is this plausible?

[restornator]

claims to reach 50mph with a 15hp outboard...

http://www.dngoodchild.com/5859.htm

[sparky_wap]

Light (total) weight and proper outboard with piercing prop. That hull design? I was expecting to see a hydroplane style.

I'll leave the hull evaluation to the experts!

[Submarine Tom]

Well, I'm 200 pounds, that leaves 100 for the boat and motor, so, no.

-Tom

[PAR]

Yes, the boat will do 50 MPH with an average size (160 lbs.) skipper aboard. The stepped scow will scoot along pretty fast in very smooth water, but once the chop appears, then the fun goes away quickly.

Using taped seam building methods, yes, even you Tom can do 50 MPH. The hull will be in the 120 - 130 pound range as a plank over frame build and very likely under 100 as a taped seam build. Naturally, light weight racing hulls don't hold up all that well long term, but this isn't the point of the design. It's intent is to scare the crap out of you, with the first few runs then get pounded to death, in fairly short order afterward.

[daiquiri]

It is plausible for a hydroplane of that size.

However, 300 lbs weight with motor and pilot included, means 135 kilograms. A 15 HP motor, two stroke, typically weighs between 35 and 45 kg - take 40 kg as average. Add some fuel (3-5 kgs is enough?) A light-weight person weighs around 75 kg, but 80 kg is more probable.
So, 45 + 80 kg = 125 kg for motor, fuel and the pilot, which leaves 10 kgs (22 lbs) for everything else in the boat - hull, command cables, wheel, windshield etc. That's not possible, imho.
So the 300 lbs weight estimate is too optimistic - something like 375-400 lbs would be more realistic.

[sparky_wap]

Are standard 15 hp outboards available with the correct pitch prop and gearing to make 50 mph?

My old 2-stroke 20 hp Johnson has a 9" pitch prop with a maximum available pitch of 12" and a 1.75:1 gear ratio. Even with the 12" prop and 4800 rpm, I'm estimating around 30 mph with little 'slip'.

With these gears (1.75:1) and holding 4800 rpm, I think a 21" prop would be required to make 50 mph.

[PAR]

At 500 pounds I'm getting high 30's MPH range with no slip, suggesting the 50 MPH target is possible (at a lighter weight), of course assuming all the variables are sorted.

As kids we fooled around with this sort of thing. No shifter at the helm, just bang it into gear and hop on the seat. Some used simple lever steering, though most did employ a simple drum, cable and pulley steering system. Throttle often was a set and forget thing as well, but again many did use a lawn mower or bicycle cable to work the go fast lever. I can't tell you how many "manual hand choke kits" I used as the throttle adjustment on a little boat.

[tinkz]

yes plausible..
http://www.svensons.com/boat/?p=HydroPlanes/spitfire
(most similar and free plan I've seen)
but, you have to keep in mind they're talking about 10-15hp with a "quickie" lower end unit for taller gear ratio AND a steep pitched racing prop on it.

100-120 lb hull, 100 lb engine and fuel, 165 lb pilot, yup.. 365-385 lbs all up.
on glassy water (with the gears and prop) it'll scream, the wetted surface becomes next to nothing because its made to "air out" and ride on just the trailing edge (midship) and the rear edge of the transom. same concept, but probably a better design.
the spitfire has the arc shape forward, and the non-trip chines aft.

finding gears and props to make it fly right might be a real challenge for whatever engine you're thinking about playing with for a project like this, havent looked myself.

[u4ea32]

http://continuouswave.com/cgi-bin/crouchcalc.pl

A stepped hydroplane, as the referenced boat, could hit 50 even at a total weight of 350 lbs.

Will require a lot of screwing around with gears, props, etc to pull this off of course.

[Typhoon]

Why not?
http://www.youtube.com/watch?v=z5k-CRH4_CM