# Outdoors > Other outdoors, sports, huts and tracks >  Power boat for the young fulla

## big_foot

So I know this question would be better posted on a boating forum but to save me signing up for one question I thought I might as well post it here in case anybody knows something about boating.

I have recently acquired a 15hp Yamaha 2 stroke with the intention of building a small hull to mount the motor on for the kids to burn around the lakes on.

Now I don't want to build the boat over 2.5m long and its only guna be carrying a child up to 40-50kg, is the 15hp motor going to be to much an back flip the little suckers if they give it to much juice or is that about right?

I've considered offering the 15hp on TM to swap for something round 7-10hp but I want them to be able to get up on the plane.

Any advise would be greatly appreciated, once I get the motor sorted I can get started building the hull on AutoCad  :Pacman:

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## Toby

I can get a plane in a small smartwave with a 4hp so 15hp should be plently. Maybe just limit how much power they can take it to. My nephews motorbike is limited to how much gas he can give it

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## big_foot

yeah I'm worried about the weight bogging the ass at low speed so the trade-in might be the option. Haha chuck her in reverse an it might turn into a submarine!

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## Toby

Yeah, better safe then sorry too.

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## gadgetman

I've got a 5HP on the back of my 3.1m ali dinghy. The previous owner had 15HP on the back but had to weight the front down a fair bit. Have only used it about 4 times with the outboard on.

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## big_foot

Does the 5HP get you up on the plane?

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## Splash

My 15hp merc gets 3 adults up to 25knots on my 3.6 m inflatable so yea I think it might give you greif.

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## Munsey

B F I've had a few boats , Had a 15hp on a 12 ft alloy dingy that planed with 3 adults in it . Don't worry too much about the weight as it is normal that a 15 hp is same as 10 hp just bigger boar.   I'd say6 to 8 hp would be perfect but not essential .

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## veitnamcam

25 odd years ago the family had a 12 foot alloy dingy with Im sure it was a 9 horse on it. It wouldn't quite plane no matter where i sat my skinny white Arse. Then it got a tohatsu 15 on it and was stupid fast with just me in it.
I heard older motors were crank rated power and new ones are prop rated? Perhaps why that 9 wouldn't make it plane. Maybe if it was a 10 foot it would have.

Sent from my GT-S5360T using Tapatalk 2

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## gadgetman

> Does the 5HP get you up on the plane?


Haven't had in anywhere that I could yet. Most of the time I just pull the thing around Ellesmere during duck season.

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## Twoshotkill

My brother has a 15 hp on a 2.5m inflatable (rigid hull) and it will tow a wakeboard and have even towed a 16ft fiberglass boat with 3 people on it on the plane (when they broke down.)
a lot will depend on the prop pitch wether you want top speed  and slow takeoff or quick takeoff and less top speed.
Im thinking a 4 hp is plenty on a light boat... but this depend on how old your kids are and how safe they play and how heavy the boat is.

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## big_foot

Cheers for the info guys yea I think the 15 might be a bit of an overkill but I got it at a sweet price.

Next question, anybody have a 4-8HP they looking to upgrade to 15?

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## leathel

I have a 9.9 that used to plain with a 12ft tinny with 2 pob and all the nets and fish etc.... 

what type of boat are you looking at building? If it has a pontoon type construction 15 would be fun. down the pitch will up the take off but drop the speed but also let the motor over rev if they hold it hard open... if it was me I would keep the 15....but for my son 6-8hp as he would hold it pinned given half the chance  :XD: 


I have a mate that has a 40 on a deeper V 12ft tinny and its great but you wouldn't want to put that in the hands of someone who way want to use full throttle all the time as you could get in to trouble....big time as it gets rigth out of the water at speed.

If the young fulla is quite mature about speed it may be OK but ...... if not mmm not the best I guess

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## Twoshotkill

Can you throw a pic of the motor on please....   what age is it?

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## Chris

Some of the modern motors are rated at the prop ,where the older type was rated at the motor. I would think 15 horses at the prop on a 2.5m is crazy.Even if its rated at the motor its way to much. I'd look at 10' - 12' for that motor.
Another thing to consider is the motor a long or short shaft ?

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## big_foot

I'm not sure how old I'll go out to the shed soon an run the number. Its quite old mid-late 70's I think. I'll put a pick up on the weekend when I get it out, its in quite tidy condition for its ages last owner reckons she runs like a dream.

Short shaft I think but no doubt you guys will be able to tell when I put a pic up.

What are the advantages/disadvantages of short/long shaft?

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## mucko

if you build a deep vee hull it will take more power to plan and give you more grip in the turns 25-30 degree should be sweet and about 4m loa should suit the 15hp nicely have a look at pelin plans they may have something to suit Pelin Boat Plans - Build Your Own Boat

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## Chris

Think regards the shaft length the longer the shaft the deeper the V but not positive on that. 
Think Mariner ,Yamaha are rated at prop ,don't know about all modern motors.
From memory the prop rated motors like adding another 5 hp .?

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## mucko

> Think regards the shaft length the longer the shaft the deeper the V but not positive on that. 
> Think Mariner ,Yamaha are rated at prop ,don't know about all modern motors.
> From memory the prop rated motors like adding another 5 hp .?


Johnson/evinrude changed from flywheel rating to prop rating around 1988   j140hp at the flywheel now equals 115 shaft hp. the trim plate at the top of the skeg needs to be parallel to the keel at half trim the height of the transom will be determinded by the length of the leg (mounting bracket down to trim plate) the trim plate may only need to be 10-20mm lower then the keel. if set to high you will lose trim and also you will ventilate the prop losing power and posablly depriving the leg of water and running your cooling impeller dry.

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## big_foot

> Johnson/evinrude changed from flywheel rating to prop rating around 1988   j140hp at the flywheel now equals 115 shaft hp. the trim plate at the top of the skeg needs to be parallel to the keel at half trim the height of the transom will be determinded by the length of the leg (mounting bracket down to trim plate) the trim plate may only need to be 10-20mm lower then the keel. if set to high you will lose trim and also you will ventilate the prop losing power and posablly depriving the leg of water and running your cooling impeller dry.


Your a legend Mucko, I'll be able to design the whole ass end of the boat off 1 paragraph.

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## Maca49

Only rule with boats is the same as cars HORSEPOWER ----------- lots and lots or you will be disappointed long term

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## big_foot

Listed outboard on TM. Yamaha 15H 2 stroke short shaft | Trade Me

Since I'm such a nice guy 10% discount off final bid for forum members :Thumbsup:

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## mucko

> Listed outboard on TM. Yamaha 15H 2 stroke short shaft | Trade Me
> 
> Since I'm such a nice guy 10% discount off final bid for forum members


Is that not F.N.R selector on the side of the engine in pic 1

looks like someone has spent shitloads on it.

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## big_foot

> Is that not F.N.R selector on the side of the engine in pic 1
> 
> looks like someone has spent shitloads on it.


Lucky I have the wisdom of you guys behind me  :O O: 

Yea shes tidy alright aye almost vintage

Cheers mucko

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## crzyman

Done abit of fishing in a 12ft marco with a 9.9 on it, it had no problem getting 2 adults on the plane.

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## mucko

> Lucky I have the wisdom of you guys behind me 
> 
> Yea shes tidy alright aye almost vintage
> 
> Cheers mucko


just thinking about it at milking thats a 70's 15hp that will = about 10 shaft hp in a post 1990 outboard

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## big_foot

So I've scored a motor for the boat, 4hp evinrude, it was the smaller end of the power range I had in mind it just means I'll be able to get the boy in it at a younger age and always have the option of a re-power.

I've been collecting old pairs of chainsaw chaps from my workmates, why you ask? each leg has 6 layers of kevlar cloth, this shit is tough as nails and seems to be an awsum substitute for fiber glass mat. Each pair of chaps has about equivalant to $30 worth of cloth of I was to buy from Bunnings, I bought 3 pair home tonight and already got a few others. Some people call me cheap, I prefer "resourceful".

However Ive encountered a couple of issues, firstly I need to get all of the oil residue out, I was thinking soaking in a bucket of degreaser?

Secondly, cutting the shit is insane. I'm using a Tajima utility knife to cut the kevlar out of the legs and I have to break the tip off about every 500-700mm. Any suggestions? (how many smart asses are gunna suggest a chainsaw???)

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## Toby

Grinder with cutting disc?

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## big_foot

> Grinder with cutting disc?


Like a blade cutting disc? one of those steel cutting discs would pick up the thread an leave me in an almighty tangle :Bouaaaaah:

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## Toby

Yeah, second thoughts haha.

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## mucko

the best way is to use carbide or ceramic shears i googled it LOL i am picking they wont be cheap but will last ages.

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## big_foot

Thought I'd give the thread and update.

I recently scored a Kawasaki jetski hull off trade me for $1 (dumping the unwanted parts cost $30), it was a shit load harder than expected but I finaly managed to remove the top half and rear third, leaving me with this which is to be the hull of the boat, havent measured it but its around 2m.



I sold the yamaha 15h and scored a 4hp evinrude, This is a bit less grunt than I wanted but it just mean I can get him in it younger then upgrade later  :Thumbsup: 

The guy I bought it off said it pumped water sweet, I got it home and chucked it in a barrel of water and cranked it up, started fine but I wasnt convinced it was passing enough water. I shut it of and ended up rippin it to bits. Long story short the water jacket was chocker block with shit but there didnt appear to be any heat damage so I decided to throw a new seal kit and water pump impeller in and a good clean up.just gotta get the prop gear case filled and I'll spark her up again.

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## big_foot

Im also doin a test run with the kevlar.

Building a cover for my air horn solenoid on my Hauler.

Just put the second layer of cloth on...aint guna be a pretty job I might try ironing the cloth before I do the top/rear of the boat.

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## mucko

> Thought I'd give the thread and update.
> 
> I recently scored a Kawasaki jetski hull off trade me for $1 (dumping the unwanted parts cost $30), it was a shit load harder than expected but I finaly managed to remove the top half and rear third, leaving me with this which is to be the hull of the boat, havent measured it but its around 2m.
> 
> Attachment 9954
> 
> I sold the yamaha 15h and scored a 4hp evinrude, This is a bit less grunt than I wanted but it just mean I can get him in it younger then upgrade later 
> 
> The guy I bought it off said it pumped water sweet, I got it home and chucked it in a barrel of water and cranked it up, started fine but I wasnt convinced it was passing enough water. I shut it of and ended up rippin it to bits. Long story short the water jacket was chocker block with shit but there didnt appear to be any heat damage so I decided to throw a new seal kit and water pump impeller in and a good clean up.just gotta get the prop gear case filled and I'll spark her up again.
> ...


when you fill the leg with oil fill from the bottom plug until it comes out the top one heaps quicker. have you rang pete lately theres a few hares about.

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## big_foot

Hey Mucko yeah I gotta take it down the boat shop get him to fill it save buying a whole bottle.

I rang him a while back, lifes been crazy busy ill make it a priority in the next few weeks. how long till you move on?

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## veitnamcam

I just had a thought...... Have you tryed it in the water with the outboard on yet?

I reckon building some small sides on top of the hull could be prudent, my ski used to sit at that line and granted it had 60l of fuel a bloody big twostroke injected triple and a jet unit but all that weight was in board not hanging off the arse.

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## mucko

> Hey Mucko yeah I gotta take it down the boat shop get him to fill it save buying a whole bottle.
> 
> I rang him a while back, lifes been crazy busy ill make it a priority in the next few weeks. how long till you move on?


I pick up the truck tomorrow be gone by lunchtime Saturday.

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## big_foot

> I just had a thought...... Have you tryed it in the water with the outboard on yet?
> 
> I reckon building some small sides on top of the hull could be prudent, my ski used to sit at that line and granted it had 60l of fuel a bloody big twostroke injected triple and a jet unit but all that weight was in board not hanging off the arse.


I havent had the chance to do the fiber glassing yet so the arse end is open, Ive removed a shit load of weight consider Ive taken off the top layer and the middle structure and only going to be adding a light weight top and arse, the motor is about 20-30kg and driver wont be over 50kg and it will only carry about 5l of gas. I'm prety confindent by the time im finished with driver and all it wont be heavier than the plain hull that didnt even have motor, jet unit, large feul tank, 2 adults.

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## veitnamcam

Get cracking then i want to see a Vid of its maiden voyage  :Have A Nice Day: 

Sent from my GT-S5360T using Tapatalk 2

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## mohawk

Kleinste scheepje ter wereld komt uit Nieuwe Pekela - YouTube

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## big_foot

> Kleinste scheepje ter wereld komt uit Nieuwe Pekela - YouTube


Haha yea thats very similar to what Im goin for....hopefully I'm guna get a bit more speed though.

Cheers for the info mate hopefully I'll have the arse end in and a float/power test within a month or two

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## big_foot

Making progress, got the arse end and transom braces in. Sourced a couple of fuel tanks from Bunnings at $10 a pop saved a bit of coin, thought I was gunna have to get a little custom tank built.

Anyway motor fits good coupla small jobs and she'll be in the drink for a test run.

Long way from complete yet, but the kids can't say "power boat" let alone operate one so plenty of time.  :Thumbsup:

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## mohawk

Me thinks you might be using the kids as an excuse too make neat little boats for dad to test drive .
looking good so far .

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## big_foot

> Me thinks you might be using the kids as an excuse too make neat little boats for dad to test drive .
> looking good so far .


Busted! just don't tell the mrs. its a whole lot easier applying for funds for the kids than myself.

We'll see, if the 4 horse has enough herbs to plain my 85kg, when it comes time to build the seat it may just be big enough for me  :ORLY:

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## big_foot

Test day finally come around today, my brother came over and fitted a camera on the front pointing back and also set up on the jetty with another camera.

Took me ages to get going as I still havent quite got the fucked up throttle/tuning system susd but we got there in the end.



Didnt have the balls to get me on the plane, I did a timed run along the top of the lake and she clocked in at around 8kph or 4.5 knots.

Might look into upgrading to 8-10hp.

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## big_foot

Got my brother to whip this up for me

https://vimeo.com/74698899

Password: imonaboat

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## Toby

Hell yeah! Great video great song

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## gadgetman

That worked, ... eventually. Good stuff.

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## Gibo

Ha ha ha ha ha and your flippy floppies!!! Classic

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## ishoot10s

That's a laugh, well done!  :Thumbsup:

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## big_foot

Been a busy little beaver in my spare time lately, cockpits starting to take shape. Back of the seat needs finishing and painting then i'll be gluing on padding. steering column is all but fully fitted, wheel needs to be shaped and fitted.

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## Joe_90

to dig this out of the distant past. Did it get finished and running?

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