3 blade vs 4 blade prop

[kellyb]

It was suggested that I come over here and ask for your thoughts regarding a 3 blade vs. 4 blade propeller for my ski boat. I ski and wakeboard about 50/50 of the time. Also what brand do you recommend.

Thanks
Kelly

[Fat Old Sun]

There isn't as much difference as there used to be. In the past, four blade props were quieter and did not vibrate as much. They were not as efficient as a three blade though. Advances in design and production have narrowed the performance gap between the two.

Acme props are good. Many manufacturers ship their tow boats with Acme props. What kind of boat do you have?

[Majikthise]

I believe the four blade gives you a better thrust to rpm ratio,and you have to factor in if your talking two or four stroke.As a marine mechanic, most of the four blades I've seen are on sterndrives, usually on Merc Bravo drives with 5.0 or better. I worked on a nice Baja ski boat this week, 21 cuddy with a 7.4 Efi Bravo 2. He had a three blade stainless Viper, I didn't notice the pitch. And I've only seen four blades on larger 4 stroke outboards.

[Fat Old Sun]

A higher end inboard or V-drive tow boat, especially one that carries a lot of ballast for wakeboarding, will usually have a four blade. Newer three blade props designed for the same purpose have almost the same surface area as a four blade, so the difference can be minimal with a higher quality prop.

There are several other things that many people don't take into account. The number of RPM's you will be turning makes a difference. A tow boat is geared for bottom end, and will rarely travel faster than 40 mph. The focus is on the hole-shot. Top end is almost worthless when wakeboarding speed is 18-22 mph. Elevation also makes a difference. You won't get the same performance from your prop at Lake Havasu as you would in Orlando.

Where did kelly go anyway? I can be a lot more specific if I know what kind of boat we're dealing with.

[Majikthise]

Maybe he found the fishing forum at the bottom of the page. I haven't had the time to check it out thoroughly yet. I work on boat engines and drive systems and have a very limited practical knowledge of prop effects. I'm usually too busy getting everyone else in the water than getting out there myself. I just get where I'm going and fish all day when I get the chance. I don't own a boat, but my friend does. Speed isn't a factor with most of the boats I work on anyway, mostly cruisers and day fishermen, some cop and EPA boats.I'm currently moving away from sterndrives into 4 stroke outboards, much nicer to work on.

[Fat Old Sun]

I was a manufacturer rep for several boat companies before I decided to go back to being a computer geek. I handled two different ski/wakeboard lines, so I'm used to the specifics of that particular market. It's a completely different world...lol

I can sympathize with being too busy getting everyone else in the water. I had three demo boats at one time last year, but almost never got to put them in the water. They spent most of the time behind my truck looking for new dealers.

[Scuba Pete]

It was my understanding that increasing the number of blades and lowering the rpms reduces cavitation which will increase the bite of the blades. There is also a new lower unit which puts the blade in front which gives the blade non-disturbed water which increases the efficiency even more.

But then I could be wrong.

[Majikthise]

Your right ND ,There is a new drive out from Volvo Penta that mounts a type of lower drive unit backwards on the bottom of the hull. They introduced it earlier this year.I think they are calling it a V-drive, and it's only on larger cruisers with deisels right now.There is an older concept of this that was used on sailboats way back when, but the prop was oriented normally.

[Majikthise]

Sweet.

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