which one to use. 2.5hp foot or 5hp foot. for 8ft boat

hello all and good evening. my names mark and i have a small problem that i cant realy make my mind up on.

i have a 4STROKE virtical shaft 5hp race tuned briggs and stratton running at 4000rpm and about 6.3hp. its tuned for high end power. I would like to connect this to an outboard foot to power my 8foot boat so nothing new there. Briggs and stratton already sell a 4stroke 5hp OHV outboard with a 1:1.75 gearbox reduction but alas my pockets are kinda empty.

soooo..

the thing i cant decide on is wether to use a 5hp clutched foot (FN) and have it connected directly. foot been driven at 4000rpm @6.3hp or......

to use a clutched (FN)2.5hp foot and gear it 2:1 from the engine.

the later would mean that the 2.5hp foot would be driven at 8000rpm and 3.15hp.
normal outboard range
2stroke outboard factory rpm range 4500-6500 5hp
4stroke outboard factory rpm range 4000-6000 5hp

basic boat spec.

monocoque plaining hull
LOA 94inches
BLW 24inches
DEPTH 12inches
weight 25kg
engine 15kg
rider 55kg
fully loaded weight 110kg
volume not calculated
above water turning stabalizer fins 12inches

im trying to get maximun speed as my other craft is a 4ft wide inflatable using a seagull lol.

so which ones it too be. 2.5hp or 5hp. would the 2.5hp prop spin to fast and and cavitate so the gearbox blows or would the 5hp just give too much static trust and not enough top end.

as for price of the foot there realy doesnt seem to be much between the 2.5hp and 5hp option.

i havent even tried to think about the prop yet as i need to decide on the foot/gear ratio.

sorry for rambling on and thanks for any and all the help you can give me..

 

I ran a 5hp on an 8ft boat and it was a perfect match, at least for me. 2.5hp would have been too weak. It was planing hull(s). It was a flat bottom cat with 2 2ft beam hulls.

 

Get a mercury 4.5,7.5,or 9.8 lower unit from a junkyard. They are all the same unit just different props on them, with your engine I think the mercury 7.5 prop would work the best. If it is too much for the engine you can just shave material off the prop.

 

hey there and thanks for the advice.

iv been umming and arring and after searching YOUTUBE and seeing what sort of horsepower pushes different craft at differnt speeds. i went on the materials hunt again and iv had a few lucky finds this week. the first being an electric start 13.5hp mountfield engine for 70 quid ( its exhaust exits from underside of the engine) i have all the electrics for this also inc, tiny TACH (40 quid ebay will help me tweek the govoner a bit and finalise the prop pitch), key start. The second is a nice blower unit ill run directly from the flywheel side of the crankshaft to use as a supercharger. (should help the engine breath a lil bit) and thirdly i was chatting to a fiberglass repair guy at the local boat yard about my project, he said his mate has pallets of scrapped outboards of which he had one in mind for me. ill be back first thing monday morning to see what hes been able to get. He also said itll have FN gears a prop and it FREE OMG, there are some nice people left out there.

Rangerspeedboat. great idea mate and i hope the fiberglass guy comes back with a merc 15hp so ill have the freedome of the different props to try. iv read that a 200rpm change in rpm can be made with an inch adjustment in pitch. as to shaveing material off the prop, i dont have the tools to do this acuratly as have spent a few weeks balancing standard seagull props for a different project and ill tell you a well balanced prop is hard to get by hand but makes a massive difference. modern props shapes and profiles have been finetuned by computers a little smarter than me. but we will see lol.

hoytedow. i bet ya had good fun, how fast did you manage to go by the way. iv designed my instrument pannel to take my car satnav and tiny TACH. i dont intend on getting lost but ill know my exact speed in mph at all times.

i only have my Iphone for a camera but ill take some pictures of my boat, engine etc. ill try and post them this weekend.

 

Its a reduction gear .. prop half engine speed...you cannot loose power but you can multiply torque...looks like this when its done....throttle and noise is th biggest problem

 

hi there guys. what a result, the dude down at the boat yard came back with a whole evinrude outboard. it looks in spanking condition. has compression and spark. wow. i havent had it running yet as i need a remote fuel tank. if the engines good ill use it as its alot smaller and lighter than the 13.5hp mountfield. theres no electric start tho and this might be a problem as i wont be able to reach the start cord if it conks out on me.



pistnbroke: nice set up there mate. ill be using either a 13.5hp or 15hp engine like i said. ill also be using the gearbox exhaust outlet to quieten my engine exhaust note. where as you have added your 3.5hp briggs to where the origional powerhead sat, ill be mounting my engine directly onto the gearbox/foot. engine height from bottom of skeg to top of engine cowling should be just over 24inches. i want remote throttle and start. also ill be raising my max rpm to about 5000 if i use the mountfield and if i use the evirude then there WOT rpm is 4500-5000rpm.

i cant wait to get to the workshop tomorow...

 

you are a very brave person Good luck with you project and I hope you stay dry...

 

Brave. I was hoping not to read that because being brave in the boating world is akin to being stupid lol. I have 122.72hp a ton which is more than my car. I ran some calculations as to max speed and they did seem to be very high. I'll run them again with some new data as to the new total craft weight, new prop and gearbox ratio and some other bits and get back to ya.

Do ya think I'll end up wet, or need a crash helmet lol

 

I didn't have a speedometer but it went a good clip when I was alone, probably well over 10 mph.

I didn't stay dry.

 

hoytedow: lol lucky iv got a wetsuit. i think ill save the actual test run for the spring then. 10mph is a nice speed mate. at eight foot long, how much was your total weight/displacment? and also do you remember the gear ratio and prop dimesions you were running? should help me tweek my calculations a bit. cheers

 

Ha! The only reason we got wet was because we tried to fit a 3rd person on the boat.
It was built to hold 2. A boat went by fast and sunk us with its wake.

It displaced about 6.5 cubic feet of fresh water properly loaded. Resting the draft was maybe 4 inches.

The motor was a Sears (Eska) 5 hp outboard, whatever was the standard at the time.(1960's mfg.)