Hulls v HP

[polyFrank]

I can understand all boat manufacturers wanting to restrict the maximum speed of the boats they have built, to what they feel is a safe limit. They express this in a maximum HP rating for the hull. HP being brake horsepower, which is the power needed to stall the motor at max. rpm.

Now to the point of the post, I have a 115 Suzuki and the owners manual covers the 140-115-90 hp motors. The 140 has the bore increased to give it a higher cc of 2044, the 115 and 90 have a 1950cc motor.
These motor are otherwise identical with the same final gear ratio of 2.59-1

On page 11 of the manual it says and I quote. "PROPELLER SELECTION. It is essential to use a propeller on your outboard motor that is properly matched to your boats operating characteristics. The speed of the engine when you operate your boat at full throttle depends on the propeller used". Unquote.

Then it goes on to give full throttle operating ranges for the 3 motors.
4500-5500 rpm for the 90hp, 5000-6000 for the 115 and 5600-6200 for the 140.

Am I reading this correctly, are they restricting the max rpm and therefore the hp rating by putting different props on the motor?
What I see is this motor has a max rpm of 6200.

Any thoughts.

[chin]

Hard to say what other differences exist between the motors, like camshafts and engine management mapping etc.
The max rpm for those 3 motors is odd though, it's usually the smaller motors that rev higher, and bigger motors rev a little slower.

[BigFella]

A lot of the new engines run identical capacity blocks,
In the Etec line up their 75=90 and 115=130.
In the old days before electronics a simple re-jet and spark advance would give you the higher HP.
These days is more complexed with the level of electronics used, often the base engines are identical with the bolt on stuff like injectors, ignition different.

When we were racing our Jet Sprint boat we had two 400 SB Chev motors, both identical in build, both with 412cubic inches of capacity.
One ran in the 400hp class and the other ran in the open 600hp class.
The changes were subtle but effective, carburetor (600CFM/880CFM), distributor (HDI-3000/HDI-5000), exhaust (tuned length over transom/tuned length ported with sonic rings over transom), cam shaft, inlet manifold (Wiend Tarantula/Victor Junior High Rise) were the major changes to achieve the high horse power.
The smaller engine would only develop usable power to 6250rpm, while the 612hp developed mountains of grunt to 8500rpm.

This is why most out board manufacturers have an acceptable "rev range" and will display their power being gained at an appropriate rpm.

I need a lye down now!

[polyFrank]

Thanks for that BigFella, I was beginning to wonder why I paid extra for the 115.
I do think however that 115 is to low a maximum for the 5.3. My last boat was a 4.75 Quintrex, that model had a 2mm bottom and 1.6mm sides. The hull weighed 390kg, I bought it second hand and it had the maximum size motor on it. A 75 Mercury 2 stroke, giving that boat a power to weight ratio of 192bhp/ton. The cc poly weighs in at 850kg, giving it a power to weight ratio with the 115 of 135bhp/ton. A lot less power on a more stable hull.

[BarryJ]

There was some discussion on the old forum about this. It's related to LOA and beam at the transom:

This formula is courtesy of Zone5

The formula laid down in AS1799 (This is the standard for recreational boating.)

Anyone building a boat, be it commercial or amateur, doesn't have a choice if they don't want to invite horrendous future legal liability.

To issue an Australian Builders Plate with non-compliant data would be a huge risk. The formula is:

Max HP = ((LOA x waterline length @ transom x 16) - 67) x 1.34

For example the 4.5 a waterline length on the transom of 1.55 m will give a max of 60 hp.


You can download this file for easy conversion, simply enter the data of your vessel and it will calculate the hp.

http://www.polyboatowners.com/forumdownloads/hullcalc0307.xls (http://www.polyboatowners.com/2010/vb/../../forumdownloads/hullcalc0307.xls)

This is Microsoft Excel file.

[polyFrank]

BarryJ. OAL. I take is overall length, just went out in the yard and measured the transom at the waterline. 2.1 metres = 149hp. Or we will say 150hp giving it a power to weight ratio of 176bhp/ton. A bit more like it.

[BarryJ]

Yep, LOA = Length Over All

[RATFish]

the old hull length - horsepower thingy. It's gold. I like polys but too underpowered. Polycraft should/could rerate the hulls no worries. A 4.8 with a max 80? Come on.

[BarryJ]

I have done a little more research and apparently AS1799 is not a MANDATORY standard; if a manufacturer/builder is able to demonstrate otherwise (by doing stability tests etc), it is permissible to depart from the standard.

[RATFish]

sorry ratfish, you were, as always right.
no worries, apology accepted. now polycraft, rerate the brumby to 115 and I'll get one (if the donk doesn't tear the little plastic transom off)

[geobec]

A lot of the new engines run identical capacity blocks,
In the Etec line up their 75=90 and 115=130.
In the old days before electronics a simple re-jet and spark advance would give you the higher HP.
These days is more complexed with the level of electronics used, often the base engines are identical with the bolt on stuff like injectors, ignition different.

When we were racing our Jet Sprint boat we had two 400 SB Chev motors, both identical in build, both with 412cubic inches of capacity.
One ran in the 400hp class and the other ran in the open 600hp class.
The changes were subtle but effective, carburetor (600CFM/880CFM), distributor (HDI-3000/HDI-5000), exhaust (tuned length over transom/tuned length ported with sonic rings over transom), cam shaft, inlet manifold (Wiend Tarantula/Victor Junior High Rise) were the major changes to achieve the high horse power.
The smaller engine would only develop usable power to 6250rpm, while the 612hp developed mountains of grunt to 8500rpm.

This is why most out board manufacturers have an acceptable "rev range" and will display their power being gained at an appropriate rpm.

I need a lye down now!

BigFella your input as always is valued and certainly cleared a few things up!! It is appreciated.

Thanks

Geobec(pirate)

[zone5]

Power = torque x rpm, so in the chase for higher horsepower in various motors, while I'm sure they do things that increase the torque, I'm also sure that to get the rating they're after they have to increase the max operating rpm as well.

This would probably be why the higher hp motors using the same "engine" would have a higher operating rpm range, and you'd have to be up there to get it all... some of the power gain would be able available through the rev range from increased torque tho...

I imagine it is controlled by intake/exhaust parameters and/or electronically, not just by prop size.

Of course, that's just what BigFella said!

[Giffo]

So how do these ridiculous Bream/Bass boats, have HP on the transom that will put them into orbit.All to catch a Cockroach of the sea (Bream).

[McGyver]

yea no idea giffo but when im rich and infamous i will get my self one, my first choice a 21ft yellowfin with a yammy 350hp V8 (custom black paint job to match the boat)

Now piccies. Last two are the 21 yellowfin (inshore boat)

Next two are of the yellowfin 36 (offshore boat)
http://i1009.photobucket.com/albums/af219/t-bone_123/Boats/P3160410.jpg
http://i1009.photobucket.com/albums/af219/t-bone_123/Boats/P3160412.jpg

And this one is why you guys need more HP
http://www.thehulltruth.com/attachments/boating-forum/107399d1269511756-contender-waranty-must-suck-fatchickplane1.jpg

[DvrDve]

mcgyver thats gold m8
boat name "flying fatties"

[McGyver]

yea i had a good little laugh, recon those guys better be carefull or the sea sheppard will be after them