best prop for 16.5' 70 hp

[mopar6791]

i have a 16.5' sport craft with a 1986 70 hp evinrude. i ususally have at least 4 people on my boat and some fising gear. i would also like to be able to tow a raft also. and would like to know what the best prop size for my application would be? i had a 13.75x15 on it. i just cracked this prop and need a new one should i get the same size? i can only seem to find 13.25" diameter so should i stay with the 15 pitch if i get 13.25 or should i move to a 13 pitch or 17? im new to this so i dont know where to begin at all. by the way i dont care too much about top speed too much but would like for it to do at least 30 mph. you tell me what you think i need. thanks.

 

[BonairII]

Re: best prop for 16.5' 70 hp

Get yourself a tachometer. You need to know what rpm your motor is running to determine correct prop. If the pitch is too large, you'll lug the motor. If pitch is too small, you'll over rev the motor and end up blowing it.

You can't just guess prop sizes.

 

[mopar6791]

Re: best prop for 16.5' 70 hp

Well that's the problem now. Can't really take it out since the prop is cracked. Also should the entire diameter of the prop be completely under the bottom of the hull? My motor is all the way down and only half the prop is actually under the bottom of the boat.

 

[Maclin]

Re: best prop for 16.5' 70 hp

The plate just above the prop should be right at the bottom of the hull at the lowest point, so yes the prop should be all the way under the boat. Can you post a pic of your rig from the side?

 

[SparkieBoat]

Re: best prop for 16.5' 70 hp

get a 13 or 15 pitch prop..see if you can borrow one for a test run. some prop shops will let you return a prop if it is still in new condition. You need to ask them if they will let you test a few different props till you find the right one..but you will need a tach.

 

[steelespike]

Re: best prop for 16.5' 70 hp

If the crack isn't too bad you may be able to get it repaired.You can do some test runs with a tach and gps to evaluate your setup.
If a prop change is in order you will have spare to carry with you.
From your description you may have a short shaft motor on a long shaft boat.
A tinytrach Iis reasonable and easy to install and will work on almost any engine.

 

[mopar6791]

Re: best prop for 16.5' 70 hp

Thanks for all the help guys I will get a pic of the side of the motor and the prop so you can see the crack. Ill also get a tach for a test run next time

 

[sschefer]

Re: best prop for 16.5' 70 hp

I run a 12 on my 16' heavy gauge aluminum boat with a 60hp 4 stroke. Lightly loaded it will do almost 35mph and with the 12 I can go from low altitude to high and still get decent performance. At low altitude I hit the max RPM range and at high I'm about 400 RPM shy. With a good running 70hp, 2 stroke and power on every stroke I think you could easily run a 13-15 pitch prop but keep the diameter the same as it is now. If you aren't experiencing any prop blow out or cavitation indicated by a sudden increase in RPM but no increase in speed or loss of speed then you're engine height is probably fine for your boat. Otherwise get the cavitation plate on the motor to even or within 1" of the bottom and it should clear up that problem.