Sacrificial Aluminum Prop

[Kosmofreeze]

Got a smokin' deal on a reconditioned set of D5 (Aluminum) duoprops for my VP 5.7GXi/1:1.95 firmly attached to my 6500lb cruiser. Current prop is F3 (Stainless). I have no issues with the performance of the F3 - snappy to get on plane, tops out at 40+ mph and cruises comfortably at 20-25 @ 3000RPM . I bought the D5 as a "sacrificial spare" only to be used when I venture to remote and rock infested Isle Royale. When I head toward the island, I'll switch to the aluminum and stow the stainless as my 'get home' prop ... just in case I hit that special rock that dings the aluminum D5 - hopefully sparing any engine/drive damage. I realize that the D5 is probably too much prop for my boat but all I really need is an efficient cruise speed (20-25 is plenty fast - the D5 has a 35 mph speed limit) to get back and forth over the 22 miles of open water. While at the island cruising will be slow to wakeless.

Any thoughts or concerns operating "too big of prop" if I limit the 320 HP 5.7 GXi to 60-75% and keep the speed below the prop speed limit? What could go wrong?

 

[stresspoint]

should be ok as get home prop if you don't try to push too hard / lug the motor.

only thing is problems may arise if you get wind or seas where you need to push the throttle to keep head way. its a very heavy boat so don't push it as drives are not so forgiving , motors can handle a little more strain.

other than that keep the speed to prop speed and throttle to a easy RPM and you will do no damage to motor or gearbox .

it will beat the shyt out of paddling home if you smash a rock .

sorry i miss read , you are going to leave home with this prop on and keep the proper one as safety net , all good the above applies anyway ,

 

[Scott Danforth]

Any thoughts or concerns operating "too big of prop"

lugging the motor will kill the motor

 

[airshot]

You will end up using more fuel and get less performance...kinda like driving your vehicle in high gear at 10 mph. Problems will arrise sooner or later...

 

[Kosmofreeze]

lugging the motor will kill the motor

Help me understand in technical and quantifiable terms how to identify when my engine is "lugging". If I'm going up a hill in my manual transmission car and I hear pinging/knocking, I know my engine is lugging and I need to down-shift. A little different in a boat. In my boat I can monitor (in real time) my knock sensor spark advance. Can I assume that when my knock sensor spark advance is at a maximum that my engine is lugging?

 

[stresspoint]

ECU can only correct so much abuse and then when it cant correct anymore it will do one of 2 things depending on the set up , older set ups will usually keep running out of perimeters till the motor gives way , newer motors will tend to go to limp mode .
if you use that prop on that boat it will naturally lug as you accelerate , it will keep lugging till it reaches a speed where the motor is working easy , at that point you need to throttle back to get the ECU back into safe perimeters and cruise at that speed , NOTE : if you don't have a feel for this and are going to rely on the fuel injection system to correct the MAP and keep the thing in safe perimeters , i suggest you don't use that prop, detonating's a motor is somewhat of an expensive exercise , a lot more than a prop ,.

now lets not go into detail about damage to outdrives when they get smocking hot .

what i am saying is "yes " you can over prop a boat and still use it safely , but you need to be aware , it is doing damage if you push the motor and drive to their limits ..

 

[Kosmofreeze]

Considering that I don't have that much invested in the aluminum props, I'll give the D5 a try carefully watching for signs that I might be overloading the engine. If things don't work out as I hope, I guess I could have them re-pitched? I understand that it is acceptable to re-pitch up to +/- 2" of pitch. Does this apply to V/P Duo-Props since they are not sized by pitch/diameter?

 

[Scott Danforth]

Repitching VP props would be more expensive than a used set of D3s

Not sure the knock sensors can pull enough timing. Try them, however they will most likely be too much pitch

 

[airshot]

Considering that I don't have that much invested in the aluminum props, I'll give the D5 a try carefully watching for signs that I might be overloading the engine. If things don't work out as I hope, I guess I could have them re-pitched? I understand that it is acceptable to re-pitch up to +/- 2" of pitch. Does this apply to V/P Duo-Props since they are not sized by pitch/diameter?

So, your better at determining what is best for your engine than the engineers that designed it ?? Lugging your engine is not noticible in a boat like a street vehicle. After spending all your hard earned money on a nice boat, why woukd a few hundred on props be an issue ?? Just curious ??

 

[bruceb58]

Considering that I don't have that much invested in the aluminum props, I'll give the D5 a try carefully watching for signs that I might be overloading the engine. If things don't work out as I hope, I guess I could have them re-pitched? I understand that it is acceptable to re-pitch up to +/- 2" of pitch. Does this apply to V/P Duo-Props since they are not sized by pitch/diameter?

See what your WOT RPM is with your D5 props and report back. That will tell you the whole story

 

[Texasmark]

Quick "lugging" test is when you hit the throttle and not much happens. Proper gear ratio (prop is a big part of that) delivers quick response to throttle changes. Now, back to the what's your WOT RPM and your max engine rated RPM under your most frequent load (what's in the boat) conditions.

 

[QBhoy]

Help me understand in technical and quantifiable terms how to identify when my engine is "lugging". If I'm going up a hill in my manual transmission car and I hear pinging/knocking, I know my engine is lugging and I need to down-shift. A little different in a boat. In my boat I can monitor (in real time) my knock sensor spark advance. Can I assume that when my knock sensor spark advance is at a maximum that my engine is lugging?

it might be equivalent to having a manual (stick shift) car...and trying to drive everywhere in 5th gear...or trying to tow the QE2 about the place...
Basically it usually doesn't end well..con rods, bearings and all sorts will become under strain outwith their design tolerance.

 

[QBhoy]

Maybe a little late an hour over where I am...and a few glasses of red wine down..also a little out of touch with the VP props these days..but what is a D5 prop ? I know well, the old type B series and C series (ally and stainless) and even the newer type J and F series. Is the D series a more recent product since ?

 

[bruceb58]

Maybe a little late an hour over where I am...and a few glasses of red wine down..also a little out of touch with the VP props these days..but what is a D5 prop ? I know well, the old type B series and C series (ally and stainless) and even the newer type J and F series. Is the D series a more recent product since ?

Aluminum version of the F series for DPS drives.

 

[QBhoy]

Aluminum version of the F series for DPS drives.

Thanks Bruce

 

[Kosmofreeze]

Recently had a chance to compare some performance data (F3 SS vs F5 Alum) DuoProps on my 26' 7000lb cruiser. To recap - My 5.7GXi DP-SM 1.95 has F3 SS props. I found a reconditioned set of D5 Aluminum on MarketPlace for $300. While VP does not recommend a D5 aluminum for my GXi (only 280 hp GSi is shown - GXi has 320 hp) - see attached - I bought it anyway. I suspect the reason is that the VP aluminum D-series props have a 35 knot (40 mph) max speed limit ... presumably due to flexing and stress that the aluminum can't handle. Bottom line, I'm impressed with the D5 Aluminum - actually get a little better fuel economy compared to the SS - sweet-spot about 2.3 mpg at 25-27 mph - 3500 RPM. The downside for my installation is I can't get to WOT without exceeding the 40 mph speed limit for the aluminum prop. No big deal - I'm not into speed ... economy is more important to me and having a sacrificial aluminum prop spare that I can use when I go places with lots of prop-eating rocks (e.g. Isle Royale).

Conclusion - No signs (i.e. knock sensor knocking) of my engine working too hard with the steeper pitch. If you want speed go SS - economy go higher pitch Aluminum but mind the speed limit. See the attached plot for the real nerds.

 

[jimmbo]

40mph Speed Limit for a Prop? That is a New One
Reading the Chart, shows approx what Top Speed would be if the Engine was at top rpm, and that was taken into account for the Load
If you use a Prop that is too low in pitch for the Load, the Top Speed you will get will be higher simply because the Engine is Over Revving.
BTW the "Flex" of an Aluminum Prop is in the Range of less than 1/100" Go grab a blade and try to get it to Flex. It is not going to happen as the Water is trying to push will certainly be yielding to the Blade. That bit of Fear Mongering has sold Millions of SS Props though.

 

[bruceb58]

Go grab a blade and try to get it to Flex.

LOL....you can't be serious!

 

[jimmbo]

LOL....you can't be serious!

Yes, I am. There is some Misconception that these Blades are so weak/flimsy they flop around like the Blade is on a Pivot. While there may be a bit of Flex at Extreme Speeds(well past 100mph), it is still gonna be a very tiny fraction of an Inch. But it has been a Wonderful Marketing Sham to sell Millions of SS Props, as it Rolls Off the Tongue very well.

 

[bruceb58]

Yes, I am. There is some Misconception that these Blades are so weak/flimsy they flop around like the Blade is on a Pivot. While there may be a bit of Flex at Extreme Speeds(well past 100mph), it is still gonna be a very tiny fraction of an Inch. But it has been a Wonderful Marketing Sham to sell Millions of SS Props, as it Rolls Off the Tongue very well.

Sorry to burst your uneducated bubble but aluminum props do flex. Even stainless steel props flex but not as much as aluminum.

You aren't going to get them to flex with your hand! That was just hilarious!

Stainless steel props can be made less thick to provide the same amount of strength. Since they are less thick, they have less drag. That is the main advantage of stainless steel props. Do i have a stainless prop on my pontoon boat with a 90HP motor...that would be a NO since there is no advantage on a boat like that. On my 26' Cobalt with a 350HP dual prop drive...yes....it matters.

I am going to take a guess but you probably have a boat motor combination that would not benefit from a stainless steel prop.