Need advice on fine tuning a new pontoon (minor cavitation)

[pwiseman]

I posted last year for help selecting a new boat and recieved a lot of great info. I ended up with a 2021 Harris Cruiser 21 with a Merc 90 CT. Overall I am very happy with the boat and most of the time it is the wife and I. I noticed that when getting up around 4900 rpm (22 MPH from GPS) with just the two of use it would start to cavitate and this can't be fully eliminated with trimming. the prop is 13 pitch black max. I took out 10 people which is about max weight and noticed the boat felt good and very responsive. Originally I thought it could be that a 12 pitch might work a bit better, but now I am thinking it is more to do with being a bit heavy in back. I should also note that this boat has lifting strakes.

I attached a picture to show water lines that might help. This is more of a minor issue or annoyance than real issue. Looking for some thoughts on possible way to make adjustments like different prop, adding some weight up front. Not looking to add a jacker plate or anything extreme like that. Most of the time we just cruise around at low speed and enjoy the lake.

Thanks in advance for thoughts on what might be causing cavitation.

 

[ahicks]

It sounds like the lifting stakes are raising the boat up high enough at speed where you are getting some venting. A cupped prop might help, as would dropping the motor one hole.

That boat is sitting WAY low in the back, which is why I would avoid 75 and 90 hp motors on a boat this length.

 

[pwiseman]

Thanks.

This is my first pontoon and I did not want to end up underpowered. If I had it to do over, I would go with a high thrust 60 to knock off 100 lbs or so.

Fortunately, the cavitation is not horrible and I really don't want to be zooming around at WOT or pulling tubes. It does ride nice at about 3500-4200 RPM with a modest trim.

I will try moving some weight around and probably pick up a cupped prop as I need a spare anyway. I think the mount is already as low as I can go but will check on that too.

.

 

[captmello]

I think "way low"is a bit overstated, it looks just past halfway up the toons. it appears you have 20' logs on your 21' boat.
I agree that the strakes are lifting you out of the water, which is their purpose...but not very helpful. moving weight shouldn't be necessary but I agree a better prop, maybe a 4 blade could help. good luck!

 

[pwiseman]

TY. They are 25" logs. I believe the logs are 21 foot and the boat overall is 22 foot. I have actually been considering a 4 blade as some people have said it helps with navigating tight turns, My slip is a bit tough in and out. Maybe the 4 blade would help with both issues.

 

[pwiseman]

I am dialing in some info on what might be a good choice on a new prop for my 2021 Harris Cruiser 210 Pontoon, I did use the mercury prop finder tool which has been generally helpful in the past with my fishing boats, but also using some info I captured myself.

I have the 13P Black Max right now which is 16" diameter non cupped version on a 2021 Merc CT90 (2:38 gearing)

I get about 18 MPH at 4000 RPM (6 people) and everything feels perfect. I can get up to about 4900 but things start feeling off after about 4200. It might be slippage, but it feels more like cavitation to me. Highest RPM I can get is 4900 and speed is just at 23 mph trim is all the way down.. With 2 people the speed does not change a lot but the issue seems slightly worse be it cavitation or slippage. Motor is mounted on the lowest hole. I do have a little room left on the throttle when I back off. I don't really want to give it WOT when the RPMS are not moving up.

Boat weighs just under 2200 lbs without motor, I am using 1000 lbs. as average gear and people weight. Overall length is 22 feet and the toons are 25" diameter and 20.5 feet long. Max HP is 200

Prop finder using current pitch of 13 and top speed of 23 MPH at 4900 RMP this is the only recommendation.
SpitFire®4 blade 14" diameter 12 Pitch

• Calculated Pitch: 11.08
• Calculated Engine RPM: 5,750
• Calculated Boat Speed: 25.35 mph

Using weight in the prop tool, I get more suggestions.
Black Max 13, 14 and 15 (both the standard and cupped options) and
SpitFire®4 blade 14" diameter 14 Pitch

• Calculated Pitch: 13.98
• Calculated Weight with engine(s): 3,673.00 lbs
• Calculated Engine RPM: 5,250
• Calculated Boat Speed: 20.07 mph

There is a third option using length of the boat which calculates the weight too low and recommendations are the spitfire 14 pitch and some stainless in 15 and 16. Stainless is not really a great option due to cost and questionable added value.

I am leaning towards the 4 blade option for a couple reasons

• 2" less in diameter means the top of the blade will be 1" lower from surface than the 16"
• 4 blade should be a bit more responsive in tight quarters and windy/rough conditions

The big question here is what is the right pitch selection if I go to the 4 blade?

I am thinking 13 pitch 4 blade with smaller diameter and extra blade are going to give a better bite and then allow RPM to move up closer to target RPM of 5700-5900. It is challenging to decode the data. I don't really care if top speed is 22 or 27 MPH, just looking to know the motor and prop are well paired.

All of these are similar cost in the $250 range, so it is more about making an informed guess which option will help the most.

Anybody have thoughts on why I might want to go with a 12 or 14 pitch?

 

[ahicks]

Motor installed in lowest hole would mean the engine has been installed as high as possible, right?

That being the case, when it comes to cavitation, you should be seeing a big jump in rpm when that happens, and to reduce/eliminate that cavitation you would lower the motor.

Your tach will tell you if the prop is well paired with the motor. If your motor is turning 5000 to 6000 at wot, it's fine. The bigger question will be is it doing what you want it to do?

Most of the 4 strokes are rated to turn 6k, but at that rpm they're sounding pretty busy (too busy for my tastes). Unless you are after max pulling power, I think they work better turning about 5500. My experience has been they don't loose any speed turning 5500 max as compared to 6000 max. The DO loose some pulling power/grunt right out of the hole when limited to 5500.

For comparison purposes,
Our 24' toon, which is a pretty light one, has a Honda 90 on it. With a 3 blade 13p prop, it turns 6k at 24-25mph (GPS), and will set you right back into your seat on the hole shot. With a 14p prop, it turns 5500, and we get 26mph (GPS), and a much more graceful acceleration from a stop, while still having plenty of power for pulling the grand kids tubing. That's our prop of choice... FWIW

 

[pwiseman]

Motor installed in lowest hole would mean the engine has been installed as high as possible, right?

That being the case, when it comes to cavitation, you should be seeing a big jump in rpm when that happens, and to reduce/eliminate that cavitation you would lower the motor.

Your tach will tell you if the prop is well paired with the motor. If your motor is turning 5000 to 6000 at wot, it's fine. The bigger question will be is it doing what you want it to do?

Most of the 4 strokes are rated to turn 6k, but at that rpm they're sounding pretty busy (too busy for my tastes). Unless you are after max pulling power, I think they work better turning about 5500. My experience has been they don't loose any speed turning 5500 max as compared to 6000 max. The DO loose some pulling power/grunt right out of the hole when limited to 5500.

For comparison purposes,
Our 24' toon, which is a pretty light one, has a Honda 90 on it. With a 3 blade 13p prop, it turns 6k at 24-25mph (GPS), and will set you right back into your seat on the hole shot. With a 14p prop, it turns 5500, and we get 26mph (GPS), and a much more graceful acceleration from a stop, while still having plenty of power for pulling the grand kids tubing. That's our prop of choice... FWIW

TY for the response. It is sitting as low as it will go. bolts are in the top hole.

The 13P black max does snap me back real good if I hammer it on a hole shot, and everything seems great up to 4200 RPM.

I am about to order up a 4 blade spitfire with the hope that more aggressive cupping, smaller diameter and extra blade will help me grab more water. It is more about the boat feeling right than going fast for me.

The CT90 has steeper gearing than your honda, but it is still good to have something to compare with. I found out for CT90/CT115 the 4 blade spitfire is available in 12 and 14. They make a 13 but that is for the CT25-60 models with 13 splines.

Gut feel is 14 would be the better option, but one of those deals where you don't really know until you put it on. The good news is these are pretty reasonably priced at just over $200.

 

[jlh3rd]

24' '07 pontoon here. 25", twin tube, now equipped with a 2021 115 hp CT. It was mounted with the possibility of lowering the motor one more hole....which I did because of cavitation at higher rpm's, turning and any attempt at trimming up....... that is now all fixed.

I would like to see a pic. of where your cavitation plate is lining up...11/2 " made a difference with mine...


The dealer put a spitfire, 4 blade, 14x14 on it. Max rpm ( with just me) was around 5600, speed was around 26/27gps.....with cavitation
I pull tubes and people...so rpm's would be in the 5200 range, at max....too low for me....
.....I wanted a 14x13, 4 blade, but that's not happening....
Long story short...really long.....(which is another thread on here) (propping can be.involved).....I ended up with a black max, 3 blade, 16x12...this does put me over 6000 rpm's, I can hit 6200...alone...use the throttle, right?
I will shortly find out what my boat loaded will do as we are at a lake with teens and family.........
Also, I learned, the gearcase on our CT's is a bigger diameter than standard so some props will not work on these motors....the exhaust gas escapes around the smaller hub prop, causing cavitation and slipping ....
.... looking at your pic and estimating the travel to full level tilt, I looks like your motor may be too high......

Yes, I have two slightly used props.....
after this weekend, I might have another one.....as a spitfire, 4 blade 14x12 might be what I need after all......

 

[ahicks]

It's not always easy to come up with the "right" prop....

 

[jlh3rd]

Yeah… two prop bookends!…
Ran with 9 on board just now. Full throttle rpm was 5900…so the 3 blade, 16x12 is it for now. Slip is probably close to 20% however.

Wonder what the 4, 14x12 would do….next year maybe….

 

[pwiseman]

Posting back with results.

Just installed a new 14x14 4 blade prop today and it is a big improvement. good responsive turning at low speed and very smooth feel at all RPMs. Could go a bit higher than 5300, but I am very happy with how the boat is performing now. Still getting nice hole shot. Can't really tell the difference between the two props.

Harris Cruiser 210
two 25" tubes
weight 2140 lbs
max hp 90
Merc Command Thrust 90 is installed
beam 8.5 ft
overall length 22'

Stock prop was the 16x13 black max

Propeller Material Blades Diameter Pitch Rotation Part #
Black Max® Mercalloy 3 16 13 RH 78114A45
WOT is 4900 with top speed of 22 MPH (GPS)

SpitFire® Mercalloy 4 14 14 RH 8M0084493
WOT is 5300 and speed was 23.7 MPH (GPS)

 

[pwiseman]

24' '07 pontoon here. 25", twin tube, now equipped with a 2021 115 hp CT. It was mounted with the possibility of lowering the motor one more hole....which I did because of cavitation at higher rpm's, turning and any attempt at trimming up....... that is now all fixed.

I would like to see a pic. of where your cavitation plate is lining up...11/2 " made a difference with mine...


The dealer put a spitfire, 4 blade, 14x14 on it. Max rpm ( with just me) was around 5600, speed was around 26/27gps.....with cavitation
I pull tubes and people...so rpm's would be in the 5200 range, at max....too low for me....
.....I wanted a 14x13, 4 blade, but that's not happening....
Long story short...really long.....(which is another thread on here) (propping can be.involved).....I ended up with a black max, 3 blade, 16x12...this does put me over 6000 rpm's, I can hit 6200...alone...use the throttle, right?
I will shortly find out what my boat loaded will do as we are at a lake with teens and family.........
Also, I learned, the gearcase on our CT's is a bigger diameter than standard so some props will not work on these motors....the exhaust gas escapes around the smaller hub prop, causing cavitation and slipping ....
.... looking at your pic and estimating the travel to full level tilt, I looks like your motor may be too high......

Yes, I have two slightly used props.....
after this weekend, I might have another one.....as a spitfire, 4 blade 14x12 might be what I need after all......

I can't go any lower with the motor. I did check how cavitation plate lines up and it is pretty good. The water lines on my pic are a bit misleading because, water rolls out the back and where the tank sits and the boat rises about 2".

I am fairly convinced that what I thought was cavitation was really slippage. The 14x14 spitfire I put on yesterday is a big improvement. RPM is now capping at 5400 and 24 MP with light load. I think the 14x12 would get RPMS up to around 5900 on paper and be better for a loaded boat. Seems like you really have to pick your priority or be willing to swap props as needed. I don't pull tubes or run WOT much, so I am really feeling like the 14x14 is a huge improvement and offers smooth overall ride all the way to WOT and improved steering. I may end up buying the 14x12 just to see if there is even better option, but for now i am pretty happy.

It is a bummer that they don't make the 14x13 spitfire for CT 90/115. It sounds like the 16x12 3 blade is working pretty for you . I suspect the 14x12 spitfire might work a little better for you than the 14x14 you tried if you are really wanting the hole shot for pulling and looking to get more RPMs.

 

[jlh3rd]

yeah...what I've learned...and tried to avoid,but didn't...is that propping involves a ..balance....( so now I have 4 props total).... It doesn't matter what a spec sheet says, that's just a starting point.....some boat/motor/prop combinations can be easier to figure out than others....

I had five teenagers on two tubes, with 5 in the boat the other day....getting 20 mph at around 5700/5800 in a straight line and almost full throttle....but I bet my 3-16x12 was slipping like crazy...
reading actual results on here does help...

 

[pwiseman]

Sounds like you were having too much fun!

To some extent, you are asking a lot of a 115 HP motor which also makes it a bit more challenging to find one prop that does it all. 115 is as much weight as I would put put on a 24' twin toon boat. That pretty much forces you to pick your poison on the prop.

There is a guy from my area that does a bunch of youtube vids. He says you really want a three log with 150+ for pulling tubes and skiers. That probably has merit, but it adds 10K or more to the sticker too. Sounds like you are dialing in how to do it just fine by swapping props around. I don't suppose any dealer will tell you that trick.

 

[captmello]

no two props are the same. I was running a 4 blade 13p prop for a few years. I hit something, so I switched to a different brand, same specs, and gained 1-2 mph.
I've been running the four blade, 13 pitch, now for a long time. I don't buy stainless or fancy expensive props since I boat in shallow lakes. I can pull two tubes fine at half throttle with my 90hp. I'd rather not have to switch props between uses, just use the one that works the best 70-80% of the time.

 

[pwiseman]

no two props are the same. I was running a 4 blade 13p prop for a few years. I hit something, so I switched to a different brand, same specs, and gained 1-2 mph.
I've been running the four blade, 13 pitch, now for a long time. I don't buy stainless or fancy expensive props since I boat in shallow lakes. I can pull two tubes fine at half throttle with my 90hp. I'd rather not have to switch props between uses, just use the one that works the best 70-80% of the time.

I am with you on not wanting to swap props or run stainless on a pontoon. I am pretty happy with the 4 blade 14P 4 blade. It is much smoother at all RPM than the 3 blade 13P I had on there, so the update is a big win for me. I may eventually try the spitfire 4 blade 13Dx12P, but hard to justify that entirely. Any improvement at this point would be minor. Matter of how much I indulge my curiosity. Some of us are afflicted with the inability to call it all good and move on to the next problem.

Not sure what motor you are running, but I could not find a 13P 4 blade prop for Merc CT 90/115. I checked a number of brands.

 

[jlh3rd]

Sounds like you were having too much fun!

To some extent, you are asking a lot of a 115 HP motor which also makes it a bit more challenging to find one prop that does it all. 115 is as much weight as I would put put on a 24' twin toon boat. That pretty much forces you to pick your poison on the prop.

There is a guy from my area that does a bunch of youtube vids. He says you really want a three log with 150+ for pulling tubes and skiers. That probably has merit, but it adds 10K or more to the sticker too. Sounds like you are dialing in how to do it just fine by swapping props around. I don't suppose any dealer will tell you that trick.

the boat is only rated to 115hp, 4stroke..so thats the biggest I went..it is a CT......The boat is an '07, but with big tubes......it's teenagers I'm pulling and it is more than enough for them.....don't need to spend that money, plus the new motor was $8000 last july.....it sure beats the 60 that was on it. It's doing what I want, I'm happy.....
I'm just here to give fyi's and FWIW....and to learn some also....

 

[jlh3rd]

I am with you on not wanting to swap props or run stainless on a pontoon. I am pretty happy with the 4 blade 14P 4 blade. It is much smoother at all RPM than the 3 blade 13P I had on there, so the update is a big win for me. I may eventually try the spitfire 4 blade 13Dx12P, but hard to justify that entirely. Any improvement at this point would be minor. Matter of how much I indulge my curiosity. Some of us are afflicted with the inability to call it all good and move on to the next problem.

Not sure what motor you are running, but I could not find a 13P 4 blade prop for Merc CT 90/115. I checked a number of brands.

yep...Hustler makes a 4 blade, 14x13....BUT..as I learned, after the fact, it doesn't match a CT's gear case diameter.....live and learn...