Dockside: LETTERS FROM OUR READERS

Radio Control Boat Modeler, Aug 2004

No speed limits!

I was amazed to read in your june 2004 issue that the new world record for RC boats is 120mph and that it was electric powered! I found the "2003 NAMBA Electric Straightaway Trials" by Jeff Vasquez very informative, and I now want to try electric racing. Mr. Joerg Mrkwitschka of the German JAGS team is to be congratulated!

RALPH SIMPSON [EMAIL]

Ralph: yes, indeed; there seems to be no limit to the speeds of competition model boats! Exciting times ahead! The gauntlet has been dropped: now, who will break this record? -GY

Finding horsepower!

In your article, "Power Train Transplants" (December 2003), Matt Boyd writes that he can determine the prop horsepower by knowing the thrust and rpm. To measure thrust, he used a "digital fishing scale." So, what's a digital fishing scale? Where can I buy one? And how did he calculate the prop horsepower?

PAULO CARVALHO [UMAIL]

Paulo, power is generally defined as force applied over distance and time, and torque is just rotational force. In terms of engines, horsepower is basically torque applied at a given number of revolutions per minute. The force can be measured as thrust-the amount of "pull" that the boat exerts, measured in pounds (in the U.S.). Digital fishing scales are easy tools to measure the static thrust of a boat. I bought mine at Wal-Mart for about $3, and sporting goods and fishing stores carry them. Then, you need the rpm at which the thrust was produced and the pitch measurement of the prop in inches (usually published by the prop manufacturer).

The formula is simple: multiply prop pitch × rpm × thrust (pounds), and divide by 396,000. Let's take the best of the three pulls for the Zenoah engine in the article. The engine produced 21.14 pounds of thrust on the fishing scale at 13,160rpm with a Prather 275 prop. The Prather 275 has a pitch of 4.3 inches. Using the formula, we get 4.3×13,160×21.14÷396,000 3.021hp.

This formula works for any vehicle that produces thrust with a propeller, including electric boats and even airplanes. But you need accurate measurements for prop pitch, thrust and rpm. And, remember, always follow the strictest safety rules when you take any measurements near a spinning prop!

Where did the 396,000 constant in the formula come from? It's actually a combination of three constants. The first is the constant for converting horsepower to oz.-in. of torque at a given rpm = 5,252. Then there's the conversion of feet to inches = 12. Last is 2 × pi to convert linear to rotational measure = 6.28. Multiply 5,252×12×6.28, and you get 396,000.

Hope this helps to clear things up! -MB

Famous boats

I've noticed something very interesting. Your contributor, Pat Tritle, has published construction articles on many famous boats! First it was the Orca, from the movie "Jaws" (June 2003) and then the S.S. Minnow from "Gilligan's Island" (April 2004). 1 think this is great! Everyone, even non-modelers, knows these famous watercraft! But how does Pat get the information and plans for these floating movie stars?

Should he ever run out of ideas, here are some suggestions. How about a simplified "RMS Titanic," or the ocean liner from the 1972 movie "The Poseidon Adventure"? The super boat from that cheesy old TV show, "Thunder in Paradise," starring Hulk Hogan, would be awesome! Finally, a simple version of John F. Kennedy's FT-109 would surely be a popular plan. Tell Pat to keep up the great work.

MICHAEL ELKINS, CHICAGO, IL

Michael, we concur; our talented contributor and boat designer Pat Tritle certainly has a knack for picking popular subjects. The Orca came about after a discussion he and I had about that blockbuster movie. I had told him that the Orca was one of our all-time most requested boat designs, and he replied that he had always wanted to build it. After a while, he called me to say that he had found a computer program that could capture still images from videos. Pat spent hours and hours watching "Jaws" and downloaded enough images to develop line drawings that eventually became his plans-a true labor of love!

We'll pass your suggestions along, and, who knows? You may see one of them in a future construction article! Thanks for writing. -GY

Write to us!

We welcome your comments and suggestions. Letteis should be addressed to "Dockside," Radio Contra/ Boat Modeler, 100 East Ridge, Ridgefield, CT 06877-4606 USA; email: rcboatmodelei@airage.com. Letters may be edited for clarity and brevity, and they must include an address or telephone number so that the identity of the sender can be verified. We regret that owing to the tremendous number of letters we receive, we can't respond to every one.