3 Blade to 4 Blade?

[gholmesjr]

So I'm having a heck of a time with getting a decent holeshoot and up on plane. If I have more than two people in the boat I have to move one to the front to get it going. If I am trying to pull a skier it drags them for a bit before they can pop up. Once we get going we are fine. I currently have an aluminum 3 blade 13.25" x 17p and am thinking of going to a aluminum 4 blade 13.25" x 15p. Do you think this might help us? My boat is a 17' EBKO tri-hull running a 140hp Johnson V-4 2 stroke so it's a heavish boat. Thoughts?

Thanks,

J.T.

 

[alldodge]

It could be your motor is tired. But so as not to get into this issue but I'm back to your question.

IMO going from 17 3-blade to 15 4-blade will get you about the same hole shot. I would suggest trying a 15 3-blade and watch so you don't over rev when going WOT. If the 15 3-blade is to high and rpm then add some cup to it and it will make it about a 16 pitch. Only issue with cup props is they go great going forward, but don't back up so well

 

[Sea Rider]

When boating you have to distribute deck weight evenly, if that's moving boaters up front towards bow is the way to go, not good to have too much weight aft or towards bow. Before investing on new props, need to tach current prop on a wot run on flat calm no wind water cond as usually loaded with just boaters. Check max wot revs achieved and dial a 3 blade prop pitch that brings wot rpm near full max wot rpm factory stated for that OB. If without a tach will be wild guessing rpm...

Happy Boating

 

[gholmesjr]

Thanks for the suggestions. Wot with 4 adults tonight would get up to 4700 rpm. On chop would hit 5k on occasion. The prop does have some wear on it from previous owner. So I am going to replace it.

 

[jimmbo]

What rpm do you get with just yourself in the boat? Is your 140 a crossflow or a looper?

 

[gholmesjr]

Never been by myself. With just my wife I sit right around 5k which is the rpm range for my motor. Motor is a 1977 140 v4. Not sure on looper or crossflow as its model 140ml77s, how can I know?

 

[gholmesjr]

So I'm thinking about going to this prop that I have linked. My previous post said that I hit about 5k with just my wife and I in the boat, rethinking and talking to her the highest we got to was 4700 rpm which is 300 under recommended at WOT. Going to a 15 pitch from a 17p should increase by 200-300 rpms correct and create a better hole shot I would think...

 

[Sea Rider]

If with less 300 rpm to be at ideal 5K with a 3 blade prop, a less prop pitch will bump max wot revs, the only issue is that is not known if one less pitch will make the trick or with 2 less pitch size would over rev. If over revving with 2 as usually loaded just throttle less and watch tach, the good issue if that's going to happen is that will be able to carrry more load to compensate over revving if like rumming full hammer dowm.

Or can send that 3 blade 15 prop to a prop shop and have it modified to a 16 if too much over revving is achieved with a 15 under current 2 passenger load at wot.

Happy Boating

 

[dingbat]

I'm guessing your motor is getting tired.

A 140 HP outboard should rocket a #1700 boat up on plane in no-time. What's your compression?

 

[jimmbo]

A 1977 140 is rated @ 5000 rpm. The recommended WOT rpm range is 4500 - 5500, preferably near 5500. If you are only getting 4700 you really should be dropping about 4 inches in pitch but you are already at 17, which is what is usually works on a 17ft boat. I would be checking a few things. Is the throttle advancing the ignition fully and opening the carbs fully? I too recommend doing a compression test. Running a few cans of Combustion Chamber cleaner through the engine wouldn't do any harm, if any rings were sticky, it would free them up

 

[gholmesjr]

I'm guessing your motor is getting tired.

A 140 HP outboard should rocket a #1700 boat up on plane in no-time. What's your compression?

Compression is 125 all the way around. Had a shop confirm my findings as well.

 

[gholmesjr]

A 1977 140 is rated @ 5000 rpm. The recommended WOT rpm range is 4500 - 5500, preferably near 5500. If you are only getting 4700 you really should be dropping about 4 inches in pitch but you are already at 17, which is what is usually works on a 17ft boat. I would be checking a few things. Is the throttle advancing the ignition fully and opening the carbs fully? I too recommend doing a compression test. Running a few cans of Combustion Chamber cleaner through the engine wouldn't do any harm, if any rings were sticky, it would free them up

The boat has been modified as well. It was gutted, new floor put in and setup like a bass boat with a platform and pedalstal on the front. I'm sure that with the wood floor being marine grade it added weight. When we are trying to get going without anyone in the water and with four adults the front of the boat will come up quickly upon launch, I trim down to get it on the water until the RPMs come up and we plane out then trim up to get speed. Upon launch to it trying to settle we will sit between 2500-3200 for awhile then it basically kicks in and will fly.

 

[jimmbo]

It would 'fly' a lot better if it could get into the upper end of the rpm range

 

[gholmesjr]

I'm starting to wonder if the timing still isn't set right. I had the shop put a new timer base on it as the old one was fried. They didn't time it after they put it on. I timed it and am wondering if I am still off on that.

 

[gholmesjr]

So I messed with the timing a bit but so can't get to above 5000rpm. Mostly sat around 4800rpms. I've linked the video we shot tonight.

 

[jimmbo]

You've got the engine mounted very high, and it's not ventilating. It also looks like you are accelerating with the engine trimmed quite far out. By chance you are trying to use an engine that is 5 inches longer than the transom? A 25 inch motor on a 20 inch transom? I think you tach is wonky and you should get a different one

 

[Sea Rider]

Jimbo,

Is it an assumption or a fact that OB is sitting high, untill somebody pulls head out of transom checking lower leg, have someone ride parallel checking lower leg or maybe flying a drone at back of OB, will never know its transom/height cond. With combo well weight balanced need to trim OB to ride parallel to water level once on plane. Could start with + trim and adjust on its way to achieve that cond.

Happy Boating

 

[gholmesjr]

I usually start with the motor down then tilt/trim it out to parallel as we get going. The motor is on the last hole on the mounting plate raised up. I am planning on taking a yard stick and seeing where on the transom the motor's cavitation plate is located and decide if the motor needs moved down a bit.

 

[jimmbo]

Jimbo,

Is it an assumption or a fact that OB is sitting high, untill somebody pulls head out of transom checking lower leg, have someone ride parallel checking lower leg or maybe flying a drone at back of OB, will never know its transom/height cond. With combo well weight balanced need to trim OB to ride parallel to water level once on plane. Could start with + trim and adjust on its way to achieve that cond.

Happy Boating

Look at the video, at 37 seconds it shows the engine. Looking at the transom bracket, 2 of the mounting holes are visible. The angle of the shot prevents seeing if 3 are visible

 

[gholmesjr]

So the motor's cavitation plate is even with the keel of the boat. The motor is on the CMC tilt/trim on the lowest hole of the motor mount.