3 blade vs 4 blade prop

[big boat little motor]

Just a quick question what is different in the performance characteristics of a 3 blade prop vs a 4 blade prop.

Is there any difference in top end or is just whole-shot?

 

[df909]

Re: 3 blade vs 4 blade prop

I'm not sure about the technical info - others here will give you techie info until your eyes fall out.

I had a 21p 3-blade SS on my 20' Capri with the 4.3L Merc & Alpha 1 drive then I switched to a 4-blade 19p aluminum prop. I had quicker hole shot, faster plane, and increased top end. With the old prop I'd only hit about 4300RPM & with the new I hit about 4550 RPM.

 

[steelespike]

Re: 3 blade vs 4 blade prop

Actual results will differ depending on the original brand and design and replacement design and brand.
I think some of the changes for df909 resulted from the pitch change.
The rpm change was surely from the reduction in pitch.The change in material
also introduced some changes.I think the ss prop was a bit of a miss match which made the aluminum prop appear to actually perform better because it was a better match.Not because it was aluminum but because of the the design.
A carefully matched 4bld should improve hole shot,stay on plane with less power
and improve mid range performance.without sacrificing top end.
Sometime a slight drop in pitch is reccomended.

 

[big boat little motor]

Re: 3 blade vs 4 blade prop

Thanks for the feedback. The reason I ask is that the boat that we have is a little underpowered for water-sports but does ok fishing and just cruising. so since all we pull is tubes and knee-boards were you really are not going for top end but more about staying steady and on plain. I was wondering if that might make a difference.
Thanks Again
Jason

 

[df909]

Re: 3 blade vs 4 blade prop

Spike is right-on about my changes. If you want more ability to pull people, you can drop down in pitch and get a 4-blade aluminum here on iboats. Just make sure if you drop your pitch you don't overrev your motor with too high of RPM's. It should also help you plane quicker too.

 

[hwsiii]

Re: 3 blade vs 4 blade prop

Spike and Df are both right in what they are saying, and in my opinion 4 blade props are the perfect math for underpowered boats. You don't mention what your exact GPS MPH or WOT RPM are, but it sounds like you would be helped by a 4 blade Solas aluminum.

If you will give Spike your maximum RPM and maximum top speed I am sure that Spike will get you fixed up with just what you need.



H

 

[Dhadley]

Re: 3 blade vs 4 blade prop

Can we assume we're talking about the boat / motor combo in your signature?

 

[tawood]

Re: 3 blade vs 4 blade prop

This is the switch you are looking for.
Last year, I had a relatively big boat (19') with a 4 banger...I switched from a 3 blade 21, to a 4 blade 19, because I wanted to pull a skiier with 6 adults on board, but I also wanted my wife to drive without having to explain to her the concept of "over-rev".
The switch was perfect: the boat is much stronger out of the hole, but does not over rev at all...top rpm's stayed the same. Top speed, on the other hand, dropped 5 mph or so. Always a trade off somewhere, but I could care less about top speed.
Tim

 

[big boat little motor]

Re: 3 blade vs 4 blade prop

Hwsii,

we had talked about this set up last year and this is what we came up with.

http://forums.iboats.com/showthread.php?t=320531

I was pretty happy with how that worked except that towards the end of the season last year my prop and skeg got married up with a submerged Mississippi river tree stump. The prop that is on will be heading over to the prop shop to get trued back up but I will need to be buying a new primary prop for the season and last years will be the backup.

I have no illusion that the boat will ever be mistaken for a twin screw cigarette boat and as long as I can pull a knee-board with my wife or eldest son I will be a content man.

Since I was noticing I was happy with the performance of the prop once I got on plane I thought a 4 blade might be in order. Given my setup will I need to step down one more pitch on the 4 blade.

Thanks
Jason

 

[big boat little motor]

Re: 3 blade vs 4 blade prop

Can we assume we're talking about the boat / motor combo in your signature?

My hair is starting to resemble my boat name every year that goes by......

 

[cougar1985]

Re: 3 blade vs 4 blade prop

My hair is starting to resemble my boat name every year that goes by......

i resemble that remark!,lol.how much does that boat weight?i had a 17 foot mfg gypsy star glass boat for years with a 90 evinrude on the back ,a little or actually proably a lot underpowered as it was rated for 135hp!any who,i found that with that boat i had to use a 14x17p for best all around performance,with the solas amita 4 blader being the best of the bunch.to my suprise when i took the old gal to the junk yard for disposale she weighted in at 1975lbs.wow!slightly water logged i presume,lol.i might add that the boat was very slow to plane also untill i added the nauticus trim tabs then it was a whole new boat ,for the better.

 

[big boat little motor]

Re: 3 blade vs 4 blade prop

per the manufacturer the hull with decking and console is 1500 lbs

the motor is 288 lbs


I was running a 13.75x15 on it last season. from what I am hearing I will need to step up to a 13x17 solas amita if I wanted to get into a 4 blade.

will I have problem with overrev on this?

 

[cougar1985]

Re: 3 blade vs 4 blade prop

per the manufacturer the hull with decking and console is 1500 lbs

the motor is 288 lbs


I was running a 13.75x15 on it last season. from what I am hearing I will need to step up to a 13x17 solas amita if I wanted to get into a 4 blade.

will I have problem with overrev on this?

sorry ,your last post got me second guessing myself so i went to the garage and the solas is indeed a 12 3/4 x17 and not a 14x17 as i previously stated.my 90 was reving at approx 5650rpm(wot) with the 4 blade.i liked the 4 blade but that was for my boat and i hate to say anything about anybody elses boat as every set up is unique in many respects but at the same time i did a informal look see last summer at 90hp omc,s and the 17p was by far the most widely used for that motor.see the boat in my avatar?the 90 is on there and im using a 21p and getting 47.9 mph by gps because its so light .approx 500 lbs.

 

[hwsiii]

Re: 3 blade vs 4 blade prop

How many RPM were you turning with the old prop and at what speed.



H