what is the tinest engine i could use to plane this boat off?

I think what most would try to tell you is that weight is the critical factor here. That racing 60 I posted will weigh a couple of hundred pounds. A 150 will probably weigh 400 pounds. The Coast Guard plate is a joke. It means something to the effect that when the weight was distributed just so, the thing didn't sink. I have seen many of these placards that were absolutely ridiculous. Please believe the people when they suggest that you are asking that boat to do a lot, and when you throw yet another two hundred pounds in a bigger outboard right in the very back, you are asking for trouble.
I am going to stick with my recommendation for the sixty but I do not think you will go 70 MPH. If it were a racer, I'd say you would already know how much horsepower it would take - likely you'd want the 125 to 150 and flat smoke across the water but if you are taking other's lives in your hands, prudency dictates that you tone down your risk-taking. Plus, if you have the entire drill team and a cooler of beer and ice on there and chop the throttle or hit something, that thing is going down.
Good luck finding someone to validate putting all that weight (and responsibility) on there and seeing how fast you can go.

[racing fan]

well i have seen other pics of the same boat same company year model etc just nicer condition and different paint job with 4 people and old 125 merc. in it and it wasn't sitting that low it was sitting exactly where it should be and the former owners had that big of a motor on it. i have proved that. and i know about the coast guard plate being like that but i am not putting that much weight on it 300-400 motor and 4 times 180 or 720 lbs or just over a 1000 lbs total and this boat is good for way over that so could we please stop talking about what will sink the boat and should i go with a 125 or a piece o crap half running 30 hp

Yet another new variable! "The 30 is a piece of crap, half running"! I will say this: You won't be happy with the 30.
I'll give you $100 for the boat and you can buy one that is a little bigger and doesn't need a rebuild.

[racing fan]

the 30 hp runs kinda it dosen't like to idle for more than a couple min like 2 min. or run at just over a idle in forward for long. it does good at full throttle but every once in awhile it misses on both cylinder like not a back fire but it stops firing and runs on 1 cylinder for like 10 sec. but in a idle it runs on 1 cylinder for ever. it's a semi old engine like mid to late 80's Yamaha that has run a lot and has more hours on it then ever originally intended it's been a good motor but it is at the end of it's life. oh and i spent 300 for the boat so a hundred wont do and i bought it mostly for the trailer but i am gonna fix it up and have a good family boat i have used the trailer on another hydroplane i got.

[tunnels]

Originally Posted by racing fan

the boat is a 1970's 14 ft. ebko sprint and it has a close design or relation to the ebko ski bee but it is a sprint

Yamaha 90 hp with 200 mm set back and set up 15mm above the keel would be a awesome boat specially with one person or two . Could almost fly !!

[racing fan]

yea but i don't really care for flying with 2 people i want to fly with 4 people so that would make a 125 a god choice right?

[Joakim]

I'm assuming this is a flat bottom boat. At 680 kg weight (1500 lbs), you may get it to plane and then about 20 knots with even a 20 hp. With 70 hp you will likely get about 40 knots, which is already like flying to some people with such a small boat. If you really want 60 knots (70 mph), you need to do a lot of work with propellers, engine height and weight distribution. I would guess 125 hp is not enough for that, but you may get close. With a standard propeller and standard engine height, you will not get much more than with a 70 hp.

[Pierre R]

You would need around 175hp to 200 hp to cruise 70 mph with 4 people. I will bet you the boat that the other three people want out before you reach 35mph and I'll bet you want out before you reach 70mph in it.

[Pierre R]

Originally Posted by racing fan

the 30 hp runs kinda it dosen't like to idle for more than a couple min like 2 min. or run at just over a idle in forward for long. it does good at full throttle but every once in awhile it misses on both cylinder like not a back fire but it stops firing and runs on 1 cylinder for like 10 sec. but in a idle it runs on 1 cylinder for ever. it's a semi old engine like mid to late 80's Yamaha that has run a lot and has more hours on it then ever originally intended it's been a good motor but it is at the end of it's life. oh and i spent 300 for the boat so a hundred wont do and i bought it mostly for the trailer but i am gonna fix it up and have a good family boat i have used the trailer on another hydroplane i got.

If the 30hp is a two stroke my guess is it needs new bearings.

[racing fan]

this is a hydroplane not a flat bottom Jon boat it has sponsins that stop and tapper off and air traps and release spots like a normal hydroplane. it will plane off way easier than a v hull. and even a flat bottom swamp boat or hunting boat flat bottom in nature and the piece o crap 30 is a 2 stroke and i happen to know it needs new rings bore out etc. and most likely bearings. i am a bit of a gear head it is not worth it to fix and it is a long shaft this boat runs a short shaft and the mid section on the 30 isn't interchangeable i looked

[racing fan]

i race boats too so 70mph is not a prob. i do that in a 10 ft hydroplane

[racing fan]

i can also run a 150-175 hp but it don't weigh 1500 lbs its can carry that and not sink the boat weighs 880lbs. is the boat seats steering 5 gall. filled gas tank and motor no people so 880 ready to run. just add 4 people

[Joakim]

Originally Posted by racing fan

i can also run a 150-175 hp but it don't weigh 1500 lbs its can carry that and not sink the boat weighs 880lbs. is the boat seats steering 5 gall. filled gas tank and motor no people so 880 ready to run. just add 4 people

Wouldn't 880 + 4 people be about 1500 lbs. An OSY 400 hydroplane can make 70 mph with a 33 hp Yamato, but it only weighs 1/4 of your boat and is well optimized. With an optimized hydroplane of 1500 lbs, you could probably get 70 mph with 70 hp motor, but your hydroplane is probably not very optimized nor is a standard OB, with a huge drive drag at those speeds.

Since you already have a hydroplane capable of those speeds, you should know what it takes.

[racing fan]

yes it would be close too 1500 lbs but i checked it will hold 1500 lbs and a motor its a weird plate on the boat i made a couple of mistakes before it will hold 1500 lbs with out motor but it will hold 1950 total and at 2100 it sinks

[Pierre R]

Originally Posted by racing fan

i can also run a 150-175 hp but it don't weigh 1500 lbs its can carry that and not sink the boat weighs 880lbs. is the boat seats steering 5 gall. filled gas tank and motor no people so 880 ready to run. just add 4 people

Please note Racing Fan that I used the word Cruise and that implies no more than about 70% power to achieve the 70mph continuous cruise. 200 hp X 70% = 140hp