Porpoise problems

[29tunnel]

Hi ,

My dunlap 27 is tuned as well as I can get it. The boat has really good top end speed and corners like a race-car . I've tried many set-ups but I get a little porpoising in the straits, it settles down in the corners real good. The porpoising is minimal but when you get alittle choppy or windy this when I go airbourn. I run a O.S. stock with 440 prop, there,s 2 oz weights in each tip. How can I maintain real good speed and get rid of the hop. I'm going on holiday monday I,ll check forum in about a week. Thanks for any input------Don

 

[Jerry Dunlap]

On the last D27 I ran/raced, I had front air deflectors attached to both sides of the cowling just below the wind screen on a Dumas cowling.

I wrote an article on installing the air deflectors a few years back in RC Boat Modeler.

I'll take a photo of the deflectors on my WOF 34 and post it in the next day or so.

Jerry D.

 

[Grimracer]

Take a look at the bottom of the boat...air might not have nearly as much to do with the porpois of the hull.. Every time this has happend to me its been the bottom of the boat. (sponsons)

Grim

 

[Eric Perez]

Grim, what do you mean...Crooked spoons? Too much lift?

My Cat does a little porpoising when it's heading full steam into the wind. I think it means to much tunnel pressure. You can see the video here:

http://69.27.9.16/mikep/12/nitroaggressor23a.mpg

Little air dams near the front would probalby help to keep the nose down. What other tricks do you guys do? My boat runs flat so I don't think that I can lower the AGA without loosing a bunch of speed.

Has anyone tried to add small thin wood plates to the bottom of the spoons to break the water surface?

 

[Jerry Dunlap]

Re: thin wood plates to bottom of sponsons

On one of the WOF 28s I built, I cut notches into two places on the sponsons. Rather than add on, the notches kept the sponson surface the same. The boat ran fine, but I don't know if it's any better than just flat sponsons.

I have tried adding on plates to the sponson bottoms and found they created lift.

JD

 

[Grimracer]

Eric.

In almost every case of this i have seen it is sponson riding surface. There are some tricks to help stop it and the one that i would begin with is 600 grit sand the bottom of the sponsons.. One more thing..never,,never,,ever wax the bottom of a boat.

Grim

 

[Propjockey]

never,,never,,ever wax the bottom of a boat.

Why?

 

[Eric Perez]

I had this crazy idea last night. While updating my Aggressor CAD prints...

If I remove 1" from the back edge of the sponsons -leaving everything else the same.

Check out the picture.

Would the boat be more blow-over resistant or not?

 

[Jeff Torgalski]

That would put the rudder and thrust farther from the rear of the sponsons....hmmm, think about this....

1. It would probably be less responsive to trimming up or down (UNresponsive is probably a better word).

2. It would definitely change the handling characteristics making the boat less likely to turn as sharp as it would if the thrust and rudder were close to the sponson.

Anyone want to make a comment on my analysis?

Jeff

 

[Propjockey]

Actually, I think the opposite would be true. You would get more 'leverage' if the the rudder and prop were farther from the ride surface.

 

[Eric Perez]

That would put the rudder and thrust farther from the rear of the sponsons....hmmm, think about this....

1. It would probably be less responsive to trimming up or down (UNresponsive is probably a better word).

2. It would definitely change the handling characteristics making the boat less likely to turn as sharp as it would if the thrust and rudder were close to the sponson.

Anyone want to make a comment on my analysis?

Jeff

Well after hacking 1/2" of the back of my hull this is what I found.

The cut actually moved the CG forward -Do the math. The hull was running flat after the mods. Even when going faster that it ever had gone before (40% Nitro ;D)

The hull did not want to spin out as much as it used to when you gave it excessive rudder commands. Jeff, this sort of supports your second item. This could be usefull with a hull that is to quick to spin out.

I did not notice any better "blow-over" control appart of the running flat, which helped the problem somewhat.

The hull flipped twice during the run. I did not notice any better blow-over protection than before. The hull was going faster than on the other previous tests (Due in part to the change to a high nitro content fuel blend) So it is difficult to make a good comparison.

 

[Eric Perez]

Here are some new mods designed to help keep the front down when going over the rough stuff...

The angle of the wing can be changed and the wing itself can be removed or replaced with a smaller one depending on the wind/water conditions.

I still have to add a couple of SS 4-40 scews to the assembly -but you get the idea.

 

[29tunnel]

Hi everyone,

Just got back from racing and holidays At my last race I roughed up the trailing 6" of the sponsons. I think the boat has settled down a little to the point where I am happy. Thanks to everyone for all your suggestions And hey Jerry, I can't wait to get started on your new boat.--------Thanks Don

 

[Trickey]

Grim, I have also been haveing problems with porpoising, and have in the past waxed the bottom of the hull so i will try your sanding trick but what is the reason for not waxing ???

Trickey.

 

[Grimracer]

Waxing, shaving its all bad... just ask my wife.. ;D

Surface tension...you dont want it.. Make your boat super duper smooth on the bottom and your going to get beat.. Wax just fills in any little imperfections in the finish making the boat even slower.

If you do not sand the bottom then at least ,,what ever you do,, do not wax the bottom of the boat.

Grimgottolearntokeepmymouthshutracer..

 

[snowdog-2112]

I've read this post many times and am surprised that nobody has mentioned the number one cause of a tunnel porpoising. Why is that? Did I miss something? ???

Very often, the number one cause of a tunnel to porpoise is the prop being to far out of the water. I would recommend he lower the prop slightly 1/32 to 1/16 inch at time until the porpoising quits. Porpoising occurs when the prop "blows out" of the water tension which is caused by the weight of the hull pulling itself back down to the water surface. Set a zero degree thrust angle and adjust prop height from there. Also ensure that your C/G is right for the hull. I believe J.D. sets his at 33% measured from the rear of the sponsons.

Hope this helps,

Snowdog