Pontoon Boat & Deck Boat Forum

I just repowered my 24' twin tube pontoon boat (2006 lowes) with a new 115 mercury CT. Tubes are 25" diameter.
Th prop is a spitfire, 4 blade, 14x14. With just me on board, I get 27 mph, at 5700 rpm. Tach is a smartcraft tach. I cannot trim it up much as it begins to ventilate. The cavitation plate is slightly below the center of the rear stern and transom. With a few people on board and pulling tubes, i can be down to 5300 rpm . So I tried a spitfire 13.8x13 ( there is no 14x13, 4 blade).......with just me on board it would not bite...I thought the hub was spun...couldn't go over 1800 rpm's without it ventilating/cavitating! ( whichever).......wtf.

So, now..I'm asking for opinions. Some say bigger diameter prop.....ok...what pitch?.... how big a diameter prop can a 2021 mercury 115 hp CT take?
Mercury makes a 3 blade 14x13 extra cup prop....
I understand basically gaining/losing rpm's with diameter and pitch....
Also, i can lower the motor one more mounting hole.....but Ive been told to be careful where that puts that cavitation plate.

Generally, the cavitation plate should line up with the bottom of the toons. It sounds like you may be too high. I'd try lowering the motor first. Won't cost you anything other some labor and it's easily reversible if it doesn't help.

I really don't know if you'll do any better than what you got with your first prop.

I tried the recommended four blade on my 90hp Honda and 20' pontoon with 25" tubes and it was fine until 4500RPM but then it was blow out city. The stock three blade ran better wide open. Also the weird thing about mine is that if all the weight was in the back pushing the motor deeper it would slow down. Literally if I was pulling a tube and had a 200lb person just walk from the back bench to the front, it felt like the turbo kicked in. It would gain 4mph instantly without touching the throttle. Trim was useless, all the way down ran best.

thanks
steve... When running at speed, I can see the cavitation plate is below the water......the dealer will lower the motor free....I'm gonna recheck at the end of the season to see where the plate is in relation to the bottom of the toons....not the transom/well.............I'm getting that "oh crap" feeling now....the motor is a long shaft, 20", the same as the 60 hp mercury it replaced....but 25" was also available......there is another plate above the cavitation plate that the dealer says you really don't want submerged when running at speed..........

steiner....yeah, that's what I'm afraid of. I was confident of being able to prop the boat but now ....Dunno....
How can going from a 14" diameter,14 pitch to a 13.8" diameter, 13 pitch have such extreme, different results....
doesn't make sense to me.....
but, if going from 14 to 13.8 diameter caused extreme results.....my logic would say going to a 15" diameter prop is
the right direction.....applying the math. that means i need a 15x12 which isn't out there in a 4 blade...there is a
15x11......according to my search.......and, my 60 hp merc actually did better with the 3 blade over the
recommended 4 blade....makes me wonder about the 4 blade hype.....
let the games begin
any comments appreciated....

Not sure if this helps or not but.
I have a Verado 200.
15X15 Enertia 3 blade and its pretty dialed in.

all comments are welcome.
I see there was a thread with a propping chart ....I did read all of those...
I didn't state what I'm trying to do...I want close to 6000 rpm ( high end of recommended rpm), even a little over, when by myself so that when the boat is loaded, my rpm's will be higher than 5300....

there is a black max 14x13, 3 blade, "extra cup".....?
solas 3 blade 15.5x13
mercury 15x13, 3 blade.

wonder what rpm's to expect from those.....

Just a couple of data points:

1999/2000 25' boat with two 23" diameter tubes and a 2000 Yamaha F115 four-stroke. No lifting strakes and old school plywood frames on full furniture for 16 passengers. (She is front heavy and plows regardless of trim) Stock Yamaha aluminum prop (with dings) 13.5" x 14"

Best speed ever was 27-28MPH on smooth water with me and my dog at 5800-6,000 RPM.
Six adults on board with 2-3 being pulled on a "Super Mabel" tube = 25 MPH @ the same 5,800 to 6,000.
Same load of people and towing the same load @ 5,000 RPM = 24-25 MPH.
Same load of people and towing the same load @ 4,600 RPM = 21-22 MPH.

See the pattern? Regardless of trim angle, my boat just plows more as I apply more throttle. Not enough HP to lift the bow at all.

Marc

yep...I see that. 3 blade or 4 blade? I'm assuming 3 being a 2000. In your situation, I'd wonder if you're motor could be lowered. That would enable more of a nose up moment. That's what it's telling me, in your case. Begs the questions, is your yamah a long shaft, is the motor mounted at the correct height......Thats why i'm also thinking about having that done. I actually have more of a nose down effect with the 115 than i ever did with the 60.
The trim gauge is marked in quarters. With the 60, I could get a little over a quarter trim , and run 19 mph...tops (21 on a cold water/air temp day).
With the 115, I can hardly get 1/8 of an indication before the prop blows out.....so..that's telling me this extra power is not letting me get the nose up...I think it should.....is it the prop?...is it motor height? ...is it both?......dunno.....That unexpected result with the 4blade, 13.8x13 prop has me back to square one ....

I have a 20 ft Lowe with a 90 Merc Optimax-25" tubes. SS prop 13x16 pitch. Fishing model with fishing seats up front. No couches.

I run 27 by myself. 28 with my aluminum prop-same pitch--go figure. I have no cavitation problems. I can trim it out a long way before it breaks loose. When rig was new in 2009 I hit 29 once in early spring when water was cold.

I'm set up with my cavitation plate maybe an inch or less below the engine mounting pod. You may need to raise your engine. Too low in water and lower unit disturbs the flow of water around the prop and it blows out--sorta like stalling a wing on an airplane. Good luck.

umm...you mean lower my motor. I noticed yours is as low as it can go. Mine can go one bolt hole lower, as my cavitation plate( wish i'd taken a pic) is more in line with the pod.....After seeing your pic, Its becoming more of a choice....
but i still need to get my rpm's up another 300.
I'm guessing your hitting your max rpm's....

yes----I can redline mine at about 5850 on my tach--I get the alarm about there without it breaking loose and blowing out. Comes out of the hole well too even with motor trimmed up a bit..

I had prop repitched from 16 to 17". RPM'S dropped to around 5450 with no increase in speed so I put it back to 16p.

I thought you had a long shaft lower unit so thought cav plate was too deep. Best thing to do is to get a video of you running WOT so you can see what the water flow looks like around the motor. Shoot from side of boat and behind the boat ...Good luck...

Enclosing pic of me running 27--throwing a lot of water.

follow up...
Lowered the motor.....fantastic results. I get max trim up, no blowout.....When making a turn , it wants to throw you out of the boat..no blowout........hard to believe that 1.5 inch makes that big a difference. I might have lost 1 mph....
Still only 5700rpm........I need 300 more....

jlh3rd wrote: ↑

Thu Oct 14, 2021 4:46 pm

Still only 5700rpm........I need 300 more....

Maybe not. the analog gauge on my dash reads 5700 when the digital readout on my Simrad (NEMA2000 network) reads 6000. If you're relying on an analog gauge it might be off a bit.

I have the smart craft tach, digital .....but, that is a valid point......wonder what the % error is.........
also, that's with just me on board...so a prop that would give me 6100 by myself would be great with adults and tubing kids....

jlh3rd wrote: ↑

Thu Oct 14, 2021 4:46 pm

follow up...
Lowered the motor.....fantastic results. I get max trim up, no blowout.....When making a turn , it wants to throw you out of the boat..no blowout........hard to believe that 1.5 inch makes that big a difference. I might have lost 1 mph....
Still only 5700rpm........I need 300 more....

While keeping everything else the same; drop the pitch on the prop by one. If it’s a 16 pitch; drop it to a 15 pitch. That should give you a 200-300 rpm increase at WOT.