. Been up and down the tidal Thames, turned into Limehouse a few times and once took it from Laleham to Reading on reds and yellows without a problem.

 

A friend of mine has the same boat but 60 x 12 with an Isuzu 70 and PRM 260. We had a burn up on the Thames once and it was neck and neck all the way. I guess they're limited by the hull speed so you can put a bigger engine and gearbox into a displacement hull, but at a certain point you're not going to get any more out of the boat and you're just wasting your money.

Edited by blackrose
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10 hours ago, mrsmelly said:

Yes is the short answer. I had a new build a few years ago with a beta 38 and a prm 120 on a 57 footer. I did the tidal rent on a massive spring tide and rounded up and pushed it to get in at West Stockwith. HOWEVER whilst the unit did the job I reckon I was being a tight arse when I specified the 120 as it was rather clunky and didnt inspire me as to its longevity. It did the job and I sold the boat aged just over a year but if I had done another I would have fitted a better gearbox.

Yours though Tony is a " proper " engine. I had a dv 36 in one of my boats, they are first class bits of kit.

Yes but the power and torque curves are similar to the BMC 1.5. Peak torque comes at a slightly lower speed than the BMC and the Bukh curves give a lower peak RPM. Being direct injected the Bukh has a slightly better fuel consumption at canal speed (possibly  at lot better at high speed).

 

As Blackrose shows getting the correct prop for your boat is half the battle. the other half is honing the helmsman's skills.

  • Greenie 1
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Its the prop. that's spinning away under the water that does the shoving. Beta recommend an 18 x 12 prop for the 2 litre 43 hp motor. All of those numbers are pretty much rule of thumb but useful guides. An 18" prop might be a practical limit for some boats. There's not much difference between 38hp and 43hp but as others have said a decent strong gearbox is a good option. The only times I have been short of 'power' on waterways I have in fact been short of 'speed' and short of fitting a great big engine and getting the boat up on the plane its going as fast as it will go anyway.

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Thanks all for your considered replies. I had't understood the difference between the PRM 120 and PRM 150, thought it was just a size/rating thing. Almost all the boats we are interested in on brokerage are in the combination of 35-38 HP with a PRM 120 and 40-45 HP  with a PRM 150.  I don't think I have seen 35-38 HP with a PRM 150. I have seen a 55 ft with a 45 HP engine and a 60 ft with a 35 HP so there is a complete mix. As for a vast majority of the time the boat will be on 'ordinary' canals I am enough of an engineer to realise that a smaller engine working slightly harder will be more efficient and will be better for it than a larger engine just on tick-over.  

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1 minute ago, Bob692 said:

Thanks all for your considered replies. I had't understood the difference between the PRM 120 and PRM 150, thought it was just a size/rating thing. Almost all the boats we are interested in on brokerage are in the combination of 35-38 HP with a PRM 120 and 40-45 HP  with a PRM 150.  I don't think I have seen 35-38 HP with a PRM 150. I have seen a 55 ft with a 45 HP engine and a 60 ft with a 35 HP so there is a complete mix. As for a vast majority of the time the boat will be on 'ordinary' canals I am enough of an engineer to realise that a smaller engine working slightly harder will be more efficient and will be better for it than a larger engine just on tick-over.  

 

Even the red bit is not really true. The fact is that it only takes a very few HP to propel the boat at canal speed so for any given boat the power a large engine will produce and a smaller one will be the same. The speed the engine runs at for any given boat speed will, within reason, be dictated by the size of the prop and the gearbox reduction but at whatever speed the two engines are running the power they are delivering will be the same. Typically narrowboats are propped to absorb maximum torque rather than maximum HP for reason of maximising fuel economy so the engines can never reach their maximum HP. A small engine that has to run at above its best specific fuel consumption speed will not be more efficient than a larger engine running at its best specific fuel consumption speed.

 

As you say that you are looking at brokers it follows that you are buying second hand so you choose from what is available rather than a theoretical perfect boat. My advice is find a boat that suits your life style and then worry about the engine. I do not like being shafted on the price of spare parts so would avoid engines painted yellow but would be happy with the same base engine marinised by a different company. Other than that for technical and support reasons I would avoid Lombardini but luckily there are few of those on canal boats. I would also avoid engines with a timing belt rather than chains or gears but all the popular engines use chains or gears. As for gearbox I would avoid Hurth if possible proffering a mechanical PRM but any of the hydraulic PRMs are likely to serve the average narrowboater very well indeed.

 

There is a lot more to things than just raw BHP and much comes down to an informed view of reliability and longevity.

Just now, Stilllearning said:

A Bukh would get my vote every time with a big hydraulic PRM box, which prop you use is as has been said, a bit of a black art.

I agree but much to my surprise the Spanish ZF mechanical box that Bukh fitted to the DV36 has held up remarkably well so would not dismiss one of those boxes like I would a Hurth. (10 years in a hire fleet and then nearly 20 years with me so pretty robust). And the DV36 does not have a drive plate to worry about.

  • Greenie 2
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15 hours ago, OldGoat said:

Note to self - prepare to be shot at....

 

My boat's a 60 footer on the Thames. I fitted a 50HP Beta with a PRM 260......

 

Others will doubtless disagree....

My 60ft NB also has a Beta 2203 50hp still going strong 25 years on, it does give me an edge when needed ;)

 

ETA 50 hp can't be run on skin tanks as they are too big to fit most NB, mine is indirect cooled using canal/river water and I don't have to clean the filters every 5 minutes.

 

Edited by Loddon
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5 minutes ago, Tony Brooks said:

Even the red bit is not really true. The fact is that it only takes a very few HP to propel the boat at canal speed so for any given boat the power a large engine will produce and a smaller one will be the same. The speed the engine runs at for any given boat speed will, within reason, be dictated by the size of the prop and the gearbox reduction but at whatever speed the two engines are running the power they are delivering will be the same.

Very, very rarely are boats using their full potential HP.

 

The top-speed on my boat is about 8 knots.

I have 2x 140hp 6-cylinder engines

 

At 5 knots I am at 1500rpm and 'using' 27.9hp and consuming 5.4 litres of fuel per hour (per engine)

 

At 8 knots I am at 2000rpm and 'using' 65hp and consuming 12.7 litres per hour (per engine)

 

If I go to WOT (Wide Open Throttle) at 2600rpm there is no increase in speed (due the hull design) the power developed is 140hp and the fuel consumption goes to 27 litres per hour, (per engine).

 

5 knots is the 'quietest' and most economical cruising speed.

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1 hour ago, Loddon said:

My 60ft NB also has a Beta 2203 50hp still going strong 25 years on, it does give me an edge when needed ;)

 

ETA 50 hp can't be run on skin tanks as they are too big to fit most NB, mine is indirect cooled using canal/river water and I don't have to clean the filters every 5 minutes.

 

Yea!

'Fresh water' cooling is so much nicer and quieter than a tin resonator that cooks the engine bay.

The former halos in cleaning the lock walls as well....

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3 minutes ago, OldGoat said:

Yea!

'Fresh water' cooling is so much nicer and quieter than a tin resonator that cooks the engine bay.

The former halos in cleaning the lock walls as well....

 

Except when raw water cooling sucks weed and other crap in and gets blocked. Seems to happen to my neighbour every time he goes out and he's always having to clear the intake to stop the engine cooking. Also raw water cooling has to be winterised. In comparison keel cooling is virtually maintenance free. Change the antifreeze every 5 years. I'm not sure what the "tin resonator" comment is all about? If it's properly sized for the engine then keel cooling wins hands down every time on a canal boat.

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If it blocks its badly designed! On canals I check filter daily, on the Nene, as you are,  its weekly. As for winterising its about a 5minute job and only done if below freezing is predicted for a few days and if living on board I don't bother.

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Experience, hard earned I might add, suggests that whatever engine you decide on you need to fit the biggest gearbox that will within reason fit. I've just fitted a new PRM 280D to a Ford BSD3 which is 42H.P. @ 2200R.P.M. The previous PRM 160 lasted for thirty years much of this time spent towing. I did replace the clutches twice in this time which leads me to think that were I starting out now I would go for the PRM500. Gearboxes are not in general rated for canal use with constant gear and speed changes so bigger generally means better. Regards, HughC.

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1 minute ago, hughc said:

Experience, hard earned I might add, suggests that whatever engine you decide on you need to fit the biggest gearbox that will within reason fit. I've just fitted a new PRM 280D to a Ford BSD3 which is 42H.P. @ 2200R.P.M. The previous PRM 160 lasted for thirty years much of this time spent towing. I did replace the clutches twice in this time which leads me to think that were I starting out now I would go for the PRM500. Gearboxes are not in general rated for canal use with constant gear and speed changes so bigger generally means better. Regards, HughC.

Which is why you should use the commercial ratings for the gearbox and not the leisure ones.

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11 hours ago, blackrose said:

 

 

 

A friend of mine has the same boat but 60 x 12 with an Isuzu 70 and PRM 260. We had a burn up on the Thames once and it was neck and neck all the way. I guess they're limited by the hull speed so you can put a bigger engine and gearbox into a displacement hull, but at a certain point you're not going to get any more out of the boat and you're just wasting your money.

so you're the bloke what spilled my cuppa when I passed the other way in my yoghurt pot.   shame on you !!

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21 hours ago, Bob692 said:

We are about to buy our first second hand narrow boat after many years of hiring. We are after a 55 - 60 ft for extended cruising (almost continuous) and would like to do rivers as well as canals, ie Thames, and Ribble link to the Lancaster etc. Is a 38HP with PRM 120 or equivalent man enough for the job or do we need a 42 HP with PRM 150.

Yes it is man enough. I have a 55ft boat with a Barrus Shire with just 30 hp. It has done the job for many years, including rivers. I don't think it's the ideal match though it's much better since fitting a larger prop. My preference would be a Beta Marine 43.

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