Slow plane at high altitude 5.0L MPI, please help

[jlrosine]

Hi everyone. I just purchased a new to me boat, I've owned boats before and probably should have done more due diligence on this one (more on that later), but here's the story.

Boat details
2004 Tahoe 204 WT Deck Boat, basically a bowrider, 20' 4" long
Drive: 5.0L Mercruiser MPI V8 / Alpha 1 with 1.47 gear ratio (yikes) / Stainless Apollo XHS 4 blade prop 14 3/8 x 18
Hydrofoil installed

Typical Boating Elevation: 4800ft - 6,500ft

The boat seems to take a while to plane, quite a bit longer than I'd expect for a V8. It doesn't seem to rev very fast under load. Once on plane, if I run it at WOT I can trim up to just below 1/2 trim and it does increase the speed and seem to open up/RPMs sound more normal. Even if I'm not towing a tube or anything behind it, it seems to take a bit longer to plane than I'd expect. Once I'm up to speed and trim up a bit, with WOT I can hit about 45mph on the GPS at 6300ft, which doesn't really seem bad and I'm guessing that my tach is off, it shows about 3800rpm but my ear says I'm probably running higher RPMs (see video links below).

Things I have done so far.
1: Replaced sterndrive oil, drain/fill (not really relevant but general stuff I do getting a new boat)
2: Replaced the Fuel/Water separator filter, it was old
3: Had it down to less than 1/4 tank and filled with ethanol free fuel, about 40 gallons. At the time of the videos I've gone through a 1/4 of the new fuel.

When The boat is on plane, it feels much more normal to me and similar to my previous boat for power (probably faster than my old boat top speed), however at low speed it's nowhere even close. My previous boat was about the same size/weight, but it had an older 5.7L Mercruiser with throttle body injection, however the big difference is it had a Bravo one drive and I believe it was a 1.65 ratio if memory serves. That boat topped out around 37mph at the same elevation with a 19p prop, but you could get on plane so fast it was incredible for towing.

My guess is that this 1.47 drive ratio on the alpha is just killing this 5.0L even with the 4 blade 18p prop and the hydrofoil. Looking for recommendations here. Do you think it's the drive ratio that is making the hole shot terrible? If you watch my video/acceleration, does it sound sluggish for a 5.0L? Could I just drop down to a much lower prop pitch to see if it solve the holeshot? I'm not really someone that cares about top speed, I don't care to go 45mph on the boat frequently, I'd rather be out of the hole while towing wake/skiers/tubers in very shot time then hover around 20-30mph. I appreciate any help.

Here are some videos I took tonight, around 6300ft elevation.

Video 1 (Just the wake trying to plane)

Video 2 (Tach and trim while planing and WOT)

 

[Scott Danforth]

You are down almost 20% on power from altitude 3%/1000 ft

Ditch the foil

Check compression ,(should be 150 psi on all cylinders,)

Check fuel pressure,(43psi)

Verify your tach

Your drive should be 1.62 at that elevation

 

[jlrosine]

Thanks for the reply Scott.

Since changing the sterndrive ratio is quite costly, do you think that lowering my prop pitch might help? If so, what might you suggest there?

If I'm having hole shot issues, do you think removing the foil will help that, or just to experiment?

 

[Scott Danforth]

foils do nothing except lighten your wallet and add drag.

 

[racerone]

Run the motor on a marine dynamometer.------If motor is producing it's rated power , then experiment with props can be done.

 

[alldodge]

See if you can borrow a 3 blade 17

 

[jlrosine]

See if you can borrow a 3 blade 17

Do you think the 3 blade props are better than the 4/5 blade for hole shot, or just an experiment at elevation? I'll take off the foil today and test it, then try to get a 3 blade at some point. My last boat had a 3 blade, I think a black max and it was great for hole shots but it also had the 1.65 ratio bravo.

 

[alldodge]

Really don't want to get into the 3, 4 5 blade argument that always happens. My point is a 4B 18 is equivalent to a 3B 20. Going to a 3B 17 drops it down to just the 17. To do the same thing with a 4B you would need a 4B 15

 

[jlrosine]

Really don't want to get into the 3, 4 5 blade argument that always happens. My point is a 4B 18 is equivalent to a 3B 20. Going to a 3B 17 drops it down to just the 17. To do the same thing with a 4B you would need a 4B 15

Oh gotchya. Yeah I don't think I should have anywhere near a 20p at this elevation. I've only had 3 blades in the past so the 4-5 blade trend is pretty new to me. I'll try to find a 3 blade to test local, it's definitely the "cheap" thing for me to test at this point. I've got one boat dealer that has a service shop and lake on premise, they might let me test out different props if I buy from them.

Thanks all!

 

[jimmbo]

The 5 liter is not known as Torque Monster at the best of times, and 5000+ft isn't helping. At least with FI, the ECM should take the thinner Air into account.
Are the Props 'Vented'? That can help the weaker in the Torque dept. engines

 

[QBhoy]

18” is too much prop at altitude for sure. Likely too much at sea level to see its best too. I’d go down to a 17” and see what happens. Even with a 17”…you’d do well to see the top of the Rpms I’d say.

 

[jlrosine]

Thanks all I appreciate the help. I'll try to get ahold of a 17 for a test.

 

[QBhoy]

Thanks all I appreciate the help. I'll try to get ahold of a 17 for a test.

Excellent. Let us know how you get on. I think these things run at their best when propped near the top end of the rpms. Definitely need to be above 4700/4800 top end, to be at their best.
You should also buy a VesselView mobile device for your mpi too. Amazing for many reasons…especially for propping well. Exact pin point accuracy on rpms.

 

[Scott06]

Thanks for the reply Scott.

Since changing the sterndrive ratio is quite costly, do you think that lowering my prop pitch might help? If so, what might you suggest there?

If I'm having hole shot issues, do you think removing the foil will help that, or just to experiment?

Yes dropping pitch 2” will be basically the same as swapping a 1.62 drive.
go through all the checks suggested and verify tach if all is good drop pitch .

 

[airshot]

Those foils are a last result, or should be....adjust weight, trim whatever you can do to get the best results, if all else fails then the foil can be used. When I bought my 22' years back the previous owner had a "fin" on the outdrive because everyone thought he should... After my first water test, I removed the fin, then I rearrainged some items in the boat for better balance and passengers as well !! My neighbor is a big guy, 6'6" and 330 lbs, yes it made a big difference where he sits!!! After all was said and done the boat perforned much better without the fin, trim reacted much better, higher top end and hole shotvwas better once I found the right trim adjustment to get on top quicker

 

[jlrosine]

I figured I'd close this thread up with what feels like a successful change. As stated before, I don't care too much about my top speed so I ended up dropping down to a very low pitched prop.

I called a local shop here (Jim's prop shop) in Denver, gave him the specs of my drive ratio/engine and typical boating elevation, he recommended that I go with a 4 blade aluminum, 15 pitch. I went ahead and trusted him and it turned out great. Here's a quick video of the boat planing while pulling a tube, we only had 5 people total on the boat today (sometimes 2 in tube, sometimes 3), but it was very noticeable how easy it was to pull/plane compared to the previous prop. I was able to typically just go 1/2 to 3/4 throttle very briefly, it would plane and I'd back it off and cruise around 20-25mph, extremely easy to drive with the wind and rollers today.

Thanks to all that helped, we'll see how the boat does at higher elevation but I think this will have to do. If I struggle pulling the wakeboarders/skiiers out then I guess my last resort is an outdrive swap, but I don't think it will be necessary.

 

[airshot]

That is why they call them experts!

 

[Scott06]

I figured I'd close this thread up with what feels like a successful change. As stated before, I don't care too much about my top speed so I ended up dropping down to a very low pitched prop.

I called a local shop here (Jim's prop shop) in Denver, gave him the specs of my drive ratio/engine and typical boating elevation, he recommended that I go with a 4 blade aluminum, 15 pitch. I went ahead and trusted him and it turned out great. Here's a quick video of the boat planing while pulling a tube, we only had 5 people total on the boat today (sometimes 2 in tube, sometimes 3), but it was very noticeable how easy it was to pull/plane compared to the previous prop. I was able to typically just go 1/2 to 3/4 throttle very briefly, it would plane and I'd back it off and cruise around 20-25mph, extremely easy to drive with the wind and rollers today.

Thanks to all that helped, we'll see how the boat does at higher elevation but I think this will have to do. If I struggle pulling the wakeboarders/skiiers out then I guess my last resort is an outdrive swap, but I don't think it will be necessary.

That’s the best if you can work with a shop and try them out. Looks good from my perspective



What is your wot rpm now ? I figured if your wot was accurate you’d end up around 15” pitch

 

[QBhoy]

Good result if it suits your needs. I suspect you might hit the limiter Bert quickly if you let it perhaps ? Doesn’t sound too bad there though, for high rpms. Happy boating

 

[jlrosine]

Good result if it suits your needs. I suspect you might hit the limiter Bert quickly if you let it perhaps ? Doesn’t sound too bad there though, for high rpms. Happy boating

I'm not sure what RPMs I'm at, the tach while I'm on water never says anything above 3000, but my ear tells me it's probably higher than that, though I could be wrong.

I purchased the RPM laser tool, I'll try to get an accurate reading to see how far off the tach is if any. I can't imagine this boat doing 45mph at 6300ft elevation and only being at 2800rpm (with the 4 blade 18p), but I suppose anything is possible? I doubt it though.....