3 blade vs 4 blade props

Bob Giaier · Post by **Bob Giaier** » Sun Aug 12, 2012 10:35 pm

Last week I learned the hard way (literally) with my 73 Tri Cabin. With water levels in Lake Huron continuing to slide, I hit bottom and bent the tips of the starboard prop. I made the mistake of not looking behind me coming out of the river onto the lake on a very windy day (believe me, I know what to do now when it's windy like that). I believe these are original 3 blade 20 x 21s. Well as luck would have it a few weeks earlier I came upon a set of 4 blade 18 x 18s, so I installed them today and went on a sea trial.

The 3x20x21s ran about 19mph at 3000rpm, the 4x18x18s run about 16mph at 3000rpm and about 20mph at 3500rpm. The engines feel like there's hardly any load on them and there's less vibration which I guess is expected with the smaller dia. The engines can run at any rpm with these smaller props not like the 20x21s.

I have 280 Chryslers (M400) with 1.1:1 velvet drives and 2.09:1 Walters V drives.

I'm looking for some input from tri cabin owners and any others with experience on the following:

-Any suggestions on repitching the 4x18x18s to 19 or even 20s? I was thinking the 4 blades would be more efficient than the 3 blade originals.

-I was planning to take her on a 8 hour cruise over Labor Day and would be running at about 3200 rpm to run at about 18mph. With the other props I'd be running 2900rpm at about 18mph. I'm concerned about stressing the 39 year old original engines.

-Any input on what other tri cabin owners have for props and what's the cruising rpm vs. speed?

-With the 20x21s I was getting about 1 mpg in calm seas. Just curious what others are getting.

I went thru the forum history and there's a lot of info on other Trojans, but not much on the tri cabin.

Thank you all in advance for your help.

prowlersfish · Post by **prowlersfish** » Sun Aug 12, 2012 10:44 pm

If the 3 blade 20x21 were right then a 20x20 4 blade would be about the same . what was the max rpm with 3 blade props and what is it with the 4 blade props ?

1 mpg ? at what speed ?

There is a lot of tri cabin info here and a lot of tri cabin owners

aaronbocknek · Post by **aaronbocknek** » Mon Aug 13, 2012 2:54 am

when i get the $$ to do it, i really want to upgrade to 4 bladed props. my mechanic tells me that low rpm maneuverability is enhanced, and, you are indeed correct, there is less vibration with the 4 vs the 3.
aaron

prowlersfish · Post by **prowlersfish** » Mon Aug 13, 2012 7:02 am

4 blades are a lot smother for sure , but I have found my boat preforms better with 3 blade props ( faster cruse and top end) others feel the 4 blade will do better .

The most important factor is you prop them so the engine gets to its maximum rpm range . right now I believe your under propped ( will turn to much rpm ) . Thats why I was asking the max rpm before and after .

Bob Giaier · Post by **Bob Giaier** » Mon Aug 13, 2012 8:21 am

Thanks Aaron for the info.

As for the mileage, the 1 mpg (maybe a little less actually, but I'm an optimist). That's at 2900rpm.

prowlersfish · Post by **prowlersfish** » Mon Aug 13, 2012 8:26 am

The reason I asked about the mileage was I get just over 1 mpg ( at cruse ) and I am running diesels .

Nothing wrong with being a optimist

Bob Giaier · Post by **Bob Giaier** » Mon Aug 13, 2012 8:27 am

Prowlerfish, I don't know the max rpm for the 3 blades. I never took the engines over 3500rpm (just didn't).

With the 4 blades I stopped at around 4200rpm. I just don't want to push her too hard. I feels like they could red line with the 4 blades. I do agree with the 18x 18s it is underproped.

prowlersfish · Post by **prowlersfish** » Mon Aug 13, 2012 9:42 am

Sounds like it to me too

jhalb · Post by **jhalb** » Mon Aug 13, 2012 2:55 pm

I hit a log a few weeks ago and had to send my 4 blade, 18 X 20 props tho the repair shop. I had my 3 blade 18 X 22 props put on by boat so I could enjoy the weekend. I like the feel of the 3 blades better and they seem to have more get up and go than the 4 blades. I did notice that I topped out at 4000 rpms at 30 mpg. I remember the 4 blades going a little faster at top spead. I usually stay between 1800 and 2800 rpms so I am thinking about keeping the 3 blades on.

k9th · Post by **k9th** » Mon Aug 13, 2012 8:31 pm

My concern is that you may be in the 4bbls at 3200. Have you noticed this at all? It will definitely affect your fuel burn if you are.

Other than that, I agree with Paul, you may be under propped.

k9th · Post by **k9th** » Mon Aug 13, 2012 8:34 pm

jhalb wrote:I did notice that I topped out at 4000 rpms at 30 mpg.

Wow John - 30 mpg - I would take that any day- LOL

Bob Giaier · Post by **Bob Giaier** » Mon Aug 13, 2012 9:06 pm

Thanks everyone for the input. Good point about the secondaries. I'm pretty sure they're not opening, but I'll check that on the next run.

There's a good prop shop in Detroit called Johnny's props that computer balances and repitches props. $450/prop to repitch. Seems like allot. Not sure yet if I want to spend that. The 4 bladers are probably going to be my back up props, but it is pretty cool to feel how much less vibration you get with them.