Speed and efficiency, single or twins I/O same HP total

[67-LS1]

Theoretically what is more efficient, one engine/outdrive or two engines/outdrives that total the same HP. This question came up between a buddy and I. He has a 1970 Searay 240 utility with twin 270 hp Volvo Penta with duoprops. We have come across the exact same hull but came from the factory as a single, hence the question.

 

[jehardiman]

It depends....
First, what do you mean by "efficient"? In marine engineer speak, "efficiency" has multiple meanings. Everything from converting the thermal energy in the fuel to the effect of the hull wake on the prop.

Second, generally, twin props are only fitted for two reasons: redundancy or your draft limit does not allow fitting a single larger prop for the horsepower you need to go the speed you want at the propeller efficiency you want.

See this thread.

M

Thread 'Prop efficiency?'

Sep 6, 2006

Hi folks,

may anybody has precise information about the prop efficiency factor of a standard 3 blade outboard propeller (or similar) of a stock outboard engine, or simply common efficiency factors of outboard-, sterndrive-, and surface drive-props.
Or described with other words: how many hp are brought into the water if there is a shaft hp of 60hp? What I heard is that the efficiency varies with speed, what seems to be logical because the prop is calculated for just one speed and weight which is usual the top speed. I have heard values from 20% up to 90%. What is a typical factor for a...

• MagicMike

 

[Barry]

Courtesy of AI

That is an absolute monster of a classic setup! A 1970 Sea Ray SRV 240 packing twin 270 hp Volvo Penta engines with Duoprops is an incredibly rare, heavily customized, and high-performance rig for a 24-foot vintage boat. [1]
The mechanics of this unique combination stand out for several reasons:

Massive Power-to-Weight Ratio

• The Original Blueprint: In 1970, the Sea Ray SRV 240 was considered the brand's large flagship model, typically powered by a single 165 hp to 260 hp MerCruiser or OMC sterndrive. [1, 2]
• The Upgrade: Shoving 540 total horsepower into a 24-foot deep-V hull is mind-blowing. It essentially doubles the hull's original maximum factory power design. This boat must jump out of the water instantly and likely has a terrifyingly fun top-end speed. [1]

 

[67-LS1]

My friends dad bought it in 71.

It came new from Searay with twin 302 small block Fords and OMC outdrives. He repowered in the early 90s with new Volvos (carbureted) with Duoprops. He wore those out and repowered it again with new Volvos (efi) and the latest duoprops in 2018.
The other one I found is exactly the same but a single engine (and not near as nice). The stringer layout is different in the engine compartment.

 

[HelmutSheina]

Searays were obviously much nicer boats back then. The sheer line is a work of art with that bit of reverse.

 

[kapnD]

Ironic that it’s parked at the gas pump!

 

[jehardiman]

Ironic that it’s parked at the gas pump!

That's the other side of the "efficiency" coin.

 

[fallguy]

The most common reality is two engines are used because one ain’t enough to meet the hole shot or top speed wishes of buyers.

In general terms. More engine, more speed results in lower efficiency. As long as the single engine can plane the boat out without trouble; it wins.

Should add, this is not horsepower vs speed, so you need to define efficiency a bit as well as hardan says. We usually think fuel.