Prop slipping or?

[prawln]

Hey all,

So i took my boat out today and noticed when turning sharp and above 3500rpm the RPM would jump up to around 5000 and the boat would start to slow down in speed.

I've done some googleing and searching on the forums and I think I have put it down to a "slipping prop". Does this sound right? What is the best fix for this? Replacing the prop completely or is there a kit to fix it?

Engine is a 1995 Mercury Force 120hp

Many thanks
Hayden

 

[tazrig]

Re: Prop slipping or?

Has it ever done this before? Most likely it's just "cavitating". (air mixing with the water) If the prop has sustained any damage lately or is bent or nicked on its outer edge that would do it. The only other possibility is the splines on the propeller shaft or on the hub of the propeller are worn down. Remove the prop and bring it to a boat yard. They should be able to tell you just by looking at it.

 

[jestor68]

Re: Prop slipping or?

If it's the "stock" aluminum prop, what you are experiencing is "ventilation" in turns. This is quite common with aluminum(and some SS) propellers.

When you turn, the boats lists(leans) in the direction you are turning. This causes the prop to come closer to the surface and suck in air; causing ventilation.

Putting on a prop with cupped blades will usually reduce this tendency.

Also check the motor height on the transom. The AV plate should be about even with the boat bottom. If the motor is mounted too high, this will increase the tendency to ventilate (commonly known as cavitate) in turns.

Trimming down(in) when(before) turning sharply helps too, as that puts the prop a little deeper in the water.

The fact is all boats will ventilate the prop in a sharp turn; some more than others.

What kind of boat do you have?

 

[prawln]

Re: Prop slipping or?

I just did some more reading with trusty google about Cavitating and Ventilation and it sounds like it's ventilation. What is usually the cause of this? I read that it can be the hull or transducer?

The engine is on a 1995 1702LS Bayliner Capri Cuddycabin

 

[jestor68]

Re: Prop slipping or?

There is a 99.9% probability that there is nothing wrong with your boat/motor.

If nobody has messed with the motor height, it should be OK(mounted with the AV plate even with the boat bottom).

When using the power trim, don't leave it trimmed too high when trying to turn "sharply".

If it has the "standard" mercury aluminum prop on it, that may be part of the problem. You need a prop with "cup" on the trailing edges of the blades. Solas makes outstanding aluminum (cupped) props that will perform much better.

Remember; ALL boats will ventilate the prop at some point in a turn. When the hull leans into the turn, air is drawn under the hull and into the prop. The cupping of the prop blades helps it to "hold" on in those situations.

Putting a Doel Fin on the motor "covers" the prop and aids in reducing or eliminating ventilation issues, as it prevents air from being drawn into the prop.

I used to have the exact same boat. Two things will make your issues go away; a better (cupped)prop and a Doel Fin.

 

[prawln]

Re: Prop slipping or?

Just went and had a look, the engine is about even with the bottom of the boat. But it definatly seems like ventilation. If i take a sharp corner slow its fine but if i take it fast/higher rpm it does it and I have to pull back on the throttle then push it back forward to stop it. Saw that the engine isnt all the way down also, it has 2 bolt holes to be right down on the transom.

Also was wondering what sort of RPM at WOT should the engine be doing. Mines at arond 6500 maybe 6600 at 47MPH.

 

[jestor68]

Re: Prop slipping or?

Just went and had a look, the engine is about even with the bottom of the boat. But it definatly seems like ventilation. If i take a sharp corner slow its fine but if i take it fast/higher rpm it does it and I have to pull back on the throttle then push it back forward to stop it.

Also was wondering what sort of RPM at WOT should the engine be doing. Mines at arond 6500 maybe 6600 at 47MPH.

Verify the prop size; diameter and pitch. While the speed sounds about right, I suspect the tach is off, depending on what size prop is on there.

You don't want to run it over 5300-5400 rpm. 6000 rpm will quickly destroy the motor.

The recommended WOT rpm range for the 1995 Force 120 is 4750-5250 rpm.

 

[prawln]

Re: Prop slipping or?

I'm unsure of the prop size (new to this boating stuff) but I found some numbers on the prop. 48 77346 a40 19p

Edit: Just found someone selling these and they have it has a 13" x 19" pitch

 

[jestor68]

Re: Prop slipping or?

That is a Mercury/Quicksilver 19" pitch prop. That boat would have come equipped with a 21" pitch aluminum prop. It sounds like a previous owner reduced the pitch for better acceleration for pulling water toys, skiers, etc.

But that would not cause that high tachometer reading. With a 19", it should run about 5500-5600 rpm. With a 21", expect 5000-5300 rpm at WOT.

I would consider getting a better prop to help the ventilation issue; the size depends upon what sort of activities you plan on doing. You might consider a 4 blade design for better holding in all conditions. A Solas Rubex 4 blade aluminum works very well.

Check the connector on the back of the tachometer. If the pins get gummed up, it will cause strange readings. There should be a black plastic connector you can pull off to clean the three pins. I have had to do this a couple of times over the years with my Bayliner's Tachometer. In each case, it was reading too high on rpm. After cleaning the pins, readings returned to normal.

 

[prawln]

Re: Prop slipping or?

ahh! well it did come with ski pole/water toys which is what I'm looking at using it for mainly.
Hmm I did replace the old gauges with new ones as the old ones were looking tired so perhaps I did something wrong or the gauges are just out. Might look at running a new wire for it perhaps. All the wires are quite tacky and yuck.

Next on the list of purchases is a cupped 13x19 pitch prop and a doel fine. Will have to find somewhere here in New Zealand that sell them! Thanks for all your help jestor! Have helped a huge amount!

 

[jestor68]

Re: Prop slipping or?

There is/are settings on the back of the tachometer for 2 stroke/4 stroke. You might verify that it is set properly for 2 stroke; as most come set for 4 stroke(?).

Since Bayliner uses the same tachometer in their outboard and I/O boats, it can be set for either application.

 

[coolbri70]

Re: Prop slipping or?

you have to throttle back for a sharp turn, when the boat leans into the turn it is pulling your prop out of the water, it gets a pocket of bubbles around it and loses thrust. anything over 3500 rpm in my boat and it takes the width of the river 100-150 yards to make a u turn without ventilating. back off, turn sharp, throttle up, is the quickest way i turn around

 

[steelespike]

Re: Prop slipping or?

It's normal to have to trim down and reduce speed for sharp corners.
Before doing any props we need to confirm your rpm.At 6600 I think your motor would be in pieces.
Also 6600 rpm would produce about 51 mph I think unlikely with a Force 120 on a 17 ft cuddy.
Also I think its unlikely that a 19" prop would produce 6600 rpm when a 21 is supposed to be within normal range.
I think your speed probably is closer to 35-40 mph.
The best way to check speed is with a gps.We need accurate speed to help tell if the tach is accurate.
I do have a feeling that your tach is set wrong producing the greatly inflated rpm.
I would also suggest you avoid the foil until your sure your setup is working.