Streamlining sponson tubes

Here is a testbed I made for testing drag on sponson tube. I connect a leaf blower that has a velocity rating of about 100 mph. I checked the speed of the airstream with the pressure gauge from my flow meter ,100 mph equals 4.92 in water. Here are the results

Round tube= 140g

Streamlined tube=20g

Round tube with streamlined trailing surface=40g

Streamlined trailing egde only

very interesting! how about checking this...... the section where the tube meets the tub or sponson. have it straight in and also with a fillet.... rounded corners at the tub/tub joint.

Are there any concerns about aerodynamic lift using the trailing edge stock?

Hi Mike

I have often wondered why nobody seems to bother with this easy area to improve. Have you thought about trimming the trailing edge stock so it is an inverted airfoil. This should still provide the streamlining but with added downforce.

I have been waiting for somebody to do this. It is a fact that an airfoil shape is 10x less drag than the same size round shape. I just never did it on a boat because it would be more things to adjust.

What would be nice is an airfoil sleeve that slides over the existing tube. Any light weight material would do. I had thought of using the CNC to make a mold for the top/bottom of such a sleeve and vacuum some carbon mat. Just need to come up with a simple way of measuring and adjusting the angles. The airplanes use an incidence meter so there is half the battle.

A gentleman by the name of Steve Powers who used to write the Power Boating column for RCM in the early 80's. He has a design called Merlin and he used an inverted airfoil section for boom support. So this has been done before.

I have tried/borrowed his idea by using broken heli rotor blades for boom support in my scratch designs and it has worked very well for me.

Regards,

Kez

Interesting setup. I have talked with Tom v.d. Brink about this subject before. He has done some CFD calculations about the sponsons tube, so i think he can add some thoughts about it.

I will take some photos of how I make the airfoils. I slide them over the round tube and stop them from moving with two small wood screws. I thought about using heli blades but then you have a problem fastening and adjusting sponsons and tub plus they are way expensive . I am not sure how much these airfoils will help but I will have them on both boats for the Legge Lake event at the end of this month.

Preston_Hall said:

What would be nice is an airfoil sleeve that slides over the existing tube. Any light weight material would do. I had thought of using the CNC to make a mold for the top/bottom of such a sleeve and vacuum some carbon mat. Just need to come up with a simple way of measuring and adjusting the angles. The airplanes use an incidence meter so there is half the battle.

Click to expand...

Preston,

What if you just let the airfoil float on the sponson tube? Would you get the aero benefit without the lift problems?

yes i have done some cfd testing with this on a whole boat and found a 23% drag reduction on a whole boat by placing the right foils over the boat.

a point of causion is that if you place the foils on the boat you will create additional lift (with some foils this can give you ten fold of the orignal lift value of the boat)

even a naca elipse where the length is four times the height (sort of what you tried i think micheal?) it will give you about three times the lift value.

so think wisely before you place just any profile on your boat. they are allready near the mark of aeroplanes but with that sort of lift i don't think you will be able to classify it as a HYDROplane

Tom,

Given your testing results - do you use foils on your boom tubes at all?

All,

My concern with a foil shape would be if the boat hit a wake or bounced a sponson would the change to the foil angle relative to horizontal cause the boat to generate more lift than normal and increase the chances of a blowover. It's probably not an issue for SAW but not so good for heat racing.

Tim.

What if you just let the airfoil float on the sponson tube? Would you get the aero benefit without the lift problems?

Click to expand...

I think what you would get would be 'flutter', followed by a sudden lack of sponson tubes.

<_< i think i dont matter too much what you use because the small amount of tube that is used would not intrefere with the boat at all for instance most boats that run 100 mph + the dont use anything special realy its mind over matter ?

Mike B,

Can you adapt your testbed to get some ideas on lift comparisons on the three different shapes that you tested for drag? Rotate the pivot and scales 90 degrees?

Tim.

Propjockey said:

What if you just let the airfoil float on the sponson tube? Would you get the aero benefit without the lift problems?

Click to expand...

I think what you would get would be 'flutter', followed by a sudden lack of sponson tubes.

Click to expand...

I agree about the flutter. If you have ever flown planes and lost a pushrod then you know.

If you used a symmetrical foil I wouldn't think you would gain lift if set properly. What you may experience is a reduction in the current drag that would allow the current lifting ability of the hull/sponson design to be overly effective. If that is the case, some of the lifting ability could be removed from the hull/sponson design and still benefit from the reduced drag.

Lift isn't really a problem if you can control it. I noticed that the Eagles and Crapshooters don't seem to get loose enogh to kite off the water. My CS seems to fly nice and level when it comes up. Jumping up off the water in a vertical movement and pointing towards the moon when light are two diffrent things.

Great topic guys.

Here is the real skinny on the subject. While anything you do to streamline your hull and appendages will lead to less drag on the boat, the Reynolds numbers on a airfoil around a 1/2" tube are so small, that calculating them to less than 2 decimal places is almost impossible with any equipment available to the average modeler.

Point is that the lift or negative lift (downforce) they create is miniscule.

Form drag on the other hand is certainly reduced. Again though, this number is so small due to the close proximity of the foil to the surface of the water, and the effect the uneven surface has on the foil.

A tube with a cross section profile of 1/2" is only slightly more aerodynamic than a square bar. The laminar flow on a tube is horrible. This is the whole point of streamlining a tube on an aircraft. The drag created around the tube can lead to significant gains in airspeed. You have to remember that we are also talking about tubes that are several inches in diameter, and in airflow that is generally well over a 100-MPH.

On my full sized aircraft, adding wheel pants over the wheels give a 3-5MPH gain over having them hang out in mid air. The plane cruises at 130 MPH and the tires have several square feet of surface area.

As you can see, the numbers are not huge even in this scale.

Long and the short of it is that one small change in prop dynamics will lead to a huge increase or decrease in speed. Making a foil or filet around your sponson booms will only change the speed by a few hundredths of a mile per hour.

Look way bigger. Change the tub and sponson shape and now you will be playing with numbers that are visible.

Peter R.

www.climatemodels.com

Thanks, Peter.. Excellent post!!

Charley

Set aside the theories for a minute and tell me if I am correct. Mike set up a test stand and performed an experiment which yielded a 120g reduction in force. The control area he used seems to be about the length of 1 of the 4 exposed boom tube sections on a boat. 120g X 4 tubes = 480g = 1.06lb

A while back we came up with a number of 17lb to accelerate a boat from 30-100mph. So we are talking about 6.2% of the force required being removed. Wouldn’t this mean that I should be able to go 6.2% faster? 70mph X 62% = 4.34 mph gain, 80mph – 5mph gain, 100mph – 6.2mph gain

If this is somewhat correct then it would be well worth the effort.

wel first of all no i haven't actually put them on a boat yet just simulated them on a model. i am going to though because i see there is a gain even if it is small.

I have done some more calculating with the data i have allready and found the following

all data @45m/s or roughly 100mph

all the drag data seemed not to vary to much from different profiles as the maximum difference in drag of the profiles was 6.2%

the best profile I found reduced boat drag by 22.7% (purely aerodynamic drag)

if you go back to the weight having to be pushed forward (so you add the boat weight to the standard boat drag and to the improved boat drag) the difference is 6.4% in weight difference.

worsed profile was a 5% gain (i'd say this is still pretty good figures)

now on lift i definatly disagree what has been said and wrong profiles can definatly produce to much lift even if they are very small.

now a good profile will only give a -156% of the original lift (the profiles i choose)

as the orginal lift on the round boom boat was allready small this only gives a 2.3% difference in boat weight.

two bad profiles I tried had a 1246% reduction in weight or -789% increase in lift.

on the total boat weight this would be 17% reduction and 13% increase.

this i think will definatly be noticed when you are running your boat.

and you can definatly mess up the handling.