change in prop pitch on a 2002 380 DA

[bluebelly]

Can somebody tell me how much my wot rpm will change if I increase the pitch on a 380 da with 8.1 horizons 2.5 reduction gear? Right now I am getting 4680 rpm wot under full load. I would like to go from 22x23 props to 22x24 but am concerned that 1 inch of pitch may be over kill. Maybe 1/2 inch would do.

 

[Hampton]

Isn't the WOT supposed to be 4600-4800? If so, 1" would be overkill. If they are currently matched, I wouldn't mess with perfection. In addition, these props are not identical port to stbd. They are mismatched to make the RPMs very close to each other with one transmission running in forward and one in reverse. They do this to make counter-rotating props from similarly rotating engines. (If your engines run opposite, disregard last part - don't have the year, but judging from the engines, I believe the transmissions run opposite each other in different gear ratios).

 

[bluebelly]

The wot is supposed to be 4600. The problem is that when the boat is normally loaded I am getting about 4750 rpm. If my memory is correct I will see about a 150 rpm reduction with 1 inch of pitch increase on transmissions that are 2.5 reduction. I know that the higher the reduction gearing the less impact changing prop pitch will have. I just want to get this one right the first time since it will be next summer before the boat is hauled again.

 

[brimanst]

I see you are from HH. You can search some old postings for my dilemma about my 02 340DA. SR had it all wrong, and I finally got it right with some 4-blade props that made it feel like a new boat. The guy to call is Nelson at Palmetto props. He is in Johns Island and knows more about props than any guy I have talked to. He is still my prop guy on my current boat.

 

[fwebster]

Most experienced prop guys won't mess with the pitch on props that are that close, unlesss they just need to pay for a new propscan computer.......assuming your boat was fully loaded and had a clean bottom when you turned up the 4680, most would just add a little cup.

This is one of those cases where it might not be the best course to try to fix something that isn't broke.

 

[bluebelly]

Most experienced prop guys won't mess with the pitch on props that are that close, unlesss they just need to pay for a new propscan computer.......assuming your boat was fully loaded and had a clean bottom when you turned up the 4680, most would just add a little cup.

This is one of those cases where it might not be the best course to try to fix something that isn't broke.

These props have a #3 cup. I wonder what would be the next # cup to take off about 100 rpm. The reason I am doing this is the boat is currently out of the water and I took the props off to service the cutlass. So right now it is no trouble to tweak them a bit. I do know that if I successfully reduce the wot rpm by about 100 then that will make the boat a little more efficient at 3500 rpm which is the best cruise. I figure I would pick up about 1/2 to 3/4 mph at 3500 rpm.

 

[Hampton]

I really doubt you'll see that much of a difference. You may get that speed at that RPM, but the engine will be working a little harder to achieve that RPM, and will therefore be comsuming a little more gas at that same RPM. I would bet that your economy changed right about 0.

What you want to try to avoid is running your boat too loaded up. I wouldn't mess with it.

 

[fwebster]

If you are going to change the props, don't try to get in the prop engineering business. Just tell the prop guy you need to reduce your rpms by 100 on each side and let him decide what to do based on the blade profiie your props have. It could be that increasing the cup on your particular prop reduces efficiency. My earleir comment was just that an inch of pitch is worth about 300 +- rpm and finding 80 rpm is sort of like splitting hair for a prop guy. Most find it easier and less prone to cause other problems to alter the cup when the change is that small.

 

[bluebelly]

Update on this thread. Changing the pitch by one inch on this boat made a difference of 220 rpm at wot. This increased the cruise speed by 1.25 mph at 3500 rpm. Now with half load of fuel and half load of water with 2 people on board we hit 4520 rpm wot. So we achieved almost exactly what we wanted.

 

[Hampton]

Update on this thread. Changing the pitch by one inch on this boat made a difference of 220 rpm at wot. This increased the cruise speed by 1.25 mph at 3500 rpm. Now with half load of fuel and half load of water with 2 people on board we hit 4520 rpm wot. So we achieved almost exactly what we wanted.

Good job. What's the increase in fuel flow at 3500 RPM with the bigger bite?

 

[bluebelly]

Fuel flow stays the same based on engine rpm but fuel economy changes based on a increase or decrease in speed. The fuel flow is still 31.5 gph.

 

[osd9]

Fuel flow stays the same based on engine rpm .......

How can that be? You've increased the load...the engine has to be working harder, burning more fuel, at say 3500 RPM after added pitch compared to 3500 RPM before added pitch....right????:huh:

 

[Hampton]

Fuel flow stays the same based on engine rpm but fuel economy changes based on a increase or decrease in speed. The fuel flow is still 31.5 gph.

Have you ever heard of a constant speed prop? Many aircraft use them. I understand that there are trucks/buses that use transmissions to obtain the same effect - constant RPM for the engine. Here's how it works (like your generator): Motor turns at same RPM regardless of load. As load increases, more coal is poured on (more gas injected) to increase the power output to compensate for the increased load while maintaining constant RPM. In airplanes, the pitch of the prop changes as fuel is added to increase power at the same RPM. It's the same concept, working backwards.

In all cases, you don't get something for nothing. If you increase the pitch or cup of your prop to push your boat faster through the water for a given RPM, you will have to advance your throttles to a higher setting which will command more fuel flow in order to provide the additional power to push your boat through the water faster at that constant RPM.

Granted, there are more efficient prop shapes than others. Adding a little pitch or cup to bring your max RPM down 100 clicks does not change the efficiency of the prop by any measurable margin.

Here's an example where it will make a difference: Take a bent/gouged, damaged prop that is driven by and engine at 3500 RPM and uses 25 gph per side to do it, yet only drives the boat 20 mph. You change the shape of the prop to look like a prop and now, all of the sudden, you get 3500 RPM at only 15 gph per side, and the boat accelerates to 25 mph. You have increased the efficiency of the prop.

What you have done, while admirably anal, may bring your boat closer to the sweet spot, but you will not gain 1.5 mph for nothing.

This is all about intellectual discussion and education. I am not trying to rain on anybody's parade or be negative. Please forgive me if it appears that way. It's hard to address the differences and type with a smiling flare to the message, so I'll add this : )

Note: No edits to this post.

 

[bluebelly]

How can that be? You've increased the load...the engine has to be working harder, burning more fuel, at say 3500 RPM after added pitch compared to 3500 RPM before added pitch....right????:huh:

I don't know. All I know is that the wot is closer to the optimum (4600) and the mph is up 1.25 at 3500rpm. The gph stayed the same but I do not believe that the smartcraft instrument is that accurate to begin with. The goal was not to improve fuel economy but rather to improve performance. This boat was clearly under-propped when it was operated with a light load. Maybe this boat was originally propped correctly for fresh water instead of salt water?

 

[Hampton]

I don't know. All I know is that the wot is closer to the optimum (4600) and the mph is up 1.25 at 3500rpm. The gph stayed the same but I do not believe that the smartcraft instrument is that accurate to begin with. The goal was not to improve fuel economy but rather to improve performance. This boat was clearly under-propped when it was operated with a light load. Maybe this boat was originally propped correctly for fresh water instead of salt water?

Perhaps. For the crowd, what you want to avoid is running over-propped. If you should be running 4600 RPM at WOT, but with your boat load, props, bottom fouling..., your actual WOT is only 3800 RPM, then when you cruise around at 3500 RPM you are not running below the recommended 75% max load (give or take), but are running around much closer to the limit. This puts extended, undue wear on the engines.

At the other end of the spectrum which is the area Bluebelly was working to perfect, if you have significantly less prop than you can throw, it's like running down the highway in 2nd gear instead of over-drive. It's a waste of gas. Your engine will not be over-worked as long as you don't push the power up to make up time, but you will not go as far or as fast as you would with the right prop. Note: This is with a significantly uner-sized prop, not minor adjustments. I'm tired.

Bottom line, Bluebelly is in the sweet spot. Congrats. I did the same 20 months ago. Here's a smiley : )

 

[dc380]

I don't know. All I know is that the wot is closer to the optimum (4600) and the mph is up 1.25 at 3500rpm. The gph stayed the same but I do not believe that the smartcraft instrument is that accurate to begin with. The goal was not to improve fuel economy but rather to improve performance. This boat was clearly under-propped when it was operated with a light load. Maybe this boat was originally propped correctly for fresh water instead of salt water?

So now at 3500 RPM how fast are you cruising at?