ill informed boat knowledge

[agallant80]

Figured I would start this thread because I see allot of people saying things that are just wrong.

Fuel injection produces more HP than carburetor
4.3MPI puts out 220HP VS 4.3TKS puts out 190HP
5.0MPI puts out 260HP VS 5.0TKS puts out 220HP
5.7MPI puts out 300HP VS 5.7TKS puts out 250HP

Dual prop drives are slower not faster than single prop

SS is more durable then aluminium

When going from aluminium to SS go down a pitch
When going from three blade prop to 4 blade prop go down a pitch

 

[achris]

Re: ill informed boat knowledge

Figured I would start this thread because I see allot of people saying things that are just wrong.

Fuel injection produces more HP than carburetor
4.3MPI puts out 220HP VS 4.3TKS puts out 190HP
5.0MPI puts out 260HP VS 5.0TKS puts out 220HP
5.7MPI puts out 300HP VS 5.7TKS puts out 250HP

Nothing wrong here. All correct...

Dual prop drives are slower not faster than single prop

I don't know if this is correct or not. I suspect not.

SS is more durable then aluminium

Completely correct... The only reason Ali props last as long as they do is because they are thicker, making them LESS efficient..

When going from aluminium to SS go down a pitch

Ok, this one is definitely wrong. When moving from 3 blade Ali to the same 'family' SS, you should use the same pitch....

When going from three blade prop to 4 blade prop go down a pitch

Depends on where the engine is at WOT with the original prop. The main reason people say this is because 3 blade props are made in odd numbers of inches of pitch, and 4 blades are made in even numbers of inches... So there is no direct swap available...

Chris......

 

[Home Cookin']

Re: ill informed boat knowledge

If it weren't for "ill informed boat knowledge" this forum woudn't exist!

 

[tpenfield]

Re: ill informed boat knowledge

Figured I would start this thread because I see allot of people saying things that are just wrong.

Fuel injection produces more HP than carburetor
4.3MPI puts out 220HP VS 4.3TKS puts out 190HP
5.0MPI puts out 260HP VS 5.0TKS puts out 220HP
5.7MPI puts out 300HP VS 5.7TKS puts out 250HP

Dual prop drives are slower not faster than single prop

SS is more durable then aluminium

When going from aluminium to SS go down a pitch
When going from three blade prop to 4 blade prop go down a pitch

If it weren't for "ill informed boat knowledge" this forum woudn't exist!

:thumb:

My ill informed understanding . . .

The dual props versus single prop is that at the high-end, a single prop will get you more speed ( WOT ) due to less 'drag'. The dual prop will beat the single out of the hole. This spring I will have my first dual prop drives to see first hand.

My 1991 7.4L carbed mercruiser was rated at 330 hp . . . my 1996 7.4 LX MPI is rated at 310 hp. :noidea: there is more to the HP output than just the fueling method. (intake, valves, camshaft, etc)

 

[emilsr]

Re: ill informed boat knowledge

Originally Posted by agallant80
When going from three blade prop to 4 blade prop go down a pitch

Depends on where the engine is at WOT with the original prop. The main reason people say this is because 3 blade props are made in odd numbers of inches of pitch, and 4 blades are made in even numbers of inches... So there is no direct swap available...

Chris......

The answer to this one is actually "it depends". Some 4-blade props (Merc's Revolution 4 for example) come in odd pitch increments. Merc states that going from an equivalent pitch Mirage + (3 blades) to a Rev 4 you will lose about 300rpm at WOT. In my limited testing I found this to be accurate.

 

[smokeonthewater]

Re: ill informed boat knowledge

I'd imagine that the end function of this thread will be to consolidate many true and false blanket statements into one thread... the opening post contains both and I'm sure many more will follow LOL

 

[Mischief Managed]

Re: ill informed boat knowledge

My 1991 7.4L carbed mercruiser was rated at 330 hp . . . my 1996 7.4 LX MPI is rated at 310 hp. :noidea: there is more to the HP output than just the fueling method. (intake, valves, camshaft, etc)

The 91 engine was rated for crankshaft HP, the 96 engine is rated at the prop shaft. The 91 was good for 300 HP at the prop and the 96 is good for 340 at the crank.

 

[achris]

Re: ill informed boat knowledge

The answer to this one is actually "it depends". Some 4-blade props (Merc's Revolution 4 for example) come in odd pitch increments. Merc states that going from an equivalent pitch Mirage + (3 blades) to a Rev 4 you will lose about 300rpm at WOT. In my limited testing I found this to be accurate.

And there in lies the problem. That is switching from one family of propellers to another. The Mirage and Rev 4 are large diameter props...

The Merc Alpha 4 and the Offshore series are both 4 blade, (and in the same diameter class as the Black Max and Vengence) and are in even inches of pitch....

Basically, when working with props... all bets are off, on anything...

Chris........

(Hey, this is just like an oil thread.... Just more fun )

 

[achris]

Re: ill informed boat knowledge

My 1991 7.4L carbed mercruiser was rated at 330 hp . . . my 1996 7.4 LX MPI is rated at 310 hp. :noidea: there is more to the HP output than just the fueling method. (intake, valves, camshaft, etc)

The 91 engine was rated for crankshaft HP, the 96 engine is rated at the prop shaft. The 91 was good for 300 HP at the prop and the 96 is good for 340 at the crank.

Yep, and in 2003 they went back to rating at the crank... :facepalm:

 

[Grandad]

Re: ill informed boat knowledge

opcorn:

 

[tpenfield]

Re: ill informed boat knowledge

The 91 engine was rated for crankshaft HP, the 96 engine is rated at the prop shaft. The 91 was good for 300 HP at the prop and the 96 is good for 340 at the crank.

. . . I had a feeling the 1991 boat was a little bit slow

 

[QC]

Re: ill informed boat knowledge

Basically, when working with props... all bets are off, on anything...

Yup. But I will say that sometimes you need less pitch with a SS, even like for like. Aluminum's will flex and behave like a lower pitch in many cases. But back to your quote . . .

Yep, and in 2003 they went back to rating at the crank... :facepalm:

Double check that, not here they didn't. I/Os and OBs are all prop rated. Inboards, crank rated.

Where do we want to go with this thread? I do agree could end up like an oil thread although no one has got all wadded up yet.

 

[H20Rat]

Re: ill informed boat knowledge

Fuel injection, in a typical marine boat engine of the boats we are talking about here, will ALWAYS produce more power. Carbs are a trade off, able to handle most conditions, master of none. They have limited or no temperature/density compensation, so they have to run rich enough that they don't burn down an engine at 40 degrees. That same mixture is going to be used in the middle of summer, at 100 degrees. The engine will be quite a ways into rich territory.

People often don't compare apples to apples. There are MANY other variables, some of which are intake, exhaust, cams, valvetrain, etc... Displacement is only a very small, and somewhat insignificant number, in the horsepower equation. (My little 2.0 liter car engine is over 300 hp. Many 5.7l car engines, which has twice as many cylinders and 3x as much displacement, don't put out that power. Again, not apples to apples, lots of other factors.

lastly, you have no idea where the manufacturer measured the peak horsepower, you are simply taking their word for it. I have yet to see an OEM marine manufacturer include an engine dyno slip with their boat. (again, typical marine boat, think glastron bowrider. Yes, some exotic racer probably does do this) That horsepower rating might be on the absolute razor edge of engine failure RPM, it might be at redline, or it might be closer to typical cruise rpm. Those three horsepower numbers for the exact same engine are all going to be quite a bit different.

 

[emilsr]

Re: ill informed boat knowledge

And there in lies the problem. That is switching from one family of propellers to another. The Mirage and Rev 4 are large diameter props...

The Merc Alpha 4 and the Offshore series are both 4 blade, (and in the same diameter class as the Black Max and Vengence) and are in even inches of pitch....

Basically, when working with props... all bets are off, on anything...

Chris........

(Hey, this is just like an oil thread.... Just more fun )

Way more fun for me as I've got a baaaaad prop fetish. :facepalm:

Bravo I props are even larger in diameter but they come in even pitches. Adding to the confusion is they generally measure out to 1" or so less than how they're marked. For example, the 22" pitch Bravo I I'm running right now is actually a little less than 21" in pitch...and turns + or - 150 rpm MORE than a 21"p Rev 4 (which is just about exactly 21" pitch according to my prop jig), even though it's slightly larger in diameter.

.....so, yea, all bets are off with props.

.....and the BIII drive can actually be faster than a BI on a heavy, slower boat....so there. :laugh:

And the winners are:

Amsoil
Mequiar's
Chevy
Mercruiser I/O

That about cover it? :lol:

 

[WIMUSKY]

Re: ill informed boat knowledge

opcorn:

x2!

QC - The keyword is "yet"..........

 

[emilsr]

Re: ill informed boat knowledge

I/Os and OBs are all prop rated. Inboards, crank rated.

When I was researching this last time I bought a boat I was told the following by someone I consider to be a reliable source:

The Mercury Racing 525efi (that I wanted but couldn't afford) is rated 525hp at the prop. The 496magHO 425hp (that I wound up getting) is rated 425hp at the crankshaft.

The actual performance of the boat with each engine tends to confirm this as the 525 powered version is about 10mph faster. An extra 100hp won't push that hull 10mph faster; there has to be more to it.

That the Merc racing 500efi was rated 470hp at the prop tells me that the "rules" are somewhat fluid.

Using different dyno correction factors probably has a bigger impact than crank vs prop measurements, and I suspect there's some of that going on as well.

 

[QC]

Re: ill informed boat knowledge

MerCruiser | Mercury Marine

This shows all of the "standard" units as Propshaft rated. I can see Merc Racing being different, but I have seen nothing to indicate that Merc or Volvo have rated their off the shelf stuff as other than propshaft since 1991. This is a NMMA deal I think . . .

One thing for sure is that it needs to be understood when discussing this stuff as it makes a big difference. Around 35 horsepower on an I/O depending on the model and the rating.

 

[RogersJetboat454]

Re: ill informed boat knowledge

op2:

 

[QC]

Re: ill informed boat knowledge

This discussion can be had nicely!!! It can, it can, it can . . .

 

[3rdtimesthecharm]

Re: ill informed boat knowledge

I agree

If it weren't for "ill informed boat knowledge" this forum woudn't exist!