The theoretical maximum speed that a displacement hull can move efficiently through the water is determined by it's waterline length and displacement. It may be unable to reach this speed if the boat is underpowered or heavily loaded, though it may exceed this speed given enough power. Read more.
Classic hull speed formula:
Hull Speed = 1.34 x √LWLA more accurate formula devised by Dave Gerr in The Propeller Handbook replaces the Speed/Length ratio constant of 1.34 with a calculation based on the Displacement/Length ratio.
Max Speed/Length ratio = 8.26 ÷ Displacement/Length ratio.311
Hull Speed = Max Speed/Length ratio x √LWL
A measure of the power of the sails relative to the weight of the boat. The higher the number, the higher the performance, but the harder the boat will be to handle. This ratio is a "non-dimensional" value that facilitates comparisons between boats of different types and sizes. Read more.
SA/D = SA ÷ (D ÷ 64)2/3
A measure of the stability of a boat's hull that suggests how well a monohull will stand up to its sails. The ballast displacement ratio indicates how much of the weight of a boat is placed for maximum stability against capsizing and is an indicator of stiffness and resistance to capsize.
Ballast / Displacement * 100
A measure of the weight of the boat relative to it's length at the waterline. The higher a boat’s D/L ratio, the more easily it will carry a load and the more comfortable its motion will be. The lower a boat's ratio is, the less power it takes to drive the boat to its nominal hull speed or beyond. Read more.
D/L = (D ÷ 2240) ÷ (0.01 x LWL)³
This ratio assess how quickly and abruptly a boat’s hull reacts to waves in a significant seaway, these being the elements of a boat’s motion most likely to cause seasickness. Read more.
Comfort ratio = D ÷ (.65 x (.7 LWL + .3 LOA) x Beam1.33)
This formula attempts to indicate whether a given boat might be too wide and light to readily right itself after being overturned in extreme conditions. Read more.
CSV = Beam ÷ ³√(D / 64)
The Tom Thumb 24, designed by Grahame Shannon sits at the extreme for steel boat designs. At 24 feet of length this little boat by conventional standards would be too heavy to work, but Shannon shed weight with clever use of steel as a skin over a frameless interior, stiffened by only interior furnishings much like the interior lining in modern fiberglass designs. And so the wee Tom Thumb 24 makes an appearance as the smallest of steel go anywhere cruisers.
She’s a sturdy little boat and capable at sea. In spite of her short hull, heavy displacement and wide beam, the Tom Thumb 24 has a reputation for surprising speed. She can easily surpass her theoretical maximum hull speed of 6.2 knots, can point reasonably well, and thanks to her weight her motion remains comfortable.
Interior space has been described as “roomy”, owners under 6 feet in height will have standing headroom.
Many have been built worldwide, quite economically, by amateur boatbuilders. As with the nature of home build projects you can expect interesting variations between boats. The design has enjoyed unexpected success and has inspired a range of siblings including the Tom Thumb 26, 28, 305 and 330. They can be built in multi-chine steel or aluminium. Plans and kits are available from Bruce Roberts Yacht Design.
I designed the Tom Thumb 24 in about 1983 as a bit of a joke. I had designed other steel boats and used to get a lot of people asking “why are there no small steel boat plans?”. I used to give the standard answers (too heavy, can’t make the plating thin enough without distortion, etc.). Then I had the idea that if we designed her like a GRP boat with a monocoque hull, frameless, using interior plywood furniture as stiffening, that the weight would be within reason. After a few calculations, I saw that this would work.
I thought “Let’s give them what they want” and drew up a list of requirements, including full keel, bowsprit, standing headroom, and inboard diesel, with a “shippy” look.
I drew up the plans over about 8 weeks with assistance from John Woods, who also had worked for Bruce Roberts when I did. Then I placed an advert in Cruising World offering plans for $99 with a small line drawing. Response was amazing and within a year we had sold 300+ sets of plans. Then I sold the rights to Bruce Roberts, and he has sold many hundreds more, I don’t know how many exactly. He also designed the similar Tom Thumb 26 based on the 24.
I designed other frameless steel boats, notably the Amazon 29 and the Opal 28, neither of which came anywhere near the 24 in popularity. In fact until I designed the Walker Bay 8 dinghy, the Tom Thumb 24 outnumbered all of my other designs put together in terms of boats built.
Not only did the plans sell better than expected, the boats turned out well and we received many reports of her good sailing characteristics.”
Thanks goes to Tony Fountain (co-writing and research), and also designer Grahame Shannon (designer comments used under permission). Additional owner feedback from Bruno Caroit. Use of line drawings kindly granted by Bruce Roberts.
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