BartW
Well-Known Member
yes indeed,
especially while knowing that there are still some victims inside,
and all these private belongings of 4000 people
SOF Cassis to Rome delivery trip
- Thread starter BartW
- Start date
yes indeed,
especially while knowing that there are still some victims inside,
and all these private belongings of 4000 people
AndieMac
Well-Known Member
And a typical local dish prepared by Alfonso
...you couldn't resist putting the food picture into the post Bart!
and picked the right crew member to take along 
Nice trip report mate, the conditions looked fabulous. Lots of work ahead on BA...good luck with her.
BartW
Well-Known Member
I have been offered a "1992 Versilcraft Challenger 80" (that Ital broker from BA is still sending me offers)
very similar boat like mine, slightly bigger but superstructure is GRP (so probably similar weight like BA), same MAN engines
they quote cruising speed 19kn, max 22kn
where BA is Cruising 20,5kn (2000RPM) max 26 kn(2300RPM)
This Versilcraft has Koopnautic (Naiad) Stabilizers.
So these figures could give an indication of what we can expect on BA ?
I have asked the broker for the weight and fuel consumption of that boat.
edit,
just got a mail answer from the broker,
weight is approx 60 tons
consumption is 330l/h
drag from the stabilisers is 0.3kn he claims (this is a bit optimistic I think, how could they know)
so the boats are compare-able, and a perfermance loss of about >10% can be expected ?
or are Sleipner stabs so mùch better
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MapisM
Well-Known Member
I very much doubt that such boat was originally built with the stabs.
And if she wasn't, the owner could have a very accurate idea of the difference they made when they were fitted.
That said, I agree that 0.3 kts is optimistic.
It would be interesting to speak with whoever owned the boat when the stabs were installed.
jfm
Well-Known Member
The sleipners do not cost 10% on top speed. Fairline have seatrial test data on every Sq78, ie 79 boats launched, 2 of which have Sleipner stabs, and they show a top speed loss of about 1 knot. Princess report the same based on 10 or so sleipner installs on their 72 and 78 (V and fly)Idrag from the stabilisers is 0.3kn he claims (this is a bit optimistic I think, how could they know)
so the boats are compare-able, and a perfermance loss of about >10% can be expected ?
or are Sleipner stabs so mùch better
BartW
Well-Known Member
interesting !
at what top speed is that ?
do they have test data at cruising speed,
or at 20kn ?
I assume that drag is almost a lineair function from speed ?
BartW
Well-Known Member
I have asked the question
MapisM
Well-Known Member
Nope, it definitely isn't. As anything drag-related, the effect is rather exponential.
The actual curve obviously depends on each hull, but in principle, if you double the speed, you much more than double the drag.
rafiki_
Well-Known Member
Bart, you are dealing with forces (drag) here, so there is a squared law that comes into the equation.
jfm
Well-Known Member
They have zillions of spreadsheets of data, of which I have some. The -1kt that I referred to is top speed; 32 kts in a standard high spec boat, 33 in a very light boat, 31 in a stabbed boat (mine and one other)
At cruise speed say 20kts you can compare the litres/mile or lph of a standard boat vs a stabbed boat to see the power absorbed by the stabs and it is in the order of 100hp at fast cruise (with 2 x 1m squared fins)
I think drag/speed for the whole hull is close to flat line once boat is fully on the plane. Whether or not it's linear, what matters is that the gradient is close to zero. If you study the data you see that litres per mile is almost constant from say 20-30kts. IE you burn the same joules of diesel to do 100nm at 20 knots as at 30kts (not exactly the same, but very low gradient). Drag/speed for the fins in isolation will be steeper though and curved, as rafiki says, but that is only part of the whole hull
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jfm
Well-Known Member
Only for the appendages MapisM, not the hull surface which is waterskiing
BartW
Well-Known Member
Well, I still don't see the point in reducing the pleasure time...
...even more so when you think that you could make it at 4,6 l/nm, as we recently did in a similar size/weight boat!
yes MapisM I know where you want to get me
The owner of that Versilcraft bought the boat in 2006, with the stabs installed, so no data available without stabs.
very interesting info from Jfm,
but lets postpone the planning of stab's on BA until later,....NEXT year
as I have just asked for a quotation for the Hi lo platform, (last friday In Dusseldorf) eventyually to be installed THIS year,
while already so many works have been put in to action.
so first things first I should say.
MapisM
Well-Known Member
Yup, agreed. By "as anything drag-related", I meant any sort of u/w gear.
For hulls as such, when planing, the ball game is different.
Though I suspect that it would still take more than double of the horsepower to push a Sq78 at 60kts... But that's just a wild guess.
jfm
Well-Known Member
Agreed! You would have a cubed-law on the appendage drag, or you could Arneson it, plus at 70mph you begin to get some (2-300hp maybe) air resistance unlike at 20kts. Yup, you'd want 8000hp rather than 2x2x1600hp. For my next project MapisM...
BartW
Well-Known Member
I'm mainly interested in efficiency loss at slower speed, 20kn
in stead of double speed 60kn.
just discussed here with my eldest son, (5th year at univ for civil engineer)
he thinks that the appendage drag is mainly friction,
and friction is a complex curve, but in a certain range, linear for 2 different speeds....
(but he has little interest in my hobby, so might not have put all his brain power on it
)
jfm
Well-Known Member
I'm mainly interested in efficiency loss at slower speed, 20kn
in stead of double speed 60kn.
just discussed here with my eldest son, (5th year at univ for civil engineer)
he thinks that the appendage drag is mainly friction,
and friction is a complex curve, but in a certain range, linear for 2 different speeds....
(but he has little interest in my hobby, so might not have put all his brain power on it
Fluid drag is quite different from surface-to-surface friction which does indeed show linear relationships
At 20kts I'd work on 100hp loss. On 2x 1200hp motors, that's just about significant. My boat, which has stabs AND 3 tonnes extra of fuel and gear was launched with the stnadard sq78 props and the C32s wouldn't reach full rpm when heavy. The props were changed by iirc 2 inches, 51inch P were fitted to replace the standard 53P. Then it was ok. So overall the stabs are not a big deal in terms of drag, they might require a lift out and prop adjustment, and they will increase fuel burn maybe by 5% or so at 20kts AOTBE
MapisM
Well-Known Member
Surface drives and 70mph? Now that would be a setup deserving the name Match 2...
Looking forward to the new thread!
vas
Well-Known Member
Surface drives and 70mph? Now that would be a setup deserving the name Match 2...
Looking forward to the new thread!
or could justify dropping the "t"
MapisM
Well-Known Member
Yep, that's what I meant, but since #1 has the t, for consistency...
BartW
Well-Known Member
the MAN D2842LE402 (1100HP) data sheet tells me:
at 2000RPM, (prop curve) fuel consumption is 145l x2= 290l This is very close to what we actually observe !!!
so adding 75Kw (100Hp) we would need 2100RPM for same speed
at 2100RPM, fuel consumption is 165l x 2 = 330l
290l to 330l = 14% more fuel
alle very aproximate !
but gives a good indication of what we would acheeve, I think.
and I'm still considering..
