What determines max motor HP on boat?

[BaitThrower]

I gots me an old fibreglass 3.4m Nautiglass - 1981 model i think - (see pics in the "False Floor Help" thread) and the build plate says its rated to 6HP max.
What determines this maximum rating? Is it illegal or safe to go beyond this? Ideally i'd like to put a 9.9HP on it. Currently has an '83 Suzi DT5.

[Kleyny]

I think most of your dramas is with insurance (if you dicide to get any) exceed the max horse power and your insuance is wiped.
have an incident and the someone inspects your rig to find that the excess hp/weight could be to blame and you are in a world of trouble.
I'm sure you can talk to a boat builder and get it re-rated to suit your 9.9 with a few mods like strengthen your stern etc.

neil

[Blackened]

G'day

I do know there's an australian standard, a mathematical formula you're sposed to to whilst drinkng a rum standing on your head and you will get the answer..... It's been posted here a few times

Insurance is your biggest hurdle...... In reality, a 11' glass should handle a 15 well

Dave

[BaitThrower]

As Honky mentioned, maybe I can just take it to a boat builder or someone similarly authorized and they can take a quick look and say, "yep 9.9hp is fine for this hull",perhaps without any mods needed at all and gimme a new build plate??

[troy]

Does the boat builder decide the max hp the boat will take.
The boat i am hoping to get built which is 5.3 m he has been fitting with 200 Etecs.
To much for me.
Troy

[gar26lw]

i had a little 8ft glass cathedral hull once, max was a 6 but we put a 9.9 suzuki on which back then was more like a 15. planed at just over idle. top speed was faster and you could have 3 in with no diffrence to performance but the weight made a difference to handling. i think an 8 would have been the limit really but propped right would have got 50kph out of it. with a 5hp it did 40kph
i think the builder had set the hp rating at a safe limit

[Lovey80]

I'm in the same boat as you. My boat is currently rated at 50hp max and I would like to put a 60hp on it as the new boats are currently rated to 60hp or 120kg max but my plate says otherwise. Who can actually re plate a boat legally anyway.

Cheers Chris

[gar26lw]

i think its the boat builder