Yanmar 370HP V8 Diesel

[Bostjak]

Hey guys, looking at buying a power cat and the company I'm in comms with says that the 53ft cat will go 25 knots easy with these engines and 4 blade props. They sent me some graphs that show near identical fuel consumption and range from 12 to 24 knots... is this even feasible??

[Tin Tin]

Sorry don't understand the question. Are you saying the consumption is the same at 12knots as at 24 knots?

[Bostjak]

Well...near the same. The performance apparently levels out over this range. I've uploaded a snapshot of the graph in question

[jimbunyard]

Noting the 'theoretical' nature of the graph, not too fishy, but would be more interested in seeing what happens to fuel consumption when the speed goes over 25.

More 'fishy' to me is not seeing the complete curve, from zero to top speed.

[Bostjak]

Originally Posted by jimbunyard

Noting the 'theoretical' nature of the graph, not too fishy, but would be more interested in seeing what happens to fuel consumption when the speed goes over 25.

More 'fishy' to me is not seeing the complete curve, from zero to top speed.

Along with theoretical however they have a curve which they label as being a model they took out on the water to test. And the result is super close to the theoretical curve they made. Surely the engines would need to be spinning more just to get that higher speed, and the higher the RPM the higher the fuel consumption? I'm just confused here in general

[billknny]

Before we get our knickers in a twist let's look to see if the numbers make sense...

0.2 miles/liter at 25 knots== 125 liter/hour

According to Yanmar, these engines at full load use ~75 liters per hour , times 2==150 liters/hr

Seems their fuel consumption number are a bit light but not insanely so...

But as for the shape of the curve, remember these are not displacement hulls, and as such do not follow "normal" rules of resistance increasing very rapidly with speed.

[a64pilot]

If this vessel can indeed double speed and not increase fuel usage, then it’s the first vehicle I’ve ever seen that can, from automobiles, to airplanes and helicopters. In order for that to happen the drag has to not increase or the engine to become much more efficient. Never seen an engine that does that, so pretty much have to assume the drag curve is flat?
I assume the higher speed is full tilt or nearly so?

[rslifkin]

That curve looks reasonable enough to me for a boat that planes. More efficient at low speed, then once you start to push hull speed, fuel use goes up. From there, it depends on weight and hull design for how the curve is shaped as it goes over the hump and gets up on plane.

Generally, the lighter the boat, the less penalty for going faster. It's not unheard of for the reduction in wetted area (and corresponding drag) at higher speeds and increased engine efficiency at higher load to lead to a flat spot in the curve like that. Small, light, high performance boats sometimes hit peak fuel economy (relative to distance) at 30+ kts.

These engines (the Yanmar 8LV) interest me, as they're 370hp diesels at the same weight as a gas big block and in a similar V8 footprint. But other than a couple mentions of fuel economy being good, I've seen very little about them online. Doesn't seem like there are a lot of them out there.

[Cadence]

Originally Posted by Bostjak

Hey guys, looking at buying a power cat and the company I'm in comms with says that the 53ft cat will go 25 knots easy with these engines and 4 blade props. They sent me some graphs that show near identical fuel consumption and range from 12 to 24 knots... is this even feasible??

I hope you have deep pockets when the price per barrel goes up.

[rslifkin]

Originally Posted by Cadence

I hope you have deep pockets when the price per barrel goes up.

The graph doesn't look too bad on the low end. It looks like 0.73 nm per liter at about 8 kts, so about 2.75 nm per gallon. That's not bad at all for a boat that size. And dropping the speed to 7 kts would likely improve it a bit further. So slowing down would always be an option to save fuel.

[Cadence]

Originally Posted by Bostjak

Hey guys, looking at buying a power cat and the company I'm in comms with says that the 53ft cat will go 25 knots easy with these engines and 4 blade props. They sent me some graphs that show near identical fuel consumption and range from 12 to 24 knots... is this even feasible??

I would guess so, if she planes.

[Crusty Joker]

Suggest you look at the performance of the "Speed Twin" which was built for fishing down in San Diego. She had or possibly still has the record for pulling the most water skiers. (25?) I think that she was captained by the same guy that fished her and went on to have a famous long range boat, "Spirit of Adventure" built to fish the islands off Mexico. You may also want to research "Double Bullet" a cat sailboat that once had the speed record around England. She was put together in Marina del Rey at one of the yards there. No crushed fingers. Those speeds seem slow now but in their day those vessels performed at the cutting edge. Seems like the ultra-lights have the speed potential, so I voted something fishy, I deem things possible. Even 'speeds' that appear to be above that of light. Perhaps.

[a64pilot]

Originally Posted by rslifkin

That curve looks reasonable enough to me for a boat that planes. More efficient at low speed, then once you start to push hull speed, fuel use goes up. From there, it depends on weight and hull design for how the curve is shaped as it goes over the hump and gets up on plane.

Generally, the lighter the boat, the less penalty for going faster. It's not unheard of for the reduction in wetted area (and corresponding drag) at higher speeds and increased engine efficiency at higher load to lead to a flat spot in the curve like that. Small, light, high performance boats sometimes hit peak fuel economy (relative to distance) at 30+ kts.

These engines (the Yanmar 8LV) interest me, as they're 370hp diesels at the same weight as a gas big block and in a similar V8 footprint. But other than a couple mentions of fuel economy being good, I've seen very little about them online. Doesn't seem like there are a lot of them out there.

I’d like to see data supporting best mileage at 30 kts. I believe that’s right up there with many, many people who are certain that their car gets better mileage at 70 mph than lower speeds cause they are in a more efficient operating RPM.
Engines do have “sweet spots” but it’s a small increase in efficiency, not at all dramatic.

To double speeds with no hit in MPG would mean negligible drag increase, and no increased propellor slippage etc.

Years ago I graphed three different planing boats on fuel consumption vs speed, certain that I would find a sweet spot just after plane, on the theory of that’s where you can back off a little on the throttle, but maintain speed.
It was a twin engine Diesel 45’ Sportfishing boat, a 36’ twin engine gas Sportfishing boat, and a single engine 21’ CC outboard.
All had nearly identical mileage graphs, and no sweet spot, there were speeds that the increased fuel wasn’t as steep as others, but there was no speed that the MPG was less or the same as a lower speed, the faster they went, the worse the MPG.
By a wide margin the best MPG was idle, with the curve going up until about hull speed where it went almost vertical, then after plane the curve flattened out quite a bit, but the curve was always increasing.

[rslifkin]

Yes, idle will typically be best, but the faster the boat, the less that will be the case. Mind you, most cases of not losing efficiency at high speed will be gas, not diesel, as gassers lose a ton of efficiency at light load.

Usually that effect is only seen on very light hulls where prop slip and wetted area drop as speed increases. Running just up on plane has a lot more hull in the water compared to running 15 kts faster. And depending on the engines, you'll still often get best mpg at idle, but once you get on plane, going faster isn't necessarily worse.

As an example, here's a test of a Mako 284CC with a pair of 300hp outboards on the back: https://www.boattest.com/review/mako/3767_284-cc

That test shows best mpg at idle (2.1 nmpg). It drops from there, but then goes back up once on plane. Peak planing mpg shows 1.8 nmpg at 25 kts. Going faster costs more fuel, but so does going slower, at least until you're down to 5.2 kts (2 nmpg).

[Bostjak]

Thanks for all your interest guys. I definitely agree with the curve always increasing even if it "flattens out" a bit. I'm starting to think these hulls are duds in that the amount of nautical miles you get per litre of fuel is really low at any speed higher than 8 knots. As Cadence said, "deep pockets when the price per barrel goes up."