Weight vs horsepower

I am looking at Chris Craft Corsair 28s for sale and came across one with a single engine. All others I have seen have been twin small blocks, Mercruiser or Volvo with each engine being between 280 and 320 HP apiece.
The single engine one I found has a single 380 Volvo bib block. The guy selling the boat swears the single engine boat has the same cruise and top speeds as the twin engine versions due to the lighter weight. I’m inclined to call BS.
I would guesstimate the single engine version is approx. 1000 lbs lighter. To me that doesn’t seem like enough to offset the loss of at least 180 HP and a second set of props pushing water.
The single engine version is appealing (one engine to maintain, more room in the bilge, possibly less fuel burn) but it’s a LONG, LONG way away so I’d have to travel to see/sea trial it. I don’t need a speed demon but I don’t want a dog either.
Any thoughts?

 

Yeah. That's BS. The speed calculation formulas, which are many, generally have at their core square root of power to weight ratio. All other things being equal, which is truish for different versions of the same hull, it will come down to that.

 

He's probably being truthful, and absolute fuel consumption is probably slightly higher with two engines; but there is a lot more going on.

In simplistic terms: In order to generate thrust at a specific speed, the prop needs to push water aft faster than the boat is moving forward, this is called slip; the more slip the more thrust but less efficiency of the prop (lots of caveats here for real props, but we said we will keep it simple). Slip times prop diameter gives a mass flow rate that generates thrust, and the needed thrust is only dependent on the hull; doesn't matter if are using 1 or 2 props. This means that for props of similar shapes, the single shaft motor needs to have twice the slip of the dual shafts....or twice the area (i.e. dia(1)= 1.41 dia(2)). Since thrust hp at any speed is constant, and more slip gives less efficiency; a single shaft motor is working more than twice as hard as the motors on twin shafts.

This brings up two other topics; Maximum Continuous Rating or MCR, and redundancy. MCR is the maximum power output engine can produce while running continuously at safe limits and conditions. It is significantly less Full Load Rating or Brake Horsepower which is the typical advertised hp of an engine. So at the speed a twin motor vessel will be at MCR, a single motor will be significantly above MCR leading to increased maintenance and reduced service life. This is where redundancy comes into play, if a single motor has reduced service life, it is typically more likely to fail in operations. Independent twin motors significantly improve overall reliability for operations.

 

Thank you for the replies. So I can understand that the current single engine boat owner may be able to get a similar cruise speed as a boat with twins but the single engine RPM/load factor will be much higher, correct?
Top speed, in my mind, has to be substantially lower though.
And time-to-planing has got to be way worse.

 

A lot depends on props...and weight. Corsair 28's aren't that big or heavy and can use a CR set on the single... so depending on how the props are pitched...hole shot vs top end.

 

What drive setup is the single engine? Single prop or double prop leg? Or straight shaft? The weight distribution has a major influence and is often better with a single engine. I'd say the owner's statement may well be reasonable. If drive is a Volvo DP it is worth nearly 15% higher prop efficiency than a corresponding single prop installation. If we can get better all-up weight estimates, prediction quality increases.

 

With a tremendous amount of respect to @jehardiman I must disagree. The single engine boat has a power to weight ratio that is best case 22% lower. And with a single prop, reduced propulsive efficiency. With a much larger prop, and therefore comparable disk loading, it would still probably only go 88% as fast.

Edit: I'm late to the party again. Basically irrelevant. Sources say the CC 28 in question had a dry weight of 7500#. Less a thousand # plus normal fuel pax etc, the percentage drop in weight is not proportional with a drop from 560hp to 380hp.

 

Hmm. Let me walk that back. Is it possible that the reduction in parasitic drag going to single prop counterbalances a 22% drop in power to weight ratio?

 

I can speak of Bertram 25s, which generally came out with twin ~165hp inlines (4 or 6 cylinder), but occasionally somebody would order one with a single 228hp 350 CuIn V8. The 28' Bertram had a single (generally diesel) option also, but I can't remember seeing one.

They lost a little on acceleration and top speed and used a bit less fuel, but the biggest loss by far on a small flybridge boat was the poor man's bow thruster you gain from twin counter rotating propellers, especially during low speed work when the windage is having the biggest effect, like every time it is manoeuvred to a trailer, marina pen or boat lifter.

 

Not really John, if total power output is similar then top speed will be similar (assuming proper setup), what you will gain from twins is impressive acceleration (because the power is converted to thrust more efficiently).

You will also gain a huge manouverability advantage as many boats with twins can pretty much rotate on the spot.

 

That was what I was thinking. Now with outboards there really is a lot of parasitic drag from the legs and a 300 single could be faster than a pair of 150's. But in this, a large drop in power.

 

Pretty much all of the 28 Corsair's I’ve seen, twins and the single, are Duoprops. Volvo or Mercruiser. The only weight figure I’ve been able to find is 7500 lbs and that appears to be for the twin small block version.