Tunnel questions

[Kez]

Hi All,

I have been following this forum for a couple of months and have searched old threads but could not find related topics.

Back in the 80's I bought four Dumas HotShot .21. The fiberglass tunnel, I believe, was the next generation after the original blunt nose Hot Shot. I have built only one and the other three are still in their original boxes. I understand this tunnel hull is very outdated but I am not a serious racer and I would be happy with a sport boat with decent performance.

Can you share your experience on this boat? What is the best set up (prop, engine height and angle etc)?

Also, what dictate the top speed potential of a tunnel hull other than engine power? Is it hydro/aero dynamic? Can this hull be modified in some way to be competitive? It has a tendency to blow over even with about 3degrees of negative angle and 4oz of lead at the nose. I am wondering if making the tunnel shallower would help.

Thanks,

Kez

 

[Jerry Dunlap]

Here's what we did to help a guy in our club wiht a fiberglass Hot Shot like yours:

1. Ran an Octura X440 prop.

2. Set the prop depth 3/16" above the bottom of the sponsons.

3. Set the prop thrust angle at neutral.

4. Added 6 oz. of lead to the tips of the sponsons - that's 3 oz. on each sponson tip.

5. Installed "stumble blocks under the front of the hull."

6. Suggested the guy consider purchasing a newer design if he wanted to compete - he now has a Dunlap 27.

In my opinion, there isn't much of anything you can do to modify the boat to make it competitive with today's designs. Last year some one gave me one of these fiberglass Hot Shots, I thanked them, stuck it in my truck, and then depositied it in the garbage can when I got home. There was no reason to waste any of my time setting the boat up.

But, it does run - it just has limitations.

Jerry Dunlap

 

[Jeff Torgalski]

Hello everyone,

If you have a Tunnel Hull that is in perfectly good condition and you plan on throwing it out, please contact me. My email address is

[email protected]

Thank you,

Jeff Torgalski

 

[Kez]

Jerry,

Thanks for your input. My next project is the Dunlap 27 (the all wood version, not the WOF), which I have the frames already cut and the cowl ordered.

However, I still enjoy modifying the old Hotshot and see how each changes improve its performance.

I am already running an X440 (have also tried 442 and JGE20, Prather etc) and at the height you suggested only with 3 degree of negative thrust to help it stay on the water. It is running about 35mph (using a stop watch) on 25% fuel and will blow over if I lean it out. Perhaps I should add more weight to the front?

Can you tell me what makes a modern design (the Dulnlap 27, VillainS1)run faster? Is it because of a smaller wet area? Better Aerodynamics?

Thanks,

Kez

 

[Atious]

Add me to that freebie-boat list too ;D ;D ;D ;D

[email protected]

 

[Jeff Torgalski]

Modern design of model tunnels is starting to gravitate towards tunnels of less width, and sponsons of greater length. That makes sense though, the higher top speed, the longer you need to make the boat for stability. And, as always, the lighter, the better.

I have seen some interesting ideas on a Villain S1 which the inside of the sponsons had hooks. Check it out...it's in the Villain S1 section of our site.

Jeffrey T.

 

[Jerry Dunlap]

Kez,

There are a couple of major design changes that have made the model tunnel boats faster and more stable. Jeff hit on the main ones. But, here they are again:

1. Less tunnel width - reducing the amount of air being trapped under the hull.

2. Greater sponson dihederal - allowing for better cornering without a turn fin.

3. Less width on the sponson runniring surface - less wetted area, less drag.

4. Shape of the center section foil - thinner foil to reduce lift.

Jerry D.

 

[Jimboat]

Check out the Secrets of Tunnel boat Design book. It outlines many of the design factors influencing tunnel boat performance.

Actually, less tunnel width will not make the boat more stable - nor will it be more efficient. A wider tunnel will provide more lateral stability, and will also provide a more efficient lift generator. The appropriate amount of "lift" should be designed into the tunnel by changing the chord length of the "wing" itself. Shortening the "wing" section will move the center of pressure aftwards, and provide an improved dynamic stabilty. Other efficiency improvements include thicker aerofoil.

A wider sponson running surface (has a more efficient aspect ratio) will have better acceleration, but may suffer in heavier water with lower top speed.

We find it easier to see the effects of all such design changes using performance design software.