IPS or not re Prestige 58

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interest the new Prestige 58 project here
but this got me thinking about the IPS advantage and if really does so much to speed and better economy or are we being fooled....
the Prestige 58 with IPS850 with 670hp engines and a displacemnt of about 23t has a top speed of 31 knots and cruising of 25
I am sure that the AZ55 with Catpillar 669hp shaft propulsion and a similar displacement 28t loaded had a similar preformance 31 knots max and 27/28 cruising
I think SQ58 and Fer 53 with the same weight and more or less similar engines do more or less the same
Fer 53 which has that weight peaked 32/33 knots really and cruised at 29/30 knots with the Cats..
for the Squadron may be JFM can give us some better figures
 
if boats achieve the same max and cruising speed, with the same size engines, they'll burn roughly the same amount of fuel, regardless of the drive system.
 
yep Nick H is right infact the title IPS850 means that altough it has officially 670hp it gives an equivalent 850hp with shafts
I dont think it is the case for Prestige 58
an engineer from a regarded builder told me that while initial forecast of 40% better performace was forcasted to reach with IPS, 20% is very difficult and to achieve this many builders have to start a new hull from scratch, he said the average are achieving 10% versus a shaft propulsion
I remember that he pointed to the Absolutes as among the most clever users of IPS at the time which was about a year ago
he also made me notice that unlike a sterndrive once over 30knots+ the dynamics have to be much efficent in the vee hull, meaning less vee in the centre of the boat, he also told that while he did not try it yet he thinks ZF Zeus at the time esclusive for Mercruiser should give better and easier to achieve results for most of today hull shapes
 
.. but that's not the same point, your graph only shows that if you DO make the drive more efficient it will use less fuel at a given engine rpm and boat speed, because it requires less power. That's not disputed, in fact it's obvious. Now, if you re-propped the boat to take advantage of the improved efficiency of the drive, then you would use the same amount of fuel at a given rpm, but the boat would be going faster, and would have a higher top speed.

Now consider that in reverse. If the Jeanneau with IPS can not achieve a higher speed than an equivalent shaft boat with the same engine hp, and assuming the props are optimised on both, then the hull/drive combination on the Jeanneau is no more efficient than the shaft boat.
 
the Seatorque is very efficient and I am still surprised how it is not used so much by the industry
is that graph from the Ocean Yachts or Donzi Rosciolli sportfish?
I know someone who seatrialled a Donzi with these and he was impressed, the owner also said that they did 12% more performance
now this might be a happy owner really but it is interesting his point of view...
 
Interesting PY, and i think you're right. Sq58 with 2x715 on shafts was 34kts new in UK (cold february) waters, light (say 24tonnes). In med with a few years of use and 3/4 loaded (27 tonnes) it is 31kts

So the IPS/Prestige 58 combo offers no advantage over shafts, and as nick has explained there is no fuel saving either.

Thing is, that's kinda what you'd expect, if it weren't for the volvo hype. A shaft boat has to drag a shaft and P bracket thru the water, plus a small slender rudder. An IPS has to drag that huuuuge gearbox thru the water. Anyone looked at the big IPS at LIBS or any of the recent boatshows? It's enormous. It looks like it'll have the same drag as, er, a shaft/P bracket/rudder.
 
By the way, that's an odd way of showing a graph. The scale on the x axis is all over the place, so the line appears different to a normal consumption curve. I can see how it's ended up like that, the trials have obviously centred around the normal cruising rpm, but then the results have been plotted on a graph as if the x axis scale is linear, which it isn't.

I guess it's not a major problem, it shows your product improves efficiency, and I suppose that's what your after, but it's not entirely professional IMO.
 
It is a horrible graph (speaking as an engineer), but the idea was to make it easy in a compact graph to show the differences over a speed rpm range. Most non engineers will be able to pick out a couple of speeds or rpm and see the differences, which are significant.

But yes the scale is all over the place!
 
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