Prop selection question

[vr4rhoads]

I have a 2000 70 hp 2 stroke, currently the prob is a 13.5 by 17 pitch, thats what it came with when I bought it, anyways my boat is a 16 foot predator with a pretty big tunnel hull. question I have is when im in a turn it tends to cavitate, a sharp turn, I had heard a 4 bladed prop works really well for this. Just wondering if anyone can help me pick one out. That being said i run at about 6200 rpm right now, what is the ideal rpm for my 70? I not looking for speed, this is a bay boat and will be in many shallow area's so getting up fast is the main priority.

[DennisG01]

I'm not sure on the RPM's, but it sounds a bit high. You can download your manual from Yamaha's website. It might be printed on a sticker somewhere on the engine, but I'm not positive on that.

Yes, a 4-blade can help... a little. But, it could also be that the motor is mounted too high. Is there a "wing" mounted to your anti-ventilation plate? Sometimes those can actually cause ventilation. If so, try it without it on.

[throrope]

That motor doesn't want to spin faster than 5500. I suggest you check Yamaha's performance bullitins before you blow it up. An easier way - call PowerTech Props directly and ask for a selection. Their gurus use proprietary software and their wheels are about the best you can get.

A prop may not help cavitation during a hard turn. Crazy physics happening.

[vr4rhoads]

ill give them a call, thought that was a little too high which is why I asked, so I will have to do some research on proper engine rpm and prop selection, the way it was explained to me about the cavitation during turning for my particular boat is. its a 16 predator and it has a pretty big tunnel hull, a friend of mine has a 19 ft predator hull with the exact same thing, the way he explained it is when your moving forward or in a not so sharp turn all the nice and clean funnel water from the tunnel hull is hitting the lower unit perfectly, when you go into a sharp turn it can come out of that clean funnel water from the tunnel and that can cause it to cavitate a little. its nothing big just something that ive heard can be corrected from a four bladed prop. I plan on called a bluewave dealership because they have similar designs in terms of the hull and tunnel.

[vr4rhoads]

I'm not sure on the RPM's, but it sounds a bit high. You can download your manual from Yamaha's website. It might be printed on a sticker somewhere on the engine, but I'm not positive on that.

Yes, a 4-blade can help... a little. But, it could also be that the motor is mounted too high. Is there a "wing" mounted to your anti-ventilation plate? Sometimes those can actually cause ventilation. If so, try it without it on.

Also forgot to say I have no wing on my anti vent plate

[boscoe99]

The red line RPM for your motor is 6000. So, you are not far off.

It is not abnormal for a propeller to ventilate during a sharp turn, particularly if the motor is mounted high for optimum performance and particularly if the motor is trimmed well up during a turn.

After getting my boat set up the way that I wanted it, I knew that if and when I needed to make a sharp turn it was necessary to trim the motor down. Coming out of the turn I trimmed the motor back to its optimum trim angle.

Now keep in mind one mans sharp turn is another mans gentle turn and still yet another mans crazy turn. Sharp will mean different things to different people.

Ideal RPM depends upon what your objective is. Maximum speed or most efficient cruise. You can run at WOT all day long if you want, recognizing that you will be paying for speed in the form of increased fuel consumption and reduced engine life.

Optimum propeller selection is trial and error. It will take several different brands and styles of four blade propellers being tested to find out which, if any, will work for you.

[vr4rhoads]

The red line RPM for your motor is 6000. So, you are not far off.

It is not abnormal for a propeller to ventilate during a sharp turn, particularly if the motor is mounted high for optimum performance and particularly if the motor is trimmed well up during a turn.

After getting my boat set up the way that I wanted it, I knew that if and when I needed to make a sharp turn it was necessary to trim the motor down. Coming out of the turn I trimmed the motor back to its optimum trim angle.

Now keep in mind one mans sharp turn is another mans gentle turn and still yet another mans crazy turn. Sharp will mean different things to different people.

Ideal RPM depends upon what your objective is. Maximum speed or most efficient cruise. You can run at WOT all day long if you want, recognizing that you will be paying for speed in the form of increased fuel consumption and reduced engine life.

Optimum propeller selection is trial and error. It will take several different brands and styles of four blade propellers being tested to find out which, if any, will work for you.

Hm so another thing I heard is when you go to a four blade from a three blade your suppose to drop down 2 pitch, sound about right? So being that im sitting around 6200 at a 17 pitch i should prob drop down to a 14 to start with?

I have heard that props are one of those things you have to keep trying till you find one you like. I am not looking for top speed, wanting something good out of the hole and for cruising. thanks for the response, wish there was a way you could try props out before buying them

[boscoe99]

Hm so another thing I heard is when you go to a four blade from a three blade your suppose to drop down 2 pitch, sound about right? So being that im sitting around 6200 at a 17 pitch i should prob drop down to a 14 to start with?

I have heard that props are one of those things you have to keep trying till you find one you like. I am not looking for top speed, wanting something good out of the hole and for cruising. thanks for the response, wish there was a way you could try props out before buying them

Ruff rule of thumb when changing from a three blade to a four blade is to drop one inch of pitch. But, there are so many different types of blade geometry that this is a ruff ruff rule at best.

Where are you located? There is a propeller dealer in Florida that will let you test as many as you like.

Ken at propgods.com

[vr4rhoads]

Im in corpus christi texas, pretty far away haha ill have to remember that though thanks! ill call around locally and see if anyone here will let me do that

[DennisG01]

You should be going UP to an 18". If you go down, you'll still be spinning to high.

[boscoe99]

If he can turn 6200 now he won't be able to come the high temperature and humidity of summer time on the Texas coast. He is going to be losing considerable horse power and WOT RPM.

Switching to a four blade is going to further reduce the WOT RPM.

Worse thing one can do to an outboard motor IMO is to over prop it. I prefer to see a motor propped so that the engine can run at or near its red line, with the boat loaded. If the motor can go over the red line at any time then it is the operators responsibility to not let that happen.

But just because one can exceed the engine red line does not mean that it should be done. That is why there is a throttle in ones hand and a tachometer in front of ones eyes.

[99yam40]

being as old as that rig is there is no way of telling if the prop has been reworked before or wore down from running through the sand and mud in shallow water.
You loose cup and you drop speed and/or gain RPM.
Only way to tell is running a new prop to make sure what it does.

Not sure anyone would let you run a prop in shallow water and then return it for a different one.
But in deep water you might

I did not realize a 70 2 stroke that old was red lined a 6K

[DennisG01]

Good points above -- high temps/humidty, overpropping, better to be in the upper RPM range, control the RPM with your hand, flattened blades... agree with all of that. I doubt he'll lose 200RPM's due just to temps/humidty, but it's a good call to wait and see. I definitely can't see going all the way down to 14", though, - great hole shot, but when cruising the RPM's will be up pretty high. I know you said your main goal was hole shot, but don't overdo it. There was also good advice about needing to trim down in a tight turn - do you do that? Can you lower the engine?

[99yam40]

being a tunnel it should have a jack plate on it so he can adjust as he runs

[boscoe99]

I did not realize a 70 2 stroke that old was red lined a 6K

It surprised me also.