Horsepower rating

Can anyone explain how they decide on rating horsepower for a hull?

I ve hear a hundred rumours - so I would like to get the hard facts.

Thanks!

Gary

The hull designer has some say as to what max Hp he specifies Gary, and usually if they have any sense they err on the side of caution for public liability reasons.

Then they also have something as a "legal" guide which is the relevant Australian Standard.

As an example, My own Hull came originally with a specified 30 HP Max from the designer according to the original plans.

I started out with 25 hp - but once I added all the required stuff to pass survey - the amount of weight meant that 25HP wasn't enough to plane with one passenger and myself in many instances - like freshwater or with a stiff head breeze.

I had opportunity to get a 50HP 4 stroke...so asked the designer what was the chance of increasing the max rated HP for my transom?

He said - no sweat - it's Max under the Australian Standard is 60HP, but he just used 30hp as the max on the plans to keep his butt outta the fire in case of any liability claims ever eventuated.

So for survey purposes he provided me with a set of "uprated plans" with a 60 max HP rating which complies with the relevant Australian standard.

Now having said that - I've seen a vessel very much like mine in length and weight and design - in the Northern Territory, sporting a 200HP Yama donk - so how the fricken hell they get away with that I have no idea!!

I can only suggest they just ignore ALL the rules - OR the manufacturer designer is dumb enough to leave himself liable by specifying something so high and obviously "overpowered" and accept the public liability risk, hoping a claim never eventuates (remembering that it's well outside the Max Australian standard for all but a ski boat)

I'm led to believe that same hull later flipped doing 70+ MPH on a bend in the east alligator R and ended up wrapped in a "U" shape around a bankside melalueca about 15 ft above water level.....story is that the occupants skipped / bounced across the water and ended up on dry land with injuries (which is better than ending up in the water and becomming crock turds).

So..

You have designers Max and you have Australian Standards Max.

Lastly, you have transom weight.

Some hulls just won't float a heavey OB back there on the azz end and so sometimes the "Max" Hp can be determined by the manufacturers weight specified for the transom as well.

A good example is some of the NEW 4 stroke 150 Yams just released are a LOT lighter than the equivalent old technology Hondas 4's of same/similar HP (130?).....so in say a big boat with twins, 2 x 130 Honda's might exceed the transom weight - while 2 x new release 150 Yams might not! Capiche?

In Fact, I hear some of the new 4 strokes are now lighter than their older 2 stroke counterparts - such are the advances in 4 stroke engineering..

Transom weight - might therefore be a consideration - you will recall manufacturers like Qunitrex comming out with the Milennium hull - with a full width transom pod to get the extra bouyancy to carry the 4 strokes additional weight a couple years back.

Some people mistakenly think putting flotation foam in their hull will magically make it "float higher" and therefore somehow support the added weight of a 4 stroke - and this is wrong - theres no extra bouyancy from foam until your boats swamped and full of water, THEN it will help out - but while the boats empty of water - the added weight of foam only robs bouyancy to carry the heavier 4 strokes..

Designer Max (Liability)
Australian Standards (Max capacity)
Hull Transom Weight (Bouyancy consideration)

Lastly, transom strength & bracing design also determine what max thrust torquing the transom can cope with...and this comes back to the designer. More triangular bracing off the keelson and gunnels along with gussets/brackets off the floor stringers (assuming you have some) will also increase what punishment the transom can take.

Thats about as best as I understand it Gary, hope it helps.

Cheers!

Trouty

Thanks for your knowledge and experience

and for taking the time. Just spot on!


The scan of the Standards is hard to read - but I get the idea

Cheers mate!

Gary

Hi Gary, having had quite a bit to do with boats and survey, I would agree with trouty on this. He is pretty spot on with what he's saying

Cheers Samurai