Replacing Engine

[Noggin]

I've currently got an Isuzu 35 in my 57ft boat which is fine for canals, but I'd like a little more "oomph" on rivers - is it a straight forward matter to replace with a reconditioned Isuzu 42 - this was an option when the boat was built - also the gearbox is a PRM120 - I assume this will have to be upgraded also?

[peterboat]

Depends whether its from the same family, I had in my boat a 50 hp barrus shire which shares the same block as other hp engines in the range, however the smaller hp engines were different. I do have a low hours 10 year old Barrus Shire for sale ? complete with gearbox ready to go.

However I am a little confused because my earlier boat had a 35 hp kubota in it and it was up the job on the Trent after it had been fitted with the correct propeller.

[Sea Dog]

54 minutes ago, Noggin said:

I've currently got an Isuzu 35 in my 57ft boat which is fine for canals, but I'd like a little more "oomph" on rivers - is it a straight forward matter to replace with a reconditioned Isuzu 42 - this was an option when the boat was built - also the gearbox is a PRM120 - I assume this will have to be upgraded also?

Whilst it possibly wouldn't be everyone's first choice of engine for river work, have you done some rivers and been  concerned about it or has conventional wisdom caused your doubt?

[Machpoint005]

21 minutes ago, peterboat said:

after it had been fitted with the correct propeller.

 

I think that may be the key to getting more "oomph" (by which I presume the OP means more speed through the water for given engine/prop revolutions).

[Sea Dog]

Just now, Machpoint005 said:

 

I think that may be the key to getting more "oomph" (by which I presume the OP means more speed through the water for given engine/prop revolutions).

Or, if he's only tried stretching his boat on a canal where he'll suffer shallow water effect, it might perform perfectly adequately in a deeper river. Hence my earlier question.

[Tony Brooks]

Maybe even trying to drive the boat faster than the "hull design speed", but I doubt it. No Idea how I can be more than happy with my similar HP Bukh in a 54ft boat on the tidal Trent, the Tideway and Thames in close to flood conditions. Mind you I have had Crowthers make me a suitable prop. I think the demand for more and more powerful engines may be rather misplaced.

[Richard T]

Another thing to check if replacing with a larger engine is the size of the skin tank. A rule of thumb is 1 sq ft of area for each 4hp of engine. So a 35hp engine requires 9sq ft and a 42hp one 10,5sq ft.

[Machpoint005]

I'm not good at obsolete measurement systems but I'm pretty sure my skin tank, for a 43hp Beta, isn't as big as that. The engine was a replacement about 15 years ago (previous owner) but I don't know what the original was. 

[Tony Brooks]

31 minutes ago, Machpoint005 said:

I'm not good at obsolete measurement systems but I'm pretty sure my skin tank, for a 43hp Beta, isn't as big as that. The engine was a replacement about 15 years ago (previous owner) but I don't know what the original was. 

The 1 sq ft per 4 HP is/was stated in Beta documentation and its fairly close to calculations I got done. 10.5 sq ft = about 1 sq metre.

 

I suspect that majority of narrowboats have a theoretically undersized skin tank because the skin tank is sized for sustained full power and the highest likely canal/river water temperature plus a safety margin so in canal and summer river use that only demands a few HP from the engine somewhat undersized skin tanks perform well enough. However we regularly get reports of engines that have been fine for years boiling when they go and play chicken with the cruisers on rivers, especially going upstream and more often than not it turns out to be an undersized skin tank that is at fault.

 

It is not just the size that is important, it needs to be baffled, should be as thin as possible and ideally not horizontal. Ignoring any of these things means a larger area tank may be required.

[blackrose]

As others have said, check that your prop is properly matched to your engine and gearbox before doing anything else. 

 

I have a 55hp Isuzu on a 57 x 12ft, 30 tonne widebeam and I used to think it would always be underpowered, but I had the prop repitched and now I've got loads of power. I had a burn up with a passenger/restaurant boat on the river Avon and when he caught up with me at the next lock the skipper couldn't believe it was only a 55hp engine. The pilot from portishead to sharpness said it had plenty of power as well. However, as Tony says, you may have to install an additional skin tank. I did. 

Edited March 12, 2019 by blackrose