Vespro 55 test drive - JetDrive

[marcochi76]

Hello
I had the pleasure to test drive the Vespro 55 from Centouno Navi.
Thanks Marco Arnaboldi for his kindness.

What a boat!
I was very impressed by the handling, the agility of the water jet propulsion.
No vibrations, only the siffle of the MAN turbos.

What are your opinions on this type of propulsion?
They are supposed to be more efficient than shaft, Arnesons or IPS.
But they require a carefull engineering in the design of the hull, fluids, ... to ensure that the appropriate amount of water is "sucked" by the jets system.
Definetly, a more expensive R&D is needed.

 

[julians]

They are supposed to be more efficient than shaft, Arnesons or IPS.

I was always under the impression that jet drives were the least efficient of all the different drive systems - but I'm no expert.

 

[jointventureII]

Much like Arnesons, they suffer with even a tiny bit of growth and they're a bit more complicated to clean

Although once you've got the hang of manouvering, you can do literally whatever you want

 

[marcochi76]

OK, but the impeller that suck and accelerate water has a higher efficiency than a propeller, so maybe less affected by marine growth?

 

[jointventureII]

OK, but the impeller that suck and accelerate water has a higher efficiency than a propeller, so maybe less affected by marine growth?

Well the impeller is in the dark (ie. it is in the tunnel where no sunlight can reach), so certainly less than surface drives (which have the props hanging off the transom, often in full view of the sun). In the end, if something is left uncoated in a warm sea, then stuff is going to grow. I've never run a jet boat but I can't imagine it could be any worse than surface drives - I work on a surface drive boat that will have a noticeable difference in performance after as little as 8 days if it doesn't move! (in the Med summer)

 

[marcochi76]

Concerning consumption:
237l/h per engine @ 46.7 knots, @ 2250rpm -> 10,1 L/nm
143l/h per engine @ 29.6 knots, @ 1910rpm -> 9,7 L/nm

 

[Portofino]

Would have thought the 1900 rpm is cruise so those numbers are good , But it’s not being bought for fuel economy?
Obviously helps …..a bit !
What’s its deadrise , let’s start with the basics if you want to really use that speed .

Btw you need to know the “ load “ MAN screens will tell you and see what it does at 80 % , speed wise .
As the season progresses set the load @80 % every day and watch ( hopefully) a slight reduction in speed as the impellers foul .

Cleaning them involves a diver assuming he can reach under enough the grills ? .
With jet skis we used to flip them on there sides on the beach , unbolt the grill / take it off to access the impellers to clean up .Also rope removal…..Yeh ropes get sucked in .How much potential bearing damage and engine stall is don’t know , but worth you enquiring before flashing the check book .

I personally in the interests of mechanical sympathy wouldn’t be happy running my engines all day over 2000 rpm .

As said can the hull take it ? That’s a big “ if “ because ( happy to stand corrected ) jets usually have flat aft sections .
If so then it ain’t gonna go well in bigger seas as the trad Ray Hunt deep V s of Itama , Magnum , Otam et al .

Sure if you thrash it on a pancake flat sea it will have the bragging rights .

But I always say “ I lll race you from Cap Ferat to Calvi ….not one day but 10 consecutive days back to back” , to get everything sea state wise .

If it’s a slammer you will be disappointed with it , bumping along on lap 8 while the true deep Vs have finished ( 10 laps ) and relaxing in port .

 

[marcochi76]

Deadrise 18°
Load in both cases was 50%, but I am not certain if I am reading the correct number.

 

[marcochi76]

52 or 72?

 

[MapisM]

Did you ask Arnaboldi about the rationale behind claiming 56/50 kts max/cruise respectively on their website?
Even assuming they were just numbers hoped for at design stage, when compared to real life 47/30, they give to wishful thinking an entirely new meaning...!

Not to mention that they also add a map with several examples of cruising distances, suggesting (for instance) that the 130-ish Nm from Portofino to St.Trop can be covered in 2.15 hours - i.e. flat out at the (theoretical) 56kts speed.
Borderline to fraudulent, 'fiuaskme.

 

[MapisM]

52 or 72?

Undoubtedly the latter.

 

[marcochi76]

He told the boat was anti-fouled before the summer. So now the hull is a little dirty.

 

[MapisM]

Load in both cases was 50%, but I am not certain if I am reading the correct number.

Just FYI, when an engine doesn't reach the rated max RPM (as in this case - 2250 vs. 2300), you know for a fact that it's running at 100% load, even without having ANY instrument.

That aside, the "little dirty" hull may indeed justify those missing 50rpm.
BUT, there's a snowball's chance in hell to get 9 more kts out of that.

 

[marcochi76]

I am not certain that we can think that way for a jet driven boat.
The engine rpms are not linked with the speed of the boat. Or at least not the same way as a propeller propulsion.

 

[Portofino]

72 % load ( if that’s the correct No ? ) is good .
This could mean cruise all day low 30 s .Which is realistic fully tanked up .I note it only appears to have 194 L of diesel so pretty light .

Yeh 18 % deadrise and 55 ftr is ok .
If you are serious blag a test in rough * seas next .

* you definition of what you regularly get caught in .

 

[MapisM]

The engine rpms are not linked with the speed of the boat.

 

[marcochi76]

In a shaft propulsion boat, the charge (load) of the engine is directly linked to the force needed to spin the propeller.
In a jet drive system, the boat moves according to the principal of action/reaction. The force that moves the boat is proportional to the difference in speed between the water entering and exiting the system (and its weight).
The effort of the engine is not to push the boat, but to suck and eject water.

 

[MapisM]

By charge you mean load, I suppose? Italian for load being carico, it's not so hard to guess for me...
Regardless, how and why should an impeller make any difference?

 

[marcochi76]

wait, I ask M. Arnaboldi

 

[marcochi76]

if I understood his answer, I was correct.
in a shaft boat, it is the propeller rotation that pushes the boat. if there is resistance in the forward movement of the boat, the engine's load increases due to more effort to spin the propeller.
in a jet boat, the role of the engine is only to spin the impeller to eject water, not to move the boat. it is the action/reaction force that moves the boat.
but I stop here, as I have no knowledge in this field, and I could be terribly wrong.

anyway, it is a very interesting topic. but so complex, that I understand why it is so rarely employed.