im planning something here … and ive seen occasional reference to something about how theres a “maximum hp to weight” ratio for boats (unless its used strictly for racing)
anyone know where i can find this info ?
im trying to talk someone into scratch building a inboard powered step hydro, and need to know if theres a HP limit 
This Might help you a bit. I built a 12 foot outboard powered speed boat a few years ago, got the plans from Glen L. Glen L also has some pretty sweet plans for hydroplanes, well worth your time to check out their site.
PLANING BOAT SPEED CHARTS
“How fast will it go?” is a frequently asked question. There isn’t a simple answer, as many factors must be considered. However Chart 1 will provide a rough estimate of the potential speeds possible for a typical planing boat.
Chart 1
The chart is based on pounds per horsepower, a factor obtained by dividing the gross weight by the shaft horsepower. Note that “gross weight” is the total weight of the boat as it would be fully equipped with passenger(s) on board ready to go. Don’t overlook the fuel (6.2 lbs. per gallon) and ANY extra gear carried. After figuring the total weights add a 10% safety factor.
To use the chart, locate the gross weight divided by the shaft horsepower factor along the bottom of the chart. Go upwards from this line to the “average planing hull” curve and read the speed in mph.
Note how important the weight factor is in a planing boat, and the smaller the boat the more important it becomes.
Chart 2
As an assist in determining gross weight Chart 2 indicates the weight of a typical outboard motor based on the horsepower. The weights are an average taken from several outboard motor manufactures and included four cycle types.
glen-l is the shit but i donno how i missed that page on their site
www.svensons.com/boats also has some decent free plans, im thinkingthat the “rocket” would make a hell of a fun boat. a 250hp car engine should do do well
i have alot of planning to do … lol
the rocket looks similar to the boat I built, but I only have a 25 HP outboard, and thats scary enough for me. putting 250 HP would be guaranteed brown pants, fun nun the less. . Ill try and post a few pics of my boat for you. I would love to see something like the rocket on the water. Ever been to the grand island classic boat show? from the sounds of it it is right up your alley.
goodluck
haha, i wasnt serious about 250hp. im thinking that an all aluminium 4 cylinder would work best, something that makes around 100 hp should work well
ive never been to the grand island classic boat show, but heard about it before and its sounded interesting
and i would love to see pics of your boat
ok, so ive found out that the horsepower laws only apply to outboard boats, inboard boats are unlimited
after looking over our options here, im thinking of suggesting that we purchase a complete boat, something like THIS and gut it for parts.
that way we have a 4 cylinder engine that meets code, all the wiring ill need, lighting, gauges, steering, fuel system ect. but the serndrive would get the boot , i dont expect it to be light
well if you have too big of a motor the boat will sit in the water funny with the stern down to far and the bow end up to high, so when you give it gas the bow will rise real high and then how are you supposed to see where your going. And with to much power it might not plane out, and you also run the risk of making it flip over completly or when you stop you will be swamped by your own wake
Dont know if it helps but i guess its just a word of caution
yea to much power wouldnt be good. if we have to much power ill put a stop or detent on it, so we can hammer the gas and then throttle back to “safe” for some high speed cruising, but i dont expect that to be a problem, as the plans for the boat mention the possibility of installing a vord v8 for power
altho the plans for the rocket on svensons.com dont mention a finished hull weight, the “hot rod” on glen-l is a similar boat in size and design. on that boat, hull weight is approx 600, and is ment for v-8 power
the difference is that the “hotrod” is v-drive, and the rocket is a simple prop on a propshaft arrangement
Buying that boat sounds like a pretty good plan, that will definently save you money on all the accessories. and allow you to focus more on the hull. Try searching for boat salvage yards/ hurricane boats. I know there are some websites that sell flooded/damaged boats at dirt cheap that would make a good parts boat.
attached is a pic of the boat I built from its first launch,It is the glen-l flying saucer design. Wasnt registered/totally finished yet. As you can tell I was pretty happy with it, so happy that I forgot to take the bumpers up, a little sloppy seamanship on my part 
since the first pic I added a small wind-shield, more for looks than function. I will try and dig up a newer pic today sometime and post it.
I also will try and get a picture of one of the racing boats from the grand Island boat show. they really do have motors over 250 HP in them,
quite the thing to see first hand.
The Rocket looks like a good deal, the step in the hull might be a challenge to get right, but nothing impossible. the prop on shaft would be much easier than the v-drive. the boat builders forum on Glen-l will provide you with a lot of answers to your questions. the people on there love to help out when they can. One thing I can tell you is that you will have a blast building it.
nice boat :tup:any idea what the top speed is?
the plans for the rocket are rather … um … abbreviated, so they will require some serious planning before even the first part is purchased, im currently starting to draw them out in a 2d drafting program to figure out placemnet of everything, patterns for all the shit, and engine placement, prop shaft routing, exhaust routing ect
not totally sure about the top speed, but I have raced a couple of jet skis and pulled away from them. I would guestimate that it is around 50 to 55, which is more than fast enough for me in a 12 foot boat.
Glen L has a book out that might help you with the lofting process and interpretation of the plans for the rocket if you need help. sounds like you are on the right path, goodluck
some progress
curves drawn to within .005" of perfectly round, which should be exact enough for the hull to avoid hydrodynamic problems
and the side & top views
top view was one w/ staight sides just to keep track of chine / sheer points when using my leet 2d drafting skills to develop the frames seen above
next i have to figure out the cuts for the keel , and the outer keel, as per the plans the plywood sheeting doesnt meet in the middle (obviously)
frames 5 & 6 might be modified slightly in order to increase the seating area from the current 24" to something more like 30 -32, for comfort & for easy entrance / exit of the boat
edit: oh yea, small block chevy engine, drawn perfectly to scale (except for the oil pan) prop shaft angle is exactly 12*, drawn in just for the sake of having a general reference
im considering the option of making a upper prop shaft bearing, then using a CV joint attached to the fly wheel so i can level the engine in the hull, allowing for proper oil pan clearance without having any problems with oil drainage / oil pickup … just something im mentaly toying with tho
Gotta be a little faster than that if you are pulling on Jet Skis… Mine claimed 58mph on the speedo when I had it…
I’d be very wary about building a boat from scratch. There are considerations with regards to venting the gas fumes, making sure you have flame arrestors on the carbs, cooling, making sure no oil or fuel can get into the water, etc. If you do it, you really need to make sure and do your homework.
ive already started to do my homework on the subject, ive ben searching for sites w/ homebuilts, ive started to study up on all the terms, studying different designs, learning about differnt hull shapes and why they work, ive been lurking on a few forums and reading threads, looking at how the boat companies build THIER boats …etc etc
i figure i have plenty to learn, and plenty of drafting to do on the boat i want to do before i can even start to ask questions on forums without sounding like a total noob
im not to worried about the venting / flame arrestors / using marine spec equipment because i plan on using a parts boat, which will give me most (if not all) of the equipment i need, and will also give me a good visual of how THEY did it
if i can handle building cars, im sure i can handle building boats