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The Evinrude Lightfour is 15 cubic inches – 9.2 to 9.7 horse power. Johnson built the KD model. At 14+ cubic inches – 9.8 horse power. There KR model is built for racing with anti-fraction bearings. That model is hard to find & expensive if in good shape. The Mercury KG4H was a 15 Cubic inch class "A" racer. They were rated 7.5+ horse power. A good one would double that. A more modern "A" racer is based on the OMC 13.2 cubic inch 9.9 – 15 horse model. These modified models with there slim lower units run over 50 MPH. . . 😀
I have a Mini Mac, another easily built from plans boat. Not as advanced as a real hydroplane but still fun. I have run 3 hp Johnsons up to KF-7 Mercurys. In this pic is a 1956 15hp. The stopped pic is with a1940 Johnson KA-10.
quote SquierKA39:I have a Mini Mac, another easily built from plans boat. Not as advanced as a real hydroplane but still fun. I have run 3 hp Johnsons up to KF-7 Mercurys. In this pic is a 1956 15hp. The stopped pic is with a1940 Johnson KA-10.I built a Minimost with my son when he was 14, and another like it with my grandson when he was 14 (still ahve that one) similar to the one you posted photos of. How much motor depends on how fast you want to go and how close to the edge you want to run. I started my grandson with a 6 Hp Evinrude until he got the hang of it, watching waves, etc, then moved him to a 9.5, then a 10. The 6 did plane it off, once you put your hands out on the deck to hold it down until it built up speed. He’s grown now and in the Air Force, but Grandpa has plans to drop a 15 on it next summer to run himself. Al these are stock, and with a stock 15, it should do around 40 MPH – fast enough for me at age 70, but I still water ski too. My son raan a KE7 Mercury (10 HP advertised but put out 12 to 14 when the RPMs get revving high). Note that motors set up for racing these light boats have a skinny lower unit with special gears and props to get the speeds mentioned in the post about the various classes of hydros. When you are a few inches off the water in a boat this small, picking your way through the waves, and leaning into corners, 20 MPH can feel like 50. With boats like these, you learn pretty fast how to get motors going again after they get dunked. On the smaller hydros like the Minimax, keeping motor weight down is desirable.
There are many boat plans online. The two main sites are below – both have several hydroplanes, and just about every other boat you can build with wood. Most of these plans originally appeared in magazines like Popular Mechanics in the 1950s.:
Dave
quote Tubs:
Some people might think its time for you to grow up.They just do get it. Too bad for them.
Cool Photo!
"Some people want to know how a watch works, others just want to know what time it is"
Robbie RobertsonAll of Hal Kelly’s plans, both Hydroplane and Runabout are / can be obtained for free; download PDF forms, right here-
http://boatsport.org/kelly.html
just sayin’ ! 🙄
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