Displacement Speed Question

[mydauphin]

Would creating a curtain of air on bottom and sides of hull reduce friction or just make ship sink in air bubbles.

[daiquiri]

Originally Posted by mydauphin

Would creating a curtain of air on bottom and sides of hull reduce friction or just make ship sink in air bubbles.

Diverging results from various authors:
http://www.dtic.mil/cgi-bin/GetTRDoc...c=GetTRDoc.pdf
http://www.nmri.go.jp/turbulence/gro...crobubbles.pdf
http://wasub2.tudelft.nl/download/mi...0submarine.pdf
http://www.newscientist.com/article/mg18925391.600
http://www.nmri.go.jp/spd/drag/taka99/taka99e.htm
I also recall (but I'm unable to retrieve that paper) that one japanese author (Kawashima? some 6-7 yrs ago) has reported big problems with maintaining the layer of bubbles attached to the hull, because they tended to escape upwards at some distance from air injectors.

[mydauphin]

Wow thanks Daiquiri... Everything except a working boat...

[liki]

Here some displacement boats in previous life have been converted into speed monsters. See what has been done for the bottom shape as an example here:

http://www.friddemartin.net/fanny/8161.html
http://www.friddemartin.net/fanny/IMG_3708.html

[Ilan Voyager]

Many Thanks Daiquiri, very interesting links. Lot of research since years but none full size commercial boat has been made...between research claims and real life there is a gap.
For example the model used by the Japanese in the last link is rather unreal; 12m long, 1m wide, 0.045m draft (??!!). That gives at best a displacement of 0.340m3; the ultra-ultra-ultra light boat.

Originally Posted by Ilan Voyager

Many Thanks Daiquiri, very interesting links. Lot of research since years but none full size commercial boat has been made...between research claims and real life there is a gap.
For example the model used by the Japanese in the last link is rather unreal; 12m long, 1m wide, 0.045m draft (??!!). That gives at best a displacement of 0.340m3; the ultra-ultra-ultra light boat.

There have been many vessels built using that air bubble technique! Although (to my knowledge) only icebreakers. The bubble "curtain" reduces the friction between ice and vessel. As far as I know all of them have been replaced by water injectors to gain the same advantage with less power and structural issues (hullform).

[loups1]

[loups1]

Τεχνικά χαρακτηριστικά 6.20
Το Saronic 620 είναι ένα σκάφος παραδοσιακό, τύπου τρεχαντήρι.
Fishing μέγιστου μήκους γάστρας 6,12 μέτρων και μέγιστου πλάτους 2,20 μέτρων.
Δέχεται ένα κινητήρα έσω ντίζελ από 8 έως 35 ΗΡ και έχει maximum ταχύτητα με τη βοήθεια δύο παρατροπιδίων 9,5 knots. Το βάρος του με κινητήρα και καύσιμα αγγίζει τα 1100 κιλά.

Κατασκευάστηκε για να καλύψει τις ανάγκες της ελαφριάς παράκτιας αλιείας αλλά και του μικρού οικογενειακού σκάφους που θα μπορεί να χρησιμοποιηθεί ασφαλέστερα για ψάρεμα και αναψυχή από μια απλή βάρκα χάρη στη βαριά κατασκευή του.
Συνδυάζει τα σύγχρονα πολυεστερικά υλικά με το παραδοσιακό στιλ. Διαθέτει κλειστό χώρο με καντράν έτοιμο να υποδεχθεί κάθε ηλεκτρονικό όργανο.

Η διακυβέρνηση του σκάφους γίνεται από το κατάστρωμα με λαγουδέρα το οποίο μάλιστα είναι αυτοστραγγιζόμενο, μηδενίζοντας έτσι το άγχος του κυβερνήτη στις δύσκολες καταστάσεις. Το σκάφος επίσης χαρακτηρίζεται για την ιδιαίτερα καλή για τα μέτρα του πλεύση αλλά και τους αποθηκευτικούς του χώρους. Εύκολος στην πρόσβασή του για συντήρηση είναι και ο κινητήρας του.
Το υλικό κατασκευής του είναι ενισχυμένος πολυεστέρας με υαλoίνες (GRP). H γάστρα του διαιρείται σε τρία στεγανά διαμερίσματα από ισάριθμους μπουλμέδες (φράχτες) ενδυναμωμένο με ξυλεία ναυτικού τύπου ακόμη και για βαριά επαγγελματική χρήση.

Στο κατάστρωμα και στην καμπίνα πλοήγησης ανάμεσα στο GRP υπάρχει κόντρα πλακέ θαλάσσης υπό μορφή σάντουιτς για να μπορεί να δεχθεί βίντσι υδραυλικού τύπου.
Eξοπλισμός
Specifications:the saronic 6,20 is a traditional boat, trehandiri type fishing length 6,12m and 2,20m wide.it takes one inboard diesel engine from 8 to 35hp and has maximum speed with the help of two “chines”? [I don’t know the exact translation maby its something else] of 9,5 knots.it weights with engine and fuel about 1,1tons.it is build to cover the needs of the near shore commercial fishing also the small family boat which will be used safely for pleasure fishing than a small boat by its heavy construction. it combines grp with traditional design and it has a closed space where c take any electronic organ .the control of the boat is done from the deck which is self draining with a paddle? The boat also has very good -for it length-flotation and good storage. Easy to acces is the engine.

I tried to translate the specifications of a boat similar to mine from a greek boat building company. I once tried to ask them but no response.
Thanks for your time .
Also this morning I took a photo of another boat similar to mine.as you can see it has something like flaps at the rear and left and right at the middle has something like a keel? I don’t know the speed of that boat but I would like to know the use of that structure. Thanks again for your time

[Guest625101138]

lou
It should do better than 6.2mph with 60HP.

Do you have any detail on the prop - diameter and pitch?

Do you know what the engine speed is at full throttle - maximum rpm?

The various photos shown are different. Some of the boats have the aft trim tabs but the first photo you posted does not. These will reduce the squat when the hull is pressed beyond hull speed. They should reduce the required power to get above 5.5kts. Does your boat have the aft trim tabs?

Rick W

[loups1]

hi rick thank you.you are right but as i mentioned are boats similar to mine in dimentios6,20x2.00 and the hull is exactly the same exept those differences which i noticed.if aft trim tabs you mean those which i call flaps no my boat had not but last year a friend of mine automotive engineer told me to put two piece of metal 30x45 under and a litle forward of the prop. after that the bow does not rise as did and the stability is better but no more speed.the maximum rpm is 4200 as the engine is from a nissan car used as a taxi. the diameter of the prop is 36cm in diameter but how i can meaure the pitch? the boat is at the marina.is it any pratical way to do it without taking off the prop?

[Guest625101138]

Is the engine reaching 4200rpm or is it loaded down too much to get to this rpm?

Is there a gearbox? If so do you know the ratio from the engine to the prop?

You need to be able to get at the prop to find markings on it or measure the blade angle at a known radius. Is the boat in the water? If so is it easy to get it out?

To measure the blade angle you need to place a straight edge across the back of the blade at say 14cm from the shaft centre. This should be 4cm from the outer tip. Then determine the angle between the straight edge and the shaft.

Rick W

[ancient kayaker]

Originally Posted by mydauphin

Would creating a curtain of air on bottom and sides of hull reduce friction or just make ship sink in air bubbles.

That sounds awfully close to a hovercraft!

Enough pressure has to be exerted somewhere on the bottom of the hull to support the boat's weight. I suspect if enough of the bottom area is covered in bubbles the boat would just sink enough to push the bubbles out of the way. Under way, the bubbles would be swept back into the wake before they could do much good. To have the bubbles stick together sufficiently to stay put and exert pressure on the enclosed air the liquid would have to contain a lot of detergent or be something other than water. But I could be wrong.

[Village_Idiot]

Originally Posted by ancient kayaker

That sounds awfully close to a hovercraft!

Enough pressure has to be exerted somewhere on the bottom of the hull to support the boat's weight.

OK, this is totally off-topic, but one of the leading theories to disappearances of vessels in the Bermuda Triangle is bubbles. Some scientists believe huge amounts of gases (nitrogen?) are occasionally released from fissures in the ocean floor, and this creates enough "bubbles" in the water to immediately sink ships, and even adds enough instability in the atmosphere above it to bring down airplanes.

Now, back to the OP's topic...

Originally Posted by ancient kayaker

Enough pressure has to be exerted somewhere on the bottom of the hull to support the boat's weight. I suspect if enough of the bottom area is covered in bubbles the boat would just sink enough to push the bubbles out of the way. Under way, the bubbles would be swept back into the wake before they could do much good. To have the bubbles stick together sufficiently to stay put and exert pressure on the enclosed air the liquid would have to contain a lot of detergent or be something other than water. But I could be wrong.

Nahh, Terry it will not sink. The air "curtain" is too small (by volume) to have a big effect on displacement value. But in a turbulent flow, those tiny bubbles at the boundary layer are under some very strong shear forces, and become deformed and rather flat, like lentils: and this change in shape seems to reduce turbulence, and holds them in the layer quite a while, therefore frictional drag drops.
But there will be much more to research before we see this in every day use. If ever..

Regards
Richard

[ancient kayaker]

True. I didn't expect the boat to sink to the bottom of course, just enough to compress the bubbles, just as you describe in fact. Bearing in mind the power needed to pressurize the air, much more for a gas than for a liquid, and the complexity and weight of the delivery system required to define the bubbles and get them to the right place, it is probably the sort of thing that would only benefit larger vessels such as tankers.

There was a TV program a few months back that looked at highspeed projectiles entering water; by designing so the projectile traps and carries with it a bubble of air, it was found to travel a much greater distance and be less likely to break up on entry.