Power cat performance advice

[Bostjak]

Hey guys, been looking at buying a 53ft power cat and the company sent me some performance figures for the cat that have me a little bit suspicious.
According to them, with two Yanmar 370HP diesel engines and 4 blade props, the boat goes 25 knots quite easily and has near identical
fuel consumption and range from 12 to 24 knot boat speed. Is this even feasible?? I'll attach a snapshot of one of the graphs but
I'm just really unsure about whether or not this is as fishy as it seems to me

 

[Bobc]

Graph looks upside down to me

 

[ithet]

Once on the plane speed increases more proportionally to RPM. The graph shows NM per litre which flattens, but as you are going faster the rate of fuel burn increases dramatically.

Converting the graph figures to litres/hour:

10Kn - 26.7 lph
15kn - 67 lph
20kn - 100 lph
24kn - 137lph

 

[boatmike]

You have to be cautious when looking at cats. You don't say what boat it is but hull design is critical. I know far more about sailing cats than motor versions but with two long slender hulls they are typically much more easily driven than monohulls and cut through the water with a much higher limit on hull speed. at 53 ft it is entirely feasible that consumption would not increase as dramatically between 12 and 24 knots as it would with a monohull but no one can comment on the figures without knowing more about the hydrodynamic design of the hulls themselves. I would also discount any talk of planing as such unless the hulls are specifically designed to do so. It is quite feasible to drive a well designed cat of this length to those speeds without planing and the energy expended to keep a monohull up on the plane is not relevant.

 

[boatmike]

Once on the plane speed increases more proportionally to RPM. The graph shows NM per litre which flattens, but as you are going faster the rate of fuel burn increases dramatically.

Converting the graph figures to litres/hour:

10Kn - 26.7 lph
15kn - 67 lph
20kn - 100 lph
24kn - 137lph

if these are the figures it is more relevant to talk about litres per mile than per hour.
at 10 Kn this would be 2.6litres per mile
15 kn 4.47 Litres
20 kn 5 Litres
24 kn 5.7 Litres

Its much more interesting to talk litres per mile because this indicates the fuel usage of a trip over any distance while you could happily sit at tick over and consume fuel in litres per hour without going anywhere! And as I said in my other post this is a cat so talk of planing is probably irrelevant. But as I said no one can tell the OP if these figures are reasonable or not without knowing more but they don't represent an impossible scenario with the right design of hull.

 

[prv]

Graph looks upside down to me

Why? Obviously the number of miles you will do on a litre will go down as you increase to planing speed from plodding.

Pete

 

[davidej]

This really ought to be on the Motor Boat forum but how can anybody condone buying a pleasure boat that burns 5.7 litres a mile?

 

[boatmike]

This really ought to be on the Motor Boat forum but how can anybody condone buying a pleasure boat that burns 5.7 litres a mile?

You are showing a degree of prejudice I fear my friend. The title of this forum is "Practical Boat Owner" Not "Only for sailing purists" and for a 56ft boat these figures are relatively modest. Or are you saying that no one should be permitted to have a boat that big??

 

[bluenomads]

We have a 50' powercat. At those lengths, there aren't many planing hulls, mostly semi displacement (like ours). We're purposely underpowered with 2x135hp (derated to 100hp max for longevity) and can max out about 12-13kn. We are about 2.6 litre per nm at that point.

I did a little study of consumption for all cruising powercats I could find good figures for (bluenomads.blog).