4 Blade prop or 3 blade prop??

[tmitchell5188]

I have a 1996 Bayliner Tropy 2502 with twin 175 Merc carb's 2 strokes. I have a 3 blade SST prop (don't remember the diameter or pitch right now). I could use more out of the hole power, better fuel economy, but don't need the top end of 45-48 MPH that I get now.

Would going to a 4 blade help me? If I go 4 blade SST, how should I size it? Same diameter and pitch? Drop the pitch by 1"? Anyone do this already?

Anyone know of a good source for these?

Thanks!

Tom

 

[rndn]

Re: 4 Blade prop or 3 blade prop??

Welcome to the forum. Have you considered smart tabs to help with exactly what you're looking to improve?

 

[tmitchell5188]

Re: 4 Blade prop or 3 blade prop??

I have trim tabs. I setg them so the bow is down, tabs down, and trim the engine down. I can hold 190 gallons of fuel, and when the tank is full, it's a dog. At 1/2 tank, it's a lot better. Since I burn most of my fuel getting up on a plane, a 4 or even 5 blade prop might help me.

 

[Silvertip]

Re: 4 Blade prop or 3 blade prop??

Not knowing what prop you have now, and what your wide open throttle RPM is we can't be of much help. Whatever prop you end up with should be one that puts the engines at the very top of their recommended wide open throttle rpm with an average load. Whatever prop you choose, you will not see a significant improvement in fuel economy unless your boat is so over propped now that you lugging the engines to death. Finally, if the boat is propped incorrectly at the top end it is propped incorrectly across the board so your logic of looking for cruise economy is not valid. You prop for wide open throttle. Everything else takes care of itself and what you get is what you get. Get us something to work with (WOT rpm, prop diameter, and pitch) and we can make some suggestions. Get your props right here on iBoats. They even have an exchange program if you miss the mark the first time around.

 

[tmitchell5188]

Re: 4 Blade prop or 3 blade prop??

Thanks. The WOT was at max engine RPM's. I think right around 5800-6000 RPM. Since I burn so much fuel getting up on plane, I thought I could save some by improving the hole shot.

The boats in storage now, abd it comes out in a few weeks, so I'll be able to get the prop numbers to see what I have.

With a full tank of gas (190 gallons), it might take 20 seconds (going from memory) to get on plane. If I can cut that down by 20-40%, I'll save gas. When I try and get up on plane, my fuel flow meters jump way up for a while, then come down to normal.

I also have a 18' sea ray bowrider, with a 3.0L I4. I had a 3 blade aluminum prop, and I could not pull my daughter out of the water on 1 ski. I picked up a 5 blade SST, and I can get her up without too much problem. With only 1 person in the boat, and not pulling anyone, the boat jumps out of the hole a lot faster. A lot better low end torque.

A 4 or 5 blade prop might help me there.

 

[LeftFeild]

Re: 4 Blade prop or 3 blade prop??

1.Nauticus, Inc
8080 Snowville Rd.
Brecksville, OH 44141
Toll Free: 800-233-0194
Fax: 440-746-1291

Call them and see if they have a good solution for a boat that is that stern heavy

2. post your prop's pitch and yes a 4 blade will give you better low plaining speed and gph.Most probably a Michigan either a SS or alum

 

[tmitchell5188]

Re: 4 Blade prop or 3 blade prop??

Thanks! I'll give them a call. I guess my big question is, if I have a 3 blade prop, and do go to a 4 blade, should I get the same diameter and pitch? Up/downin pitch/diameter? I'll call them and ask them this question.

It would be nice if they could tell me how much of an approvement I could expect. Or at least a range.

I'm also thining that this might lower my cruising spped. Right now, my RPM's we a little above 4000 to keep on a plane. If I dropped down into the mid 3's, I would start to come off of plane.

 

[tmitchell5188]

Re: 4 Blade prop or 3 blade prop??

I checked their web site. Will these smart tabs work better than my manual tabs?

 

[LeftFeild]

Re: 4 Blade prop or 3 blade prop??

I checked their web site. Will these smart tabs work better than my manual tabs?

I was under the impression that you did not have tab's,.., id still give John a call and explain your boat's sluggish hole shot, if indeed it really is sluggish for a boat like that. His tab's are reactive which require no input.

I also assume you have hydraulic trim (Motor) on the boat and trim in or out as needed correct?

And lastly since you might get into some expense here id take a moment to verify your tach is good within a 100 rpm or so..

 

[tmitchell5188]

Re: 4 Blade prop or 3 blade prop??

I was at the local West marine and the guy there said that I should go up one pitch in a 3 blade, and not do a 4 blade. I think the Iboats prop swap is the way to go.

 

[tmitchell5188]

Re: 4 Blade prop or 3 blade prop??

Well,. I got the boat in the water and had mixed results. I had also added larger trim tabs, but that seemed to cause some problems. With the 4 blade prop, it took me 15 seconds to get up on plane, but I did a short run, and I seemed to burn 8 gallons of gas per my flowscan.

I went in and put the 3 blades back on, and seemed to do the same run in 4.5 gallons. I went back, and put the 4's back on, and I burned less than the 1st time. But I did notice the following:

1st run 4 blade 15 seconds to get on plane, 5K RPM=30MPH, 10 GPH port/9.5 GPH starboard, 8 gallons used

2nd run 3 blade 20+ seconds to get on plane, 5K RPM = 33 MPH, 12 GPH port/12 GPH starboard

3rd run 4 blade 15 seconds to get on plane, 5K=29-30 MPH, 10 GPH port/9 GPH starboard

Conclusions. I get on plane faster w/the 4 blade 19" pitch over the 21" 3 blade, but it takes me longer to get where I want to go (3 MPH less), so over all, my miles per gallon improve slightly. So, in the long run, I'll save some gas.

Next step. Good question. I'm putting it back in with the 4 blade, and I'll hang onto the 3 blades for a while. I think my flowscan might need a recalibration (fill up, run a while, fill up again & check accuracy), but that's another whole issue.

These 4 blades were 19" pitch. Does anyone have any thoughts on what going to a 21" pitch would do for me? I'm close to red line with the 4 blades now.

 

[DMAN1968]

Re: 4 Blade prop or 3 blade prop??

Is 5k rpm the highest that the engines are turning? Seems like that should be higher.

 

[tmitchell5188]

Re: 4 Blade prop or 3 blade prop??

They will go up higher. I didn't push too much past 5500 RPM

 

[Dandelion]

Re: 4 Blade prop or 3 blade prop??

I was at the local West marine and the guy there said that I should go up one pitch in a 3 blade, and not do a 4 blade. I think the Iboats prop swap is the way to go.

If you go up one pitch your boat will take even longer to get on plane. Higher pitch = higher top speed but slower hole shot. Not what you're looking for.

And, to answer your question, when swapping a 3 blade prop for a 4 blade prop, the general thinking is to go down 1" in pitch.

 

[tmitchell5188]

Re: 4 Blade prop or 3 blade prop??

thanks, I'll try the 4 blades a while before I ebay the 3 blades. I'll also have to see how much slower I go with the 3 blades, and where the payback is. If I use less fuel per hour, but takes me longer to get there, I may not save as much as I would like too.