Running Surface texture

[anthony_marquart]

Last year I was running a SP21 and had some handling problems with it. Hopping in corners mainly. I changed alot of things on it and made good progress. One of the things that I think really helped alot was getting rid of the polished surfaces on the sponson and hull bottoms.

I have heard lots about this before and I've done it. For the past few years I've just been running riggers. I had forgotten to do it for a long time. I think it doesn't have as much of an impact on riggers because there is so little wetted surface touching water.

It made a big difference in my SP21. This sounds crazy and maybe I'm totally wrong,.. but I think the outside sponson was sucking down to the water in the turns, in a sense holding he sponson down and and lifting the prop. I went back and block sanded the sponson bottoms dead flat,.. through most of the paint, put a coat of epoxy back on, scuffed it down to I think about a 300 grit, all circular motions, definitely not front to back,.. and this made a big different in how the boat ran.

I plan to shingle my new rigger bottoms,.. and scuff the same way.. what are you guys doing and what effects have to seen from this? Any real down side?

I'm thinking I may not even paint the bottoms of my new sponsons,.. the paint sits higher at the corners,. just due to surface tension,. end up sanding it off anyway..

 

[Jeff Barnhart]

No down side at all to anything you said. You want the edges as sharp as possible to keep the boat freed up. When the paint dries on the boat it tends to very slightly round the edges (which will suck the boat down), scuffing it off gets rid of this and makes the edges nice and sharp. As you found, it will make a difference.

 

[Grimracer]

Check that flat section too.. If its to short your going to run into the same issue.

Grim

 

[Neil LaGarry]

Not knowing what rigger your running, We found on the JAE ,the hopping was due to the turn-fin being off. . Try moving the rear of the fin up. all hopping stopped for me. you may need to oval the rear hole on the turn-fin.

 

[John Knight]

IMHO 300 grit paper is not near enough to finish any boat, hull as well as sponsons. I use 220, 400, 800, 1500 and finish with 2000, all WET. Each one progresses over the other. The process takes about a hour to do. You will be amazed at the difference between each level. Run your hand over it dry and wet it and do the same again. Then put a good quality wax on the entire boat. A teflon boat wax is an excellent choice.

 

[anthony_marquart]

The point was more about the running surface NOT being smooth as the surface with stick to the water.,

 

[John Knight]

The point is that you want to prep the boat NOT to be the same as the water surface, but rather to make the running surfaces MUCH more slick than the water. It releases from the water more easily and will perform better. Equal does not improve it much, being more slick does improve it greatly. Why take one step forward when you can take three?

 

[anthony_marquart]

I don't know that we are speaking the same language.. that or I don't understand..

 

[Grimracer]

Anthony.. your right.. you want to decrease surface tension. A "Smooth" surface is not the key.

Grim

 

[anthony_marquart]

thanks... I learned a bit about this back in my wind tunnel days.. my first tests (the sp21) were with very smooth painted and polished surfaces.. The scuffed surface did release better from the surface tension of the water.

Man I'll be glad when I get these boat hulls Im building done.. get back to actually burning fuel,.. before my fuel gets old!!!

 

[RonShaw]

Anthony,

I saw on another thread where someone posted a chemical that, when applied to any surface, seriously repels water. This on top of what you guys are discussing above would be something worth experimentation. I used Speedkote years ago with a noticeable increase in speed and release of surface tension. I haven't gone that route this time around as I try to simply have more fun these days. 220 and a good T sanding bar will give you the results you are looking for.

 

[Kevin Krogness]

Anthony,

I saw on another thread where someone posted a chemical that, when applied to any surface, seriously repels water. This on top of what you guys are discussing above would be something worth experimentation. I used Speedkote years ago with a noticeable increase in speed and release of surface tension. I haven't gone that route this time around as I try to simply have more fun these days. 220 and a good T sanding bar will give you the results you are looking for.

Haha is that the same stuff Clark Griswold used on the bottom of his saucer! !! I'll have to find that thread again but J think it was real spendy!!

 

[dwilfong]

I tried the shingles with a air trap in the middle on my .21 rigger. It made it very light in the front end.

Did not like the results when running it in the rough water.

 

[anthony_marquart]

I have run shingles but no air trap with very good results. I can see where the air trap would get you lots of lift... maybe needing a lower AOA in that design. good thing about shingles,.. you can sand them off and re-seal the bottom over night!

 

[Grimracer]

If you added shingles to a current bottom for the most part all you did was increase the AOA.

When adding shingles one has to take this into account.

Grim

 

[anthony_marquart]

I don't know if I could totally agree Mike. One might say you could add take a straight edge, hold against the TE of each shingle and then you have your AOA,.. but if you did not add one that TE of the sponson,.. but lets say dividing the flat running surface into 3 sections with the shingles, you may have something different. I have thought of it only as a way to get an easier launch, break free from the surface easier. With the JAE style ski, the prop frees up easily anyway... hm,.. maybe I just talked myself out of it..

 

[Grimracer]

Give it some thought.. test it.. All you are doing is adding AOA. Sure you are adding breaks to the bottom but that’s venting and not the same deal.

I got a great study in this when I did the AquaCraft Motley Crew.

Grim

 

[John Knight]

Grim is dead on. Any addition to the sponson bottoms will change the AOA. Pure physics. The shingles break up the suction of the water but changes the AOA. Acute measuring will reflect those changes.

 

[Steve P]

I do not know much about this stuff. But can you raise the rear of the sponsons to offset the extra thickness that adding shingles adds or is it the added angle of each small step of the shingle that changes it?

 

[Neil LaGarry]

The hull is a proven winner. Why would you change anything. Them 2 little riding surface on the sponsons -is just that little.