Starwind 860 design logic

"Why that?"

It has to be among the most common questions I get about the Starwind 860 trimaran. Why a tri-hull? Why that size? Why is it a powerboat?

Here's the short version of the design logic.

We already have a powerboat. Sunset Chaser is fun and versatile, but at five metres long and with a load capacity of only a few hundred kilos, she's too small for some of what we want to do, and being light and open, she's a bit too sensitive to weather.

So we want a bigger boat.

Our cruising grounds include the Rideau and Trent-Severn canals. The low bridges and confined quarters make a sailing rig impractical, so we're looking at another powerboat.

We also spend a lot of time on inland river and lake systems. That means we'll move the boat over land a lot, so she'll have to be comfortable on a trailer. Her maximum dimensions are therefore fixed by the Highway Traffic Act sec. 109: maximum beam 2.6 m, maximum length 12.5 m including engine and trailer tongue. Fully loaded, including the trailer, she must weigh less than 4600 kg, or else we'd need truckers' licences to tow her.

For economic reasons, we'd like to go smaller. A 12 metre, 4 tonne boat is just too much for us to build and keep right now, but we can probably swing an under 9 metre, under 2 tonne (loaded) boat.

Trips on the Great Lakes and coastal cruising in the Maritimes are expected, so we need a seaworthy hull form. (Sorry, pontoon boats, you're out.)

We want two people to be able to sleep on board for extended trips, up to about a month. (When cruising with 4-6 people, we're quite happy to let a few folks camp on shore.) Being pragmatic folks, we're also pretty sure this boat will be pressed into cargo service, carrying mattresses, lumber and concrete to the family cottage. The interior, then, should be spacious and simple, with an emphasis on multi-purpose space in the cockpit rather than sheltered cabin space down below.

High speed is not a priority for us. We don't want to be restricted to the 5-knot cruise of a traditional displacement monohull of this size, but we also have no desire to blast around at 50 knots with a hundred-decibel exhaust system belting away. We're happy to cruise at six to ten knots, although a 20-knot sprint would be nice to have in reserve. It's possible to build a planing hull that runs in this speed range, but it ends up being a very light, shallow-deadrise boat that would tend to pound in chop. A multihull can be designed to run nicely in displacement mode in this speed range, cutting through the waves rather than bouncing over them.

We're planning to work, dive and swim from this boat, and we want to be able to carry passengers who have mobility issues. All of that means we want a very stable platform, one that won't heel over at the dock, and that's easy to board without having to climb up or down over the gunwale.

We look around, and find that nothing currently on the market seems to fit all our requirements.

So a new boat is born.

She has a large, multi-purpose cockpit, and a small forward cabin for the head and for some dry storage. Her main hull is just 85 cm wide on the waterline, with a 9.4:1 length:beam ratio, so that we can push her to speeds in the mid-teens with a modest powerplant. Outrigger hulls provide the required stability and offer plenty of space for fishing and swimming, yet can be folded up to fit within the trailer size limits. A single engine, 50 to 60 hp, is an economical choice for main power; we've been assuming a 60 hp outboard so far but an inboard diesel is also possible. Electric trolling motors could be fitted as auxiliary power.

And there we have it. The potential design space is whittled down, by adding requirement after requirement, until only a handful of likely avenues remain. One of those avenues looks appealing, it becomes a 3D model, which becomes a stack of construction drawings, and eventually becomes a real boat.



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