Portofino
Well-Known Member
There’s a little “ step “ which I referred to as a deflector plate above directly in front of the rectangular UW hole which is larger than the actual pipe that feeds it .
The rear of the box has a leading edge say 50 cm long .
This edge is lower than the lines of the hull , another step .
Others Boats with UW exhausts have seen in the yard are completely different , just holes in the hull , no fwd step or rear edge , some are round ,some oval teardrop shaped streamlined into the hull as to not effect the water flow .
No fwd or rear appendages basically.
So why are Amati s so different ?
Firstly let’s have a look at .....
Steps
3 advantages
1- cut down wetted area thus drag .
2- break suction under the aft body of the hull .
3- increase lift from any leading edge effect .
Now the big disadvantage of conventional fan transverse shaped steps that run to the keel from the hull waterline like on a Montecarlo 30 is to eliminate at suction and employ any or all of the 3 modes listed above they must breath enormous quantities of air .
So in waves in any kind of sea that block the inflow of air to the steps a partial vacuum can be inadvertently set up which actually drags the stern down .
This is because the sides that were open to the atmosphere are now blocked by the waves - not good .
Another minor disadvantage ( for bonus points ) of conventional steps is the inherent potential structural weakness due to sudden change in sectional area , you can loose continuity of structural elements that withstand the severe longitudinal bending moments , extra framing and weight may be needed .
Rough water wave action frequently piles solid water above the side vents and thus suddenly changes the action of the step from reducing resistance to suction which enormously increases resistance .
How ever this where Amati,s design comes in .......
Alternatively if you use the exhaust gases to vent , it’s not dependant of the lateral air vent being patent and carries none of the disadvantages in waves , the real sea
So neutral in waves .In fact the more rpm to more gas esp with larger displacement engines .
Are you starting to see a theme ?
By placing the two ( Ok small steps ) either side fwd / rear of the exhaust box outlet you eliminate the suction load in that area behind as well as reduce resistance/ drag .
That’s the difference between Amati UW exhausts and others .
He uses steps
Effectively you reduce the aspect ratio of that part of the hull .
Do bear in mind the engines are in the centre so, there’s plenty of hull laterally behind to profit from this .
Quote Amati
“ for those who love the sea “
P if you can a opportunity to see a Itama on the hard , you will be shocked how different it looks underneath to the rest of the planing boats .
View attachment 72500
View attachment 72501
The rear of the box has a leading edge say 50 cm long .
This edge is lower than the lines of the hull , another step .
Others Boats with UW exhausts have seen in the yard are completely different , just holes in the hull , no fwd step or rear edge , some are round ,some oval teardrop shaped streamlined into the hull as to not effect the water flow .
No fwd or rear appendages basically.
So why are Amati s so different ?
Firstly let’s have a look at .....
Steps
3 advantages
1- cut down wetted area thus drag .
2- break suction under the aft body of the hull .
3- increase lift from any leading edge effect .
Now the big disadvantage of conventional fan transverse shaped steps that run to the keel from the hull waterline like on a Montecarlo 30 is to eliminate at suction and employ any or all of the 3 modes listed above they must breath enormous quantities of air .
So in waves in any kind of sea that block the inflow of air to the steps a partial vacuum can be inadvertently set up which actually drags the stern down .
This is because the sides that were open to the atmosphere are now blocked by the waves - not good .
Another minor disadvantage ( for bonus points ) of conventional steps is the inherent potential structural weakness due to sudden change in sectional area , you can loose continuity of structural elements that withstand the severe longitudinal bending moments , extra framing and weight may be needed .
Rough water wave action frequently piles solid water above the side vents and thus suddenly changes the action of the step from reducing resistance to suction which enormously increases resistance .
How ever this where Amati,s design comes in .......
Alternatively if you use the exhaust gases to vent , it’s not dependant of the lateral air vent being patent and carries none of the disadvantages in waves , the real sea
So neutral in waves .In fact the more rpm to more gas esp with larger displacement engines .
Are you starting to see a theme ?
By placing the two ( Ok small steps ) either side fwd / rear of the exhaust box outlet you eliminate the suction load in that area behind as well as reduce resistance/ drag .
That’s the difference between Amati UW exhausts and others .
He uses steps
Effectively you reduce the aspect ratio of that part of the hull .
Do bear in mind the engines are in the centre so, there’s plenty of hull laterally behind to profit from this .
Quote Amati
“ for those who love the sea “
P if you can a opportunity to see a Itama on the hard , you will be shocked how different it looks underneath to the rest of the planing boats .
View attachment 72500
View attachment 72501
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