Cat C-18 vs. Cat C-32 in Carolina boat

[JPH]

What is the difference in speed and fuel usage between C-18 Cats (1150 HP) and C-32 (1675 HP) Cats in a 57 – 60 Carolina sportfisherman? I already know about the I-6 and V-12 differences as well as fuel use differences at each RPM. What I am asking is about real world observations; does 70 GPH for a C-18 boat and 105 GPH for the C-32 sound about right at 1900 RPM?

For example, How much faster would a similar C-32 boat be, at say 1900 RPM, vs. the C-18 boat at that same RPM?

Or, how much more would the C-32’s burn to go the same speed as the C-18 boat in the same or similar boat?

For a given 60 – 90 mile trip offshore, how much more fuel would the C-32 boat burn vs. the C-18 boat?

If you have any observations or rules of thumb you can share, I’d sure appreciate it!

[Captain Jay Kavanagh]

if it were my boat it would have c-18's

the extra weight of the 32's really kill the extra hp's and you only loose a couple of knots and gain alot more efficiency and a more realistic usable speed.

[JPH]

Thanks for the reply Jay, I've heard variations of that before and the difference you reference is what I am trying to get as precise an answer to as possible.

[Fortunate One]

Jay is right on the money.

The weight of large motors means more HP is required and HP really needs to be increased exponentially in order to really see substantial increase in speed from one engine to the next.

When I was selecting engines for my boat I had the luxury of having various engines analyzed for performance and fuel burn.

I was comparing the C-18 cats vs. Yanmar 8Sy-STP 900 HP V8and Volvo’s 800 HP motors. I also threw in an analysis on the Yanmar 720 Hp motor.

What I found interesting was the higher HP Cat would only produce a couple of knts more on the top end and some of that was attributed to weight differences.

I liked the torque range of the Yanmar and the fuel burn rate is better so I went down that path. Range is important to me. At cruise I gained an extra 65 miles per tank full with the Yanmar vs. the Cat. If I throttle back off of cruise I can gain even more range than I would have if I were to have selected the Cat.

I shouldn't cost much money for a naval architect to do an analysis on the choices. Have him run calculations for optimum efficient cruise RPM and you can really get a feel for what mill is best.

From my experience I think you may find that the C-18 is the better choice of the two. But then again the weight of the boat will also come into the equation as well as the size of the prop you can swing.

Good luck!

[TROPHY SPORTFISHING]

if it were my boat it would have c-18's

the extra weight of the 32's really kill the extra hp's and you only loose a couple of knots and gain alot more efficiency and a more realistic usable speed.

hands down best advice. also alot cheaper to rebuild.

[Captain Jay Kavanagh]

Really the way to look at it and anaylize it mathematically is a pounds per hp computation.

Determine the weight of the package.

Look at the hp and tourqe curves for each pair of motors.

divide the total weight by the amount of power (each for HP and tourque.)

this will give you a pounds per hp number, the lower the better. this if the number of HP required to shove a pound of wieght for that particular motor package.

I also do it a where you are realistically going to run the motors not the High end, say probably about 1800 rpms or less.

Also consider other factors, you are going to have to pack alot more fuel in the boat for the 32's, and the increased weight of all of that will dramatically affect how the boat will back up, troll (cleanliness of wake), ease of running etc.

I think about a 55-57' boat with c-18's is about the ideal boat.

[JPH]

Jay and others, thanks for the replies...I had all the calculations done and wanted to see if there were any comparisons available with hard data that I could extrapolate. If available, I'd really like to see it.

Here is what the numbers showed with all the speed tricks like Veems, tubes, V-C paint, etc. figured into each package. Since Cat states in their literature that 1900 RPM is considered max cruise for extended running, that's what I used. Like you said, 1700 - 1900 will be the most often used RPM range. Weight included 400 gallons more fuel for C-32s.

Full fuel + Water, Yacht Interior:
C-18s = about 31 knots at 1900 @76 gph, C-32s = about 35.5 knots at 1900 RPM @ 107 gph. A 60 NM trip to the fish and back will use about 78 gallons more in the V12 assuming 1900 RPM cruise speeds. The C-32s also force the hull 3.5" deeper into the water. Pulling the C-32 boat back to 31 knots showed 94 gph though this calculation is the toughest to accurately project.

It's hard to argue against the C18s.

[54bullwinkle]

Paul spencer has built several 57's and 59's with 18's. He has one 57 with 32's and a few 59's with 32's.

The efficiency of the 32's is within 10 -15% of the 18 boats. Plus at the same speeds you are pushing the 18's at 75% load and the 32's are less than 50%.

The 18's are great motors, The 1150's are ACERTS and are new and dont have a track record yet. They will give you a very spacious engine room and are cheaper to buy, operate and repair. I am sure they will be as good as the standard 18's

I went with 32's. Pauls next 57 will have the 32's as well.

Its a win win, both great motors. Good luck.

[Captain Jay Kavanagh]

one thing nobody has mentioned is how the weight affects the boat and the trolling wake.

A lighter boat (less fuel lower motor weight) will not sit down as far in the water and will have a cleaner trolling wake.

that is what it is all about to me.

a couple of knots don't matter if whitey dont want to eat your flat line.

[Fritz]

[QUOTE=a couple of knots don't matter if whitey dont want to eat your flat line.[/QUOTE]

What He Said!!!!!!!!!!!