Ventalation problem

[npd4432]

I am currently running a 75 HP evinrude on a 16' open bow boat. The boat is mostly used for water skiing, I also have a ski pole on the boat and Nauticus smart tabs. The prop I am running on the boat is a Solas Amita 13.75" diamater and a 13 pitch. The lake I currently live on is a smaller lake with a "ski zone" area which is a buoyed area of the lake in an oval pattern. Here is the problem, while skiing or just crusing at WOT the prop/boat will experince a ventalation when making the turns at the end of the oval "ski zone" area. I have had other boats on this lake and never had this problem before. The turns I am making are not too extreme and other boats have no problem with the same type of turns. I am looking for some advise on how to correct this problem. The motor is on the lowest setting on the transom. Would a different type of prop help this problem, 4 blade?, stainless steel? advice please.

 

[steelespike]

Re: Ventalation problem

Are you trimming in for turns?Is the prop in good shape?Are there any devices on the boat that may be disturbing the water?There was some discusion the other day on the
smart tabs placed closer to center than recommended helping venting problems.Allowed the motor to operate at extreme height and trim.You might try different settings on your tabs.
Sometimes adding cup to your prop will help.I don't know if yours may allready have some cup.
As I understand it stainless will usually help venting problems, 4 blades are supposed to improve low end performance.Should be someone along with more detailed prop info.

 

[Texasmark]

Re: Ventalation problem

I can think of 4 things:

1. Angle of engine during turn.....trim position if you have it; but I assume when you say "the lowest setting on the transom" you are referring to the tilt hence no trim. Solves that problem.

2. Location of antivent plate vs bottom of boat. Even with or above can aggravate ventilation problems but gives better WOT performance. Probably you don't care too much about that as you are towing skis.

3. Cupping and rake on a prop can help to eliminate it....hence SS prop.

4. Hull shape in front of the prop can cause problems. Like on my alum boat there is a v shaped strip "keel if you will" running full length of the bottom.....covers up the weld where they put the sides/bottom halves together.

On turns this skids sideways as the boat turns and creates bubbles. The bubbles get into the prop slipstream and cause ventilation problems. Course in my case I have a couple of other things that contribute to that too which I doubt apply to you.

My 2c

Mark

 

[npd4432]

Re: Ventalation problem

Are you trimming in for turns?Is the prop in good shape?Are there any devices on the boat that may be disturbing the water?There was some discusion the other day on the
smart tabs placed closer to center than recommended helping venting problems.Allowed the motor to operate at extreme height and trim.You might try different settings on your tabs..

The motor does have power tnt, when turning I do have the motor in the lowest position, then trim up on the straightaway. The prop is 2 years old with no defects, dings or chips. There are no devices under the hull to disturb the water. The boat is a v hull, I will post a picture. The pics were taken before I installed the smart tabs.

 

[npd4432]

Re: Ventalation problem

I can think of 4 things:

1. Angle of engine during turn.....trim position if you have it; but I assume when you say "the lowest setting on the transom" you are referring to the tilt hence no trim. Solves that problem.

2. Location of antivent plate vs bottom of boat. Even with or above can aggravate ventilation problems but gives better WOT performance. Probably you don't care too much about that as you are towing skis.

3. Cupping and rake on a prop can help to eliminate it....hence SS prop.





Mark

When I say lowest setting I mean that the motor itself is on the transom at its lowest position with the three sets of adjustment holes on the bracket itself. Nothing to do with TNT.

The antivent plate you refer to is the plate above the prop where you would put a "whale tale" or other type of plastic or metal fin? If so it is about an inch below the the hull and cannot go any lower as I explained above.

I dont have a problem spending some money on a new prop to solve this, I just dont want to go and buy another one and spend alot of $$$ if it will not solve the problem. There are a few decent choices here on Iboats for props, I just need to know which one if any will solve my problem.

 

[Texasmark]

Re: Ventalation problem

I can't see a thing that would cause you a problem except one thing you mentioned. Having your engine as low on the transom as you do certainly helps eliminate the problem....one would think. Bad for speed at WOT but you aren't concerned about that.

You said that you tuck it in all the way on a turn. Does this put a lot of the boat in the water and put the bow down significantly? If so, you may be plowing and this could upset the water flow to the engine.

Have you tried turns with the engine vertical (90 degrees to the surface of the water)...which would mean that the trim would be about half way from prop out of the water and ventilating (on a straightaway at WOT), and tucked all the way in.

Other question is, do you have a skiier pulling on you when you execute the turn when ventilation is present?

How much boat movement (side to side) do you get when a slalom is "performing" behind you?

When you execute the turn, does the boat tilt into the turn and "hold", or does it stay pretty flat and skid sideways?

How is your ski rope attached to the boat?

Mark

 

[Silvertip]

Re: Ventalation problem

I really can't tell by the picture but it appears the transom bracket is not sitting down on the transom. Yes the motor is mounted as low as it can be on the mounting bolts. That said, if the engine isn't actually sitting on the transom the transom would need to be re-drilled to lower the engine some more.

 

[npd4432]

Re: Ventalation problem

I can't see a thing that would cause you a problem except one thing you mentioned. Having your engine as low on the transom as you do certainly helps eliminate the problem....one would think. Bad for speed at WOT but you aren't concerned about that.

You said that you tuck it in all the way on a turn. Does this put a lot of the boat in the water and put the bow down significantly? If so, you may be plowing and this could upset the water flow to the engine.

Have you tried turns with the engine vertical (90 degrees to the surface of the water)...which would mean that the trim would be about half way from prop out of the water and ventilating (on a straightaway at WOT), and tucked all the way in.

Other question is, do you have a skiier pulling on you when you execute the turn when ventilation is present?

How much boat movement (side to side) do you get when a slalom is "performing" behind you?

When you execute the turn, does the boat tilt into the turn and "hold", or does it stay pretty flat and skid sideways?

How is your ski rope attached to the boat?

Mark

The motor is sitting on the transom, as low as it will possibly go. I have tried turns at all trim positions, it seems to be least when trimmed down, but not all the way to prevent "plowing". The problem occurs when skiing and when not skiing at about the same rate. The boat side to side movement is not too bad when one is skiing behind and cutting hard,(I am 235 lbs), when turning the boat turns and holds does not skip. The ski rope is attached by a fly high ski pylon.

I have had this problem since I have owned the boat. I just want to try to eliminate or minimize it by trying a new prop if that is the problem. I just hate hearing the motor scream when it happens, I am afraid of blowing it up. A few people drive the boat and are not as careful of the problem as I am and can possibly destroy something. I will try to get some better pics, the boat is in storage for the winter, but will try to get to the lake to snap some pics.

 

[MikDee]

Re: Ventalation problem

IMO, Thinking logically about this, when you make your turns, you're scrubbing some speed, and the smart tabs come down abit by hydraulic action,,, this, as well as your bow settling a bit, causing the transom to lift just enough to ventilate your prop some (because even without the tabs, your prop is prone to do this anyway in a turn,) it's just making the conditions worse. You might try unbolting the tabs, and tying them up out of the way tempoarily, and see how it reacts. If thgis is the case, I would opt for exchanging the hydraulic cylinders for lighter pressure ones.

 

[walleyehed]

Re: Ventalation problem

The Amita does not have enough cup to hold for what you are using the boat for....it has cup, just not enough and not in the right location.

 

[guy74]

Re: Ventalation problem

I had some ventilation problems on my boat when I had my 60hp Johnson on it. I assume that your motor is very simular to my 1988 60hp, it seems that the shaft length on the 3 cylinder motors is a little shorter (about an inch). Once I installed the V4 motor the ventilation problems when away. With the 60hp, I had the best luck with my Hustler prop, it seemed to hold in turns better than that factory aluminum or SS props.

 

[npd4432]

Re: Ventalation problem

IMO, Thinking logically about this, when you make your turns, you're scrubbing some speed, and the smart tabs come down abit by hydraulic action,,, this, as well as your bow settling a bit, causing the transom to lift just enough to ventilate your prop some (because even without the tabs, your prop is prone to do this anyway in a turn,) it's just making the conditions worse. You might try unbolting the tabs, and tying them up out of the way tempoarily, and see how it reacts. If thgis is the case, I would opt for exchanging the hydraulic cylinders for lighter pressure ones.

The ventalation problem is why I put the tabs on. It helped alittle bit.

 

[npd4432]

Re: Ventalation problem

The Amita does not have enough cup to hold for what you are using the boat for....it has cup, just not enough and not in the right location.

Any suggestions for a replacement that will suit my needs?

 

[MikDee]

Re: Ventalation problem

The ventalation problem is why I put the tabs on. It helped alittle bit.

Well so much for that idea, if the ventilation was there to begin with,,, But it appears to me, that prop seems to be to be a bigger diameter, then average for that pitch.

 

[Texasmark]

Re: Ventalation problem

I really can't tell by the picture but it appears the transom bracket is not sitting down on the transom. Yes the motor is mounted as low as it can be on the mounting bolts. That said, if the engine isn't actually sitting on the transom the transom would need to be re-drilled to lower the engine some more.

He said his AV plate is already 1" below the bottom.

 

[Texasmark]

Re: Ventalation problem

Take the boat out of the water and put it on the trailer with the engine in the tilt/trim position you are in during your turn where you have vent problems.

Get directly behind the boat, down at hull level and take a pic. Go to the bow and get down under the hull just barely so that you can see to the transom and see what the water sees of the engine's prop and lower unit and shoot a pic of that. Post em.

You didn't answer my questions.

Mark

 

[npd4432]

Re: Ventalation problem

I will take some pics of the boat with the motor down. It is currently in winter storage but I will get out to the lake and do so for some pics. Give me a few days.

 

[walleyehed]

Re: Ventalation problem

First, let me add that at 13.75" diameter, this is slightly SMALLER than std diameter for the V4 JohnRude. Most of the OMC/BRP's in 13P for the 13 spline will be 14" diameter for 13 pitch.
One "slightly" dinged blade can cause the problems you have....so can a missing piece (even small) of gel-coat on the hull.
If I were to look into a prop, I'd say you'd have as good of luck with an aluminum OMC/BRP 14X13, as you would any aluminum.
Now, if you think you might want to go SS, I'd shoot for a Stiletto 13.25X13P, but we need to make sure there isn't something from the hull causing a vent pocket first.

 

[walleyehed]

Re: Ventalation problem

The ventalation problem is why I put the tabs on. It helped alittle bit.

The tabs helped because they keep the boat from leaning as much in the turns...

 

[Dolphin59]

Re: Ventalation problem

You may want to lower the pitch say around an 11, this I believe would also help on the top end. Anyone agree with this???