Virtual wind tunnel testing.

shop deals on amazon
Shop deals on ebay
Advertise with us
Intlwaters

Help Support Intlwaters:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Terry Keeley

Terry Keeley

Well-Known Member
Supporting Member
Joined
Jul 24, 2002
Messages
7,124
Finally got back to this, was a steep learning curve but I'm starting to get some usable data.

I've been running a flat bottom SAW boat for years and was wondering if rocker would create down force and if so what angles and how much. Trying to keep big pitch hooked up and moving weight back on the new boat but was hoping to take advantage of the aero as well. Air at 100+ mph has got to be doing something!

I modeled the tub in CAD and ran the simulation at 125 mph. Then added a straight 7 degree rocker/diffuser and re-ran it. The simulation shows a fairly big (relative) increase in down force. Guys using the s/w say to not worry about the numbers per se, just the differences. The flat boat showed 276 and the curved bottom 3883 units of down force.

Next I'll try 4 & 10 degrees.
default_smile.png


SAW_tub_-_CFD.jpg





Length is the X direction and +ive FX is drag. Y is side to side and Z is up and down, -ive Z is downforce:

Summary (Flat bottom)

-------------------

Total area, 397.708, mm^2

TOTAL FX, 23903.1, micro-Newton

TOTAL FY, -817.013, micro-Newton

TOTAL FZ, -276.329, micro-Newton

Center of Force about X-Axis (Y-Z), -4.70255, 1.60052, mm

Center of Force about Y-Axis (X-Z), 304.295, 1.73733, mm

Center of Force about Z-Axis (X-Y), 33.1146, -1.75049, mm

Summary (7 deg. rocker)

-------------------

Total area, 394.029, mm^2

TOTAL FX, 25334.1, micro-Newton

TOTAL FY, -389.061, micro-Newton

TOTAL FZ, -3883.42, micro-Newton

Center of Force about X-Axis (Y-Z), -1.73195, 1.69422, mm

Center of Force about Y-Axis (X-Z), 53.0675, 1.78069, mm

Center of Force about Z-Axis (X-Y), 25.8385, -1.719, mm
 
Last edited by a moderator:
Thanks for sharing. It would be interesting to see the effect of rear sponsons or a ski would have in relation to downforce and drag. It would also be interesting to see the effect of toeing the rear sponsons inward to create a diffuser as used on race cars.

diffuser_enzo.gif
 
I mainly ran ski's on the boat and ended up with one that was completely flat to try to eliminate as much lift as possible. I tried fully symmetrical rear sponsons as well. The rears would definitely form an air trap back there...
default_rolleyes.gif
 
Last edited by a moderator:
It's an interesting balance of hydrodynamic forces, aerodynamic forces and trying to get the power to the water from a grossly over powered model with nearly 1hp/pound.

Just as soon as you get everything dialed in you try a different sponson design or change the prop, turn fin, rudder or pipe to get more speed and everything needs to be dialed back in.
 
Tested flat, 4, 7, 10 & 15* rear diffusers in all.

Flat = 266 units of down force.

4* = 3571

7* = 3883

10* = 4094

15* = 3282

Pretty obvious the flow separates on the 15* model and that's what the tracing showed.




Think I'll go somewhere between 7 & 10 degrees. Will be interesting to see how it translates on the water.
default_smile.png
 
Terry..whats the goal... why add weight to the boat?

Seems to me you would want all to be neutral as much as you can.

Grim
 
Grimracer said:
Terry..whats the goal... why add weight to the boat?

Seems to me you would want all to be neutral as much as you can.

Grim
Click to expand...

The problem I've had with my SAW program is trying to keep the props from blowing out because of the huge lift caused by the high pitch. You can just add a bunch of cup and they'll stay hooked up but the boat doesn't go any faster that the LE pitch and it just lugs the motor down. The Eagle Tree data is great for confirming this.

To combat this with my existing boat I've been increasing the sponson attack and adding weight to the transom. 8* sponsons and 6 oz of lead in the trunk was definitely going in the right direction as I was able to keep props planted with just a slight cup that would blow out easily before.

The new boat will have the motor moved back 6" and I was hoping a "diffuser" at the back would add more down force, this testing sure points that way. Diffusers on race cars give lots of down force for relatively little drag. I'm hoping in the end to be able to run lower attack on the fronts as well, 8* has got to be a bunch of drag.
default_sad.png
 
Last edited by a moderator:
Terry,

When my riggers had prop blowout I just moved the sponsons forward to put more weight on the prop and that worked perfectly. Also, if the front sponsons are too narrow that causes blowout. I also found the recurve at the transom to not work as well as a flat bottom for faster speed. Just what I found on several boats. I like your scientific approach.

Lohring,

Nice article. Makes me want to join NAMBA so I can read your stuff!

I play with the same things mike does and in fact his sport gas hydro looks very much like the one I set the IMPBA record with. Funny how we are worlds apart yet doing much of the same things. Love experimenting with boats!!!

John
 
Last edited by a moderator:
John Finch said:
Terry,

When my riggers had prop blowout I just moved the sponsons forward to put more weight on the prop and that worked perfectly. Also, if the front sponsons are too narrow that causes blowout. I also found the recurve at the transom to not work as well as a flat bottom for faster speed. Just what I found on several boats. I like your scientific approach.

John
Click to expand...

Hey John:

Moving the sponsons forward would be the same as moving weight back and too narrow would be akin to too little attack, no? Sounds like we've been to the same movie.
default_tongue.png


Can you expand on "I also found the recurve at the transom to not work as well as a flat bottom for faster speed." From what I read and also my testing shows very little drag increase for rocker at the transom.
 
Last edited by a moderator:
Terry,

Same movie different theater. I tested recurve on my nitro riggers about 15 years ago and had three boats set up the same except for the recurve. I also changed the amount of recurve on the boats in 1/4 inch increments. I tested them and used the radar gun to get speeds. This does not mean that what I found is gospel but I found the less recurve the faster the speed. Not by much, but faster. When I got to 1/4 inch from no recurve the boats would sometimes stuff on occasion. They were 45 size hulls. Rather not say what manufacturer as I don't want to get into design conversations with them. When the bottom was totally flat all the way to the transom the boat would stuff more often. The manufacturer had plenty of recurve and the boats heat raced well. At the time I was just doing heat racing but it probably applies to SAW. When setting up for SAW runs with my gas sport hydro with a flat bottom boat I ran a bead of cyanoacrylate along the last 1/16 th of an inch on the bottom right at the transom to see if it would push the bow down while on plane and the boat would not launch. It would cavitate and not get up on plane at all. Just that small hooker would not even get the boat up on plane! Amazing stuff. I ended up using small spoilers like the photo of mikes sport boat that Lohring showed us on the NAMBA article to keep my sport boat on the water when I eventually changed it from a SAW boat to a heat racer with a wider tub webbing to the sponsons. I also wired a third channel to a carbon fiber wing in front of the tub suspended between the sponsons and it was super sensitive on burying the bow or lifting it. fun stuff but bottom line for me is set up a boat that does not need wings or other aerodynamic devices if at all possible. You know you have the plans to a SAW boat that will easily do 120 mph, so why have you not built it?
 
John Finch said:
Terry,

When my riggers had prop blowout I just moved the sponsons forward to put more weight on the prop and that worked perfectly. Also, if the front sponsons are too narrow that causes blowout. I also found the recurve at the transom to not work as well as a flat bottom for faster speed. Just what I found on several boats. I like your scientific approach.

Lohring,

Nice article. Makes me want to join NAMBA so I can read your stuff!

I play with the same things mike does and in fact his sport gas hydro looks very much like the one I set the IMPBA record with. Funny how we are worlds apart yet doing much of the same things. Love experimenting with boats!!!

John
Click to expand...
John, thanks. You don't need to be a NAMBA member to read the Propwash. I try to write one technical article for every issue. Feel free to read them on the NAMBA web site.

Lohring Miller
 
John Finch said:
Terry,

Same movie different theater. I tested recurve on my nitro riggers about 15 years ago and had three boats set up the same except for the recurve. I also changed the amount of recurve on the boats in 1/4 inch increments. I tested them and used the radar gun to get speeds. This does not mean that what I found is gospel but I found the less recurve the faster the speed. Not by much, but faster. When I got to 1/4 inch from no recurve the boats would sometimes stuff on occasion. They were 45 size hulls. Rather not say what manufacturer as I don't want to get into design conversations with them. When the bottom was totally flat all the way to the transom the boat would stuff more often. The manufacturer had plenty of recurve and the boats heat raced well. At the time I was just doing heat racing but it probably applies to SAW. When setting up for SAW runs with my gas sport hydro with a flat bottom boat I ran a bead of cyanoacrylate along the last 1/16 th of an inch on the bottom right at the transom to see if it would push the bow down while on plane and the boat would not launch. It would cavitate and not get up on plane at all. Just that small hooker would not even get the boat up on plane! Amazing stuff. I ended up using small spoilers like the photo of mikes sport boat that Lohring showed us on the NAMBA article to keep my sport boat on the water when I eventually changed it from a SAW boat to a heat racer with a wider tub webbing to the sponsons. I also wired a third channel to a carbon fiber wing in front of the tub suspended between the sponsons and it was super sensitive on burying the bow or lifting it. fun stuff but bottom line for me is set up a boat that does not need wings or other aerodynamic devices if at all possible. You know you have the plans to a SAW boat that will easily do 120 mph, so why have you not built it?
Click to expand...

It sounds like you tested the same setup with each hull (ie:prop, sponsons, angles etc.) but just changed the rocker (recurve as you call it)?

Lowering the rocker should raise the transom putting more "weight" on the front sponsons causing the "stuff" makes total sense, but the only reason I could see more speed is if by raising the rear it freed up the prop a bit allowing the motor to turn more rpm? As you know it's a balancing act so if you took the flat bottom boat and added a little more attack to the fronts it would probably fix the stuffing issue, right?

Interesting stuff with your Sport Hydro, I knew anything that came in contact with the water was super critical, I've seen guys put 1" of radio box tape on a mono rudder to trim it down and Scale boats that would not launch with polished propellers, rough them up a bit and away they went. Sounds like you discovered the aero is pretty powerful too at 100+ mph, that's what I'm trying to take advantage of.

Plans for a 120 mph SAW boat?
default_huh.png
 
Last edited by a moderator:
Terry,

Maybe..less rocker LOWERS the transom to some as the boat runs. if the tub runs at any AOA then the air would speed up and cause a low pressure in the back.

Grim
 
You may be right on the prop lifting more when the recurve is less. Like you said, it is a balancing act. I ran wedges under my sponsons so that a 3 degree angle of attack on the original sponson bottom turned out to be much more. I changed wedges for more or less angle and riding width. Lots of pond time. Found for turning the boat without a turn fin the width of the sponsons was very important. They had to dig in a bit so the rear of the boat would come around at slow speed. I have a whole file cabinet full of different width sponsons for SAW. Trent got smarter and built a set of sponsons where he attached different bottoms pretty easily.

Kentley gave you patterns for the SAW boat I took the 60 SAW record with before Martin took the record. Kentley built it for me as a payback for the first record boat he had that I scratch built for him. Think back....Remember E-city years ago. I thought he gave you templates for the boat.
 
John,

It's interesting reading this thread on SAW boats as the stack of old Boat Modeler magazines I got from Joe Petro included your article on your and Kentley's efforts to brake 100 mph.

A few of the points I found interesting were you removing one rear sponson and finding you needed to get the engine on the pipe while turning on to the straight to keep the hull from blowing off.
 
Grimracer said:
Terry,

Maybe..less rocker LOWERS the transom to some as the boat runs. if the tub runs at any AOA then the air would speed up and cause a low pressure in the back.

Grim
Click to expand...

Sorry Grim dude, not quite following ya there?

John Finch said:
You may be right on the prop lifting more when the recurve is less. Like you said, it is a balancing act. I ran wedges under my sponsons so that a 3 degree angle of attack on the original sponson bottom turned out to be much more. I changed wedges for more or less angle and riding width. Lots of pond time. Found for turning the boat without a turn fin the width of the sponsons was very important. They had to dig in a bit so the rear of the boat would come around at slow speed. I have a whole file cabinet full of different width sponsons for SAW. Trent got smarter and built a set of sponsons where he attached different bottoms pretty easily.

Kentley gave you patterns for the SAW boat I took the 60 SAW record with before Martin took the record. Kentley built it for me as a payback for the first record boat he had that I scratch built for him. Think back....Remember E-city years ago. I thought he gave you templates for the boat.
Click to expand...

No one I know of has had more pond time than you John, that's for sure! Have tried running w/o the fin but prefer a real small one, maybe I'll give that another shot. Could be just a bit better technique blipping the throttle to get it around the corner.

I remember it well, K Dawg just brought the template out, I never copied it. I did remember you guys running a 36" tub tho, mine was 30" at the time. Any guess how long my G2 and G3 tubs are?
default_rolleyes.gif
 
Tried a curved diffuser and the results are even better.

4155 units of down force and less drag than the 10* flat version.

You can see how the flow speeds up / slows down in this coloured trace video, it comes in at 125 mph, speeds up to over 150 mph on top of the tub and slows to around 90 mph at the back under the diffuser:




Time to go cut some parts!
default_smile.png
 
Last edited by a moderator:
Good stuff from yinz guys. Data is were it’s at.

Come to record trails in May 7th thru the 11th 2018. Portersville Pa. Set an IMPBA record and you will

have no entry fee.
default_smile.png
:)

Nails
 

Similar threads

Don Ferrette
  • Locked
IMPBA and NAMBA membership policy thread content
Replies
0
Views
1K
Don Ferrette
Don Ferrette
anthony_marquart
Interesting prop study
2
Replies
29
Views
4K
Tom Foley
Tom Foley

Latest posts

Back
Top