RPM loss at Lake Powell.

[DWRAT]

Anyone ever track RPM loss when boating at Powell vs Havasu?
If so how much RPM did you lose?

 

[HBCraig]

Yes. My old boat that was a N/A engine. High elevation coupled with not moving down a size in props

 

[rivermobster]

Yes. My old boat that was a N/A engine. High elevation coupled with not moving down a size in props

Dang. What's the elevation difference??

 

[DWRAT]

Dang. What's the elevation difference??

Powell is 3,000 foot higher than Havasu.

 

[4Waters]

Powell is 3,000 foot higher than Havasu.

3000 feet in 233 miles (by the way the crow flies), basically 13ft drop per mile

 

[Gramps]

3000 feet in 233 miles (by the way the crow flies), basically 13ft drop per mile

and that kiddies is why the water flows from Powell to Havasu

 

[4Waters]

and that kiddies is why the water flows from Powell to Havasu

I wonder what the drop per foot is by river mile? I'm seeing anything from 300 to 900 river miles between the 2. Anyone know what the distance is?

 

[I Dig It !]

Yes Powell is like 3600’

you need to drop a size in props I run a 26p at havasu and a 24p at Powell

 

[D Mac]

I don't remember RPM numbers but lost 5-7mph in 2 different boats between Havasu and Powell without prop change. Powell is our home lake. Can get away with a couple more degrees of timing if going for kill mode at Powell altitude. Don't forget to pull timing back before going to Parker and burn a piston like I did 20yrs ago. 2 pitches lower prop for Powell is the typical move.

 

[jetboatperformance]

Altitude can scrub off a 3% percent for every 1000' on NA deals

 

[HTTP404]

and that kiddies is why the water flows from Powell to Havasu

Gravity rules!

 

[DaveH]

ahhh the old density altitude myth on full display.

while its true that 1000' is around a 3% loss, so about 10% of total power at 3600 as opposed to sea level, keep in mind temperature also plays a factor as does humidity.

powell doesnt get as hot as havasu. 10' temp change gives a 1% loss of power.

i can see a 5-7% loss of power compared to Havasu.

on a 500hp motor, you are talking maybe 30hp. i doubt that enough to warrant a prop change, unless you are already over propped to being with.

I think the real reason people feel their boats are sluggish at powell is becasue they load their boats to the roof. lots of passengers, full tanks of fuel, big coolers and so forth. in other words its the weight, not the density altitude that makes the boat slower.

 

[Racer56]

ahhh the old density altitude myth on full display.

while its true that 1000' is around a 3% loss, so about 10% of total power at 3600 as opposed to sea level, keep in mind temperature also plays a factor as does humidity.

powell doesnt get as hot as havasu. 10' temp change gives a 1% loss of power.

i can see a 5-7% loss of power compared to Havasu.

on a 500hp motor, you are talking maybe 30hp. i doubt that enough to warrant a prop change, unless you are already over propped to being with.

I think the real reason people feel their boats are sluggish at powell is becasue they load their boats to the roof. lots of passengers, full tanks of fuel, big coolers and so forth. in other words its the weight, not the density altitude that makes the boat slower.

No you are completely wrong. My boat loaded the same would barely get on plane at Powell. Switched the 26 for a 24 and the boat ran like a champ. My boat is a Nordic Rage with a Mercury racing 520.

 

[Todd Mohr]

Yep, 2 inches, go to Tahoe and it's another 2 inches.

 

[DaveH]

No you are completely wrong. My boat loaded the same would barely get on plane at Powell. Switched the 26 for a 24 and the boat ran like a champ. My boat is a Nordic Rage with a Mercury racing 520.

yeesh.

take a look. this chart its for a relative humidity of 50% but that doesnt matter as it will be factored into both measurments.

Havasu is about 750' sea level. on a 120' day (hot but not unheard of), the corrected density altitude is about 5000' of elevation.

Powell at 3600' and a 100'f day is is about 7100' of density altitude elevation.

thats 2100ft difference.....so right about 6-7% power loss. if that kind of power loss wnt get your boat on plane.....something is way wrong.

 

[Racer56]

yeesh.

View attachment 1489709

take a look. this chart its for a relative humidity of 50% but that doesnt matter as it will be factored into both measurments.

Havasu is about 750' sea level. on a 120' day (hot but not unheard of), the corrected density altitude is about 5000' of elevation.

Powell at 3600' and a 100'f day is is about 7100' of density altitude elevation.

thats 2100ft difference.....so right about 6-7% power loss. if that kind of power loss wnt get your boat on plane.....something is way wrong.

I am very familiar with density altitude and the effects of temperature. What ever the books say is not reality when you go boating at Powell. If you normally boat in Havasu and don't prop down in Powell you will regret it. This obviously is for NA boats .

 

[DaveH]

I am very familiar with density altitude and the effects of temperature. What ever the books say is not reality when you go boating at Powell. If you normally boat in Havasu and don't prop down in Powell you will regret it. This obviously is for NA boats .

i boat powell all the time and dont change props on my 27 Hallett.

 

[77charger]

Its more than just the altitude yes the altitude makes a noticable difference but its also the heat combined with the altitude which really kills the power.Sure i can rejet my carn but find it easier to run one prop size smaller lose a lil mph but still go good the rest of trip.

 

[DaveH]

Its more than just the altitude yes the altitude makes a noticable difference but its also the heat combined with the altitude which really kills the power.Sure i can rejet my carn but find it easier to run one prop size smaller lose a lil mph but still go good the rest of trip.

and thats my point.......the extreme summer temp at Havasu compared to powell makes a big difference. the chart isnt "wrong"

 

[DWRAT]

Thanks everyone, gonna look for a smaller prop.

 

[Motor Boater]

I’ve gone to Powell and not changed props and been fine but the boat was loaded down and you could feel a big difference. I run a 28 on my Howard and usually put a 26 on it now when at Powell. It takes me about 3 minutes to change a prop so I don’t worry about it too much.

 

[Racer56]

i boat powell all the time and dont change props on my 27 Hallett.

I Apologize for my previous posts coming off pretty harsh. Had a employee put me in a pissed off mood today and for some reason I decided to carry it over here. Normally I'm pretty mellow. Can't wait for the day I don't have anymore employees. Happy boating

 

[DaveH]

I Apologize for my previous posts coming off pretty harsh. Had a employee put me in a pissed off mood today and for some reason I decided to carry it over here. Normally I'm pretty mellow. Can't wait for the day I don't have anymore employees. Happy boating

we all have our moments

 

[bilz]

So, after all this, have you tried a different Prop?

 

[mesquito_creek]

I have never changed my prop pitch, but my boats have always had props that favor the hole shot not top end. So if you have props that maximize top end and sacrifice the low end torque then probably need to go down in prop pitch.

The larger issue is proper fuel mix/jetting on carburetor motors…. You are just throwing away gas by running extra rich and sacrificing HP. FI solves that problem or knowing how to properly jet your carb. If you run rich of peak at havasu to protect your motor you will most likely be extra rich at 5000 DA…

 

[Bigbore500r]

I am very familiar with density altitude and the effects of temperature. What ever the books say is not reality when you go boating at Powell. If you normally boat in Havasu and don't prop down in Powell you will regret it. This obviously is for NA boats .

I 100% agree with this. Unless your N/A boat is underpropped in Havasu, you will definately want to prop-down at least 1 size to enjoy Powell.

 

[mesquito_creek]

in the middle of the summer you will see density altitudes of 6k plus…. It’s March and today at 2pm it’s going to be 5700. Prop pitch is secondary and masking some of the issues

 

[DaveH]

in the middle of the summer you will see density altitudes of 6k plus…. It’s March and today at 2pm it’s going to be 5700. Prop pitch is secondary and masking some of the issues

View attachment 1489861

Page airport is 800' higher than the lake.

today, the temps at havasu and powell are very similar, showing a density altitude difference of about 3000'.

this difference in DA will narrow during prime boating havasu months when Havasu heats up more than powell.

 

[Cobalt232]

At Powell is that we generally have more people and gear on the boat, which really zaps the performance too. I also carry extra water at Powell just in case of a remote breakdown, which adds to the load.

 

[mesquito_creek]

Page airport is 800' higher than the lake.

today, the temps at havasu and powell are very similar, showing a density altitude difference of about 3000'.

this difference in DA will narrow during prime boating havasu months when Havasu heats up more than powell.

Are you going to make me break out the eb6?… haha… you will still see DA 6k + all summer at the waterline. Throw the lower pitch prop at it and tell everyone how you would be able to go faster with a different prop or leave the prop on it and complain about the hole shot.

 

[Cobalt232]

Are you going to make me break out the eb6?… haha… you will still see DA 6k + all summer at the waterline. Throw the lower pitch prop at it and tell everyone how you would be able to go faster with a different prop or leave the prop on it and complain about the hole shot.

Yes. You just have to find it...

 

[DaveH]

Are you going to make me break out the eb6?… haha… you will still see DA 6k + all summer at the waterline. Throw the lower pitch prop at it and tell everyone how you would be able to go faster with a different prop or leave the prop on it and complain about the hole shot.

just do a DA for havasu in July for 118' for comparison.

 

[mesquito_creek]

just do a DA for havasu in July for 118' for comparison.

I agree, I believe we are saying similar things. If it runs well in peak summer at havasu and you don’t have the prop pitch on the ragged edge of top speed, assuming fuel injection it’s probably a run what you brung situation for me.

 

[FROGMAN524]

I've been on both Tahoe and Powell in this Hallett and both times noticed significant power loss despite Mercury's claim of making same power at any altitude or temperature with the SCI motors. Don't remember the specific RPM though.