Drag characteristics of RIB

I am looking for hydro drag characteristics of a 4-5m RIB.
Ideally measured data, up to 20kn, multiple payloads.
But wild guesses or simulations would also help. ;-)

 

What is your ultimate aim here Ursus?
As in what are you trying to achieve?

It might be easier to find / borrow / rent a 4 metre RIB, and do some practical experiments with it with different payloads.
You could measure the speed with a GPS, you should be able to calculate the delivered power, and hence you should be able to measure resistance (if this is what you are after?)

 

It is not exactly what you asked for but maybe it can help you. These are the power / speed curves for a 5 m RIB, at different drafts / displacements.

 

Getting measured drag for 4-5m RIB is not easy. It means towing of full size boat, likely on open water. One needs to model the outbord engine thrust vector while towing, otherwise the results would be useless.
On other side, it can done by reverse calculatiosn of propeller thrust and speed/RPM measurements, but also not accurate - exact properties of outboard props are unknown.
Thus better to rely on calcs by Savitsky or Blount-Fox methods.

 

If performing a standard model test, im not sure what is not easy in this sense. The Fn is not high, it is only 1.4.
So I wouldn't say it is not easy - it is just not so easy relating to the actual SHP needed I would suggest, since as you note outboards are not known for their exact specs!

However, the difficulty is more related to the LCG and getting this 'optimal'.

At the same Fn as this 4-5m RIB you can results with an "optimal" LCG for the Fn as :


But when compared to a less than optimal LCG at the same Fn:



Small margins lead to major discrepancies in small boats.

 

Excellent stuff, thank you so much!
So planing starts around 12.5kn; isn't that a bit high for a 5m hull?
Would you happen to have (or run) the same for a 3.5m RIB and down to 5kn?

 

Please note that the graphs show calculated results; how to validate them is another story. Some outboards with hydraulic trim can be used for thrust measurements. A pressure tap in the hydraulic circuit can be connected to a manometer, and the setup calibrated for horizontal force. BUT, these measurements will include the OB rig drag, so there must be a calibration of rig drag, fe by towing.

Hydraulic pressure is more easily avialable in I/O applications, though.

 

No, the 12.5 knot figure doesn't mean anything. It is only the point where the program has positioned the cursor. The study was made for speeds from 10 knots, which does not mean that the ship cannot start planing before that speed.

Sorry, the only calculations I have are for that RIB.

 

I think for 4-5m RIB the tank tests will cost more than building and testing the prototype boat
However, tank tests are not too accurate for small fast craft, You absolutely rigth. Say the propulsor is changing pressure field at stern, then it changes the trim, and then the resistance is wrong
Running straktes, etc. which are plenty on the RIB are not properly modelled at tank test, too.
So yes and now. When we design small fast boats we rely mostly on calculations 'anchored' to real-time performance data of our previous designs. And do CFD for strakes and steps, too. We do tank tests for some projects, but mainly if this is tender requirement.

 

Indeed!!!

 

We've used this thrust measurement plate on OB powered boats as small as 5m with good results. We calibrate the load cell (not mounted in the picture I attached) using a tension gauge and cable pulling on the lower unit at the prop shaft. The drawback to this approach is the 63mm hole that must be put through the transom to accommodate the load cell tube.

 

Is this RIB a fiberglass bottom hull with inflatables on the sides or just a fully inflatable with plywood transom plus plywood bulkheads?

If the first, would a standard Savitsky or Mercier/Savitsky method suffice to calculate hull drag/power?

 

Agree, assuming a prototype boat will be built. For a coarse estimate Savitsky could be useful. A coarse estimate of performance could also be made looking at several outboard manufacturers websites which have tests of their engines on a variety of boats. I would check any analytical estimates of resistance against the best data available for similar boats.

Another challenge with small boats is the wide range in payload weight and location for many boats.

In use in the real world the payload (people and gear) of a 4.5m RIB can vary greatly, perhaps from around 50 kg / 110 lbs to perhaps as much as 900 kg / 2000 lbs. (Based on a quick look at several websites including HIGHFIELD BOATS - Aluminium Rigid Inflatable Boats https://www.highfieldboats.com/ Falcon 420 - 13ft Premium RIB Inflatable Yacht Tender | BRIG USA https://brigusa.com/falcon420/ ) Also RIB occupants sit in different locations depending on numerous factors so the CG can vary greatly.

RIB originally meant Rigid Inflatable Boat with a rigid bottom and inflatable side tubes. But recently I've seen a variety of boats with inflatable side tubes, some with a soft floor, referred to as RIBs.

 

Our 3.2m RIB's drag characteristics change enormously depending on crew position, as others have noted. Any accurate drag prediction would have to allow for varying the C of G as speed changed. It starts planing at about 9.8 knots, and then the speed increase to 15+ is so fast that it's difficult to keep it planing but under the 10 knot speed limit. The enormous variation in speed that a slight increase or decrease in throttle causes around that speed show how hard it would be to be accurate in calculating speed and drag.

 

That behaviour is typical for a "short-and-fat" hull. It has a high hump resistance, and once it is overcome, there is an increasing thrust margin with increasing speed, until it reaches the top limit. This makes for unstable (and possibly dangerous) behaviour in the "cruising speed range"; you can not select speed to suit the wave situation.