Power cat unlikely performance?

[Bostjak]

Hey guys, I currently own a 30 something ft cat but looking at going bigger. I don't want to mention names but I was in comms with a company for a 53ft power cat and they sent me some performance figures which I must admit, made me scratch my head a little bit. Apparently this cat with two Yanmar 370hp V8 diesel engines and 4 blade props can get up to 25 knots and has relatively the same fuel consumption and range from 12 to 24 knots. Is this even feasible or is the wool being pulled over my eyes here??

I've added these snapshots of two of the graphs they sent me so you can see what I mean

 

[Mr Efficiency]

I would be a little sceptical about the halving of mpg between 8 and 10 knots, seems a bit much. I think I'd try to get used to 8 knots if that were true !

 

[Bostjak]

I would be a little sceptical about the halving of mpg between 8 and 10 knots, seems a bit much. I think I'd try to get used to 8 knots if that were true !

Agreed!

 

[Mr Efficiency]

I'm interested in what Ad Hoc has to say about those curves, the only way you'd get a huge jump in resistance like that is a "hump" effect, which suggests it has fat demi-hulls and plenty of weight.

 

[bajansailor]

Have a look at the test results chart here for a 51' Leopard power cat (scroll down a bit to find it) -
51 PC (2019-) | BoatTEST https://boattest.com/review/leopard/3869_51-pc

At 8.2 knots (1,500 rpm) she gets a range of 1.028 nautical miles, and at 9.9 knots (2,000 rpm) this goes down to 595 miles.

I wouldn't doubt these test results, and the hulls on the Leopard cat do appear to be pretty efficient.

And same as Mr E, I would be very happy to cruise at 8 knots instead of 10 knots to achieve this sort of fuel saving!

 

[Mr Efficiency]

Well, the figures quoted of a six-fold increase (or thereabouts) increase in fuel between 1500 and 3000 rpm pretty well tallies with Yanmar figures here:
https://www.yanmar.com/media/global...catalog/Yanmar-8LV-320Z-8LV-370Zdatasheet.pdf
Not so much a boat thing, as an engine one, perhaps. The guzzle sets in quickly.

 

[BlueBell]

Bostjak,

Welcome to the forum.

The graphs look normal to me.
But it is ridiculously weight sensitive in the lower speed range ( up to ~11 knots ) !

 

[gonzo]

The odd thing is that the range and fuel consumption are not exactly the same.

 

[DogCavalry]

Welcome Bostjak! Can I recommend a sea sled instead?

 

[Yellowjacket]

The curves look right. You're seeing the effect of going beyond hull speed and that's why the resistance spikes. That's normal.

 

[DogCavalry]

How much is this thing supposed to weigh, at those performance figures?

 

[Mr Efficiency]

20 tons

 

[DogCavalry]

Right.

Now I reeaallly think a sea sled would be better.

 

[DogCavalry]

Crouch's formula gives

(680hp/44000)^.5*240=29.8 knots.

Assuming good hull condition, optimum engine/prop arrangement etc.

But as everyone knows, sea sled fever is in my brain.

 

[DogCavalry]

Heck, at that size, I'd be using a couple marinized big block turbo diesels. Proper engine room space.