3 blade to 4 blade prop on 90 HP Mercury- 18 ft. Aluminum Semi V

[Space_Tex]

I recently purchased an 18 foot Aluminum semi-V boat with a 90 HP Mercury 2 stroke. It currently has a 3 blade SS Raptor 13.25 x 17 prop on it. I have yet to get to WOT on it because I hit about the 6000 rpm mark and back it off before I get to full throttle (this is three people on board, but acts about the same when its just me). I want to get an Aluminum prop since I use the boat in shallow areas and want the "flex" of the aluminum if I hit bottom/ debris.

My question is, if I go to a 4 blade prop, should I stay with the same pitch/ diameter or increase the pitch to bring the rpm's down even more? I've used the prop selector tool on multiple websites but it just gives you a 'ballpark' answer. I understand that the only way to truly know what is best is to try it out on the boat but I haven't found any places to buy an Al prop that has a return policy if it's not what I want. I see iBOATS has both 13.25 x 17 and 13 x 19 Hustler 4 blade props, so those look like my two options right now.

One other side note is that I am planning on installing a Mac's Riverrunner Prop guard. I understand there is increased drag with doing this and am fine with losing the top end speed but want to make sure I account for this factor when selecting a new prop (i.e. will this bring down RPMs some also?).

Any advice would be greatly appreciated!!

 

[ondarvr]

First, why do you want to switch to a 4 blade prop? There are good reasons, but just going with 4 doesn't mean you will gain anything for how you plan to use the boat.

Find a shop that will allow you try a few props until you get it dialed in, some shops do it, others won't, but it can get very pricy buying props that aren't what you need.

​GPS and a good tach will be your best friends during this process.

 

[Space_Tex]

Thanks for the reply!
I am looking at a 4 blade prop for its better low end torque, quicker on plane time and fuel efficiency. During duck season, I will at times have 4 people and all the guns/ decoys/ gear so I can be pretty heavily loaded.
I have both a good tach and a GPS Depth finder so I've been using those to test so far. All the local shops I've found so far have a no return policy on Aluminum props. Does anyone have a recommendation on a shop for this?

 

[ondarvr]

A prop calculator is going to be about as close as you may get, so check to see what it says about the performance with your current prop and then you'll get an idea of it's accuracy for your boat/motor combo. From the current numbers you gave the 4 blade 19" may not have enough pitch, although you could have it modified if needed, it's just more $$$$.

 

[Space_Tex]

Is there a general rule of thumb for how many RPMs I'll drop at WOT going from a 3 blade to a 4 blade? Same question for moving up in pitch two inches? The 19" pitch prop on iBoats in .25" smaller in diameter-- will this affect RPMs much?

My logic here- I'll drop a few hundred? RPMs by going from 3 blade to 4 blade. Ill drop a few hundred? more by increasing the pitch to a 19 but I'll gain some RPMs since it .25" smaller in diameter.

 

[Sea Rider]

Would say there's a theoretical rule of thumb. If say OB achieves 6 K at wot with a 3 blade prop as currently loaded, should achieve same rpm going one less pitch with a 4 blade prop. Can't possible know how much rpm reduction will have going one less pitch while maintaining same 3-4 blade prop design. Hull shape, weight, hull drag coeficient, trim, OB-HP, transom height & deck weight affects those parameters badly. To each combo it's own prop rule applies to.

In your case post max wot rpm achieved with tach and current 3 blade prop pitch as currently loaded on calm water cond. Selecting props blindly is no exact science, testing them on water is all . If in pursuit of the best top prop for combo, could result in a costly adventure..

Happy Boating

 

[ondarvr]

You currently have a 17" SS prop, this will sort of compare to an 18" or 19" aluminum prop. Since you are hitting 6000+ with the current prop, you need to get at least a 21"+ aluminum prop to bring down the rpms, but this is only a guess at best because you don't know what your actual peak RPM is, you need to check it yourself. With a 4 blade you may lose a few RPM's, how much is hard to say, there are just too many variables in prop design and set up to know.

​The diameter will have a slight affect, but don't be counting on it make much of a difference.

 

[Sea Rider]

About prop guards, which one are you interested in. The skeg protector alone or the round guard protector ? Both reduces top wot rpm due to extra water drag.. OB needs 1 pitch reduction to gain lost wot rpm back to current ones before adding prop guards. So must add that to the selecting prop equation...

If haven't tried running full hammer down with 3 up on calm water, won't know how much OB is over revving and screwing selecting best top prop for that combo. Go for it say 30 seconds and report back achieved max wot numbers.

Happy Boating

 

[Space_Tex]

Thanks for the replies!

Sea Rider, I'm going to install the Macs River Runner. It doesn't go around the prop but attaches to the keel and goes under the prop.
http://m.overtons.com/modperl/produ...id=adwords__&gclid=CLLr3vXk0dMCFQQJaQodTeQMKg

I don't think I have a whole lot of throttle left but is there a certain RPM arhat I definitely shouldn't go over? WOT should be 5500 per the manual

 

[Sea Rider]

According to some users lower skeg also achieves water drag which lessen wot rpm. Needed to go 1 less in pitch to compensate that, same as with the ones that are installed around the prop. Skegs should ride parallel to water level and round guards perpendicular to when combo is on plane to have the least drag on them.

On first post you mentioned " I have yet to get to WOT on it because I hit about the 6000 rpm mark and back it off before I get to full throttle (this is three people on board", so there's still some more throttle to go...go for it and report max wot achieved as in previous first test. I was over revving myself + 500 wot revs by going one less pitch from factory delivered one.

Happy Boating

 

[Texasmark]

Look at my Avatar photo. That is the picture of my wake at just under 50 mph gps on just about the same alum boat as yours with the same engine (2002 90 hp 2 stroke ELPT) running 5700-5800 rpms with a Ballistic 24P XL I think I bought from iboats. The PO (I bought it 2 years old) was running a 17P which was on it when I purchased it. The first time I took it out I almost had whiplash. Took all of half a second to plane out and maxed out my rpms at some rediculous speed of around 25 mph. Can't believe the guy used it like that for that long. That engine has a 2.33 gear ratio not a 2 and why dealers prop is as such I have no idea but it's wrong; not nearly enough prop for that rig!

If you boat alone and like to do what she can do most of the time, fine. If you want a general purpose prop for boating with another person or so then drop down to a 21 SS. I bought the 21 SS Ballistic first and it wasn't quite what I needed. For what I said about what you may be doing it would do fine. Running lightly loaded you would be up against the rev limiter again but would be going much faster than currently.

I see you have a jack plate. I ran mine 1 notch off the transom. Does make a difference in top speed but requires a good grabbing prop (like most SS...cupped, high rake) to keep from ventilating. If you bought a ported SS (which ballistic isn't) you probably could stay with a 24P as the ports would get you a good hole shot (with that second person aboard) and once up and on plane, trimming out till she just starts to ventilate then back a tad for sweet spot best rpm and mph. mmmmmmgood.

 

[Space_Tex]

Texasmark, thanks for the info! It's always good to hear from someone with a similar rig. So it sounds like the 17p is just way to low. Since I have times where I do run more heavily loaded, I think it makes sense to go with the 21.

I'm going to install the skeg protector this weekend and do some more test runs to see how that impacts my current setup and then make a final decision. I'll post back after that.

 

[Texasmark]

Keep us posted.

 

[Fed]

Since I have times where I do run more heavily loaded, I think it makes sense to go with the 21.

You're thinking in reverse, the extra weight would be a good reason to stay at 17".

Load it up with all the weight & take it for a WOT run to check the WOT RPM.
You could easily find the extra weight brings the RPM down enough, you may even finish up with a 17" 4 blade.
Don't be afraid to run over 6000 for testing you won't hurt it.

 

[Texasmark]

You're thinking in reverse, the extra weight would be a good reason to stay at 17".

Load it up with all the weight & take it for a WOT run to check the WOT RPM.
You could easily find the extra weight brings the RPM down enough, you may even finish up with a 17" 4 blade.
Don't be afraid to run over 6000 for testing you won't hurt it.

You running that rig? I basically have been since 2004. He does not want the 17P! If you drove the boat past the no wake buoys and punched out on your first hole shot you'd know what I'm talking about. You can hit 6k at bout 25 mph with 2 people.

 

[Fed]

You running that rig?

No I'm not Mark!

The main point of my post was to get a base line at WOT so people could offer better prop guesses!
Having said that I just saw this motor has a rev limiter on it so getting a base line could be difficult with the current prop.

 

[Texasmark]

No I'm not Mark!

The main point of my post was to get a base line at WOT so people could offer better prop guesses!
Having said that I just saw this motor has a rev limiter on it so getting a base line could be difficult with the current prop.

If he props it correctly as we discussed he will not have rev limiter problems. I'm telling you sir, I spent a couple years and about $1k in props to make that distinction.

 

[Space_Tex]

This weekend I took 3 people and a little fishing gear out, I hit 6700 rpm. I ended up getting a 13.25" x 21 AL 3 blade the next day and took it out, only had two people but WOT rpm went down to 6200. Still a little higher than I'd like but gained about 5 mph and I'm happy enough with it for now. Ideally I'd think a 4 blade 21p would be nice. I go through about a prop a year so if I can wait that long I will look for a high pitch 4 blade next time or just bump it up to the 23p. The 90 HP outboard is the max size for my boat so I think even with a high pitched prop, the motor has plenty of power.

Should I plug the ports in my new prop (there is one by each blade)? This should bring down the rpms some, right?

 

[Texasmark]

This weekend I took 3 people and a little fishing gear out, I hit 6700 rpm. I ended up getting a 13.25" x 21 AL 3 blade the next day and took it out, only had two people but WOT rpm went down to 6200. Still a little higher than I'd like but gained about 5 mph and I'm happy enough with it for now. Ideally I'd think a 4 blade 21p would be nice. I go through about a prop a year so if I can wait that long I will look for a high pitch 4 blade next time or just bump it up to the 23p. The 90 HP outboard is the max size for my boat so I think even with a high pitched prop, the motor has plenty of power.

Should I plug the ports in my new prop (there is one by each blade)? This should bring down the rpms some, right?

No on plugging the ports. They only function during the hole shot and sometimes at high trim angles and tight turns or pretty good chop you will pick up a little air and rpms will increase slightly because of them. Once bypass seals off the ports at higher speeds, depends on boat and all, 25ish to 30 ish mph, they are out of the loop.

If you got yourself a high rake, ported 21 SS which is cupped you ought to pull yourself down to around 6000. You are going to pick up mph in the process. If you want to pull it farther on down, need more prop. 3 people and gear in that boat and still pulling over 6 really says come on up another 2 inches. But, as one of the guys said, he has no problem with running 6k nor do I. I had an '88 115 tower I ran at 6 every time I could for 7 years and sold it to muy SIL who did the same thing for who knows long. It just sang to me.....as did the prop....you could hear the blades humming as they beat the water right at the surface. My boat as I said earlier ran 57-5800 at WOT most of the time with my 24 XP...was easy. The lake was about 1000 acres and just big enough to get on it but not so big as to swamp you with tidal waves. Was a lot of fun.

 

[Space_Tex]

Copy on plugging the ports. So running at 6k isn't a huge deal- that makes me more comfortable. I'm going to stick with this setup for now and eventually bump it up to a 23p. Thank you all for your help during this process!