HP Formula

I'm working on a 1956 Chris Craft 16 foot plywood kit boat. I have a 1950something 30 HP Johnson motor that goes with it that will need restored when I'm done with the boat if I decide to use it, but more power sure would be nice.<br /><br />The information guide for the boat say that it's rated for up to 40 HP but when I do the coast guard formula, ( (2 X 6 X W) -90 = RATED HORSEPOWER ), I come up with 70. I wonder if the boat is rated for 40 HP because that's as big as outboards came in 1956. The transom is strong. Would I be safe hanging a 70 HP on it?<br /><br />I want to keep the boat as orriginal as I can but I'm not sure I can live with only 30HP on this boat.

Re: HP Formula

I have a 16' fiberglass runabout that came with a 75hp Evinrude. As far as I know it's been hanging off the back of this boat since 1964. My manual refers to a 50hp V4 in 1958. I see that same size boats towing skiers with 40hp. How do plywood boats compare to fiberglass for strength and speed? Are they meant to compare to fiberglass?<br /> Boat Photos

Re: HP Formula

I wonder if that rating/recomendation had more to do with engine weight than horsepower. Today you see engines with 20 to 50 horsepower difference, but have almost the same weight. <br /><br /><br />A weak link on any boat is where the transom attaches to the sides of the hull. That may have been a concern relating to engine weight back then on your hull.<br /><br /><br />Anyway:<br />If you put the more powerful engine on it, you might think of reinforcing that corner joint between the hull & the transom-particularily at the top where the stress of the engine is trying to rip itself off the back of the boat when you're going along in FWD.<br /><br />Your application would be different, but on my hydro, you can see the bracing to keep the transom glued to the sides in the first picture with a slab of 3/16ths x 2 Alum. tying the top of the transom down to 3 or 4 frame members on the sides of the hull.<br /><br />Then in the picture of the the back end when it was being painted, you can see braces that will clamp the plywood transom boards to the frame/sides when they are slid down into place.<br /><br />I wouldn't think you'd need something that extensive on your's, but the pictures of the bracing show how the forces from the engine's thrust will run on the back of a boat and how you have to contain them.

Re: HP Formula

Thanks for the tips, guys.

Re: HP Formula

my 1960 glasspar citation came stock with a 40hp v4 and a 50hp as an option, and its 16 feet long and 6 feet wide and shows it pulling 2 skiiers. they made the 75hp in late 1960, but too late for the 1960 model year build's.