Top speed with 5 passengers?

[stevenk2]

We were on vacation last week down at Lake Powell with our AR195, and I noticed that at WOT I was topping out at about 30 mph (far shy of what I've observed in the past - normally I'd top out at 48-49 mph at WOT). Everything with the boat and motor seems to be operating normally, no issues with hesitation or funny noises or crap wrapped up in the impeller or shaft or anything like that, so I don't think there's anything wrong with the engine or impeller. I would say that we did have a couple more passengers on board this time around, bringing the total weight of passengers and crew close to 1100 pounds (normally we run with 3 passengers with a weight of just over 500 lbs). Also, I did recently install a Thrust Vector - could that be having an effect as well?

Just for grins, I may uninstall the Thrust Vector and see if my top speed returns to normal after that, but just curious if anyone else here may have some ideas about why the loss in top end speed.

 

[HangOutdoors]

Your elevation at Lake Powell causes this.

 

[stevenk2]

Wow - that's crazy.........I didn't even think about that. So at about 3500' or so, I should expect to have that much cut off the top end (19-20 mph), even with forced induction? I guess the motor is working even harder with my two extra passengers as well, plus with the elevation to boot.

 

[HangOutdoors]

@stevenk2 Yes. Results may differ. Search Altitude or Elevation or Lake Powell here on the forums and you will see a lot of threads discussing this occurrence

 

[Crob83]

Yep true story! A few weeks ago (never again) I had 7 adults and one 12 year old who is the basically the size of an adult (family from out of town) on my 2018 AR195. I am in Indiana so basically no elevation, and I was shcoked the boat still did 45

 

[anmut]

Air density is a thing!

 

[stevenk2]

Okay then, I'll stop worrying that there's something wrong with my AR. I thought for sure that I'd messed up something on my boat on the trip down there.

 

[2kwik4u]

Air density is a thing!

Whats worse here is that a supercharger doesn't help. A turbo would just spin faster to maintain intake pressure. Supercharger can't spin any faster, so you lose boost at altitude.

 

[mraz72]

if that was your normal boating lake, I believe there are different impellers you can get to help with elevation issues? I might be wrong though....

 

[stevenk2]

It's actually not - we vacation once a year down there, and this is the first year with the AR. The cut is closed due to low water levels, so we had a long stretch around Antelope Island where there were a few opportunities to open it up, plus we were over by Lone Rock as well that last day and noticed it was topping out significantly slower than our lakes here at home (which are at or near sea level). Not a huge deal per se, since we only visit down there once a year, and honestly between the water levels, the boat traffic, and the conditions out on the water, the opportunities to go at WOT are pretty few and far between. My kids were struggling a bit to get up on the wakeboard as well, but to their credit, they kept trying and eventually made it up pretty consistently after a few tries.

 

[ScottS]

@stevenk2 What is your home lake elevation? If you are coming from a higher elevation, and the boat has an impeller tuned to that elevation, you are going to have an "over-torqued" impeller. Meaning that you should have seen a better launch and more low-end grunt but lost out on higher speeds.

Largely though, my guess is that you are feeling the effects of Density Altitude. With the drought and heatwave that is hitting the west right now, conditions are not optimal to get the most out of your engine. If you look at the graph below, you will see that the density altitude makes your engine operate as if it were at 7200' instead of 3500'. I have been at Flaming Gorge and had density altitudes that put me over 10,000' - turning our AR195 into a hog with 4 adults and a kid on board.

In a nutshell RPMs = Speed, and at higher altitudes without the proper impeller, our engine cannot get the RPMs high enough to get the most power out of the supercharger.

Others can explain this better than me, and please feel free to chime in if I have misrepresented anything. Hopefully, this gives you some idea of what is likely going on.

Edit: I think a rule of thumb is 10% drop in power for every 1,000' of elevation gained. True for both actual elevation and Density Altitude.

Example calculator:

Density Altitude Calculator - English/Metric

 

[2kwik4u]

@stevenk2 What is your home lake elevation? If you are coming from a higher elevation, and the boat has an impeller tuned to that elevation, you are going to have an "over-torqued" impeller. Meaning that you should have seen a better launch and more low-end grunt but lost out on higher speeds.

Largely though, my guess is that you are feeling the effects of Density Altitude. With the drought and heatwave that is hitting the west right now, conditions are not optimal to get the most out of your engine. If you look at the graph below, you will see that the density altitude makes your engine operate as if it were at 7200' instead of 3500'. I have been at Flaming Gorge and had density altitudes that put me over 10,000' - turning our AR195 into a hog with 4 adults and a kid on board.

In a nutshell RPMs = Speed, and at higher altitudes without the proper impeller, our engine cannot get the RPMs high enough to get the most power out of the supercharger.

Others can explain this better than me, and please feel free to chime in if I have misrepresented anything. Hopefully, this gives you some idea of what is likely going on.

Edit: I think a rule of thumb is 10% drop in power for every 1,000' of elevation gained. True for both actual elevation and Density Altitude.

Example calculator:

Density Altitude Calculator - English/Metric

View attachment 157695

This calculator (linked on the page you submitted) has a relative HP % calculator on it as well.

Engine Tuning Calculator - using dew point

*edit* playing with various numbers, I can get a 10% swing in HP from spring conditions to summer conditions on various lakes around the area.

 

[stevenk2]

@stevenk2 What is your home lake elevation? If you are coming from a higher elevation, and the boat has an impeller tuned to that elevation, you are going to have an "over-torqued" impeller. Meaning that you should have seen a better launch and more low-end grunt but lost out on higher speeds.

Largely though, my guess is that you are feeling the effects of Density Altitude. With the drought and heatwave that is hitting the west right now, conditions are not optimal to get the most out of your engine. If you look at the graph below, you will see that the density altitude makes your engine operate as if it were at 7200' instead of 3500'. I have been at Flaming Gorge and had density altitudes that put me over 10,000' - turning our AR195 into a hog with 4 adults and a kid on board.

In a nutshell RPMs = Speed, and at higher altitudes without the proper impeller, our engine cannot get the RPMs high enough to get the most power out of the supercharger.

Others can explain this better than me, and please feel free to chime in if I have misrepresented anything. Hopefully, this gives you some idea of what is likely going on.

Edit: I think a rule of thumb is 10% drop in power for every 1,000' of elevation gained. True for both actual elevation and Density Altitude.

In a nutshell, we boat in Oregon (which is also where we purchased the AR), and do a majority of the time on the water at or near sea level. We do visit Central Oregon on occasion, and I believe they are around 2k feet (give or take) over there where we visit. Now, Lake Powell is an entirely different animal from that, and given the altitude, heat and humidity differences, I fully buy that we would see some difference in performance, but I just wanted to be certain there was nothing wrong with the boat.

Now, given the relatively infrequent visits down there as well as the typical conditions on the lake, I probably won't bother taking any action in terms of trying to remediate - it's just not really worth it to me. If I lived down there and spent more time on the lake, it might be a different story. Of course, given the state of things down there with water levels and such, the lake may not exist for much longer as we know it, but that's a different conversation...........

 

[ScottS]

One other comment, remember how sensitive these 19 foot Yamahas are to weight in the bow. With your added weight, I’m curious how much of it was in the bow.

 

[Crob83]

One other comment, remember how sensitive these 19 foot Yamahas are to weight in the bow. With your added weight, I’m curious how much of it was in the bow.

Very true, I usually can't go over 45 with people in the bow.

 

[stevenk2]

One other comment, remember how sensitive these 19 foot Yamahas are to weight in the bow. With your added weight, I’m curious how much of it was in the bow.

It really varied during the days on the water down there - at times, yes, there were 2 people up there, I'd estimate approx. 500 lbs. total. But there were also times when everyone was in the main part of the boat, either on the port side bench seat or at the rear. And given the varying water conditions, I wasn't necessarily going WOT during those times either, so I didn't really pay attention or note how the boat reacted to the weight distribution throughout the boat. I just know that the times I did have the throttle pegged, I topped out at 30 MPH. Given all of the comments, I would say that the loss of top end was due to a combination of the elevation, heat and combined weight of my passengers.

 

[Julian]

One other comment, remember how sensitive these 19 foot Yamahas are to weight in the bow. With your added weight, I’m curious how much of it was in the bow.

I think all our boats are sensitive to bow weight! Mine sure is. It will even impact turning ability if too many people are up front.

 

[dzeppos]

I find altitude AND bow weight to make a big difference. Putting three big guys on the bow, especially with no one in the back will slow you down like 5-6 mph. I have a 4STAR CAR 2042 (2016) model. At a lower altitude of 1000 feet i can do 47mph with 3 people and full tanks.
Put 3 people on bow and it goes down to like 41-42mph. I noticed this on the Sacramento Delta - even my wife and two girls on the bow- who are light - reduces speed by 3mph and couldn't go past 45-46pmh at sea level.

 

[2kwik4u]

Verified the bow weight sensitivity again yesterday. 2 ladies in the bow seats at ~280lbs total, top speed was 37mph on GPS. They moved to the back (despite me enjoying that particular view and slower speed) and I almost instantly picked up to 42mph. No other changes. They decided the shade wasn't all it was cracked up to be and back to the bow they went. Back to 37mph.

Also. Had a guy in the raft up a few boats down that had an absurdly loud Baja. Was giving him a hard time about how slow it must plane despite running in the 60mph range. Said "I'm all about the acceleration, so I got a jet". He said I was full of it, so we bet a frosty barely pop on a race to 30 to settle the discussion.........Our little AR190 had him by almost a full boat length from a standing start to 30. Of course once he planed out it was over for me, and he was gone. But I got a free ice cold beer back at camp.

 

[DrPhil]

No worries! Lake Powell's water level is going down, so you'll soon be at a much more super-charger friendly altitude!