Sponson Edges and Airfoils??

[Flying Pilgrim]

Back in August, Jerry Dunlap responded to a post regarding the Hot Shot, and suggested adding outside sponson edges. Are these an additional part not in the original plans? Different than sponson pads? (I know outer edge of sponsons needs to be sharp).

Also, I see some tunnels have upright thin "fins" on the inside edge of the front of the sponsons sticking up above the top of the sponson that appear to be either an airfoil or a deflecter to shed water. What are these, and what is their purpose? ???

 

[TimD]

I spoke to Jerry about that same thing way back then. They are just like strakes on a mono. Please correct me if I misunderstood you JD.

Grim is the man on the "deflector" I'm sure he can answer this better than me. (I haven't tried it - yet)

TD

 

[Jerry Dunlap]

F.P.

Tim is right about the sponson outside edges. They are the same as the outside flat surface on a vee hull. The idea is to better sheer the water as it runs under the sponsons.

I started adding front air dams(upright thin fins) to the front of the sponsons because the full size tunnel boats had them and I figured there must be a reason the "Big Boys" stick them on their big boats so J.D. started sticking them on his small boats.

Of the two ad ons, I think the sponson outer edges are more performance enhansing.

J.D.

 

[GTR]

Hi guy's,

I am pretty sure the theory behind the deflectors on the sponson tips is:

as long as the tunnel has forward speed the air travels over the tunnel causing lift. This is important as when you start to turn the air at the front of the sponsons will spill over the sides and lose tunnel lift - eg the air needs to travel along not across the tunnel to get the job done. Full size racing cars have also applied a similar idea to guarantee airflow over rather than across as it unsettles things aerodynamically.

This is my understanding of what they're there for....could be wrong???

The strakes is a different issue. As JD states they will cause the water to separate more cleanly from the sponsons......but they also generate lift as the are parallel to the water surface not unlike hydro's. Possibly combines the benefits of steeper sponson riding surfaces (grippier turns) with th increased speed of shallower angles on the sponsons.....could this be almost some kind of benefit for free???

cheers

GT ;D

 

[Flying Pilgrim]

J. D.

The theories make sense, but I still can't picture what the outside sponson edges look like. I assume they would run parallel w/ hull bottom (parallel w/ water surface as well). Do you have any pics of them for details? Did you incorporate these on the 27" Sprint plans?

Thanks,

F. P.

 

[Grimracer]

FP

Are you talking shear edges or strakes.. They are not the same.

For instance the Villain S1 uses shear edges.

Grim

 

[Jerry Dunlap]

I suppose the correct term is an out side strake. I added them to both out side edges of my Villain.

I create the strake by glueing a piece of 1/16X1/4" spruce stringer along the outer edge of the sponson. The stringer is tapered towards the front to blend into the curve of the bottom sheer.

The stringer is glued 1/4" back from the edge of the sheer and parallel with the bottom of the tunnel floor. The gap along the outside is back filled with body putty. I just leave the 1/16" inside ridge alone.

I don't think strakes are shown on the D27 plans. I don't have a set of the plans from the magazine.

Jeez, I really need to get a digital camera. Talk about Tom Prezentka of Octura being resistant to new technology!!

J.D.

 

[Grimracer]

I will send photos of strakes also.

Jerry get a Sony. I like my cybeshot.

Grim

 

[Flying Pilgrim]

J. D.

With all your magazine articles, maybe the publishers will chip in and buy one (digital camera) for you!! (Yeah, right). I have to swipe mine from work and hope no one notices it's gone!!

F. P.

 

[duc]

Yep I would like more pics on this forum to! One pic sais more then a hundred words I guess.

 

[GTR]

Hi Guy's

Here are some pics of strakes and shear edges

 

[GTR]

JD should recognise the hull design

 

[GTR]

Piggy back sprints show standard shear edges and mod one with strakes

 

[Flying Pilgrim]

Thank you all for the help and clarification!! ;D

I'll implement them on my next project.

 

[_laxmasta2039]

wow......

i've never looked @ the bottom of a tunnel very closely, it's hardly any different than my 'cat'......why is it that cats are so stable in chop when tunnels arent?

joe

 

[Jerry Dunlap]

Hey GTR,

Nice work with the photos. I think they do a good job of show the strakes. Is that a JD WOF 29?

Joe,

Most cat hulls are inboard and that makes for a more forward C/G. Also, the cats tend to be bigger boats and have more sponson depth.

My 1/4 scale tunnel will run in some pretty ugly water - but it's also 44" long.

J.D.

 

[Flying Pilgrim]

I've never owned a cat, but I always assumed their sponsons had a sharper outside sponson angle, allowing it to ride a little deeper in the water. ...but then again, maybe I'm way off base. :-

 

[_laxmasta2039]

that's probably true on alot of cats, its just that mine's got "ride pads" on the bottom, so the sponsons arent cutting through the water.

JD,

didnt think about CG......i get it now!! ;D

[b.t.w., my cat is 25" long....havent got it running yet, im not even sure if it'll float! 8)

i think it might also have to with how wide a cat is, it probably helps the stability to have the sponsons farther apart.

 

[GTR]

Hi guy's

JD the hull is a WOF 7.5 - made by a friend (he loves to work with wood - I like fiberglass).

There is heaps of difference between a tunnel and a cat hull, most obvious is that tunnels tend to be a continuous sweep from bow to stern, whereas most cats have a series of steps along the bottom. Cat hulls usually have flat riding pads along the bottom. This might stretch the imagination but....take a vee hull, slice off the keel so that it is flat along the bottom, now cut it down the centre and add a flat tunnel - roughly speaking you have a cat hull. I guess I could post a picture of a 21 cat upside down next to a 21 tunnel to illustrate my point. A picture is certainly worth a thousand words - well quite a few anyway.

cheers

GT ;D