## 2234Re: [AtkinBoats] Unsanctioned motor question

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• Feb 1, 2009
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Unsanctioned is a full-on planing boat, so she'll go faster if you put a
bigger engine on her. Playing around with George Crouch's formula* for
guessing planing boat speeds we can work back to find that Unsanctioned
probably weighs around 2,300 lb. when she goes 15.65 knots with 25 hp. Big
Twin. If we could find a 50 hp. motor that weighed 85 lb. she'd go about
22.1 knots, but a 50 hp. Honda weighs 205 lb., so with that engine we'd
only get about 21.6 knots -- 24.8 mph. A 90 hp. Honda weighs 365 lb. and
would push Unsanctioned 28 knots, but at that power you'd have to consider
carrying more fuel and beefing up the construction to handle the power.
Lets say 150 lbs. for that -- 27.25 knots, 31.3 mph. Weight matters, so if
you take a couple of corpulent friends along you'll go slower. At an
all-up weight of 3,230 lb. top speed with a 90 hp. Honda would be around
25 knots, with that same couple of big friends and the old 1951 Big Twin
speed would be around 14 knots. So, it'd be advantageous to have more than
25 hp. (the Big Twin was one of the most powerful outboards available in
1951), but going bigger than about 50 hp. would be pushing things, IMHO. A
50 hp. Honda will carry you, the boat and those two fat friends around
19.6 knots, or around 22.5 mph. Plenty fast enough for a cruiser. Of
course these are only guesses... Even with the 50 hp. motor you should pay
extra attention to making the transom area strong. You've got to consider
that the heavier motor is going to be straining the boat more while you're
trailering down the road, as well as when you're on the water.

Playing with Crouch's formula quickly drives home the point that _weight_
is the enemy of planing boat performance! Build strong, but don't add a
little extra something here and there to make it "hell for stout." It'll
add up and spoil the boat's performance. Something to think about when you
load the boat, too, or thinking about an extra helping of dessert. ;o)

* Kts = 150/(Weight/Horsepower)^0.5

On Fri, 16 Jan 2009 14:14:05 -0800, Troy wrote:

> Hello all,
> I am looking at the boat unsantioned and was wondering what would the
> upper speed limit and hp max be for this style of hull. I love the
> looks of it and the "ease" of building with plywood. It mentions a 25
> hp would get it to 18 mph.
> Thanks for any help

--
John <jkohnen@...>
Distrust any enterprise that requires new clothes. <Henry David
Thoreau>
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