80 MPH 21 hydro

[Ian Inverarity]

I would say that aerodynamic stability would be one of the key factors in getting a heat racing boat to consistently run very fast, in my opinion more important than aerodynamic drag reduction. At very high speed very little of the boat is in the water, and the effect of the hulls aerodynamics becomes very important for both lift and stability. As the speed increases the aerodynamic lift should not affect the attitude of the boat, and also the boat should not be pitch sensitive, ie if the boat pitches up a little at the front ideally the aero lift should not increase significantly.

A moving ground plane wind tunnel or some good CFD sofware would be nice!!

Ian.

 

[Terry Keeley]

I would say that aerodynamic stability would be one of the key factors in getting a heat racing boat to consistently run very fast, in my opinion more important than aerodynamic drag reduction. At very high speed very little of the boat is in the water, and the effect of the hulls aerodynamics becomes very important for both lift and stability. As the speed increases the aerodynamic lift should not affect the attitude of the boat, and also the boat should not be pitch sensitive, ie if the boat pitches up a little at the front ideally the aero lift should not increase significantly.
A moving ground plane wind tunnel or some good CFD sofware would be nice!!

Ian.

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What you're saying is the C/L has to be the same as the C/G throughout the speed range...

 

[Brad Christy]

Guys,

I think that most of us are putting "airfoils" on our imaginary 80 MPH 21 hydros that are way too big. There have been several comments about them causing lift when the boat tips up. I'm pretty sure all of us have seen a rigger or two blow off without these airfoils, so I'd say it's safe to say that the tub is already capable of creating far more lift than we want at the wrong attitude. The reason I keep saying I'm not worried about this lift from the airfoils is that the ones in my head are very small.

For example:

The booms on my BlackJack are .395" in diameter. The airfoils I have drawn up taper out at about .625" (5/8") behind the C/L of the tube, for a total length from leading radius of the tube the the trailing edge of the foil of .822" and a taper angle of about 18.5 degrees per side. With this profile, I'm thinking that by the time these foils would take over and cause a blow-over, the tub would have already seen to it. While foils of this profile would not be as efficient as ones that were, say, 1 1/2" long, but the effect on ride attitude would be minimal.

Also, we are "building" a heat racing boat, not building a SAW boat. Anything that leads to better top speed, but is undesirable during heat race conditions is of no use to us. It is safe to say we need a turn fin and rudder that enables quick turns at full speed, and to this end they must be "full sized", whatever size this may be. I have my dimensions, I'm sticking to them (for now). We also need a left front sponson. It has been shown that it is POSSIBLE to keep a boat running that has LOST it, but launching a boat without a left front sponson is going to prove to be VERY difficult and consistency is lot likely. Personally, anything that must be controlled or varied depending on speed is out of the question. I don't even like using my third channel mixture control if I don't have to. A well designed cowl will eliminate MOST of the aerodynamic drag over the engine and pipe, and most likely only add to the overall downforce of the hull.

Again, I will say that lift if a problem, not an advantage. I think if your hull has lift, it is only leading to less stability, which, in the end, is leading to poor performance. The whole point behind running an outrigger instead of a conventional hydroplane (other than reduced weight) is a dramatic reduction in lift. An outrigger is "non-air-trap" by design, not default.

Thanks. Brad.

Titan Racing Components

 

[DJT]

Brad

You said that the square transom is not a major cause of drag but you streamline your sponson tubes, seems contradictory to me, according to your thinking a square trailing edge on the tubes would not cause drag either.

 

[Joe_Knesek]

Joe,
Boom blocks, collars, adjustability........ ICK!

The round tubing is monumentally easier to work with in terms of incorperating it into the design of the boat. I'm going to concentrate on "add-on" airfoil shaped trailing edges attached to the existing booms. Dave Roach and I are working on some ideas already. News at ten...............  B)

Thanks. Brad.

Titan Racing Components

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How about going to a smaller diameter, solid carbon shaft for the booms?

 

[TimD]

I've been thinking about going to 8mm carbon tubes for the 21 boat. They seem stiff enough.

Tom VDB - during your testing did you test the effect of smaller boom tubes at all? I'm curious to know if the reduction in drag is worth making the change.

Tim.

 

[Terry Keeley]

My choice for a 20 boat would be 1/4" solid CF rods. Been using 5/16" on my 40 and 60 boats, great for the 40, a little light for the 60. Gonna go to 3/8" for the 60 new year.

 

[Mike Bontoft]

Everyone else has had a go on this one so I will throw in my 2 cents. Aero fixes are nice but there are many other things that will have a greater effect on speed. to name just one. We know the flex cable is robbing alot of power at 30,000rpm. My last two riggers have not needed any strut adjustment and my next one will probably have a none adjustable assembly. Most times, once a strut is set on a new rigger it gets left alone, Someone needs to come up with a good "U" joint assembly so a drive shaft can be supported in ball bearings, while there at it, a well designed thrust washer, "OH"and a ball bearing strut please, How about it Brad.

 

[Brad Christy]

Derek,

Square trailing edges on the booms would, indeed, be better than the round they are now. A tapered trailing edge would be even better, as long as they are tilted to a neutral attitude.

Joe,

I had that on my 7.5 Hawk. It was OK, but I didn't like not having a method for positively locking the sponsons to the tubes or the tubes to the boat. The method I use now is very good for meeting my needs. I'm going to try ideas of decreasing drag that don't require a complete redesign first.

Thanks. Brad.

Titan Racing Components

 

[Ian Inverarity]

Just how much power does a flex cable take to run at 30000 rpm? One issue which concerns me about flex cables and surface drive is the wind up of the cable, surely it is acting a bit like a spring and the engine torque varies during each revolution, and the load varies depending on blade immersion?

Mike,

I have been considering the solid shaft and bearings approach to monos as the boats I have built in recent years have has a little down angle on the strut, and I never adjust it after getting it where I want anyway. I think I could get a straight system like FSRV boats, but then would need to go to similar couplings as well.

I already run needle bearing struts and ball thrust washers! B)

But U joints rob power as well.....

Ian.

 

[Mike Bontoft]

Hi Ian

I have had no luck running the small ball thrust bearing, the cages are weak and the balls go flying, maybe your using something better. I have a FSRV boat and it runs a shaft with a ball bearing up top that also acts as a thrust bearing, My boat has a rubber star shaped coupling that is held within a aluminum collar. Very nice!

CMB also makes a pin and groove type drive coupling that is supposed to work well. Prestwich models has them I think.

Mike

 

[Ian Inverarity]

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.

 

[Ian Inverarity]

Mike,

The ball thrust bearings I use for 1/4 shafts have 13mm OD, 7mm ID, grooved washers and brass cages, I generally get about a year out of them before the cage wears out too much. I used to have some 12mm OD, 5mm ID steel caged ball thrust bearings for 3/16 shafts, but the last one I had the steel cage disintegrated after little use at the NAMBA nats. These bearings are not the Octura or Prather ones.

The mono I am building now is a hull I havent run before, so as yet I am not game enough to try solid shaft with no adjustment with it.

Could the CMB ball and pin joint be used as a universal for a rigger shaft?

Ian.

 

[GTR]

Now we're getting into some lateral thinking. How about carbon fibre driveshafts with a small length of flex shaft for a UJ just in front of the strut. You could do likewise for the UJ connection to the engine. Using soild shafts might require a slight rethink of engine placement to flatten the shaft as much as possible.

 

[DJT]

Brad

It would seem to me that if a flat transom was a low drag design, than drafting, such as what goes on in full size car racing, would not provide any speed increase. Do you follow my thinking on this

 

[EatMyShortsRacing]

Now we're getting into some lateral thinking.  How about carbon fibre driveshafts with a small length of flex shaft for a UJ just in front of the strut.  You could do likewise for the UJ connection to the engine.  Using soild shafts might require a slight rethink of engine placement to flatten the shaft as much as possible.

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Gary,

You need to have a look at my 21 hydro drivetrain....

sort of close to that.....

EMS Racing.... We try anything!!!

 

[Anders_Martinelle]

Tim, to reduce the diameter of the tubes you will have less drag.

But if that is a major gain in drag i don`t think.........

If the drag of a 21 boat is 0,5hp at 126km/h i still wonder were the rest of the hp disappers????

No one yeat have thought about that and after over 150 posts :unsure:

2 tubes on a boat is around 50% of the total drag, yes that is alot but it`s only 0,25HP at 126km/h!!

Anders

 

[Tom v.d Brink]

Tim, to reduce the diameter of the tubes you will have less drag.But if that is a major gain in drag i don`t think.........

If the drag of a 21 boat is 0,5hp at 126km/h i still wonder were the rest of the hp disappers????

No one yeat have thought about that and after over 150 posts  :unsure:

2 tubes on a boat is around 50% of the total drag, yes that is alot but it`s only 0,25HP at 126km/h!!

Anders

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anders,

did you try calculate back to what kind of force that is at that speed?

well 0,5hp should be about 367,75Watt or kg.m^2/s^3

at 126km/h or 35m/s

this would result in a force of 10.5Newtons

you want to know where the power goes? try accelerating that mass added to the boat weight and water force to the next speed.....

 

[Mike Bontoft]

Most of the drag comes from the drive train, prop, rudder and turn fin. 80 on the straight is definately do-able, and I am sure in the right conditions a few people on this forum have boats capable of this or coming close to it. A good tight turn around the buoys is going to knock at least 10 mph or more off, are we talking about 80mph average? So what can be done with the prop? 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:

 

[Anders_Martinelle]

Tom, how mutch force does it take to accelerate 1,5kg to 35m/s??

What force does it take to maintain that mass at 35m/s??

Drag at that speed i know but what about the mass?

Is that possible to calculate?

Of cource the first one with acceleration has time involved but???

Have anyone done any tests with different props how mutch thrust they generate??

Anders