What Size

[touchstone]

Is there a rule of thumb as to what size engine is ok for a specific boat size?

For example would a Volvo Penta 2003 be big enough for a 40ft steel canal boat?

[MtB]

Canal boats back in the day that sort of size would have been fitted with about a 10hp or 15hp engine. Perfectly adequate with a decent size and correctly sized prop.

 

Current trend is to fit stupidly large (in my opinion) engines in any given size of boat. 35hp is not unusual to see in a 40 ft boat.

 

MtB

P.S. What power is this Volvo 2003? Hopefully not 2003hp! :D

[nb Innisfree]

Our first engine was 2.5lt and 33bhp@ 2000rpm, we found it struggling a bit when cruising slowly (600rpm) with our large 24v 100amp alternator at full output and batts were low. Our 2nd engine is 1.85lt and 40 bhp @ 3000rpm, copes better and has a better reserve of power even though It's slightly overpropped compared to the first one which was spot on. Mind you the first one sounded better at 600rpm than the second which req. 1200rpm for the same speed.

 

60' narrowboat btw.

[touchstone]

Canal boats back in the day that sort of size would have been fitted with about a 10hp or 15hp engine. Perfectly adequate with a decent size and correctly sized prop.

 

Current trend is to fit stupidly large (in my opinion) engines in any given size of boat. 35hp is not unusual to see in a 40 ft boat.

 

MtB

P.S. What power is this Volvo 2003? Hopefully not 2003hp!

Hi Mike. The Volvo Penta 2003 is a 28hp.

[RLWP]

Current trend is to fit stupidly large (in my opinion) engines in any given size of boat. 35hp is not unusual to see in a 40 ft boat.

 

MtB

 

If you spend time on some of the 1970's boats, you'll find they too had large engines. 30 to 40 foot boats with BMC 2.2 or 2.5 engines, or Lister SR3s are quite common

 

Dunno why

 

Richard

[trackman]

Torque is more important than power. Our current boat has a 30hp engine, as did our previous shared boat.

The current engine has vastly greater torque output and is far more capable. It swings a much bigger prop and stops a heavier boat far more easily. On rivers we are far more comfortable, with much more "power" in reserve.

This is one reason why people like trad engines, which tend to have proportionately higher torque relative to HP, which also tends to be available at lower revs than with modern road vehicle based engines.

[alan_fincher]

 

If you spend time on some of the 1970's boats, you'll find they too had large engines. 30 to 40 foot boats with BMC 2.2 or 2.5 engines, or Lister SR3s are quite common

 

Dunno why

 

Richard

 

I very rarely come across the 2.2 or 2.5 BMC in narrow boats, and when I do it usually seems to be as some kind of DIY job, not as a standard fit to a "production" boat. I realise you have worked on some, but I would say they are very rare compared to the 1.5 or 1.8.

 

In my view they are usually far too overpowered when they do end up in a leisure boat, and in a 30 foot one would be mental.

 

However a Lister SR3 is only about 20HP, (I think strictly 19.5 ??), so is not really that much of a beast of an engine. However it would be overkill in a 40 foot boat, and I have rarely seen that either. (They will power a 70 foot working boat, although not with huge reserves of power, usually).

 

Initially hire fleets tended to have only the twin SR2s, which were sometimes a bit short off grunt as boats got longer and heavier. I would say it became generally more normal that boats under 50 foot regularly had the 13HP SR2, whereas for boats 55 foot and above, they increasingly branched out to SR3s.

 

When the visually similar, but faster revving ST series came in, they were considerably more power per cylinder, and IIRC, a 2 pot ST is about equivalent to a 3 pot SR.

[RLWP]

When you go back that far, it's hard to know what is a 'production' boat and what is a DIY job. Production boats may consist of batches of one, built irregularly

 

Richard

[alan_fincher]

When you go back that far, it's hard to know what is a 'production' boat and what is a DIY job. Production boats may consist of batches of one, built irregularly

 

Richard

No but when they are shoe-horned into a rivetted BCN day boat, or an old wooden butty, it is fairly obviously a "one off".

 

I can't think I have ever heard of one of the big BMCs fitted new into something like a Harborough, Fernie, Dartline, Teddesley, Rugby, H&L built hull, although I might not go as far as adding Springer to that list, as some weird and wonderful things happened sometimes with Springers.

 

What boats have you encountered them in, and do you think they were the original engine, or have gone in later? I still maintain "pretty rare"!

[bizzard]

Indeed.most inland leisure boats of years ago with overlarge engines Like 2.2 BMC's and bigger ect were in the main cobbled up and semi marinized by DIY'ers, in most cases bought cheap from breakers yards when money was much less abundant than it is these days and folk were much more practical and would ''have a go'' in happy anticipation of getting on the water. The SR3 but still only about 19hp is a slight exception usually fitted into boats of over about 50' in length.

[RLWP]

Each of the boats had been built from scratch, and still had their original engine. I'm afraid I don't have your eye or experience to be able to spot the makers of 1970's hulls. The only distinctive one would have been the vee-bottomed concrete hull with an SR3

 

Richard

[churchward]

Surely worrying about the maximum BHP output for a NB is a red herring.

 

You need an engine that will provide enough power at the cruising speeds to spin prop and alternator around 1100-1500 RPM not the top end RPM and power. On canal this is likely to be modest amount of power (10-15 BHP at when cruising) on a flowing river rather more perhaps up to 30-50% more depending on the flow/tide you are running against. To get this with a modern high RPM engine you are likely to need an output of 30-40 BHP (torque is more important though) to get the power needed give or take a bit.

 

I think a lot of the reason for increasing engine power is to do with the electricity needed to generate and provide power for the modern electrical needs on board. Do not under estimate how much power a 110amp or more alternator takes from the engine. Another factor is that people use what is available as most of these engines are not made for boats but agricultural, tractors and diggers etc.

[Neil2]

Surely on the canals it's more important to have an engine capable of stopping the boat quickly. Anyone who's ever sailed an o/b or outdrive powered boat knows that it's not difficult to maintain speed but with their small props these boats don't brake that well. Canal boats need a big prop to grab the water which generally means a bigger (higher torque) engine than would strictly be necessary to maintain cruising speed.

 

However, I think there's something in the argument that electricity demands of modern boat buyers and hire fleet owners has led to a corresponding demand for bigger engines.

[blackrose]

 

I think a lot of the reason for increasing engine power is to do with the electricity needed to generate and provide power for the modern electrical needs on board. Do not under estimate how much power a 110amp or more alternator takes from the engine. Another factor is that people use what is available as most of these engines are not made for boats but agricultural, tractors and diggers etc.

 

Perhaps another factor is that people like to be able take canal boats onto rivers and so with high-revving, lower torque modern engines they need a bit more power.

Edited July 20, 2013 by blackrose

[Dalslandia]

The topic about power versus torque always come up, does*t matter if talking cars, airplanes or boats, boats and airplane have several things in common, propellers, rudder, and drag, it works the same way. (Maybe not the rudder)

 

First torque without movement is not power, and power is simply said work don.

To do the work, it can be made in small bits, large bits, fast or slow.

 

The bigger bits and the faster, the more power, more work don in a specified time.

 

An efficient propeller will need less power to do the same job. I have designed airplane propellers that have been 25-26% more efficient then the propeller it have replaced with same power from engine, it is not sales talk or bragging, it is plane! data from user. No one is more surprised then me.

 

I am just telling that because it show that the propeller is very important, it is like going from 40 to 50 HP, it does not change speed much but take off and climb, and not least fuel consumption at cruise. 25% more power increase speed 7.7%

 

Let’s say we have an old boat with a big propeller, a single cylinder, long stroke, direct drive engine, say it cruise at 500 rpm and at 10 hp just to have a number.

 

We don't like the oil it spit out from the exhaust and leak under the engine, so we want a modern engine, we find a 25 hp engine from Taiwan, 25 hp at 4000 rpm,

 

we want to keep the old propeller, that fits the boat so well, say the engine make 15 hp at 2000 rpm, we want the propeller to do 500 rpm when engine make 2000, so it have to be geared down 4 times.

 

Just as before the propeller need 10 hp at 500 rpm, the gearbox take some 3% or more, so engine will produce 10,3 hp at 2000,

 

The torque and rpm at the propeller is the same, so is the power at the propeller.

The torque at the engine is <4 times less, but power is the same (plus the power the gear box drag down the engine with)

That little rice boiler can produce 15 hp at 2000, on the paper, so we can throttle back some, if it is a modern engine tested with all needed items like water pumps and alternators (low load) we still have some power to increase speed with, but when charging the batteries there is not enough power. But there is a 35 hp engine also. But the 45 HP engine is just £500 extra and come with a bigger alternator.

 

So we buy the bigger engine, it is also 33% more powerfull then what the guy moored next to us have.

 

It is producing 45 hp at 3000 rpm, with a 3:1 gearbox it will do 1500 with 500 on the prop

A 2:1 and we get 1000 rpm on engine and 500 on prop.

 

We still want to keep that old prop, just for nostalgic reasons, not for saving some money because the big engine was expensive.

 

With hot air in the engine room, good silencer, and some load on the bigger alternator, we might have 80% of the original 45 it say on the box, so we have maybe 35 HP to deliver to the propeller shaft.

 

Say it still need 10 hp at a 5 mph, why wouldn’t it. With 35 hp we can go 35/10= cube root out of 3,5 = 1,5 times faster ~7,5 mph

 

To do that we need about 1,5 times more RPM at the prop 500*1,5=750 rpm with the same diameter and pitch, But the engine need to turn 3000 rpm to produce the 35 net HP, so we need a gearbox ratio of 3000/750 = 4 if we want all power from engine, so it will still rev 2000 at 5 mph. and produce 10.3 HP because thats what propeller and gearbox need at 500 rpm

Or we give up those potential HIGH speed runs, with a different gearing. Or cave in and buy a smaller propeller, but it will need a new prop shaft too … what will wife and mother say?

 

So my guess is they say: it is good as it is no need to change anything.

 

Jan

Edited July 20, 2013 by Dalslandia