Porposing Four Winns

JackD - your class C racing runabout is a hull design that benefits from Lift (and drag) from hydrodynamic planing forces, motor lower unit & propeller forces (lift, drag and thrust) and aerodynamic forces (lift and drag). The balance and location of these forces is ever-changing throughout the operating velocity range. To change the onset of porpoising, the dynamic CofG (acting forces) needs to be changed so that the full curve of porpoising onset (versus velocity) is changed. The design objective is to change the design and dynamic balance of the operating hull such that operating trim curve is always below the 'porpoising onset' curve. There are both design and setup alterations that can help achieve this.

 

The boat I am speaking of is about 3.4 meters long but has a relatively wide transom,about .8 meters. The bottom is dead flat for 1.25meters along the keel forward of the transom. It has a double chine. The chine coming off the bottom is a shallow angle to prevent tripping as the boat "skids" around high speed turns. The keel curves upward toward the bow with a shallow
"V" bottom, increasing in dead rise starting at the flat. There are no strakes any where on the bottom. This boat reaches 60 mph (96kph) with only 33 hp. To remain competitive,I would like to not degrade it's performance.
It seems to me that for porpoising to happen,a boat must derive almost all of it's lift from planning (little buoancy) As such, if the trim angle were increased to lower the onset velocity to a point where sufficient buoancy is encountered it would be damped out.
I am considering reducing the width to about 1.23 meters by reworking the bottom. I believe this will increase the trim but I am hoping that it will lower the onset speed of porpoising to where it encounters damping from the shallow angle chines.
Does this make any sense?

 

You say your transom is relatively wide at .8m but then you say you are intending to reduce beam to 1.23m. I do not understand this.

There is a Savitsky calculator set up here:
http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html
It allows you to set up your boat parameters and it will estimate the power, trim and potential to porpoise based on Savitsky test data on planing hulls.

I have attached the porpoising chart with parameters that might be close to your hull although I am confused by the data you give. You will see that at trim angles lower than 3 degrees this hull is close to porpoising but when trim is higher, the margin increase a bit.

You must have a light hull if it can get to 60mph with 33HP. There may be some aerodynamic lift as well at that speed so Savitsky could yield a pessimistic result.

Rick W

 

I gave the reason poising occurs at post #2 here:
http://www.boatdesign.net/forums/powerboats/porpoising-27337.html

Rick W

 

I'm quite certain, that a flat bottom is not the fastest setup for this kind of boat. Is it an outboard? Here in Scandinavia we have a racing class T400, which uses (almost) standard 25 hp outboards and reach a top speed of 54 knots (99.3 km/h) despite being rather heavy (240 kg with driver). Here is one example: http://www.winrace.no/index_eng.php?side=hoved_eng

Does the rules limit the hull form (not in T400, except for minimum size)?

The problem of porpoising does not come from buoyancy issues, since porpoising only exists at planning region, in which buoyancy forces are very small compared to dynamic lift. When the trim angle increases the angle of attack between water and the lifting surface (hull) increases. This increases the lift coefficient and thus the same amount of lift can be achieved with less wetted area. But at the same the the lift force moves back, since the area is reduced from forward. As the trim angle increases the lift force creates an increasing moment, which pushes the bow down. Without porpoising there will be a balance, but at the porpoising region this balance is "too hard to find".

Porpoising is very hard to calculate from physics. The momentum balance is easily calculated from the Savitsky method, but porpoising is only derived from experimental data by comparing the calculated trim to curves defining the stable region. At least I haven't seen any accurate model for porpoising.

 

I am a mechanical engineer but not in the boating business. My interest in boats and racing is purely a hobby. A lot of this stuff is new to me and I am very appreciative of the help and advice you guys have given me. I have been struggling with trying to calculate the porpoising function and I think I have a reasonably good method for finding the resonant frequency based upon the physical parameters of the boat when planning but I have a problem with the speed related forcing function. I was "glad" to hear that this is also very hard for every one else.
Rick, I appologize for the typo relative to the width. The new transom will be .62 meters(not 1.23 tri) In the next week I will try to apply the material you have provided to on my problem. I will be gone on a business trip for a week so it may take a while. I will post as soon as I have something.
Joakim, to answer some questions both you and Rick have, the boat,engine and driver weigh 216 Kg.(minimum by class rules) The outboard engine is a stock Yamoto engine used for professional boat racing in Japan. The class rules require the aft bottom to be dead flat.but I will look at the Web site you have provided (thanks) I have estimated the lift derived from Wig airlift and find it to be negligible at porpoising onset,and still small at top speed.

 

JackD, I think aerodynamic lift is the reason for your boat NOT porpoising at higher speeds. According to my Savitsky program it should porpoise at all positive trim angles at top speed. Aerodynamic lift may not be important for carrying the weight of the boat, but it may cause a moment big enough to stabilize porpoising. Do you have any idea of the actual trim angle at top speed?

What do the class rules actually say about the bottom? If you can make it as narrow as you like, you could actually make a V bottom with a flat "ski". This type of bottom has also been succesfully used.

Here is a video of a Finnish boat showing the influence of aerodynamic lift. http://www.youtube.com/watch?v=efIj7UvFIRE

Here is anotherone with some porpoising as well: http://www.youtube.com/watch?v=b8qzQleHinI

It's not a racing boat although they have races for this boat type. They can reach well over 40 knots with a standard 40 hp outboard.

The original Savitsky method is not very good for racing boats due to many limitations. It does not take into account whisker spray drag, aerodynamic drag and the drag of the drive, all of which are very important for racing boats. Also it assumes that the chine allways touches water, which is far from true for light racing boats with V bottom. I have included all these to my Savitsky program, but not the aerodynamic effect on porpoising or lift. I have no idea how to implement that, but I'm quite convinced, that it is an important effect for racing boats.

 

Added whisker spray drag

Joakim
here comes a report on whisker spray drag to add to your savitsky program


hjs
www.sassdesign.net[/url]

 

Thank You, but I have already done that based on that paper about two years ago. What I don't have is the aerodynamic lift including moment and it's effect on porpoising.

The porpoising curves of http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html seem to be incorrect. The beta=0 curve is close to the real beta=20 curve from the Savitsky 1964 paper.

This paper suggests even lower angles, but only beta range 15-25 was studied: http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADB240994&Location=U2&doc=GetTRDoc.pdf

 

You both have beta=20, which should be 0 for flat bottom. Also note that the porpoising limits are incorrect.

 

The loaded deadrise was zero.. I noticed that it appeared that Rick and I both loaded a deadrise of 20 degrees, but that is not the case. If you run the program the output mirror is showing a deadrise of 20 degrees if you load in zero. I used betas of 1, 5 and 10 degrees and increasing deadrise showed an decrease in stability as one would expect. If you put in a zero deadrise it shows as 20 degrees, but if you put in 1 degree is shows 1 degree.

Edit..

Oddly enough, if you put in small deadrise values the program seems to work correctly but you can't put in a deadrise of zero and get anything but 20 degrees in the output, so Jokim is correct in that it did appear to load a deadrise of 20 degrees if you put in zero. I have rerun the analysis for a .1 degree deadrise and have edited the previous post. I guess a word of caution is appropriate for using that web site since the curves appear to be offset and inputs of zero deadrise come out at 20 degrees.

 

No, I would not expect that. Flat bottom is most prone to porpoising and deep V is least prone. That just shows, that the porpoising curves are incorrect. Take a look at the 1964 Savitsky paper: http://www.boatdesign.net/forums/boat-design/anyone-have-copy-1964-1976-savitsky-papers-10970.html

At 45 kn the predicted trim angle is about 1 degrees. The sqrt(Cl/2) is then about 0.08. These are OK, but according to Savitsky paper the limit of porpoising is already at sqrt(Cl/2) of 0.13 at about 1 degrees and still decreasing. The curve fitting I use yields -0.35 degrees for 0.08.

The www calculator claims, that porpoising limit would be 2 degrees at 0.08 and 3.5 degrees at 0.13.

According to measurements in this: http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADB240994&Location=U2&doc=GetTRDoc.pdf the beta=20 model porpoised already at 0.16 with trim angle of 3.5 dergrees, but the www calculator claims limit of over 6 degrees. Savitsky 1964 gives 4.2.

So the limits seem to be 2-3 degrees too high at this region.

 

Yes. I use 1 degree for flat surface as I have found strange things happen when it is set to zero.

I have never bothered to work through the Savitsky formulas so do not know if it is a formula problem or the MARIN version of Savitsky's work.

There are some good results being achieved for CFD code on planing hulls. Maybe there will be better models available in the future.

I expect any 200kg boat doing 60mph will have aerodynamic forces of significance. Savitsky should give a guide to what is happening but the whole picture, including aerodynamics, is going to be more complex.

Rick W

 

You can work all this out if you like but what are you going to do when you find a shallow V might help,--Change it? perhaps a longer boat?

Only thing you can do to THIS boat is change -re adjust weight, trim tabs, move fuel tanks, batteries, etc.