Choosing the right prop, trailering boat engine up or down

[td95ma]

So I after 2 years of cleaning, rebuilding, searching for used parts, I finally got everything in working order on my 1970 16 foot Corson Boat...now I can try to learn a few things about boating...first off....choosing the right prop I have a 1971 Evinrude 60 hp motor and I can top out at just below 30 mph and 4400 RPM with a 3 blade 13 1/4 by 17. Should I look at buying a different prop, I Don't know if I will get any more out of this motor...also is a stainless prop worth the money? I mostly just fly around the lake and try to pull skiers now and then. <br /><br />Next Question: when trailering the boat, motor up or down? I just assumed it was up but my father in law who actually owns the boat keeps telling me to keep it down on the highway better balance with such a heavy motor and boat.<br /><br />Finally: Someone told me to add Octane booster to my mix tank of oil and gas, I used premium gas and evinrude oil to get my 50:1 any opinions on octane booster for older engines?

 

[ZmOz]

Re: Choosing the right prop, trailering boat engine up or down

You need to find out what RPM range your motor is designed for in WOT, and get a prop that works for that. For every inch in pitch you go down your RPMs will increase about 200. 4400 RPMs sounds too low to me, although I don't really know for sure. If it is too low don't use it in WOT until you get a different prop. A stainless prop would be alot better...especially trying to pull up skiers with that small of an engine. When the boat is on the trailer, the engine should be up. Again, I don't know specifically about your motor, but my 150hp Mercury has power trim, and when fully upright it has a clip to keep the motor from coming back down while it is on the trailer. Also the majority of the boats I see out of the water have their motor up. As for octane booster, I don't think it's necessary, but you definately should be using premium gas in an old motor.

 

[JasonJ]

Re: Choosing the right prop, trailering boat engine up or down

Make sure the motor is in good running condition before you reprop. A good tune up and everything set right, if it already hasn't been done, makes all the difference. If the motor is running as good as it can, you will want to look at you setup. How the motor is trimmed, how much weight you have in the boat, where the weight is, it all affects speed and performance. If all that is in order, then you can mess with the prop. You will need to know what RPM range the motor should run in at wide open throttle, and set everything up to achieve that goal. <br /><br />With the boat on the trailer, if you do not have at least a foot from the bottom of the skeg to the ground, then trailer it in the up position. I personally feel it should always be up, you never know if you might drive over a dip or something than could cause the skeg to drag. The forces the motor puts on the transom is nothing while trailered compared to when it is in the water and under way, so don't worry too much about that. The balance is similar either up or down.

 

[td95ma]

Re: Choosing the right prop, trailering boat engine up or down

So what do you mean when you say drop pitch by 2 inches? what is the proper size i should order tommy isnt feeking very smart today

 

[JasonJ]

Re: Choosing the right prop, trailering boat engine up or down

If you are dropping 2 inches, then you are going from a 17 inch pitch to a 15 inch pitch. All the number means is that the propeller pushes forward 17 inches in one revolution, although slippage reduces the true amount a bit. Going to a lower pitch is just like gearing down.

 

[Solittle]

Re: Choosing the right prop, trailering boat engine up or down

Forget the premium gas & octane booster and run with 87 octane. Lots of info on this if you go to the search function at the top of the page.

 

[td95ma]

Re: Choosing the right prop, trailering boat engine up or down

ok so dummy tommy has one more question, what does the 13 1/4 mean, I understand the pitch 17 vs 19....so should I be looking for a prop thats 13 1/4 by 15??? <br />Also Really only run regualr octane in such an old engine? I was told the engine was built for high octane

 

[ob]

Re: Choosing the right prop, trailering boat engine up or down

13 1/4" is the overall circular diameter of the prop.Your rpm at WOT should be 5500.First off insure that the motor is at its optimal mounting height on transom.Anti ventilation plate of lower unit should be roughly even with bottom of boat when trimmed level.87 octane fuel is fine.You will notice no performance improvement using higher octane.<br /><br />I'm guessing that getting the motor at its optimal mounting height and dropping back to a 13 1/2 X 15 prop will put you where you need to be.

 

[ISkiH2O]

Re: Choosing the right prop, trailering boat engine up or down

OB's advise sounds right. Ask if the shop will let you try out a few props. Try to locate someone in your are that works on props, they can be very helpful. Many new boaters ding a prop the first trip or two and need their services anyway.<br /> A hydrofoil may also help. Probably more than a stainless prop and cheaper too.<br />Most motors have a trailering latch which will hold the motor in a position about half up (or half down). You might also want to invest in a "transom saver" to support the motor while trailering. It is a bar that attaches to the trailer and supports the back of the motor.

 

[rodbolt]

Re: Choosing the right prop, trailering boat engine up or down

Hello<br /> you have good advice from all the above. but If you were my customer and I had no clue about your boats history it would take a dyno run or at least a test wheel to let me know if your motor was capable of making the required power at WOT. if it will turn up to spec on a test wheel or dyno then start playing with props.<br /> good luck and keep us posred