How the Slo-mo-shun IV Was Designed [1950]
By Ted Jones

Naturally, the performance of the Slo-mo-shun IV in breaking the mile record June 26 was gratifying to me, as it has been my ambition since I was 12 years old to design or build the fastest boat in the world. While the Slo-mo-shun IV has been timed almost 20 miles per hour faster than any other boat yet built, the possibilities inherent in this type of design have not by any means been realized. I have currently on my drawing board the plans for a boat which will make 200 mph or faster.

The lines of the Slo-mo-shun IV were developed as the result of my experience in building and racing boats since 1927. The Slo-mo-shun IV is basically a four-pointer. She is 28 feet long and has 11 1/2 feet beam at the widest midsection. Power is an Allison Aircraft engine developing 1800 hp through a 3-to-1 step-up gear, specially built by Western Gear, which gives us 10,800 shaft rpm. Propeller used on the record-breaking run was a Hi Johnson, two-blade wheel. Total weight of the boat is around 4,000 pounds.

So it can be seen that she is not an exceptional boat as far as size, power, or appearance is concerned. In fact, it has the appearance of a conventional Gold Cupper. Her speed relies solely upon the development of its planing surfaces, its balance, and the efficiency of its propulsion unit. The weight distribution is different from any of the other Gold Cup contenders. We put the boat in a sling and moved the engine and the gas tank until it balanced correctly. Her stern comes out of the water and she planes like a 135 cubic inch class, which for its horsepower is the fastest boat built.

From 120 to 130 mph is a good cruising speed for the Slo-mo-shun IV. At that speed the hull has risen from the water, supported almost entirely by a film of air that puts her in perfect suspension at all high speeds. Only the forward fins, the propeller, and the rudder remain in the water. The motion is remarkably smooth except for the impact of the propeller which is transmitted to the hull in the form of minute, high-speed vibrations. At this speed the boat is under perfect control and the only limitations are those of the driver. As yet the throttle has never been completely open.

She also handles well and is the fastest accelerating boat I have ever driven. This is due in part to her comparatively light weight. At present she has a single rudder offset from the propeller. I have put her into a racing turn at 100 mph and she came around under perfect control.

All our speed tests this spring didn't even crack the varnish at the seams of her mahogany plywood panels, which proves the soundness of her aircraft construction. Plywood gussets were used on all corners, and none of the frames are notched for the battens or keel.

All builders of racing craft know that the sponsons must have lift to keep the boat in its proper planing angle. The sponsons support the weight of the boat at high speed, and the proper angle of attack, varying with the distribution of weight and horsepower, is very important. These factors were carefully calculated on Slo-mo-shun IV, based on data collected over previous years, and is largely responsible for her speed.

The performance of Slo-mo-shun IV has demonstrated the superiority of multiple point type boats over the conventional hydroplanes. My first multiple pointer was of the 151 cubic inch class, built in 1934. I placed the sponsons aft, which gave her stability and served her well on turns, but she was no faster than a conventional hydroplane.

In my following boat, the Wasp, built in 1936, I put the sponsons forward, and found her about 12 mph faster. By putting the boat into balance, instead of ploughing the stern through the water, I had achieved a racing model that for the first time would get right upon a film of air. However, I had trouble; if I didn't part my hair in the middle the boat would fall off to one side or the other, due to the fact that the sponsons had no dihedral or angle of attack.

After tearing off and rebuilding on the sponsons 13 times, I finally had them where I wanted them. It was through this trial and error that I learned that the angle of attack of the sponsons drives the boat out of the water into a correct planing angle, and then as speed is increased the air pressure takes over and supports the weight of the boat.

The dihedral angle can be properly described as the lifting angle on the planing surfaces when making turns, which also tends to keep the boat tracking on a straight course. This is determined by the same factor as the angle of attack, and the lessons learned on the Wasp and many other models have been applied to Slo-mo-shun IV. Now we have a boat which is not only fast on the straightaway but is also good on the turns.

The fourth point of suspension, on Slo-mo-shun IV, is primarily a safety factor, and has proven its worth several times.

At one time Stan Sayres and I were the only persons in Seattle who had 3-point boats. About 1938 he bought the Slo-mo-shun I from "Pop" Cooper, a famous Midwest racing driver, and it was the mutual interest in fast boats that brought us together first. At this time I was racing the Phantom, 225 cubic inch class.

After the Slo-mo-shun I threw a rod, burned and sank, Sayres bought a second boat from "Pop" Cooper, the Slo-mo-shun II. One sponson was damaged in transit. He called me in to help true up the new sponson. It was at this time I informed him that I had the working designs for an extremely fast boat and was looking for a sponsor.

After the war Sayres asked me to design and build a 225 cubic inch class boat, which I built in my basement in 1947. This was the Slo-mo-shun III. She is similar to others built in recent years, although built a little heavier to withstand rough usage. We found her about 10 miles per hour faster than its class at that time, but Sayres didn't bother to race it because of lack of local competition plus the fact we were already aiming at the big boat.

Construction of the Slo-mo-shun IV was begun at Jensen Motor Boat Company in the fall of 1948 following the basic design I had developed 10 years previously. Anchor Jensen was in charge of construction and did much of the work himself. He is a fine mechanic and turned out some of the neatest work I have ever seen on a racing boat.

Without Mr. Sayres' intense love for speed on water, this boat in all probabilities would still be on my drawing board gathering dust.

(reprinted from Pacific Motor Boat, August 1950)

Hydroplane History Home Page
This page was last revised Thursday, April 01, 2010.
Your comments and suggestions are appreciated. Email us at
Leslie Field, 1999