Boat struggles to get on plane... Time for a new prop? 17ft, 70hp, 13 1/4 x 21

[EriccirE]

Hey guys,

When driving solo with minimal gear my boat hops right up on plane and WOT RPM is just above 5k. Speed is 33mph bouncing to 34.

However, with only 2 other passengers and a cooler (400lbs), my boat struggles to get out of the hole and up on to plane. It can sometimes take over 30sec to get on plane.

If I have more than 2 passengers, my boat will just plow water, no planing.

I have a 1988 Merc with a 13 1/4 x 21 (3 blade, aluminum) prop. The outboard is mounted so that the cavitation plate is about an inch lower than the bottom of the transom on my deep-v hull. My boat is a fiberglass 17 footer with a deep v hull.

My research leads me to believe that a lower pitch prop should help me get on plane faster. My questions are:

What about a 4 blade prop? If so, what size and pitch should I look for?

Would raising my outboard to match the bottom of my transom (a mere inch) give me the results I am looking for?

I appreciate any comments or suggestions regarding my setup. Thanks for your time.

 

[ejthomp]

Re: Boat struggles to get on plane... Time for a new prop? 17ft, 70hp, 13 1/4 x 21

What is max rpms on that motor? I would guess 5500 rpm. So with a light load, you should be all over that at wide open throttle.

I had a 70hp 3 cylinder Yamaha on a small, but heavy 15' boat and I was running a 15 pitch prop. I think 21 is way too much for what you describe. For reference now... I run a 90hp on a deep vee 17' and my prop is a 16 pitch. For me, getting on plane quickly is much more important than top-end speed. If I were you, I'd be looking to going down to at least a 17 pitch.

Also, the cavitation plate should be an inch ABOVE the bottom of the transom so you are probably a couple inches too low.

I'd suggest raising your motor AND changing the prop. You're killing your Mercury now.

 

[EriccirE]

Re: Boat struggles to get on plane... Time for a new prop? 17ft, 70hp, 13 1/4 x 21

Will raise the motor. What size and pitch should I go with if I decide to grab a 4 blade?

 

[oldman570]

Re: Boat struggles to get on plane... Time for a new prop? 17ft, 70hp, 13 1/4 x 21

I would go to a 13 3/4 x17 pitch prop and raise the motor. You will probbly not loose much speed and it will get the boat on plane faster. Just watch the RPM's and a 3 blade prop should do the job. 4 blade prop will get it up on plane faster but you will probly have to go down in the dia of the prop some. Recommed RPM for the motor is 4750 to 5250 @ WOT, and will preform best there or cruse a little lower for best fuel savings.

 

[EriccirE]

Re: Boat struggles to get on plane... Time for a new prop? 17ft, 70hp, 13 1/4 x 21

There is a 13 5/8 x 15 stainless prop local to me on craigslist. Do you guys think lowering the pitch from 21 down to 15 will be too much?

Here is the ad local to me:

Ballistic 13 5/8 x 15 stainless prop

 

[oldman570]

Re: Boat struggles to get on plane... Time for a new prop? 17ft, 70hp, 13 1/4 x 21

Before buying a prop you need to measure the Diameter of the old prop you now have. Some of the older 70 HP motors have a smaller Dia. than the newer motors. Mine measures 4 1/4" dia. accross the hub where it goes in the LU. Also measure from the prop shaft to the cavitation plate to know what Dia. you can use and you will need enought for clearance on the tork tab also. My 1987 motor uses a 13 3/4 X 15 prop on a 17' mod flat boat. Runs out at 33 MPH at 5300 RPM. with 3 people , About 600#weight.

 

[roger3645]

Re: Boat struggles to get on plane... Time for a new prop? 17ft, 70hp, 13 1/4 x 21

If your motor is an inch below, I would raise the motor up at least one hole. I also believe a 21 pitch prop is too much, you need to drop to a 19 or 17 pitch. I don't know about the ballistic prop. I'll let someone else chime in on that. I had a 70hp Johnson and my max WOT rpms was 6000, but I don't know about your merc. That's info you need to post.