80 MPH 21 hydro

[Tom v.d Brink]

the force it takes to accelerate that 1.5kg to 35m/s depends on how quick you want to get to that speed.

but if the 1.5kg also increases with the aero resistance and water drag you will need to do this step by step with increasing mass.

in the case i gave you earlier if the boat were to weigh 1.5kg but the 0.5hp gives you 10.5N of resistance the boat would now weigh 106.5Kg that is one heavy boat to accelerate i would say........

so reducing the air resistance to say 60% would give you only 63kg to accelerate.

it doesn't take force to maintain a speed that was reached.

well almost true if there would be no resistance

the force needed to maintain a speed would be equal to the forces tryin to reduce that speed (air resistance, water resistance, mechanical drag etc)

 

[lohring]

Mike Bontoft is not mentioning the extensive testing we have done with both aerodynamic and hydrodynamic effects on gas riggers and gas sport hydros. From all this I conclude so far that aerodynamic lift is very important for stability, but aerodynamic drag doesn't matter much at 100 mph. Hydrodynamic drag is everything. Rudder and sponson riding surface changes have together added 10 mph to our riggers. Streamlining the sponson tubes didn't make a measurable difference in speed and cost Mike a lunch bet with me. Changes in the lift on a gas sport hydro allowed the boat to run 20 mph faster even though the change increased the drag.

At the same speed the water drag on an object will be about 840 times the air drag. This is true for lift as well, so water rudders are much more effective. The forces in air and water increase as the square of the speed so eventually the air drag becomes important. At our speeds, keeping water from touching surfaces gives the biggest gains.

Lohring Miller

 

[Tom v.d Brink]

yes lohring you are right about the improvement on hydro surfaces first.

the effect is indeed a lot worse with water.

hey guys,

how about a few more ideas on cornering as we need to maintain this target 80mph round the course to.....

 

[Brad Christy]

Lohring,

You're talking about an 8-10 lb GAS rigger. We're talking about a 3.5 lb 21 rigger. This really needs to be taken into consideration when balancing the effect of hydrodynamic vs. aerodynamic forces. If you watch a well designed 21 hydro at speed on calm water, you can see that a good portion of the time, the only thing on the boat with any considerable hydrodynamic interaction is the turn fin, rudder and prop (and the prop is obviously working for you, not against you).

If I'm reading this 1/2 HP thing correctly, we are consuming this power just overcoming the drag induced by the boom tubes. This is a HUGE amount of power loss. With the overall power we have at our disposal, losing 20% of it is like not running any nitro.

As for the drive-line drag, consider this: Our engines are singing along at nearly 30K RPM. At thesespeeds, with 80mm of pitch, a 1650 is propelled along at well over 90 MPH. Where did the 20 MPH go? I'd accept a 10% loss (at most), but not almost 20%. Not in a rigger. If it were driveline drag, the boat speed would be more reflective of the engine speed and prop pitch math.

Thanks. Brad.

Titan Racing Components

 

[lohring]

Brad

I think you are still missing the point. Our ruggers may weiwh 8 pounds, but we have added air traps so the boat will blow off. The only things in the water are the prop and rudder. The speed did not increase. This was also true with the sport hydro. There we decreased the lift so the boat wouldn't fly. It runs with only one sponson in the water.

At 100 mph a .36 inch diameter tube 1 foot long has around 775 pounds of drag in water and a little less than one pound of drag in air. If you streamline the tube you reduce the drag to around 390 pounds in water and less than 1/2 pound in air. In both cases the drag was reduced 50% by streamlining, but the actual difference only matters with water.

Lohring Miller

 

[Ian Inverarity]

Tom,

On the cornering speed issue, I came up with an idea to try a few years ago but haven't had the opportunity to try it. The turn fin of a rigger is located close to the CG and at racing speeds provides most of lateral force to allow the boat to turn. The rudder is actually providing a force at the back pushing out of the corner!. But the turn fin needs to run at an angle of attack relative to the path of the boat to provide the lateral force. Because the turn fin is rigidly attached to the sponsons, this means the whole boat runs with a yaw angle relative to its path in a corner. Thus all the riding surfaces are running slighlty sideways thru the water and this creates more drag.

WIth modern larger turn fin designs the angle of attack needed to provide the required lateral force is smaller, thus the yaw on the hull is smaller, thus the boats are faster in the corners.

The idea I had was to use a turn fin that was made from a rudder, and turn it the opposite direction to the rudder in the turn. By getting the relative angles right I think it should be possible to get the lateral force from the turn fin with the hull at practically no yaw, thus faster. Also the rudder style turn fin could probably be made smaller and still generate sufficient lateral force. This could provide advantages by reducing drag from the turn fin on the straights.

The turn fin also provides a force drawing it down in the water to resist the rolling moment of the boat. This could be accomplished by the angle of the turn fin pivot.

Ian.

 

[Ian Inverarity]

How about slowing it down from 30,000rpm to 20,000rpm. I have a geared A mono with a 1.5-1 ratio and its very competitive even with the friction loss thru the gears. The guys in europe pretty much all run gear drive. It would be very interesting to see how a geared CMB/MAC 21 would do in a rigger. So many neat things to try, only so much time :angry:

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Mike,

I have seen two gear drive 21 hydros, while they worked okay I didnt think they had an advantage over the conventional boats. Probably the weight difference is a bigger issue with riggers.

Ian.

 

[Ian Inverarity]

Lohring,
As for the drive-line drag, consider this: Our engines are singing along at nearly 30K RPM. At thesespeeds, with 80mm of pitch, a 1650 is propelled along at well over 90 MPH. Where did the 20 MPH go? I'd accept a 10% loss (at most), but not almost 20%. Not in a rigger. If it were driveline drag, the boat speed would be more reflective of the engine speed and prop pitch math.

Thanks. Brad.

Titan Racing Components

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Brad,

If the engine is running at 30k rpm and drive line losses are 10%, this doesnt mean the prop runs 10% slower. The prop still runs at 30k rpm. The torque available to the prop is reduced by 10%. This does not affect the advance of the prop at 30k rpm, but affects what prop can be pulled at 30k.

Ian.

 

[Tom v.d Brink]

well for those that are still interested in boom drag

i just finished some cfd testing on just boom shapes and their Cd coefficiant.

anyway

brad i started out with the boom you use at 35m/s (about 80mph)

round 10.03mm Cd of about .610

i heard some one want to know what a smaller boom does so

round 8.5mm Cd of 0.591 so there is not to much in that one

so okay

brad you wanted a fairing behind that current boom with the end at .625 behind the center line.

i thought this would be a bit short but anyway it came out to .384

so i made the fairing behind it a bit longer and came down to .295

then a bit longer still but then the difference went away and didn't go down much further.

so finally i tested a naca16 neutral profile at the height of the 8.5mm round tube.

Cd came out at 0.201

so where does this put us?

well for comparisens some Cd values

a range rover or old volvo station car: 0.5 - 0.6

a truck : 0.8 - 1.5

a bus: 0.6- 0.7

old style car ('80): 0.4 - 0.55

current cars: 0.25- 0.35

dropplet shape car: 0.15 - 0.2

 

[Tom v.d Brink]

oops one little flaw in that post:

round 8.5mm Cd of 0.591 so there is not to much in that one

not correct as the surface area is also reduced

so the gain will be slightly larger.

over a constant length the gain would be about 12% more due to the

reduced surface area

 

[Brad Christy]

Tom,

I figured that a longer taper would yield better results, but I wanted to keep the influence on overall ride attitude to a minimum, so I kept it short. How much longer did you make it before you quit getting better results?

I also realize that going to a smaller diameter will reduce drag, but it's still a round profile. The bernoulli effect is still at work, creating an enormous area of very low pressure behind the profile and this is translated into drag.

Thanks. Brad.

Titan Racing Components

 

[Tom v.d Brink]

How much longer did you make it before you quit getting better results?

i about doubled the length to 30mm from the center line

 

[Brad Christy]

Tom,

Do some figuring and tell us how much angle it would take for your taper and my taper to create enough lift to blow a 4 lb boat off the water at 80 MPH.

Thanks. Brad.

Titan Racing Components

 

[MikeP]

Maybe the addition of airfoils to the boom tubes could be done so they self center in the air flow, using heatshrink or something that would let them rotate on the shaft a little.

Would that keep them flat in the air flow when the attitude of the hull changes?

MikeP

 

[Grimracer]

Mike

Not sure but they would likely flutter.. flutter is real bad..

Grim

 

[Brian MAhoney]

Is 70 MPH enough that you could air cool the engine rather than water cool?

If so loose the water cooling and use a much thinner rudder and you should go quite a bit faster.

Brian Mahoney

 

[Don Templeton]

Brad,
I know that one of the main design features of outriggers is relatively low aero lift compared to conventional hydro designs, but from my (limited!) understanding of aerodynamics, aero lift from the front sponsons in particular would be hard to avoid.  As the sponsons move over the water the air in front of them has to get out of the way, and the shape of sponsons will try to push some of that air down- wards. Since the sponson tip is touching the water, or very close to it, little of the air can flow under the sponson, so will have to flow out sideways from underneath the ride surface. This will create a higher pressure area under the front sponsons.

If the boat is very light, and set to run very light, then at high speed controlling the lift would be quite important!  :blink:

Ian.

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Ian,

I would think that enough dihedral built into the sponsons will take care of a great deal of the lift.

Don

 

[Brad Christy]

Brian,

I don't think you could reliably air cool the engine with the load it would take to push a boat 80+ MPH. And if you did manage to get enough heat sink into the air, You've more than made up for any aerodynamic drag you've eliminated with the foils on the booms. I think the rudder pick-up is the best choice so far. We can keep making them shallower until they quit working, and thinner is better as long as you don't break into the vertical water passage. Once we've maximized this, that's about it.

Mike P,

Our current concept will allow for adjustment in foil angle without any tools, but it will not allow any movement otherwise. Dave Roach and I are hoping to still get out to the pond yet this year before we oil everything down for the winter.

Tom vd Brink,

Here's something else to consider: The foil on the rear boom could possibly be longer than the front, due to it's position relative to the CG. The closer to the CG, the less leverage it will have to influence the attitude of the boat. So we could make the front boom-foil like what I described and the rears like yours.

While you're at you calculations, see what angle it would take to make a tub (3 1/2" wide, 30" long, assume it's flat on the bottom) blow off at 80 MPH. Compare that to the angle for the same result with the foils. If the tub will blow off first, the foils should be a viable concept.

Thanks. Brad.

Titan Racing Components

 

[Tom v.d Brink]

just wondered how you wanted to qualify the wings beeing able to blow over a 4lbs boat.

they have a nice arm working on the CG indeed but it depends on the CG location to the booms.

anyway if you asume a boat to be balanced front to back any force going up added would potentially get the boat up in the air.

but it is hard to account for the effect of lost lift from the water added lift from the sponson surface etc.

i did a full boat angle test on the cfd and lift values remain a bit inconclusive.

what i did work out is that a neutral profile has no correcting power at all and only increases lift at increased angles.

i did the small profile last weekend but got some funny values from the program so i am going to have to check if i made the mistake with the input or there is something else happening.

first hand result would be that if there are two profiles about 8 3/4" wide (only booms no tub anymore i took that of)

you would have about 4lbs between 1 and 2 degrees (closer to 2deg)

quick huh?

I'll try get in the larger profile this weekend and do smaller steps between 1 and 3deg to see if i can get more exact values.

 

[Brad Christy]

Tom,

That angle seems a bit low, but engineering number crunching is not my thing, so I'll go with it for now. How about the lift of the tub alone? Can you give me some numbers there.

All I'm looking for are raw numbers. Take as many variables out as possible.

Thanks. Brad.

Titan Racing Components