Higher elevation issues.

dustyjohns

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Aug 4, 2022
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I’m sure part of this topic could belong in another forum. But I’m going to include it here too. Yes this is my first post. No, I’m not new to reading the forum for all y’all’s wealth of knowledge. I just bought a ‘18 Thunder Jet 18.5’ boat with a 115 four stroke CT. I bought the boat from a guy that lived 60 miles inland from the coast. Took it home to an elevation of around 2,700’. It had an aluminum 4 blade 14 X 19 prop. It did ok, not great. Tried fishing at 6,200’ not knowing I needed to change props. (I’ve previously only owned jet boats.) My top speed was 10pm at 3800rpm. Started reading everything I could about prop and elevation. Stop at another lake on the way home at 4,700’ elevation. Boat got up to 34mph at 5200rpm.

Swap the prop for a 14.5X17 3 blade stainless which was recommended by Mercurys prop guide. Try fishing a lake at 5,070’ and my top speed was 10.4 at 4400rpm which was WOT. What am I missing here? How does the 300’ of elevation difference change so much and why did the lower pitch prop not help? I did try putting it in neutral and revving the engine. It never hit the rev limiter and just bogged. Like the intake was plugged is how it sounded. Why is the EFI not compensating for the higher elevation? Is the Mass Air Flow sensor possibly bad and that’s causing an issue?

I just don’t know enough about propped motors to know what I’m doing wrong. So I thought I’d ask some people with more knowledge and experience.
 

Scott Danforth

Grumpy old guy who plays with boats
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Jul 23, 2011
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40,346
first, welcome aboard

next, you have a few issues going on.
for every 1000 foot of elevation, you loose 3-5% power (depending on barometric pressure)
normally, as you go up, you must drop pitch.

additionally, you are probably getting cavitation if you can turn 4400 RPM, so check your hub

you will most likely need the 3-blade 17p for your home location and a 15p or less for the 5700 foot elevation

EFI can only back-off fuel, it cant generate more air pressure. since you dont have enough air pressure to make power, the EFI is limiting fuel and you are down on power.
 

dustyjohns

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Joined
Aug 4, 2022
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13
first, welcome aboard

next, you have a few issues going on.
for every 1000 foot of elevation, you loose 3-5% power (depending on barometric pressure)
normally, as you go up, you must drop pitch.

additionally, you are probably getting cavitation if you can turn 4400 RPM, so check your hub

you will most likely need the 3-blade 17p for your home location and a 15p or less for the 5700 foot elevation

EFI can only back-off fuel, it cant generate more air pressure. since you dont have enough air pressure to make power, the EFI is limiting fuel and you are down on power.
Thank you!

Why would there be such a difference though from a lake at 4,700’ to a lake at 5,070?

I did think about cavitation. Would that be from the motor sitting too low in the water?

I’m pretty sure I’ll need different props after reading what I have.

Oh, duh, that makes sense. Thanks!
 

Scott Danforth

Grumpy old guy who plays with boats
Joined
Jul 23, 2011
Messages
40,346
Thank you!

Why would there be such a difference though from a lake at 4,700’ to a lake at 5,070?

I did think about cavitation. Would that be from the motor sitting too low in the water?

I’m pretty sure I’ll need different props after reading what I have.

Oh, duh, that makes sense. Thanks!
could have had weeds on the prop

your prop hub could be flaky
 

Dave1027

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May 25, 2010
Messages
971
He means the hub of the prop might be slipping. But I thought you tried two different props?

If I'm reading your numbers correctly, it seems your boat is struggling to get on plane. Hence the 10 MPH top speed? My opinion is that EFI engines should be able to handle altitude better than carbed engines because they have the ability to sense altitude and compensate for it.

I think you should be checking the motor for proper operation. Can you pull codes from it? Any way to read live data from it?

You might also look at the overall boat/motor setup like loading, boat weight and motor height/trim
 

dustyjohns

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Aug 4, 2022
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Thank you Dave! Yes I have tried 2 different props now. The 1st had a slip in hub. The one I have on now is a hub that looks like it doesn’t come out very easily If at all. I can get pics if it would help.

Correct, it will not get on plane at the lake with an elevation of 5,070’. I thought EFI would too. So I thought maybe the MAF sensor.

I don’t have a way but there is a boat shop around the corner from work. Maybe they would run a diagnostic on it for me.

I checked the motor height. The cavitation plate is 2 3/8” above the bottom of the hull. Which is right from my understanding. Maybe it’s just underpowered? Like I should have a 150 instead of a 115…
 

jimmbo

Supreme Mariner
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May 24, 2004
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10,663
AV Plate 2 3/8" above the hull bottom? That is the setup for a boat going over 60mph
Is this a new boat and motor, or just 'new to you'?
 

Dave1027

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May 25, 2010
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I don’t have a way but there is a boat shop around the corner from work. Maybe they would run a diagnostic on it for me.
Yeah, definitely have the motor scanned. Too bad it's not like a car that has basic OBD type port. I have a cheap little adaptor and an iphone app that I can do just about anything with.
 

dustyjohns

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Aug 4, 2022
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Jimbo, what should it be? I watched a couple setup videos and read some forum stuff and websites.

Yes it is new to me. And having a prop boat is new to me. So I’m trying to learn what I can.
 

jimmbo

Supreme Mariner
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May 24, 2004
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10,663
Make sure you are actually using the Anti-Ventilation Plate(often wrongly called Cavitation Plate) for your measurement
 

Scott Danforth

Grumpy old guy who plays with boats
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Jul 23, 2011
Messages
40,346
Yeah, definitely have the motor scanned. Too bad it's not like a car that has basic OBD type port. I have a cheap little adaptor and an iphone app that I can do just about anything with.
wouldnt work on marine engines. and the cheap little adapters only read the published OBD2 data. you cant see the BCM data or the transmission data without the proper scan tools
 

dustyjohns

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Aug 4, 2022
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Okay tonight I went to a Lake here in the Valley. With an elevation of 2531’. Running the 3 blade stainless 14 1/2”X17 prop. I had my son and I in the boat so roughly 450lbs. The boat took quite a while to get on plane. Once it did I got up to 36.1mph by gps at 5800rpm. If I touched the trim at all, it began to porpoise. I am assuming that means the engine is set too high. Is it possible that I need the 4 blade prop due to the weight of the boat? I am perfectly fine with a lower top speed if I can get it to work right. Now I’m thinking there’s nothing wrong with the engine.
 

Scott Danforth

Grumpy old guy who plays with boats
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Jul 23, 2011
Messages
40,346
Are you just leaving the trim set for WOT running?

If so, that is your problem

You should be using the trim on the holeshot. Full down when you take off, and start trimming up once the boat starts to level off. Once it starts to porpoise, tap it down a smidge
 

dustyjohns

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Aug 4, 2022
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No, I am leaving it down until it gets up on plane. Then if I touch the trim at all the porpoising starts.
 

dustyjohns

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Aug 4, 2022
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I agree. That’s what led me to believe the motor height is wrong and I need to drop it a notch. There are so many variables though, where do you start? Weight distribution is a factor as well, right?
 
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