Landracing Forum
Tech Information => Aerodynamics => Topic started by: roflhat on May 24, 2020, 11:57:04 AM
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Hi there,
not really land racing at all, but there's no dedicated water record forum...
looking to do some CFD testing on my speedboat which I've modelled up on Rhino. Just closing it now so it's a solid model.
I'm looking to improve the aerodynamics a bit, not something I know much about so open to suggestions.
Added this motorbike windscreen in front of the outboard which added around 0.57 knots.
Boat is 4 meters long, 2 persons onboard, 70hp Yamaha at around 70mph. Got a world record at Coniston 2018 at 70.10mph average, hoping to beat that again
Any help appreciated!
(https://iili.io/JWPCcx.jpg)
(https://iili.io/JW64vp.jpg)
(https://iili.io/JW66yN.png)
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Doing a fabric Tonneau deck cover from the bow & around past the two idiots :wink: to get the air flowing straight through & not down into the boat is where I'd start.
The gap between the engine cover & said two idiots :-D will be a big drag area so I'd be building a tearaway Tonneau (for when you leave unexpectedly) to fill that gap.
Question, can you guys be tandem (bobsled style) with the passenger tucked in behind the driver?
Building a tapered tail for the back of the engine cover will allow the air to close cleanly & also reduce drag.
Looks like fun, water is like concrete at speed. I know because I've hit it at about 100mph faster than you're going. :?
Sid.
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:cheers: I agree with Sid
mimic bobsleds as much as possible
I would also look in to putting a bow nose wing like Dalton did on his liner most likely going to get some lift out of that aero improvement and such.
I will leave you with this: Our friend Pork Pie's Quote
" It doesn't matter how you open the hole (in the air)), it is all about how you close the hole! What Sid is talking about behind the eng.
The other biggie IMO move the air only once ---if at all possible in other word do not let it come back and hit something else.
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Little boats going fast don't need (or want) lift. Why so many high speed record boats kill the driver. They blow over. Besides if you look at the front "crossmember" between the pontoons he has already a structure to make downforce.
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can you play with the bottom of the boat?
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Dalton wasn't lifting he was generating down force but it is still called a wing, it just inverted to an airplane---from looking at his picture he needs to move forward or change something if he moves toward tandem seating he needs to change the angle of attack
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look at the out board small hydroplane race boats that have behind the driver faired to out past the motor
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Not really possible to get both people in line unfortunately, in that photo at speed the copilot is sitting more upright than he was for the actual record run, he got flat in line the top of the boat for that, so it's only really my head in the airflow.
Good idea looking at a tail behind the outboard, I'll see if I can make something up in CAD. Making up a cover for the pilot and copilot I'd imagine would be the big improvement?
What do you think could be done on the underside of the boat, a diffuser at the rear to clean up airflow?
Finished making the CAD model one solid piece now so can start on the CFD, hopefully get some photos to show
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Look at Formula 1 boats, they are the cutting edge of tunnel hull racing. What ever they are doing in the tunnel you should do but if you don't see it there, don't waste your time with it.
Lead skeg length & shape plus trailing skeg length & shape are a huge point of drag. A sharper lead skeg has less drag. The trailing skeg is usually pretty thick on a production leg to handle rocks & $hit so you should sharpen it up & if you have to, add a piece to the front of it to get what you need. You will also want to talk to a prop manufacturer as well if you are not already. Production props are a good all-rounder but you should be working with a high speed surfacing prop assuming rules allow it.
Sid.
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back up to Sid's first statement a well designed tonneau over a frame work would be where I start because it would clean up lots of air that is very turbulent and is hitting things multiple times.
You could bobsled "skirts" past the driver and the eng.
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Have sharpened up the gearbox, and definitely got the propeller front covered.
Going to try and model a tonneau cover and then re test in CFD. Then try smooth the front and rear of the engine cowling
Finally getting somwhere with the CFD. Took a long time to get a solid model in Rhino which I could use. Cd, drag coefficient is around 0.438, now to try and reduce it
(https://iili.io/Jhben4.png)
(https://iili.io/JhbwwG.png)
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Call Woody, Design Dreams he does CFD for hire. Duke and I are just finishing up a project with him on our lakester. Does a great job and lots of experience and good practical input. 513 403 3165, www.DesignDreams.biz
Rex
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Vaguely related i.e. involves racing on water.
In my spare time I race marathon open canoe; two person, sit 'n' switch, American style / sourced Wenonah C2.
When there is a bit of side wind the canoe really gets pushed as it has two sides to the wind, inside and outside.
For the 125 mile race we made tonneau covers out of tent material velcroed in place but by design open where we sit as we also faced 77 portages. Made a huge difference to the boats tracking in cross winds but for reference we were only travelling at 5-8 MPH for a shade under 24 hours.
On the other hand marathon kayaks have a large cockpit opening (as the knees sit high and the legs also pump / drive the boat) with a lip and a spray skirt that is elasticated to attach to the boat and around the waist. White water boats have a smaller opening and really taut neoprene spray decks. With that in mind:
What about a solid tonneau (i.e. thin carbon fibre / kevlar or similar really well attached to the pontoons; sikaflex adhesive?) with cockpit opening lips built in and a spray skirt or at least deflector shield. At 70 plus mph I would imagine a full fabric one (even with reinforcing) will simply flap and potentially be worse than nothing, or tear off and I am not sure I would want a sheet in my face at 70 on water :-o
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The air flow will actually pull it up creating a natural curve just like the roof on a convertible car does or a soft tonneau on a Ute. Secured behind the bow wedge & tied down to the rail on top of the sponson through a bunch of eyelets, it'll stay parked.
A cheap source for strong material is the advertising sign board vinyl. Once the sign company takes them down you can buy them cheap & it has lots of uses like covering hay stacks & ground cover sheets for your race car at the salt. I built a 6x12 ft deck on top of my service truck & covered it with that stuff over 10 years ago & it's still hanging in there.
A hard tonneau in this application could become a people eater in a crash & as anybody that has raced boats knows, you are an accident looking for somewhere to happen.
Sid.
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I'm a litle worried about putting a cover over the boat, as both the copilot and pilot need to move around to balance the boat, there are no seats, you just sit on the floor and slide up and down to keep the boat level. The potential for getting caught under the cover if the boat goes upside down seems a little terrifying!
I could probably extend the nose of the boat back if there's a benefit to that? Been modelling an extension of the engine cowling, struggling to get accurate drag coefficient numbers out though, just want to know if it's helping or not
Would have thought it would help to round the front of the cowling and maybe extend the tail back? Can get moulding foam and make up carbon fibre pieces
These photos are as close as I could get to identical sims
(https://iili.io/Jj44V9.png)
(https://iili.io/Jj4ro7.png)
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PP says
It doesn't much matter how you open the hole( in the air) it is all about how you close the hole.
Much more to be gained by working on the back of every thing.
the engine cover fair behind it
closing the hole behind the driver and passenger
the back of the hull explore what Sid says about tunnel hulls
Lemming point of air getting inside the hull if it affects a canoe that much it is sure is having an effect on you
Also think about tandem again
let the passenger be your traveling ballast for hull trim start sitting up facing you--you between his legs he scoots forward then starts to lie back to a prone position
doing something like this the driver should not have to move very much
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I was under the impression you were doing speed runs but it sounds like you're looking for straightaway speed while circuit racing or maybe an inflatable version of Aussie dinghy racing.
You basically have three critical areas, after the top of the bow, after the two idiots (& I say that with respect because I am one, along with pretty much everybody else on this forum :-)) & after the engine. Sounds like the best plan for a front tonneau would be to bring it down to the floor ahead of you so you can't get under it.
Question. Does you ballast man go up into the bow?
Sid.
Oz https://www.youtube.com/watch?v=SjNi3qPeOrg
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It's more like that dinghy derby racing to be honest! Plenty videos on youtube, have a look at thundercat racing
www.youtube.com/watch?v=uUJJmVi1jms
Haha, I wasn't taking offence at the idiots reference, think you have to be mad to do this!
So basically keep the driver and codriver low as possible which we do already, and maybe a tail section behind the engine cowling?
Is it still worth smoothing the air behind the front nose cone, both crew are sitting on the floor so the air would have to go back over us
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Holy $hit Mate! That's some gnarly water for a little rubber boat, you guys are definitely idiot club members. :wink:
I was in my early 30's when I tried off-shore racing in big cats when I was living in Aus & one race was enough to cure me of that crap. A good Saturday night pub brawl wasn't that hard on a body!
Seems like you have a pretty good handle on where you can make improvements but I doubt any of them would be much use to you in that big chop so it might be desirable to make them removable for conditions.
Stay safe & don't grow up, I tried it once & decided it wasn't all it was cracked up to be. aktion086
Sid.