Understanding supercharger/turbo on a KAD44

[harvey38]

My boat neighbour has twin KAD44s fitted, they have done 840 hours and needs both turbos replacing so £2.5k gone. Ouch

Chatting to him, these engines also have superchargers fitted, my very basic understanding is/was that a supercharger was a mechanically driven turbo charger so why have both?

My other boat neighbour has a heavier boat with KAD41s that I understand has 'Only' turbo assistance which to my limited knowledge makes sense.

Appreciate someone enlightenment, oh and views on new turbos @ 840hours, the vane tips appear 'Chipped' with pieces missing.

 

[Tranona]

The supercharger is to fill in the power curve at revs below those when the turbocharger kicks in. It was a way of extracting more power out of the engine without increasing capacity. The base engine is 3.6l and high revving (3900) and popular with sportsboats. Not a brilliant idea as the transition can be abrupt at times when you need smooth delivery.

 

[PCUK]

840 hours is not a lot!

 

[petem]

My boat neighbour has twin KAD44s fitted, they have done 840 hours and needs both turbos replacing so £2.5k gone. Ouch

Chatting to him, these engines also have superchargers fitted, my very basic understanding is/was that a supercharger was a mechanically driven turbo charger so why have both?

My other boat neighbour has a heavier boat with KAD41s that I understand has 'Only' turbo assistance which to my limited knowledge makes sense.

Appreciate someone enlightenment, oh and views on new turbos @ 840hours, the vane tips appear 'Chipped' with pieces missing.

All KAD44's on salt water will need replacement turbos at this age if they haven't had them already.

However, he can buy exactly the same Borg Warner Turbos from an OEM supplier for £800 a piece.

New Original BorgWarner / Volvo Penta Turbocharger 5326-988-6752 / 3802105 - Check serial numbers before purchasing

The supercharger is for lower revs and the turbo is for higher revs.

 

[petem]

840 hours is not a lot!

No, but 20 years of sitting a few inches above a salty waterline is.

 

[petem]

The supercharger is to fill in the power curve at revs below those when the turbocharger kicks in. It was a way of extracting more power out of the engine without increasing capacity. The base engine is 3.6l and high revving (3900) and popular with sportsboats. Not a brilliant idea as the transition can be abrupt at times when you need smooth delivery.

That's not true. The transition is very difficult to detect (I have the same engines).

 

[penberth3]

..........my very basic understanding is/was that a supercharger was a mechanically driven turbo charger so why have both?........

You're close, it's the other way round. A turbocharger is a turbine-driven supercharger, hence the name.

 

[burgundyben]

The supercharger is to fill in the power curve at revs below those when the turbocharger kicks in. It was a way of extracting more power out of the engine without increasing capacity. The base engine is 3.6l and high revving (3900) and popular with sportsboats. Not a brilliant idea as the transition can be abrupt at times when you need smooth delivery.

Turbo chargers don't really 'kick in' their response is pretty much linear to mass flow through the exhaust.

It would be more correct to say that the super charger is there to enable the engine deliver more torque at low RPM so that the engine's ability to deliver torque is greater than propeller demand.

Background - I lead turbocharger design.

 

[ChromeDome]

By this explanation you'll understand that the charger (belt driven) will produce a boost at any rpm whereas the turbo needs some flow/heat expansion from the exhaust.

When using both the supercharger has a clutch that connect/disconnect it (kick in/out) before/when when the turbo gets up (in rpm) to produce boost.

 

[burgundyben]

the turbo needs some flow/heat expansion from the exhaust.

Enthalpy.

Enthalpy - Wikipedia