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This question arrived via email: Which boat under 60 kg weight will be the speedest ? I just finding compromise between weight, motor power and speed
For example what's speed I can reach with one 65 kg person and two 65+75 kg persons and with 5 HP and 8 HP four stroke engine ? Give me four values for both models of boats, please. Jeff

We only have a couple of dinghies that weigh less than 60 kg. None of them will plane with two persons.
Sorry.

Please do a search for "Crouch formula". It will show you a simple formula to calculate planing boats speeds.
You will see that your figures are extreme.

Dear Jeff and Jacquesmm,

thanks for Your help and suggestion "crouch formula" !
J just found simply calculator:
www.omnicalculator.com/everyday-life/boat-speed

8 HP engine, 245 kg boat displacement (boat 60 kg, person 65kg, person 75 kg, engine 25 kg, tank 10 kg, something light goods) should give nice speed 29-45 km/h !

What kind of hull do You recommend for recreation ? I'm thinking to build from plywood.

Sorry but I can't design a hull that will go at that speed with a 8 HP.

The FS12 has been tested with a 8 HP and one person onboard and the max. speed was less than 15 mph.
With 2 persons, it will not get on plane.
The GV10 and 11 are good little boats but will not plane with 2 persons and a 8 HP.

Thanks. Plane is a "sideslip" ?

So what's the minimum recommendation ?
Which boat will be quickest for example with 15 HP engine and one or two person ?

Plane is about getting on plane. For a boat, it means the transition between turbulent and laminar flow. Once you get in laminar flow (on plane), you go faster expending less energy.
I assume that google translation gets us confused.

To the question: the GF12 would be a good choice to plane with a 15 HP and one or two persons.

How about reverse engineer why you need a lightweight boat to haul a large payload with a small motor but go fast. Then suggestions can possibly be made to fit your desires. With compromise, possible a lot.

Displacement hulls can haul more weight with less power, but hull length limits their speed.

OrangeQuest wrote: ↑Tue Jan 17, 2023 7:36 pm How about reverse engineer why you need a lightweight boat to haul a large payload with a small motor but go fast. Then suggestions can possibly be made to fit your desires. With compromise, possible a lot.

Displacement hulls can haul more weight with less power, but hull length limits their speed.

It is not just hull length but also the overall hull design that prohibits a displacement hull from planning. Tom

Hmm. I did have a 2 stroke 9.9 on a GV10 for some time when our old 15 died. It planed very easily with 2 big (100+80 kg) adults and 1 kid.

So... maybe the GV10 would work with 8 HP but it would be close.

As another point of reference I once had a 2.7m Honwave inflatable with a 5hp four stroke Honda. At 27kg (OB) +10kg(fuel and gear)+ 33kg (boat) + 85kg (me) it would often, but not always get out of the hole and plane. That's 155kg for a 5hp outboard. Add even a small child at 30kg and it never did, not once.