Vespro 55 test drive - JetDrive

[PowerYachtBlog]

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.

They touched 57/58 last year when new. As it happens with speeds above forty knots, hull growth tends to make a bigger impact when you go above that number.
As does wind and current on the day.
I think also seeing numbers from sea trials they where doing 6 litres per nautical miles with a clean hull and 26 knots, and a new boat.

 

[MapisM]

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.

Far from pretending to know better than Arnaboldi, whose AB experience alone is remarkable, when it comes to waterjets - not to mention that he's also a naval architect, IIRC.
BUT, the resistance to the forward movement (AKA drag), for any given hull, is almost identical between shafts and jets.
And I'm saying "almost" because shafts and brackets do add some drag, not because of props per se.
It's the whole boat, with its shape and weight, that creates a drag that increases more than proportionally with speed, and can only be overcome with power - applied in whatever shape or form, even jet engines, with nothing at all in the water.
OTOH, if the engine load wouldn't increase with RPM in waterjets, those engines should have reached their rated RPM without sweating, regardless of the reasons which restricted the boat from reaching the declared speed.
To my simple mind, there's only one plausible reason why they didn't, ant it's that all of their total 2400hp output was just enough to overcome the 47kts drag, and that's all the work they could do, being unable to spin the impeller faster, even if theoretically they should have reached 50rpm more.

PS: note that I'm saying 2400hp total because I just checked the manufacturer's curves of those engines, and their full 1200hp output is available already starting from 2100rpm.

 

[MapisM]

As it happens with speeds above forty knots, hull growth tends to make a bigger impact when you go above that number.

Point taken, but while in principle I don't disagree, the differences are off the scale.
Consider also that during this seatrial, the boat was more than 2.5T lighter (on a 55 footer, FFS!) than she would have been with full tanks.
Anyway, even assuming that the boat did at some point reach the declared speed, how would you call the claim that she can reach St.Trop from Portofino in 2.15 hours averaging 56 knots?
If that isn't marketing BS, I don't know what else is.

 

[PowerYachtBlog]

Point taken, but while in principle I don't disagree, the differences are off the scale.
Consider also that during this seatrial, the boat was more than 2.5T lighter (on a 55 footer, FFS!) than she would have been with full tanks.
Anyway, even assuming that the boat did at some point reach the declared speed, how would you call the claim that she can reach St.Trop from Portofino in 2.15 hours averaging 56 knots?
If that isn't marketing BS, I don't know what else is.

I am not saying he did not put any marketing spin in it, unfortunately everyone does, some more others less. He know he cannot drive at full speed....

 

[Rappey]

User CDK on boatdesign.net posted this. From my experiences with jet drives i would kind of go along with what he claims.




Safety- When I started the port engine for the very first time, the jet immediately gobbled up my mooring line, wrapped it around it's shaft and pulled the boat's bow against the jetty. It took the better part of the day to remove the rope from the pump housing. I had to install intake grates to prevent such events, which further reduced the already poor efficiency. And even with the grates, the jets suck up ropes, fishing lines, wood and in shallow waters also large amounts of mud with stones in it. A prop can be cleaned in minutes, cleaning the jet involves diving, cutting and groping around in the dark.

Maneuverability- Jet-driven boats have extremely poor maneuverability, especially at low speeds, because there is only vectored thrust, no grip. Rudders under the nozzles are a marginal improvement only. Even with a twin installation, not hitting other vessels in a narrow waterway is pure luck. Steering response is so slow that I regularly went to the stern to check if the nozzles were still there.
The large catamaran I mentioned earlier performs just as badly as my 26 ft. cruiser.

Economy- Fuel economy is about half compared to props. Where you can cruise doing 8 knots at 1500 rpm with props, the jet-driven boat hardly moves at all. Optimum throttle setting is around 4000 rpm with the sea boiling around you and jetplane-like noise from the exhausts. Using low tide on a beach to do service is an option only if you turn the boat upside down or dig a tunnel to the intake holes. To remove the numerous clams, oysters and other shells from the impellers and pump housings, you must remove steering levers, nozzles, bowls and pull the impellers. The flood will surprise you long before everything is reassembled (with new gaskets).

Comfort- Exhausts are not through the propeller hub, but through the transom. Plus: more revs means more noise. Vibration from oysters using the impellers as a merry-go-round is felt throughout the boat because the pump housing transmits it directly to the hull.

 

[PowerYachtBlog]

User CDK on boatdesign.net posted this. From my experiences with jet drives i would kind of go along with what he claims.




Safety- When I started the port engine for the very first time, the jet immediately gobbled up my mooring line, wrapped it around it's shaft and pulled the boat's bow against the jetty. It took the better part of the day to remove the rope from the pump housing. I had to install intake grates to prevent such events, which further reduced the already poor efficiency. And even with the grates, the jets suck up ropes, fishing lines, wood and in shallow waters also large amounts of mud with stones in it. A prop can be cleaned in minutes, cleaning the jet involves diving, cutting and groping around in the dark.

Maneuverability- Jet-driven boats have extremely poor maneuverability, especially at low speeds, because there is only vectored thrust, no grip. Rudders under the nozzles are a marginal improvement only. Even with a twin installation, not hitting other vessels in a narrow waterway is pure luck. Steering response is so slow that I regularly went to the stern to check if the nozzles were still there.
The large catamaran I mentioned earlier performs just as badly as my 26 ft. cruiser.

Economy- Fuel economy is about half compared to props. Where you can cruise doing 8 knots at 1500 rpm with props, the jet-driven boat hardly moves at all. Optimum throttle setting is around 4000 rpm with the sea boiling around you and jetplane-like noise from the exhausts. Using low tide on a beach to do service is an option only if you turn the boat upside down or dig a tunnel to the intake holes. To remove the numerous clams, oysters and other shells from the impellers and pump housings, you must remove steering levers, nozzles, bowls and pull the impellers. The flood will surprise you long before everything is reassembled (with new gaskets).

Comfort- Exhausts are not through the propeller hub, but through the transom. Plus: more revs means more noise. Vibration from oysters using the impellers as a merry-go-round is felt throughout the boat because the pump housing transmits it directly to the hull.

Nice troll....

 

[MapisM]

Actually W, what Rappey said about CDK's post on Boatdesign is true - you can check that out here.
Not saying that I agree with all of what CDK says, mind.
And I'm not familiar enough with that forum to qualify him as a reliable source or not.
But I usually smell trolls at some distance, and in this case I don't.

 

[PowerYachtBlog]

Actually W, what Rappey said about CDK's post on Boatdesign is true - you can check that out here.
Not saying that I agree with all of what CDK says, mind.
And I'm not familiar enough with that forum to qualify him as a reliable source or not.
But I usually smell trolls at some distance, and in this case I don't.

I have never driven jets, so I cannot really comment on them.

But his example reminds me of the early Castoldi, and since then the system has improved, first with KaMeWa and now with MJP, which are even better.
I think back in the end of the nineties I always saw a KaMeWa at the Genoa boat show which responded rather easily, and this notion is followed in the Hinckley who introduced the first joystick for manoeuvring.
So without having tried them I understand that nowadays Jets are fairly easy. Yes they might need a different thinking then a shaft drive, but that is the notion between all systems, even from a single to a twin.

This notion reminds me of surface drives, when everyone says how painful they are, but really put the drives down, understand the trust they and how they produce it (especially backwards) and really are not that different from shafts in the way they handle, and are much easier for example to a stern drive boat as they obey more,

 

[jointventureII]

Jets are amazing to manoeuvre once you get the hang of it. Though (and this is the hard bit) you have to completely disregard what you are used to with twin screws. I have to say, even on our tender we've managed to suck up plenty, though you soon learn to avoid the more neglected corners of the harbour where all manner of fishing line / old rope / netting etc congregate.

Agree with @PowerYachtBlog about surface drives. I've been on a surface drive boat for 6 years, you need to have confidence in them and that what you're doing will have an effect on the boat (it's not immediate when in astern but once they grab.....). I don't put them all the way down, normally somewhere around the neutral mark, as I find that all the way down just makes then drag the stern down / kick water into the hull, then it responds by "bouncing" back up. However each boat will be different. And the steering needs to be used heavily too.

 

[Rappey]

Jets work well in large catamaran ferry hulls and for extremely powerfull small boats in shallow water. Some say at higher speeds they become more efficient ?
I didnt like that one had to use fairly high rpm at low speeds just to move and a straight line was virtually impossible.
I would imagine they are easier to control in a larger boat than a small one .
jetskis seem to have Lots of slow speed control..

 

[Portofino]

The manoeuvre thing is a cul d sac in the decision making . I have seen plenty of Leopards / Magusta and AB s docking perfectly fine . Remember they have reversing buckets ….. as indeed larger , albeit single engined jet skis . But these are twins + bow thruster . Fuel efficiency ( less drag ) at speed and to some extent shallow draft is there forte .
If you intend to do a lot of miles - fast - then they make sense .
I mean cruise between 30-35 knots as opposed to 25 ish like fairprinseekers

Additionally jets are easier on the engines than Arnesons - quite idiot proof you can’t overload the engines getting it to plane . You just wind up the rpm.

 

[Greg2]

Having operated a Redbay Stormforce 12m cabin RIB with twin Hamilton waterjets for nine years on the east coast of the U.K., sometimes in shallow waters, I don’t recognise what is described in the post by CDK quoted by Rappey and I would summarise our experience as very good overall.

On the points described by CDK in Rappey’s post:
- Safety - not had major issues with stuff being sucked into the intakes. Not saying it doesn’t happen but I can only recall one piece of rope and never a mooring line and we haven’t had major/persistent problems with anything else being sucked up.
- Manoeuvrability - with twins you can literally get the boat to do whatever you want at close quarters. Sure you have to learn all over again but it isn’t difficult - took me less than half an hour to get the hang of moving it in any direction, around fixed points and into what might otherwise be very tricky moorings. At low speeds you have to work a bit on the wheel to to keep a straight track - I would describe as being a bit more than on some outdrive boats - but it is what I would describe as a characteristic as opposed to a problem.
- Economy - less efficient than props no question but I don’t think it is anything like 50%. I can’t be sure but I have around 20% - 25% in my head but don’t have figures to hand and it might be higher. No issues with going at slow speeds. We do survey work with an ROV at 2-4 knots.
- Comfort - no issues with exhaust noise or vibration.

One downside is that our boat has no ‘clutch’ so the jets are always pumping water out so if the buckets are in precisely the right position the boat wants to move so need to be aware when starting up. Not what I would describe as a problem but the ability stop the drive to the impellers would be a bonus. We sea trailed a catamaran with jets that had a ‘clutch’ and it was useful.
.