Presiding wisdom on HP! What range of HP is best for 62ft narrowboat?

[DuncanB]

Mooring sorted. Land based garage for storage, sorted. Now about to make an offer on a 62ft narrow for permanent live-aboard. What is the optimal Horse Ower (HP) for a boat of this length? Some ads say 35Hp and others say 80HP....What is sensible for river CC cruising for a boat of this length?

Thanks in advance for the helpful replies!

[MtB]

1 minute ago, DuncanB said:

Mooring sorted. Land based garage for storage, sorted. Now about to make an offer on a 62ft narrow for permanent live-aboard. What is the optimal Horse Ower (HP) for a boat of this length? Some ads say 35Hp and others say 80HP....What is sensible for river CC cruising for a boat of this length?

Thanks in advance for the helpful replies!

 

Back in the day 1hp was enough.

 

 

[David Mack]

And a working narrow boat pair would carry 40 tons plus with a single engine of 15-20 hp.

[ditchcrawler]

It depends a bit on the engine, slow reving with low down torque , high reving modern and so on. Do you plan on punching the flow on rivers on red boards? Lots of engines are too big and never work hard enough

[MtB]

More helpfully, I found a 30HP engine in a 40ft narrowboat with a 16" blade seemed not enough. Yet a 30hp in my 68ft boat with a 21" blade was blisteringly powerful boat. So in my experience the blade size and depth of swim is just as, if not more important than the engine power. 

[DuncanB]

Thanks you! Food for thought. My question is based on current Thames situation where flow is fast. Trying to establish which HP powers a boat forward/upstream. Don't want to offer on a boat where we go backwards when flow is sub red boards.

 

2 minutes ago, ditchcrawler said:

It depends a bit on the engine, slow reving with low down torque , high reving modern and so on. Do you plan on punching the flow on rivers on red boards? Lots of engines are too big and never work hard enough

Thats really useful. I am not intending on punching above red boards, but also don't want to be taking so long to move a mile or two.

[MtB]

1 minute ago, DuncanB said:

Thanks you! Food for thought. My question is based on current Thames situation where flow is fast. Trying to establish which HP powers a boat forward/upstream. Don't want to offer on a boat where we go backwards when flow is sub red boards.

 

 

That won't happen. 

 

Post details of the actual engine, reduction gear and blade size on this 62ft boat for informed opinions on how it will perform.

 

 

 

[DuncanB]

3 minutes ago, MtB said:

More helpfully, I found a 30HP engine in a 40ft narrowboat with a 16" blade seemed not enough. Yet a 30hp in my 68ft boat with a 21" blade was blisteringly powerful boat. So in my experience the blade size and depth of swim is just as, if not more important than the engine power. 

Thank you - selecting blade seems as important as finding correct HP!

 

Just now, MtB said:

 

That won't happen. 

 

Post details of the actual engine, reduction gear and blade size on this 62ft boat for informed opinions on how it will perform.

 

I don't have this yet. Seller has promised to provide to tomorrow.

 

 

 

[MtB]

1 minute ago, DuncanB said:

Thank you - selecting blade seems as important as finding correct HP!

 

 

Yep, and reduction gear.

 

 

[GUMPY]

Loddon is best in deep river water

60ft Malcolm Pearson hull

50hp  Beta

19x14 prop ( I think) 

2:1 box

Comfortable cruising at 1500rpm 5mph

Max about 7mph at 2100rpm

You can never have to much power🤔

[nicknorman]

I think it’s a cube root equation that relates engine power to cruising speed. In other words, you need a hell of a lot more power to go not very much more quickly. So the difference between a 30 bhp and a 50 bhp engine is probably not that much in terms of top speed on a narrowboat hull. Certainly no difference at all on the average canal, and maybe 1mph at most on a river.

 

As other have said, probably hull shape and prop size have as much bearing. I doubt a longer narrowboat needs a bigger engine to achieve the same speed, a bigger engine on a longer boat just makes the moving off and stopping a bit easier due to the increased weight of the longer boat. In terms of the drag from the hull, I don’t think length makes much difference.

 

One fact which can be relevant on a modern boat is the electrical demand. If you have good batteries and a large alternator and maybe a travelpower, a smallish engine can struggle to come off idle with the load (or more probably, the mariniser will fit a smaller alternator). This is why we went for a Beta 43 rather than the 38 - even though there isn’t a big difference in max power, there is a fair bit of difference in max torque at low rpm.

[Stroudwater1]

Is it possible to talk with the previous owners of the boat to find out what use they made of the boat? That may help. 
 

Given that you seem to be specific in your needs perhaps the boat seller would allow a slightly longer test drive. 
 

Draft I believe is a factor too, if it’s a deep boat more water has to pass it when traveling. This is more relevant in a shallow canal. I see a few people especially on  hire boats wearing captains hats thrash the boat which isn’t going much faster then a boat with half the power bring used.  

[David Mack]

4 minutes ago, Stroudwater1 said:

 

Draft I believe is a factor too, if it’s a deep boat more water has to pass it when traveling. This is more relevant in a shallow canal. I see a few people especially on  hire boats wearing captains hats thrash the boat which isn’t going much faster then a boat with half the power bring used.

Most of my boating over the last 30 years has been on ex-working boats drawing around 3 ft. I soon learned that when you are struggling on shallow canals it is best to reduce engine revs. You end up with less engine noise, less wash, lower fuel consumption and the boat goes a bit faster, as it is not being sucked down onto the bottom.

[Peanut]

Beta Marine offers a rough guide to boat size for their “Greenline” range of engines, as “Installation Recommendations,” on their website.

[Captain Pegg]

Not sure why this is an issue with what appears to be an existing boat. If it ain’t broke don’t fix it might be a good starting point.

 

However I’d expect a modern 62’ narrowboat to be fitted with something like a 4 cylinder Beta 43 or Isuzu 42.

Older engines rev slower and turn larger props but if you go the same speed over the same distance then you’ve developed the same power. And at canal speeds it’ll average out at something like 5 or 6 hp.

 

Modern fast revving engines develop their peak power way above the range you’ll use on a canal boat other than in short bursts in potential difficult conditions.

 

 

 

 

[Captain Pegg]

A narrowboat configured for river cruising might have a slightly larger engine (say a Beta 50), a bigger propellor and a higher reduction gearbox (3:1 vice the usual 2:1).

 

The problem is the shell has to be able to accommodate the larger propellor or otherwise have a curved or dropped skeg.

 

I’ve moved boats configured like this on canals and they churn up a lot of crap off the bottom and if they have a dropped skeg are prone to hitting things in bridgeholes and when out of the deep channel on canals.

 

They may also use a lot of fuel on a canal because they will be revving toward 2000rpm.

 

One such boat I moved twice, either side of it having the dropped skeg cut off and a conventional set up installed. The problem then was that by retaining the 3:1 reduction box it was then under propped and whereas before it was hard to go slow enough, afterwards it was hard to go quick enough.

[Tony Brooks]

9 hours ago, nicknorman said:

I think it’s a cube root equation that relates engine power to cruising speed. In other words, you need a hell of a lot more power to go not very much more quickly. So the difference between a 30 bhp and a 50 bhp engine is probably not that much in terms of top speed on a narrowboat hull. Certainly no difference at all on the average canal, and maybe 1mph at most on a river.

 

 

In addition to this a narrowboat is a displacement hull so will never climb up on its bow wave to plane so that limits the maximum speed, however much power you apply. All you do when you try to exceed that hull speed on DEEP WIDE waters is make an ever larger wash. On canals, you just push more and more water in front of you and out from under the boat so the effective depth goes down and you drag along the bottom as has been described above.

 

In theory, the naval architect who designed the hull will have calculated the hull constant that can be used to calculate the maximum hull speed - yeah, all narrowboat hulls have been properly designed and the constant calculated - NOT. So taking a typical constant of 1.35 and applying it to a 62ft hull gives a theoretical maximum speed of just under 6 knots or a tad under 7 mph. Be aware the maximum hull speed may well be lower than this.

 

So if the flow is greater than 7 MPH you will go backwards unless you can find slower water, probably nearer the bank.

 

The engine power has much less effect than many think, and trying to keep a 62ft narrowboat safe whilst going upstream in a flood needs a fair bit of experience and skill. It is all too easy to get the bow pushed across a bend into the bank. Just forget about moving upstream on red boards

 

Edited to add, I found 30 to 35 BHP on a 54 ft boat perfectly adequate on the Thames, downstream on red, upstream on yellow and tied up when the current was too high.

Edited May 15, 2023 by Tony Brooks

[Mike Adams]

Your speed on the Thames is most likely to be limited by the hull shape and waterline length. The shape of the stern/swim will limit the power input that can be applied to increase the speed without causing excessive wash and power consumption. 40Hp would be fine with a 2:1 reduction gearbox. Unless you have a very short run ( less than 6ft) in to the propeller you should be able to cope with most river conditions. You don't need 40Hp to achieve this performance but because of modern engine design with higher speed/less cylinder capacity anything smaller is going to sound irritating screaming away all day.

[Tonka]

But if the Thames is on red boards you just stay tied up and stay safe. It doesn't matter how much HP your engine has when a plastic bag grabs your prop.

[Bee]

The thing that makes the boat move is the prop. and the thing that makes it work well is the water flow. Look at some of the commercial barges - bows like barn doors but a back end of grace and and beauty under the water.  Most boat builders are OK in this respect but some really were / are not. If you have a modern 4 cyl engine you will need to match its power with a prop that will absorb that power without spinning the prop like a mad whizzy thing or labouring along with clouds of black smoke and a too big prop.  Beta for instance would recommend their 43 HP motor for your boat and I think an 18X12 (?) prop. That s a roughly 2 litre motor and there are many boats with this combination. For a 2.5 lire motor and approx 50 HP you would fit a slightly bigger prop perhaps with a bit more twist in the blades. It all sounds terribly vague and very few boats have the 'correct' prop as there are so many compromises but most seem to get along fine. You do not need the expense or fuel consumption of a great big engine and all boats have a max hull speed above which they simply cannot go any faster. And then there is the pitch of the blades and vintage engines....

[blackrose]

Also... make sure that the boat in question has enough skin tank area (assuming it's keel cooled) to ensure that whatever hp engine it has does not overheat - otherwise you cannot use that powerful engine. The basic rule of thumb is 1ftsq of skin tank / 4hp of engine power. Like many narrowboat owners, the sellers of this boat may never have taken it onto a river and pushed it hard against the current, so they may not know it's under-cooled. Unfortunately lots of builders under-spec keel cooling and nobody finds out until the boat is taken upstream on a river. I know this from my own experience.

Edited May 15, 2023 by blackrose

[IanD]

As has been said above, from the POV of maximum speed what actually matters -- assuming the boat is correctly propped -- is power and hull shape/displacement, it doesn't really make much difference whether the engine is modern and higher-revving or older and low-revving. A Beta 43 or similar (modern engine) is normally fitted to boats this size and is perfectly adequate.

 

Having said that, there are few modern engine installations where you'd want to run at maximum rpm/power for any length of time (e.g. 2800rpm for a Beta 43) because of noise and vibration and also possibly an undersized skin tank leading to overheating -- old-style engines don't make such a racket (and are further away in an engine room not right under your feet) and are usually happy to run all day at full power because they were designed for this.

 

Which means that in real life you can have a lower-power trad engine (e.g. 27hp) than a modern one. Also note that because power rises as the cube of speed for both propeller/engine rpm and boat speed, more power doesn't make as much difference as you might think -- for a 62' boat with a Beta 43 and a 2:1 gearbox, here are some numbers from the Vicprop calculator (deep water) at https://vicprop.com/displacement_size_new.php

 

2800rpm 43hp 8.2kts -- flat out while going deaf

2400rpm 27hp 7.0kts -- typical 2-cyl trad engine power/speed? (1400rpm with 1:1 gearbox?)

2000rpm 16hp 5.8kts -- 1-cyl Bolinder 🙂

1600rpm 8hp 4.7kts

1400rpm 5.4hp 4.1kts -- half maximum rpm and speed, 1/8 maximum power

 

But the only time you'll ever need to run close to flat out anyway is going upstream on a river with a strong current...

 

P.S. 62' hull speed is about 10.5kts but you need about 100hp to get there... 😉

Edited May 15, 2023 by IanD

[Tonka]

14 hours ago, Stroudwater1 said:

Is it possible to talk with the previous owners of the boat to find out what use they made of the boat? That may help. 
 

Given that you seem to be specific in your needs perhaps the boat seller would allow a slightly longer test drive. 
 

Draft I believe is a factor too, if it’s a deep boat more water has to pass it when traveling. This is more relevant in a shallow canal. I see a few people especially on  hire boats wearing captains hats thrash the boat which isn’t going much faster then a boat with half the power bring used.  

A desperate seller will tell it is OK just to get a sale. A longer test drive will tell you nothing unless the boat is on a river

[blackrose]

20 hours ago, MtB said:

 

Back in the day 1hp was enough.

 

I never really understand why this old cliché is trotted out so often (pardon the pun). I don't know much about the history of the waterways but were single horses ever used to pull canal boats upstream for any distance? If they were I imagine it can only have happened on very gentle rivers so not really what the OP is asking about.

 

On a canal you can push (or pull) a 25 tonne boat along with the equivalent of 1hp of propulsion, but even the most experienced boater would struggle with that on most rivers because unlike canals rivers are dynamic and what might be a gentle current one day can become a strong stream the next. People say it's foolish to move in such conditions and while I agree with that, I have been caught in flood situations where I thought the mooring was safe but water was let down from upstream unexpectedly and several boats made the decision to move to a safer location further upstream. In those situations there's no substitute for power and it's an underpowered boat that would look foolish.

[ditchcrawler]

1 hour ago, blackrose said:

 

I never really understand why this old cliché is trotted out so often (pardon the pun). I don't know much about the history of the waterways but were single horses ever used to pull canal boats upstream for any distance? If they were I imagine it can only have happened on very gentle rivers so not really what the OP is asking about.

 

On a canal you can push (or pull) a 25 tonne boat along with the equivalent of 1hp of propulsion, but even the most experienced boater would struggle with that on most rivers because unlike canals rivers are dynamic and what might be a gentle current one day can become a strong stream the next. People say it's foolish to move in such conditions and while I agree with that, I have been caught in flood situations where I thought the mooring was safe but water was let down from upstream unexpectedly and several boats made the decision to move to a safer location further upstream. In those situations there's no substitute for power and it's an underpowered boat that would look foolish.

This looks like 2 horsepower