Planing boat and Bottom surface loadings

I would disagree with this explanation. Froude number is just a parameter showing possibility to plane; it is using full weight of craft wherever it is supported by lifting surfaces such as bottom or foils.

Otherwise, with 99% of weight of craft supported by foils what would be planing speed?

So, cat with foils will start planing at lower speeds because its static load factor is lower (i.e. weight per unit of lifting surface is lower); i.e. lifting surface is bigger and will generate more lift at lower speeds thus enough to lit it up. The same will happen of we just increase area of bottom of craft.

 

I agree, Froude number indicates possibility to plane. Whether planing occurs or not depends on the hull shape, running trim etc. One can for example run a round blige hull well into planing Froude numbers without any planing lift taking place.

If you have a typical planing hull, it will plane if it is operating the right Froude number and trim. if it was possible to carry 99% of the vessel weight at a displacement Froude number of the boat, the small portion of the hull remaining in the water would be producing planing lift for the last 1% - somewhat like a scaled model does.

Attached is graph of typical Hysucat resistance in relation to the same hull without foils and also relative to typical trends for different types of craft.

 

How can happen that resistance of Hysucat is lower than that of hydrofoil craft? And second, in my understanding the resistance of Hysucat should start growing again from certain speed; now it is only decreasing after hump.

 

In general the best efficiency is obtained at an attack angle of about 5 degrees.
The aspect ratio also figures in but to a smaller degree.
As an example if you need 100 lb./sq.ft. lift and you have a deadrise angle of 12.5 and an A/R of 0.2 you would get your best efficiency at a speed of 66 ft./sec.
The most important part is the angle of attack for max. efficiency (a little bit better than 5:1 Lift/Drag at the most).
The heavier your boat / sq.ft. of planing area the faster you have to go to get that 5 degree attack angle, which you could call your cruising speed.

Colin
http://www.ww.xbug.ca

 

Good questions. Hydrofoil craft have many vertical struts in the water which add resistance. At really high speed, with the correct hull, a HYSUCAT can be designed to have only a very small hull wetted area making them very efficient. I however do haveto admit that for most applications the resistance curve is not quite that good. It is normally flat after the displacement hump and increases slightly once past the design speed. This is due to the foil operating too close to the watersurface where it loses efficiency. Oten also associated with instability in reality.

 

I dont agree with this. Optimal angle of attack for the foil profile in terms of lift to drag is usually about -1 deg. Considering the foil and hull together, the optimal angle of attack is dependent on the setup as the boat changes trim continiously with speed...

 

It should be close to quadratic of speed if area of appendages is same. So it should increase, not decrease...

The question is how You defined FnV for this graph.

 

Typical is too have flat resistance curve - not downward sloping. As speed increases more of the boat weight is carried o nthe foils and less on the hull. Keeping in mind that a planing hull L/D = 4-5 and hyddrofoil L/D = 20 it is easily possible to have a flat resistance curves because lift of the hydrofoil is proportional to the square of the speed. Also note that the foils sit between the hulls so there is an end-plate effect which increases their efficiency compared to standard hydrofoils.

If you combine Savitsky's method with typical hydrofoil theory (e.g. Egorov & Sokolov method - I am sure you are familiar with this book) you will see the basic tendencies.

Decreasing resistance curve is usually only found for heavily loaded hulls that have high hump resistance. Conversely if you put hydrofoils on an extremely light hull then it is more normal to have an up ward sloping resistance curve.

Fv = standard volumetric dispalcement ratio based on displacement of craft (not hull only).

 

We learn many of these spots. Congratulations

 

Don't go chop up helicopter rotors just yet

 

How can area divided by displacement be dimensionless?

 

It has been already addressed and corrected in the post #13: http://www.boatdesign.net/forums/bo...bottom-surface-loadings-42887.html#post550681

 

Well, lets put it this way.

When you mean block coefficient, do you say

The volume divided by the length times the breadth times the beam....or, just simply say the Cb?

The WPA ratio is the waterplane area divided by displacement to the power of two thirds, thus dimensionless.

Abbreviations are used for a reason.

Do you say length displacement ratio or do you say length divided by the displacement to the power of one third?...i know which I use.

If the recipient is confused, its because they are unaware of its derivation and its abbreviation for brevity. Doesn't make it any less valid though...