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Little Tinny, BIG Motor


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What's the biggest motor you can put on a little tinny? Are there any legal restrictions on boat size/engine size? Thinking 3/4 mount V8 tinny;) .......You can take the racer fishing, but you can't take the racer out of the fisho......

Who's got the smallest boat, biggest motor here?

Still impressed how (possibly?) yellow peril's bright yellow tinny took off out of Colmslie the other weekB) B) B) (Feel free not to entertain me, just talking myself into a boat rather than a Harley:) )

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As far as I am aware there are no laws governing what size motor you can place on a hull.Some manufactures will upgrade a transom to take a higher hp motor.

The only thing is that you have a duty of care to operate the craft in a safe manner.

Just look at some of the race boats and thundercats out there to see what they hang off the back of relatively small craft.

Cheers

Ray

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i can say that dealers are obligated to go to the max hp recomended by the manufacturer, but will not fit a larger hp motor to the boat in question, and if buying new, the motor has to be fitted by an authorised brand dealer, or risk fitting it yourself and no warranties. the police have the power to give you a whopping fine if they find out the boat is overpowered, all due to a boat crash years ago where lives were lost in a single boat crash in the brisbane river. then all new laws were rushed through to try and eleviate this from happening again, and totally agree. after watching people try and drive the ski race boat i had years ago, i can understand why, driving a boat at speed requires more than pointing it where you want to go and push the throttle down.

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I like the idea, would be a bucketload of fun to build but i am not so sure about the engineering side of it though. Small boats like that are not quite designed to handle great speeds. With the work that would be involved to make it safe and useable it maybe cheaper to buy a purpose built machine.

I would love to see it if you have a go at it.

(And a harley would just be a waste of good money)

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here is a little race boat we had a long time ago, it had a 55 chrysler and the boat was 3 metres, the problem was no trim/tilt our jacking plate, it use to haul arse to the point of scary, never found out what the top speed was as no one that we knew had a boat with a speedo that could keep up [img size=486

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if you want to buy a package from a dealer be prepared to be bored

however if youre asking to put a v8 into a tinny then transom ratings are completly irellevant itll come down to picking a hull thatll give some stabillity at high speed and working out a mount system for a drive either jet propshaft or leg id go for a jet meself getting someone to knock it up get a gestimate on hp pay reg for length of boat vs hp maybe pay 70 bucks for an engineers certificate, then go and kill yourself some place scenic after letting as many afo's know first of course so they can bring cameras .

i had a tuned 65 on the back of my 14ft quintrex and it was very quick it could pull a knee boarder out the water with 8 people in the boat and would chase down the old jetskis in flat water in the bay it was a bit of a handfull but still good fun :)

jason

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noname wrote:

...... the police have the power to give you a whopping fine if they find out the boat is overpowered, all due to a boat crash years ago where lives were lost in a single boat crash in the brisbane river.

Not sure if this is completely correct, as you will probably find that the skipper involved in the boat crash was charged with something more along the lines of dangerous operation causing death. But will follow up in infringement issue soley for overpowering froma personal level too as my little 4.2 runabout is now out of warranty and the 40hp Mercury on the back, might just get some workshop tuning with next service to release it to its 60hp capacity.

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wasn't that crash at the river mouth booty, another 1 years before that 1, up near indroopilly somewhere, driver and 2 occupants were killed. oh by the way, the driver was 16 and the boat, i believe had a 200 on like a 15/16 foot boat, i personally believe it was inexperience that caused the accident. the driver was female. :huh:. :unsure: . not the way the boat was set up. i personally believe it is better to overpower a boat rather than vice versa.

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  • 2 weeks later...

well dude, legally you can only put on what the boats compliance plate specifies. for instance i gotta 13ft tinny which specifies 25hp as its limit and mines gotta a 30 but thats not to bad considering iv seen a smaller tinny with a 60!!! on the back, and iv had water police pull me up and the only thing they cared about was necessary safety gear, registration, licencing and illegal fish sizes/ species onboard

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gonefishin wrote:

well dude, legally you can only put on what the boats compliance plate specifies. for instance i gotta 13ft tinny which specifies 25hp as its limit and mines gotta a 30 but thats not to bad considering iv seen a smaller tinny with a 60!!! on the back, and iv had water police pull me up and the only thing they cared about was necessary safety gear, registration, licencing and illegal fish sizes/ species onboard

i agree with your first comment mate about the legal size motor in comlpiance with the plate but as for water police asking about size and limit of your catch you sure it wasnt the fishing and boating patrol...

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Just a wee bit dangerous. Re max hp another factor to be considered is the transom weight.

All the literature and the compliance plate on my boat says max recommended hp and weight.

As Booty pointed out there is no legistation in place to enforce this.

Some manufactures offer a reinforced transom and additional boyancy to enable larger motors to be fitted.

As long as you can prove that you have taken consideration your duty of care obligations you will never have any problems.

Some boats are absolute dogs when they are only fitted with the max recommended hp/weight motors.

Graham Pflaum has a 3.9 allycraft shadow it only had a 20hp 2 stroke yamaha on it when he got it and as it would only just plane with 2 up.He wanted to go to a 30hp 4 stroke and one salesman said no probs even though the hull was only rated to 25 hp and another said he could only go to 25hp so he went to a 25 hp and it still will only just plane with 2 up.He has tried several props and no difference. He has seen plenty of others with a 30 and they absolutely hoot with 3 up.

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  • 10 years later...

I have a 4.09 Stessco normal hull, with 30 HP four stroke Mercury, tiller/electric/power tilt n trim, boat is rated to 30 HP and 4 persons. Depending on the size of 2 adults it will get to plane, but 3 or 4 in her and it's 4 knots at best. So how is this? I only mostly use it by myself and dog, imagine if you paid nearly $12,000 like i did and wanted to take a few mates fishing, load of ****ing CRAP! Sure motor and battery are heavier BUT? For 4 adults it would need a 60 HP or at least a bloody good 50 HP. Was purchased as a tender for 18 m cruiser i had, otherwise i would have bought something with a 60 HP to 115 HP and a boat of around 6 M in length. 

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I had a 4.2m Stacer with an old 30hp Johnson tiller steer - it struggled to get on the plane coming back from Mud Island years ago with myself and two mates (each of us 105-110kg) but from memory it was also taking on a little bit of water. Mine had a hydro foil when I acquired it so I don't know how much difference they make but would be worth trying.

Someone more knowledgeable than me may also be able to advise whether 2 strokes produce more power than 4 strokes?

Is there much extra weight in the boat?

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How long have you had the rig? I personally would be taking it back to the dealer and get them to put motor on computer and if no issues found then water test may be something as simple as kink in fuel line or wrong size prop or trim issue. Let us know how you go.If you were in Brisbane I would recommend taking to Coorparoo marine 

Cheers Ray

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  • 7 months later...
35 minutes ago, Paul01 said:

hi guys a bit of advice

i have a 4m deep side tinnie currently old 80hp merc, i am wanting to put johnson 115hp. only 40kg heavier, do you think this be ok.

 

That is a LOT of motor for a 4m boat. We have a 115 on our 8.5m. 

I don't think anyone here could give you an answer as we have no idea what your boat or transom is like. 40kg extra is a lot right at the back of the boat. How does the 80 go with you at the back? 

There are marine engineers that could give you help.

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2 hours ago, Paul01 said:

hi guys a bit of advice

i have a 4m deep side tinnie currently old 80hp merc, i am wanting to put johnson 115hp. only 40kg heavier, do you think this be ok.

 

There should be a compliance plate on the boat specifying the maximum allowable engine size. I suspect that it doesn’t say 115hp! Don’t exceed the stated maximum otherwise you void insurance and risk everyone’s life every time you go out.

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On 13/09/2020 at 10:51 AM, Paul01 said:

hi guys a bit of advice

i have a 4m deep side tinnie currently old 80hp merc, i am wanting to put johnson 115hp. only 40kg heavier, do you think this be ok.

 

Hey Paul01

Thought I'd just welcome you to the forum. Hope you can find some good information on here 🙂 

That's a pretty big motor - GregOug gives some great advice above 🙂 

Cheers Hamish

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