24' pontoon prop slip issue

[Maped]

I have an old 24' Crest pontoon with a 90hp v4 Evinrude that has a Max wot of 5500. I was battling ventilation issues that I have finally cured and need to raise my motor an inch or two now to get the ventilation plate skimming the water again.

The problem I am having is that I have three different props for this motor that all over rev and give me unbelievable slip.

14x13 turning point prop 11 mph at 5500 rpm
13x19 unknown prop 15 mph at 5500 rpm
13x21 (maybe) Quicksilver prop 17 mph 5500 rpm

All these readings are about an inch from full throttle beings I don't want to rev past the 5500 with gps for speed. What I don't understand is how all three of these props are giving me slip readings of 60%+. I have always had wife and two small children on with me and making them move around boat doesn't change any performance.

 

[HotTommy]

Another forum member recently brought us a problem that involved high slip. His prop was also 14x13 or so. I suggested his prop sounded about right so his boat might be experiencing extra drag from unseen weight. He subsequently found he had a leak in one log and was carrying a lot of extra water weight. I suggest you start by checking for any hidden weight that might be causing extra drag.

 

[clemsonfor]

For comparison I have a 1998 Crest II pontoon with a 1998 60hp Evinrude. I have a 14" 11p prop and with just me on the boat with full tank of fuel I will I think spin about 5600 rpms and I will hit around 21mph. I can say for sure that a 21p is too much prop. I would think you would need to be at either a 13 or 15p prop max.

 

[Maped]

I will pull plugs and check tubes later today. I forgot to add in my original post that it is a 2:1 gearcase. The other scenario that has me worried is that the prop isn't what's slipping and instead something in the gearcase is giving me a 6:1 ratio under load causing the low speeds.

 

[Maped]

Just got done checking best I could, pulled 4 different plugs on top side of each tube and dropped 3ft of rope in. Lowered and raised jack to move any water inside and all ropes came out dry.

 

[Maped]

Well, believe I've gotten a little bit closer. Looked over electrical stuff again and found tach was on 4p instead of 6p which changes my throttle position to 3650 rpms instead of 5500. That changes my slip calculations by 10%.. could having the motor about 5" to low be causing the rest of my excessive slip?

 

[HotTommy]

Anything that increases drag will increase slip. Imaging dragging something that size (5" tall and width of motor leg) through the water to get a sense of how much. ..... I once ran a 150 HP motor that was too long and it performed about the same as my 115 HP motor with the correct leg length.

 

[wahlejim]

Do a full WOT run with correct RPM readings. You mentioned that with the tach adjustment you are now at 3650 but didn't mention if that was at WOT or if you still had more throttle to play with, as previously mentioned. You may be overpropped and the prop is slipping because it can't push the boat.

By comparison, I have a 1991 25 foot pontoon with a 50hp Merc, 1.83 ratio. It came with an 11 pitch that would only go 4200 RPMs. The slip percentage was off the charts at 40 or so. Came accross a 8 pitch and now I am spinning 5200 at 17mph. Brought it down to 20ish. These old pontoons are not efficient when it comes to propulsion.

 

[Maped]

Do a full WOT run with correct RPM readings. You mentioned that with the tach adjustment you are now at 3650 but didn't mention if that was at WOT or if you still had more throttle to play with, as previously mentioned. You may be overpropped and the prop is slipping because it can't push the boat.

Hopefully I will get back on the water Monday to get updated readings. My mention of 3650 is just calculated from switching the tach from 4p to 6p.

 

[Maped]

By comparison, I have a 1991 25 foot pontoon with a 50hp Merc, 1.83 ratio. It came with an 11 pitch that would only go 4200 RPMs. The slip percentage was off the charts at 40 or so. Came accross a 8 pitch and now I am spinning 5200 at 17mph. Brought it down to 20ish. These old pontoons are not efficient when it comes to propulsion.

Got the pontoon out today, although I had more people than I woulda liked for testing, 8 ppl, and it felt like I still need to raise my motor a couple more inches.

With the 13x 19p prop wot was 4000 rpm at 16 mph

With 14x13p 3 blade turning point prop at 5000 rpm I would lose speed like it was ventilating but it wasn't which I need to look into, but at 4500rpm I was hitting 18mph (fastest I've recorder on the big girl) which is 35% slip.

Am I crazy to think I should try a 14x11p 4 blade prop and if so what brand? This is a 4 1/4 gearcase so I can't use turning point for 4 blades

 

[wahlejim]

I think you have enough numbers to work with, the next step is figuring out how you are going to use the pontoon to dial it in. More often than not, will you have a lot of people or just a couple of people?

I understand the reasoning behind doing a wide open throttle run alone on calm water, but that has never been how I dial in my props. I am never alone on my ski boat or pontoon boat doing WOT runs and my water is almost never calm. I dial in my prop for daily use. I tow behind the bow rider I have with a typical load of 7-8 people and I am propped accordingly. I cruise in the pontoon with anywhere from 4-15 people on board and am propped accordingly.

Take a look at the history of use on the pontoon and prop accordingly. Understand that with 8 people on board, your RPMs dropped significantly. If you are typically loaded like that, I think an 11 pitch 4 blade would work well. If you are lighter than that, try a 13p 4 blade. With 4 blade props, you will probably want to drop diameter from 14. Look at prop finder tools online and go with their diameter suggestions.

 

[Maped]

Most of the time the boat will have two adults with two children on it, which is when I'd like my wot to be very close to the 5500 limit and we'll just never rev to wot.

However my hope/goal is to be able to hit 20 mph at wot pulling a tube with 2 kids on it and probably 2 kids and 5 adults on board.

Now that I'm saying that, I almost think raising the motor a little more might just put me at those numbers beings I was able to hit 18 mph with 8 adults and not quite wot. Unfortunately there is a good chance I won't get to test the boat again this year but I probably need to get one more run with this prop and light weight just to see where I am at.

 

[HotTommy]

I offer the following as one person's experience with three different setups on one boat. Here are the basics. The boat is a 25 year old pontoon boat with two U-shaped logs and three sofas. My usual load is 4-5 adults plus ice chests and gear. I use it primarily for cruising, swimming and towing a three-person tube.

The first configuration used a 90 HP two cycle engine with three bladed aluminum prop (unknown pitch) and a 20" shaft. At that time I had a large canvas top that likely produced considerable drag. Top speed (GPS) with a light load was 18 MPH. Towing a tube in a hard turn it would slow to 12 MPH. RPM was unknown due to inop tach, but the engine sounded like I expected it to sound.

The second configuration was with a 150 HP two cycle engine with the same three bladed prop as before and a 25" shaft. The bottom of the boat was skinned, the engine pod was replaced with a stronger one and I replaced the canvas top with a more streamlined flat aluminum top. Top speed with a light load was about 27 MPH. Towing a tube in a hard turn would slow to 21 MPH. RPM was again unknown. I'm sure some speed was lost due to the engine sitting too deep in the water.

The current configuration is a 115 HP four cycle engine with a 20" shaft and a three bladed aluminum prop (16" dia x 14" pitch). Top speed with a light load is 28 MPH at 5900 RPM. Yesterday with five adults and three toddlers on the boat, and one adult and one child on the tube I could reach 21 MPH WOT and slow to 18 or so in turns.

I would be surprised if you reach 20 MPH with seven on board AND pulling a tube.

 

[wahlejim]

I agree that 20 while pulling a tube isn't very realistic, much less with that many adults on board. The tube creates a tremendous amount of drag, even if it is 2 kids that might tip the scales at 100lbs combined.

Based on the numbers you have given, an 11 pitch 4 blade may be the best for your application. I am not sure if your motor height is going to make much of a difference on this one. You have mentioned the prop already acting like it is ventilating, so I am not sure you have anything to gain.

 

[Maped]

The current configuration is a 115 HP four cycle engine with a 20" shaft and a three bladed aluminum prop (16" dia x 14" pitch). Top speed with a light load is 28 MPH at 5900 RPM. Yesterday with five adults and three toddlers on the boat, and one adult and one child on the tube I could reach 21 MPH WOT and slow to 18 or so in turns.

you went from a 150 hp motor back down to a 115 and didn't see significant speed loss? Do you believe that's just because of the xl shaft on the 150?

 

[Maped]

Based on the numbers you have given, an 11 pitch 4 blade may be the best for your application. I am not sure if your motor height is going to make much of a difference on this one. You have mentioned the prop already acting like it is ventilating, so I am not sure you have anything to gain.

The motor height has been a struggle. I'm not sure what was happening this last week at 5000 rpm, may have been rev limit or ventilation or that I was low on fuel. I didn't want to take time to look at it with people on board so I switched tanks and went back to running at 4500 rpm.

To expand on the motor height tho, when I got the boat it was ventilating at half throttle. It was in the lowest hole and previous owners had wedged the motor pod to lower the back 3 inches. I bought a shaft extension from Bay mfg to get 5" deeper and still had ventilation issues. The problem ended up being the fish finder mounted on a 4"x3" bracket in front of the motor.

Now that I have removed the sensor the water flows smoothly and ventilation is gone so I removed the wedges from the motor pod to raise motor about 3 inches but it's still in lowest holes with that Bay mfg extension so the ventilation plate is roughly 6" below bottom of pod and about 1-2" above bottom of toons so I believe I should be able to raise it at least two holes without ventilation.

Also I had water splashing up the back of the boat which I had read means the motor is probably to low.

 

[HotTommy]

"you went from a 150 hp motor back down to a 115 and didn't see significant speed loss? Do you believe that's just because of the xl shaft on the 150?"

The engine weights were similar, so the only significant differences were the leg length and the props. The physics of drag are that the force inceases as the square of the speed. So the drag from that extra length of shaft is four times more at 20 MPH than at 10 MPH. That means a small amount of extra drag surface can have a large effect as speed increases.

 

[wahlejim]

To expand on the motor height tho, when I got the boat it was ventilating at half throttle. It was in the lowest hole and previous owners had wedged the motor pod to lower the back 3 inches. I bought a shaft extension from Bay mfg to get 5" deeper and still had ventilation issues. The problem ended up being the fish finder mounted on a 4"x3" bracket in front of the motor.

Now that I have removed the sensor the water flows smoothly and ventilation is gone so I removed the wedges from the motor pod to raise motor about 3 inches but it's still in lowest holes with that Bay mfg extension so the ventilation plate is roughly 6" below bottom of pod and about 1-2" above bottom of toons so I believe I should be able to raise it at least two holes without ventilation.

Also I had water splashing up the back of the boat which I had read means the motor is probably to low.

That is some pertinent information that renders every other post a moot point right now. Before you go through the trouble of removing the extension, see if you can mount the motor at the correct height with the extension in place. As soon as you have that dialed in, restart your prop testing as all of your numbers will change.

 

[Maped]

That is some pertinent information that renders every other post a moot point right now. Before you go through the trouble of removing the extension, see if you can mount the motor at the correct height with the extension in place. As soon as you have that dialed in, restart your prop testing as all of your numbers will change.

I agree, and I do not plan to remove the extension unless absolutely necessary. I have four holes of adjustment that I can raise the motor and I also have a jack plate with 3" setback that can give me another 5" in height if needed.

I'm just glad that Ifound that crucial problem and am now able to start raising the motor back up.

And for those wondering about leg height, just raising my motor 3" added about 4 mph top speed to that 13p prop

 

[Maped]

Well got some new numbers today with my normal load.

14x13 turning point prop wot 5000 rpm at 22 mph gps.

Suggested wot throttle is 4500-5500 so I'm right in the middle of that.

Using those numbers I get %28.5 slip. I do however think I can raise my motor even higher yet because there were a lot of waves and it never ventilated.

Does this amount of slip seem reasonable now?

Thanks everyone for helping get me this close!