Prop Not "Biting" The Water

[Coast Ranger]

I have a 28' AquaPatio pontoon boat with a 70 hp Mercury on it. The prop is a 3 blade, 13 pitch. The motor seems to be running good, with a couple of boat people agreeing to that. Periodically, I'll have a problem where the prop seems like it's not "biting" the water properly. Yesterday, we ran with the current a couple of miles to our favorite spot. The motor was turning at 3800 rmp which pushes at a comfortable speed - not fast but not slow. On the way back, against the current, the boat felt like it would loose power. There was no difference in the sound of the motor, like would accommodate a loss in propulsion. Throttling up did not help. We were up to 4500 rpm and it still felt like the boat wasn't moving like it should. Throttling down to 1500 to 2000 rpm produced the desired forward momentum - what you would expect for that level of throttle. When the "problem" is happening, I can look behind the boat and still see a lot of water being pushed behind the boat. Lastly, I don't feel any undue vibration.

I've tried every practical trim without result. Could I be churning air into the water under some conditions that decrease the bite? Maybe the prop is not far enough in the water? Maybe mounting it lower on the transom or (heaven forbid) a motor with a longer mid-section? Would I benefit from a 4 blade prop? I'm not interested in speed, but I do want the boat to move accordingly as I throttle up.

Thanks in advance for any ideas or suggestions,

CR

Mr. Moderator, If this would be better suited in the Props forum, please move it there for me. Thanks, CR

 

[QC]

Re: Prop Not "Biting" The Water

I am going to move this to the 'toon section as they are somewhat of a unique animal. With that said, we need all the info you can come up with. Diameter of the prop, part number, make. Pictures of the mounting height in the water would be great. You said "no difference in the sound of the motor", but you also say you varied RPM. That had to sound different, right? Has the boat always performed like this? Do you have history with it? New to you?

Edit: When you bury the throttle, does the bow dig in deeper? I have heard that from some 'toon owners/threads.

 

[Coast Ranger]

Re: Prop Not "Biting" The Water

Thanks QC,

I'm going to put the boat in a lake this weekend and see if water current makes any difference. I'll get pictures in and out of the water and post with prop measurements.

There is a difference in the sound of the motor at different rpm's. But it's what you expect as you throttle up and down. I don't recall a difference during the condition.

My brother described the condition as though the prop was "slipping". However, we saw a lot of water being pushed behind the boat at the time. This type of prop uses a square plastic sleeve. I put on a new Quicksilver Plastic Drive Sleeve #835290Q1, purchased from iboats. It looked exactly like the previous sleeve and fit perfectly. I replaced the sleeve because it was cracked. Unfortunately, that didn't stop the condition.

Full throttle doesn't change the level of the boat much. Several people fore or aft will, but just bit. Yesterday though, we had 6 in the back when it happened so the tilt was towards the rear with the foot of the motor deeper in the water.

BTW, the boat and motor are 1989 models. The tubes are enormous. They throw up a lot of water themselves and I wonder if they somehow cause air to get into the props supply of water.

Thanks again,

CR

 

[QC]

Re: Prop Not "Biting" The Water

What you describe sounds like Ventilation. The prop is getting air from somewhere. Very common actually. Current should make no difference, unless it as literally rapids and a lot of air was churned up. Water speed is water speed. GPS shows speed over the bottom, but in your case we are concerned specifically on the speed across water.

I think you should try and find the largest diameter prop you can find that will fit your gearcase. Better yet, you should talk to a local prop shop. Ask some guys around where you boat. They'll know someone. If the hulls are causing some turbulence, then I think that needs to be addressed first. Are they clean? No dents? No growth? We need some 'toon guys to show up. I have no experience with them.

 

[Coast Ranger]

Re: Prop Not "Biting" The Water

Ventilation - I'm glad there's a term for what I'm thinking. The logs are in great shape and very clean. I'll keep at it. I appreciate the wealth of knowledge shared across this forum. Whatever I find out I'll post in case someone else has a similar problem.

CR

 

[craigmack]

Re: Prop Not "Biting" The Water

Wow 28'. That's a long boat. Is the prop in good condition? Check for dings. A 4 bladed prop would definitely bite better but you'll have to drop a size or two in pitch. Check engine height too. When I first got mine I had similar issues and lowering it one hole made a difference though I still had issues when turning. The Enertia fixed that; boy does that prop grab the water!

 

[The Rooster]

Re: Prop Not "Biting" The Water

Could be you just spun the bushing in the hub of the prop. There is an easy procedure for checking this involving using a grease pen or similar marker. Do a search for it. Not uncommon for props w/ spun bushings to accept some throttle but, under too much strain, lose their "bite" Good luck !!!

P.S. Usually +/-$50.00 to get a new bushing pressed in from a shop.

 

[Bamaman1]

Re: Prop Not "Biting" The Water

70 hp for a 28' boat is very underpowered.

You should be looking for a propeller in the lowest pitch available. A 13 pitch prop will simply not cut it.

And as has been said, look for a good prop shop. Their expertise comes free if you're buying a propeller from them.

 

[MH Hawker]

Re: Prop Not "Biting" The Water

I think you need to run a prop test before you do much of any thing, from your description it sounds like it may be slipping.

Take a sharpie and draw line across all 3 parts on the prop and hub, you may have to take the prop nut off t o do this. Then go and make your run and enjoy the day. and after your home take a close look at the line you drew on the prop, if its intact then your good if its not then you may of found your problem, or at least a big part of it.

 

[Coast Ranger]

Re: Prop Not "Biting" The Water

I don't think the prop can "slip". It doesn't have the rubber bushing that seems to be most common. It has a square plastic sleeve. The sleeve has "ribs" that mate directly to the spline of the shaft. For the prop to slip, it would have to strip off all the plastic ribs. Then the prop would spin freely - something you would notice right off.

70 hp for a 28' boat is very underpowered.

I think so too. In the next year or so, I'd like to put a 110 or 140 four stroke on it. The fuel efficiency would be much better and it would be much, much quieter.

And as has been said, look for a good prop shop. Their expertise comes free if you're buying a propeller from them.

Would it be appropriate to ask a shop to borrow a prop to see how it works? Or would that be asking too much? I suppose it couldn't hurt - unless he threw the prop at me.

 

[MH Hawker]

Re: Prop Not "Biting" The Water

Running a prop test costs you nothing but a few minutes time. and then you know and if its not that then you need to start dropping the motor lower till it stays hooked up to the water, once it is then you can work out prop pitch.

 

[crb478]

Re: Prop Not "Biting" The Water

A four bladed prop will bite better and back up better, but that may not be your problem. If you have a hole it can be dropped you can do that for free. You can also raise it back up a hole in the parking lot if that did not fix it or improve it. Next what is your current wot rpm's? Pick the prop to get the most rpm you can in that range. There is a lot of turbulence coming off the back of pontoons, it is possible that these is more going into the current, all boats are different. I am not a big fan of hydro foils, but on my boat I use one to help flatten out the water coming off the pontoons. It works better with the hydro foil than without. The one I as using was on the boat when I bought it and is basically a flat piece of steel/aluminum angled from front to back. It is wider than most plastic type hydro foils. I also run a four bladed prop which was a huge improvement over the 3 bladed one.

 

[QC]

Re: Prop Not "Biting" The Water

it is possible that these is more going into the current,

I agree with literally everything in your post except this. And I have a ton of experience in 2 - 4 MPH current. Not with a 'toon, but my experience is this is why you have a pitot speedo and a GPS. The pitot speedo knows no difference either direction

What I do to verify this is float in the current, no wind. Measure GPS speed. Say it's 2.5 MPH. Upriver my GPS is 2.5 slower than my speedo, downriver it is 2.5 MPH faster. Done.

 

[Coast Ranger]

Re: Prop Not "Biting" The Water

We fished all day Saturday. I let my friend drive the boat. He works on boats and has experience with pontoons. He's convinced that more exercise with the trim will remedy the ventilation. All day, we had no problem. As I ferried the boat a mile from the dock to the landing, it ran great. It appears that load plays a big role. The difference between 1 person and 8, and whether they're forward or back. How much beer and fish you have on the boat (although one should increase as the other decreases). Then factor in the speed that you want to maintain. I'm going to postpone any changes until I have more experience with different conditions.

Thanks for everyone's help.

CR

 

[MH Hawker]

Re: Prop Not "Biting" The Water

I think thats a wise choice, get used to the boat.