Duckworks - Projects
The Online Magazine For Amateur Boat Builders

Designing the Osprey

by Tom Elliott

The Goal

I have been a sailor for nearly 30 years. But I have been feeling a bit constrained by the limits of a moored boat lately, and fed up with the trailer boat’s whole set-up-the-boat-every-time-I want-to-go-sailing routine. I eventually figured out that if I eliminated the mast, rudder, and keel, the whole launch, daytrip, and retrieve cycle would go much smoother. What a novel idea! But I don’t want to go blasting across the water either. I am still interested in the process, not just the destination. Checking out the margins of the water, looking at all the small sights, and finding the perfect beach all intrigue me. And if the boat really can be launched easily, I can use a ramp close to my destination. This will make an outing into a no-hassle afternoon’s adventure.

But on the other hand, a light boat with a small four cycle engine is capable of long range without a lot of fuel. If the load and supplies are sized to the task, a suitable small motor boat can cover a lot of water in 24 hours. And do this faster than the same size sail boat ever could. I have long dreamed of cruising the inside of Vancouver island. A small trawler could be just the boat for the job. I would not need to seek the high wind areas, like the sail boats do. And the areas that are often windless would be just fine.

This cartoon represents one version of the conversion I have been designing. It represents an 18 foot, 1000 Lbs trailer weight trawler, (1800 Lbs at design waterline) if you can call something so light a trawler. The intent is to create a trailer boat suitable for day trips, and also for cruising the protected waters of the Pacific Northwest, at a minimum cost.

This would be a great gunk holer, much like the Redwing 18, but with more shelter, more attuned to the North west weather. Easy to trailer and launch, I can see the Osprey being a great day boat and also a great camp cruiser. Tents and duffel for a shore based family over-nighter would fit on the berth..

But I also see a second use for the boat. With a Honda 9.9 driving the Osprey, and two people on board, I expect to get about 335 miles from 30 gallons of gas, running at 8 Mph or 7 knots. This run would take 41 hours or 1.7 days of engine time. In sailboat terms, a daily run of 197 miles would be quite impressive. This boat, this 18 foot boat, can do that every day.

So what could I do with such a range and speed? How about running the inside passage between the San Juans and Prince Rupert during a realistic vacation? This 300 mile inside passage is beautiful fjord country and mostly protected water. I always have wanted to run the length of Vancouver Island. Perhaps now, I will.

But traveling by boat in this region is not for the faint of heart or the unprepared. Communities are few and far in between, harbors are scarce, and the bottom is often deep enough to prevent anchoring. One choke point, Johnston strait, has tidal currents of 16 knots. It’s important to have some way of controlling your motion if the motor gives up the ghost. Running off to the leeward is not an option with cliffs to all sides. That’s where the small mast and a get-home sail comes in. That’s where the oar ports, (not shown) like the Dovekee and Bird Watcher have, come in. I want to have multiple ways of moving the boat come what may.

So since reliability is an issue for the cruise, let’s take a look at the engine. The Honda 9.9 will be running at 8 Hp at 7 knots, which is 80% of its rated power. But since it’s actually a 15 Hp engine already de-rated by the manufacture to 9.9 (with a limit screw and smaller jets) it could be said that it’s running at 53% of 15 Hp. That sounds pretty reliable, but I will need to check with Honda to be sure. And if I need to extend my range, look what happens when I slow down. 300 miles should be a piece of cake. George Bueller would like this sort of thinking.

To get to this place in the adaptation design process, I played with many different boat ideas and looked at a lot of West Coast boats. I finally ended up with what you see in the first drawing. It’s a foot longer and somewhat deeper aft than the original hull to support the weight of a heavier motor and provide a place for the motor well without eating up the cockpit. (Last night, I decided that the transom needs to be deeper still for best running at a speed/length ratio of 1.7.)

The pilot house has good sitting headroom at 4 foot 6 inches high, which is better than my old Tanzer 22’s cabin. I will give the pilot house a hatch, or perhaps a canvas center, so I can stand up to see better while docking. The cabin has a nice V berth for two with sitting headroom. I will have a tent over the cockpit for rainy days and for sleeping. I might put a canvas cloth between the existing sail boat style cockpit seats to give me a place to sleep when my wife and daughter are in the cabin. So the tent had better be good!

I will likely have a shallow keel, for control at slow speeds, but before I draw it, I need to think about how it will affect the ease of trailering. Perhaps the inertia of water in a Bolger style hollow keel will help slow down the motion of the boat?

Osprey looks promising to me. So after outgrowing a Tanzer 22, and enjoying a Luger 16 in the mountain lakes for a few years, it’s time to go back to cruising.

The Project

My familiarity with the boat is what gave me the confidence to go ahead with the modifications. I am no stranger to the hull, that of a “Loa 17.” I don’t know who designed it, but I have owned the only two I have ever seen. The first was a 3/8 plywood boat with glass and polyester sheathing. Built in the 60’s, that boat taught our family a lot about sailing, cruising, and how to fix dry rot. Interestingly enough, we kept the first Loa 17 at Vancouver Lake Sailing club (Vancouver USA, not Vancouver BC), where unknown to me, Pat Patteson, saw it, and was impressed. As a result, he bought a fiberglass Loa 17 and had a lot of fun with it, used it well, and used it up. What a good end for a boat!

18 years after I sold the first Loa, I met Pat at the Depot Bay Boat Show last April and we both found out that I was the owner of the first Loa. One thing led to another, and in a few weeks, I was the happy owner of the hulk of a fiberglass Loa 17, my second Loa 17. We had put a lot of miles on the first Loa and understood what it was capable of and how it would work for us. Some times we do get a second chance.

You see, when I met Pat, I had been looking for a small sailing cruiser to convert to power. In our area, small sail boats are a dime a dozen. You can find them really cheap if they don’t have a trailer and are eating up moorage fees. But on the other hand, I had been reading about a number of common faults with such a conversion. First, if the mast is removed or the keel shortened, stability and roll timing can suffer. This can make for a uncomfortable or dangerous boat. Second, sail boats are also often slow under power. Slow, that is, compared to what the same power could do in a faster hull form with a straighter run. But what would happen, I reasoned, if the sailboat was a centerboarder like the Loa 17 I owned so many years ago? If it could stand up to sails with just form stability, and if I liked its action with the sails furled, surely it would be a low risk conversion. Perhaps straightening out the run would make it faster? And then, I found only the second Loa 17 I have ever heard of!

I bought the boat just because it was available, thinking that if the motor boat option did not work out, I could always fix it up as a sailboat and sell it. I had plenty of time to think about it, read about the design process and draw, since I could not start work right away anyway. Being of an analytical frame of mind, I decided to see what I could learn about boat design. A number of books later, I dug out my old drafting board and got to work.

When I started working on the modifications, my general idea was to straighten the run out by installing a stitch and glue plywood wedge to the bottom. As long as I was modifying the boat, perhaps an extension to the stern would hide a motor and keep it quieter. I found that this was possible, after some analysis.

To start the analysis, the first thing I did was to take the lines off the hull of the existing boat. It’s a nice, shallow V hull, actually, an arc bottom, much like the Lighting. The Star, the Blue Jay, and the Snipe are other examples of this type of hull. It does not pound much upwind in a chop when sailing heeled, but will slap when flat. So I’ll live with a bit of slapping or pounding upwind. (But I don’t need to seek out afternoon winds now, either.) It’s a bit like the optional V bottom offered by Chesapeake Marine Design on the larger Redwings.

The Redwing 18, by Karl Stambaugh, was a major influence on my thoughts during this time. It just looks so good! George Bueller, Phil Bolger, and David Gerr’s ideas also played major rolls in my thinking. George Bueller has some good ideas for long range, rugged cruisers. The ideas, and his art, attracted me, but I did not want to make building a large boat the center of my life for three or four years, nor is he thinking of trailer boats. Phil Bolger’s ideas have long been in my thoughts. How did he get those long, light, and narrow power boats to go so fast on so little power? David Gerr has some of the best explanations of boat speed and power requirements I have seen anywhere. The whole idea of calculating hull form, weight, and power requirements for a semi-plaining boats seemed swathed in mystery, but Dave’s writings cleared it up. I recommend the “Propeller Handbook,” chapter 2, and “The Nature of Boats,” chapter 20 and 21.

In addition, I found several books and magazine articles on boat design that covered displacement calculations, center of buoyancy, center of gravity, and stability. I tried many of the formula out first on my drawing of a shortened version of Bolger’s Topaz. But I was not quite satisfied with the layout and a bit uneasy about the stability of the scaled down boat. So I decided to see what I could do with the Loa hull. After all, this was a known quantity, and even better, it would not require a boatload of plywood , fiberglass and epoxy to build! (Sorry about the pun ;-) If there is interest, I can write about the design process that has lead me to the place where I can have some confidence in the result.

So that’s where I am. I think I will end up with a nice day boat, and also, a river and sound cruiser, one that can live by the house, one that I can trailer to a ramp near my destination whether that be near or far, one that has the range for some serious inside passage cruising. I don’t know just how it will turn out, but that’s part of the fun. Wish me luck!

Tom Elliott