3 blade to 4 blade?

[dougncrew]

I have a 26' Tri toon with a 5.7 Volvo IO. I have a 14.8 x 17 three blade on it now and from what I have read my rev limit is 5000RPM. I can hit that at WOT with 6 people on board with a full take. I am not too concerned with top speed I just want to be able to pull my kids on rafts or wake board. I was told that if I go to a 4 blade I should reduce the pitch. So would a 14.8x15 be the correct answer?

 

[Bob_VT]

Welcome aboard to iboats.

First I would try the most cost efficient way......... try the watersports w/o changing the prop. The Volvo IO are really strong and you might not have to change at all. You might be pleasantly surprised.

The basic rule of thumb is to drop 1" of pitch when moving to a 4 blade.

 

[dougncrew]

OK I will give it a shot, when it warms up a bit. But if I go that route would this be a good choice? And does this come with the hub or do I need to buy one? [h=1]Quicksilver Diamond 4 QA2182X 15" x 16"[/h]

 

[QBhoy]

But surely if you can hit the limiter with such a high load, you don’t want to drop in pitch ?
it would make more sense to either go to a higher pitch 3 blade or at least stay the same with a 4 blade (

 

[Texasmark]

Welcome aboard to iboats.

First I would try the most cost efficient way......... try the watersports w/o changing the prop. The Volvo IO are really strong and you might not have to change at all. You might be pleasantly surprised.

The basic rule of thumb is to drop 1" of pitch when moving to a 4 blade.

Reason being is that as blade number increases prop efficiency decreases due to blade to blade turbulence. But, thrust increases with the added paddle which is what you need with heavy loads like water toys. I always ran less than optimum pitch because I wanted plenty of thrust for water toys and the boat loaded up with folks when playing.

 

[dougncrew]

OK I am going to stay with a 3 blade for now. Went out this weekend and figured this will work for now. But I am going to buy a new prop and need to know the difference between the Rubex and the Amita. I thought I saw this on here but I cant find it. is it only one has the hub installed and the other you put it in your self? What are the pros and cons?

 

[Texasmark]

OK I am going to stay with a 3 blade for now. Went out this weekend and figured this will work for now. But I am going to buy a new prop and need to know the difference between the Rubex and the Amita. I thought I saw this on here but I cant find it. is it only one has the hub installed and the other you put it in your self? What are the pros and cons?

I ran rubber, pressed in hubs my entire 50+ years of boating. Only during the last couple of years did I buy a prop with the removable hub. I went through the days of shear (not drive, but shear as that's what they liked to do) pins that locked the rubber hubbed props to the prop shaft. Get to fishing the good spots, where the fish feed, windward side of rip rap, wind blowing you into the rocks, get ready to get out and BANG goes the pin. No thanks.

In thinking about the design, the removable hub allows a dealer to stock the props and depending on the engine requirements, select a hub that fits the spline of that engine....result, fewer props to stock and better selection for the customer.

The problem I personally have with the hub is that it's made of plastic and brass/bronze. The rubber hub slips and reseats itself when striking an object. On the plastic/metal hub, there is nothing to give as the outer diameter is locked to the prop and the inner locked to the prop shaft.......walla, plastic all over everything.......dead in the water........just like back in the days of the shear pin. No thanks!!!!!!!

That and a buck might get you a cup of coffee! Just my opinion.

 

[dougncrew]

OK now I tried to change the prop but I can not get the thrust washer off. I hit it with 5 pound Dead blow and it will not move. Any ideas?

 

[QBhoy]

Wd 40 or penetrating oil over night and tilted so it will soak in. Then have a go tapping each side evenly ? That’s what I would try. If not a wee bit of carefully applied heat to the washer.

 

[Texasmark]

It's ID and the prop shaft OD are a wedge (to ensure a tight fit and prevent any looseness or vibration) so besides possible corrosion, you have to overcome the fact that the washer is in compression. Like Qboy said, good penetrating oil and tap from both sides simultaneously. I would not use heat as you have a double lip prop shaft seal just inboard of that washer and you surely don't want to damage that.

 

[dougncrew]

thanks. I will be trying that tonight.

 

[dougncrew]

Also in my never ending thirst for knowledge (things I don't know or understand), while removing my prop last weekend I noticed something. When I watched videos on how to change the prop people were using a board (2x4 lets say) and putting it against the cavitation plate to hold the blade still. So in my mind this means a blade should be under 1 1/2" from that plate. I tried this I had to use the head of a deadblow that is probably about 4" long. With my prop being a 14.8 x 17 and I can hit 5000 rpm easy ( i think that is the limit) I was thinking of a higher pitch. But with what I said above should I try a bigger prop with the same 17 pitch?
Sorry for the long read. And I know I should be checking speed and RPM with GPS but I have not figured out how to remove the old Garmin 95 so I cant put the new one on.... but that's for another thread.

 

[ahicks]

If you are hitting your max rpm (5000) with the prop you have now, what is it you hope to gain with a change?

 

[dougncrew]

I put the new blade on (14.8 x 17") and with 6 adults and 4 kids going up river I went WOT and hit 5000 RPM. When I started to back off the throttle I noticed as I was pulling back I staid at 5000 rpm for a while. All I am really trying to do is get full potential out of my motor. I am new so I do not know what i am doing and this is why I am all over with my thought process. Now I was thinking of going to a 14.5 x 19". I would like more speed if possible but again I know nothing. Am I way off base?

 

[dingbat]

When I started to back off the throttle I noticed as I was pulling back I staid at 5000 rpm for a while.

There is almost always slack in the throttle cable. Probably stayed WOT until the slack in the cable was taken up

 

[MaPaHa]

I've got a bass boat with a 175 and a triton with a 150 and I wouldn't have anything but a 4 blade for pulling, maintaining a bite in the water and mid range rpm performance. With the pontoon, it's a big ear - off shore prop that connects with the water. I agree, if you're hitting max prm with a load then maybe stick with the same size but go 4 blade

 

[dougncrew]

I have a 2009 with the 5.7 Volvo Penta. I have been asking about prop size and blade count on here for a while and have read some good comments and have started to learn some things. Well I finally have my GPS installed and here is what I have found.
With 4 adults and about 3/4 of a tank (45 Gallons) and the prop I am running is a 14.8 x 17 I hit and maintain 38.4 MPH at WOT 5000RPM.
I took my hull number and sent it to Bennington and asked what prop came with it new and they said a 15.5 x 17.
My questions are;
Does this top speed sound right?
Would going to the bigger prop increase my speed?
What are the negatives of going to that bigger prop?
My thought always seem to be that it seems too easy for me to hit the 5000 RPM that I am under propped, am I?

 

[ahicks]

I'd be pretty happy with a boat getting to 5000rpm quickly while able to run 39mph myself.

If you aren't pulling tubers or skiers, and wanted to see if you could nurse another couple miles an hour out of it, I'd try increasing the pitch a hair, or maybe increasing the cup. Increasing the diameter will likely just slow you down at this point. See if you can find a good prop repair facility in your area, and talk with them. There are absolute magicians out there when it comes to tweaking a prop like this when you are as close as you are now.

If you're pulling anything regularly, I'd leave it alone.