Svelte Speed; SC-1 Subchaser upgrade

Hudson River Maritime Museum has a great blog that regularly puts out interesting Hudson River historical tidbits.

One of those blogs had a story about World War I subchasers.


It provided some drawings for the vessel, but Wikipedia provided an even more complete drawing with a lines plan.


They have the following particulars:

    • Displacement:  85 tons full load, 77 tons normal load
    • Length:            110 ft (34 m) oa 105 ft (32 m) pp
    • Beam:              14 ft 9 in (4.50 m)
    • Draft:                5 ft 7 in (1.70 m)
    • Propulsion:       Three 220 bhp (160 kW) gasoline engines, 2400 gallons fuel
    • Speed:             18 kn (33 km/h)
    • Range:             1,000 nmi (1,900 km) at 12 kn (22 km/h)


I had recently watched the PBS Roosevelt documentary and there was a reference to FDR as Assistant Secretary of the Navy battling the Secretary of the Navy in trying to procure many smaller lower cost vessels and it turned out these subchasers were the result of FDR’s efforts.

About 450 of these boats were built. They were often manned by volunteer yachtsmen and nicknamed the Splinter Fleet.

After the War many went to naval service in other countries and to be able to get rid of them, the Navy even provided conversion plans to turn them into fishing vessels. Not many fishermen appear to have taken the Navy up on the offer.

These are very slender boats and would not be the most comfortable vessels, but with only 660 installed horsepower they could hit 18 knots. Just quick enough to keep up with WWI submarines which would top out at about 17 knots on the surface.

World War I was a funny period in small craft higher speed vessel design. The US Navy was actually involved in the development of offshore planing hulls, but that work was still in the early stages. One particular design that showed some success was the Hickman Seasled where some prototypes even functioned as single plane aircraft carriers that were quick enough to launch a biplane and recover it.

These were 55 foot boats that carried a single biplane. They could run over 40 knots and with a few knots headwind, and the biplane prop pulling along, they were quick enough to let the airplane lift pretty much straight off the deck and to recover the plane by simply clipping a hook over the axle when the plane passed over the deck (Hopefully without decapitating the guy with the hook). Many years ago, I had my hands on an original print that showed the recovery of an airplane aboard one of those Seasleds.

I designed a 28 foot Seasled in the late 80’s and it was a very capable boat, and technically very competitive with other high speed hull types in the 80’s,but has not really caught on again.

I did find a picture of one of these aircraft carriers at speed on the internet.


However, these were expensive boats, really high tech stuff for the day, with very limited endurance and range.

These subchasers were on the other side of the spectrum, simple and low cost, and to get speed it would require a slender hull.

While beam is cheap, the inverse of that maxim is: “If there is no way to get on plane, speed is slender.”

Conventionally one would think that the hull speed for a vessel like this is 1.34 * Square Root (105 feet), which would be a bit less than 14 knots, and therefore it would take a lot of power to go any faster than that on a hull like this.

But these vessels are only 14.75 feet wide and using the slender hull speed equation of L/3B * Square Root (105 feet) it indicates the hull speed is closer to 24 knots!

In other words, 18 knots is probably not even hull speed.

Regardless, slender hulls are far from ideal. They can maintain speed in head seas, but in other headings can become quite uncomfortable. They can be designed as snap rollers with very high stability or they can be designed with lower initial stability and a high range of positive stability with high freeboard, and be annoying drunken lurchers.

There is a median path, but it is not much better. And while they like to go straight, turning is another matter.

Still I like the look of these hulls and with modern technologies it would be fun to consider how they could be improved without losing that lovely, low wake, low horsepower speed.

Instead of three engines they could be fitted with one 250 hp diesel generator and around 300kW hour batteries, with about 450 kW total electric drive motors.

Instead of a triple screw arrangement, it would probably be better to use a single vectored thruster arrangement. (Electric drives have ample torque, so propeller sizing becomes much easier)

Those changes will result in a similar displacement to the triple engine gasoline set-up.

If the boat was built in cold molded epoxy, there could be even more battery capacity, but even with an original weight hull, this boat would effortlessly and almost wakelessly move along at 7 knots for quite a number of hours on the batteries. The long range data indicates these boats can do about 12 knots on one engine, and that would mean that she could run electric alone at 12 knots for a good 2 hours. (It should also be noted that at 12 knots this 110 foot boat burns less than 2 gallons per mile)

If the owner wanted to show off for a bit, she could hit the throttle at the full 450 kW and hit 18 knots for about 40 minutes all electric, or for a bit over an hour if she kept the generator running.

On the generator alone she would still do 12 knots, and possibly a little more.

But what about the rolling?

Here another bit of modern technology could help out. Even before WWI there were experiments with gyroscopic stabilizers, but they never worked out. However, today, gyroscopic stabilizers are readily available.

For chuckles I asked Seakeeper to give me an idea as to what a gyrostabilizer could do for this vessel. While we have the lines, I am not sure what the GM of the vessel is. We assumed a GM of 1 m, which provides a roll period of 3.6 sec.

That input provided the following responses with the various Seakeeper models.


Seakeeper recommends a Seakeeper 40 for this vessel and it is easy to see why. In a beam sea it will reduce an unstabilized roll from 15 degrees to 3 degrees! Very impressive. And a 25 degree roll will be reduced to about 13 degrees. Quite an improvement.

Since I don’t powerboat in bad weather, I may choose to go with a smaller unit like the Seakeeper 26. It would save weight and money. One thing that really makes a Seakeeper nice is that it will keep a boat like this upright when making a sharp turn in flat water (but not in a long sustained turn like that).

The Seakeeper 26 draws 1500 W (one hair dryer) in light conditions and up to 3000 W in heavy use. It weighs 3200 pounds and is presently priced at $240,000. I am really impressed that Seakeeper list the price of their units. No beating around the bush and wasting my time. $240,000 is not small change, but for a boat like this I would not want to do without it.

It would really be a lovely boat to ride on. It would be nicer than an equivalent cost catamaran (shorter but wider). Catamarans do not roll to high angles but can be incredibly snappy both in roll and and in pitch. This boat would be really smooth and actually use less power than a catamaran in the speeds we are talking about. In most conditions a gin and tonic would stay where it was put down, and in electric mode the tinkling of the ice cubes would be almost disturbingly loud.

Slender vessels tend to be inefficient with regard to the use of deck space and hull volume, but since the vessel is all electric, the generator and batteries can be placed anywhere in the vessel which makes it much easier to design an interesting hull arrangement. Even without putting pencil to paper, I can envision an arrangement that offers much more useable accommodations than the original vessels. With batteries low, it can certainly have a larger deck house, while still maintain a lovely profile.

And then all we have to do is deal with the limited low speed maneuverability of this boat by fitting an electric bow thruster.

This would be a fun boat for all kinds of interesting uses. To be green it will need to rely on shore power though, because slender vessels simply do not have any deck space for useful amounts of solar panels (unless they are sidewheelers). Once green methanol becomes available, this super efficient boat will really start to shine from a sustainability point of view, especially if fuels like this are more expensive than diesel.

A conventional power boater would not consider this hybrid design to be an improvement over a conventional IC arrangement, since most powerboaters find it difficult to reconcile their actual boating experience with full speed range. While sustained top speed is a central concern for long distance commercial vessels (and often those speeds are relatively low), on smaller vessels sustained top speed is nowhere near as important. Most powerboats do not run for long periods at top speed and often powerboaters go fast because they can, and not because it provides a better boating experience.

To glide through the water on a boat like this at speeds that would throw a wake on larger and smaller boats alike would be a unique joy and envy to all.