Atkin Plan Roaring Forty - Engine Help

[Michael R.]

What are the engine specs Atkin is referring to? He mentions 320 cubic inches and 950 Lbs? Is this only 55 HP at a speed of 12 MPH? Any help appreciated.

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Roaring Forty
A 30' 6" Raised-Deck Fishing Cruiser
By William Atkin A Seaworthy and SpeedyCruiser for Fishing in Rough Water Roaring Forty was designed with a view to seaworthiness and good speed and should be a very useful kind of boat for limited cruising and for serious fishing in rough water. The principal dimensions are: length over all, 30 feet 6 inches; length on water line, 29 feet; breadth, 9 feet 1 inch; and draft, 2 feet 8 inches. The freeboard at the bow to the top of raised deck is 5 feet 2 inches and to the sheer line at the bow is 4 feet 1/8 inch. The least freeboard is 2 feet 8 inches, and the freeboard at the stern 2 feet 10 inches. http://www.boat-links.com/Atkinco/Cruisers/images/RoaringForty-1.gif The cabin and cockpit are laid out with a view to fishing and limited cruising for a party of two. Providing the construction is kept light in weight, sashes might be fitted under the pilot house top and so provide additional indoor accommodations. Personally I do not like closed in cockpits or hermetically sealed-up closed automobiles; however I am the exception rather than the rule in this matter of being outdoors for pleasure. There is 6 feet of headroom under the cabin top carlins, and full headroom under the pilot house deck. I would rather not see these heights increased for several very good reasons. http://www.boat-links.com/Atkinco/Cruisers/images/RoaringForty-2.gif It will be noticed that the sections show rather more deadrise than is usually associated with small cruising motor boats. My experience has been that the correct degree of deadrise contributes to the comfort and ability of small boats more than any other feature and that proper deadrise and correct balance produce exceptional boats. http://www.boat-links.com/Atkinco/Cruisers/images/RoaringForty-3.gif The motor shown on the plans is a six cylinder model having a cylinder displacement of 320 cubic inches and a weight of 950 pounds. A boat like this needs 55 horsepower at 1,600 r.p.m. and with this kind of power plant will comfortably give Roaring Forty a speed between 12 and 13 miles an hour, the speed in degree depending upon how nearly the hull is built to the lines and the smoothness of its entire bottom.

[mmd]

With that hull form I suspect that 11 knots (13 mph)would require a lot more horsepower than a mere 55. However, the good news is that a lot more horsepower will weigh less than 950 lbs, so it balances out.

I am away from my design computer so I can't run my power-estimating spreadsheet, but I'd guess that to move that hull at 11 knots you'd better install a motor with about 125 horsepower.

[David G]

John K is the most knowledgable person I know regarding Atkin designs. He put together the Atkin website, and sells the plans. He started the Yahoo Group, Atkin Boats.

http://groups.yahoo.com/group/AtkinBoats/

He is also the genius behind The Mother of all Maritime Links

http://www.boat-links.com/boatlink.html

He's also an enthusiast of small engines, esp. the vintage appropriate to Atkin boats. He can probably tell you what older engine would be appropriate for this design... and what modern substitute would be (barely) acceptable (hrrrmmmph!) <G>

You can reach him at either of the links above.

"Knowledge is not simply another comodity. On the contrary. Knowledge is never used up. It increases by diffusion and grows by dispersion." -- Daniel J. Boorstin

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[Michael R.]

The hp you came up with seems more reasonable to me. How do you think the hull would perform at 8 or 9 knots? I am taking a close look for a possible build. Thank you.

[mmd]

Without the displacement and details from the linesplan I am lacking adequate info to do a proper initial calculation, but by making a few assumpions I can come up with a ballpark figure. Assuming a displacement of 12,500 lbs (LxBxDxCb where Cb = 0.35) and a couple of other factors, I think that she would achieve 9 kts on about 75-80 hp through an almost 1:1 gearbox and a 12Dx8P prop. But don't take this to the bank - it's only a guess. Eleven knots (13mph) calls for 105 hp; thirteen knots would need about 170 hp.