Merc 425's prop advice needed

Grady Smith · Post by **Grady Smith** » Fri Apr 20, 2007 5:53 am

Hey Guys, I need help. I have a 1978 33 Convertible that had 454's. I have bought and then sold 2 different sets of rebuildable 6BTA's.......and now have decided I want to ride immediately and will be able to afford a set of those nice high powered 6BTA's next winter. I had been looking at the 425 hp Cummins 5.9's....... So I bought a couple of .5 hr takeout's from a Fountain. They are 8.1 425 Merc's........and reused my old 72C's that are 1.9 to 1. Bam of Florida had blueprinted and maxed the clutch pacs to handle a couple of 572's I was going to put in several years ago. Anyway the original props are 21 x 18's. The Bertram manual says the boat came with 21 x 20's. My question is can you guys give me an idea of what prop to use? I was going to see if these props could be reworked but when I checked thinking I had 21x20's they were 21x18's which seem way short. I want to use the same 1 3/8th's shafts and intend to baby them this year as next winter the fore/aft placement might be different and I will need to change shaft lengths then. Joey Griffin says the WOT of these engines is 5,000 rpm and they should be cruised at 3500 to 3800 to be in the peak torque zone. Any help would be appreciated. Thanks, Grady

Post by **dougl33** » Fri Apr 20, 2007 7:39 am

Grady, Check out this thread on Tim Stamm's re-power: http://www.bertram33.com/forum/topic.asp?TOPIC_ID=190 At one point I sent Tim my old spare props (which are currently my new props!). At the time they were 3 blade 21x20's with no cup. He was seeing a WOT of 4600rpm with these. You can change the pitch of your 21x18's up to 20 and still be ok. BTW, I wouldn't go with the 425hp Cummins if I were you. Way more HP than needed to move these boats at a reasonable turn of speed (meaining a cruise of 22-26 knots). I think you'd be fine with the 330B's or 370B's. Regards, Doug L. 1986 33 Bertram FBC Queen Elizabeth Marblehead, MA [img]http://www.bertram33.com/photogallery/p ... sabeth.jpg[/img]

photo finish · Post by **photo finish** » Fri Apr 20, 2007 8:09 am

I think you should be able to use your props. I would leave them as is until your first sea trial . I am not sure how you would compare them to my 385hp Crusaders you have to get to 5000rpm where i am at 4600 rpm but you have 40 more hp so that is why i would just wait and try them first as is.As far as your shafts you may not be able to reuse them because they may be too short. I know mine were.I wonder if Genes old shafts were too short?

lobsta1 · Post by **lobsta1** » Fri Apr 20, 2007 1:47 pm

grady, How can you reuse your old trannys without a counterrotating engine? Like Tim says, new trannys are about 6" shorter. So, how can you reuse your old shafts? Al

Grady Smith · Post by **Grady Smith** » Sun Apr 22, 2007 9:54 am

Al, My engine builder said loosen four bolts up on the 72C and he showed me by pointing it out. It reverses the pump. The four bolts are arond the plate at the output shaft. If you look closely you can see forward and reverse. Loosen them and turn to new bolt hole pattern and retighten and it reverses the rotation. I had completely forgotten about this and it would have really messed me up. Big Rock Propeller suggested to test then send them down and said I should be close anyway from what he could tell. Thanks for the help.... Doug, I know it's too much.......we run the Pamlico and it's always relatively calm there.....about 30 miles or so of nice water before the sound. I know you are right but I'd love 26-28 kts when it's calm. Grady

inspector · Post by **inspector** » Sun Apr 22, 2007 10:08 am

Grady, that won't work. What you are talking about just re-indexes the pumps to accept a different direction input from the motor; it does not cause the gear to reverse it's rotation. Rather it allows the tranny hydraulic pump to match the revs of the motor so the fluid goes in the proper direction through the tranny, the direction of rotation is mechanical. The trannys that can reverse the rotation; as in the newer motors that only spin in standard rotation, have the ability to reverse rotation internally like shifting into reverse, but the reverse gears in the newer setups have the same duty rating as the forward gear. The 72c running in reverse all the time will last only a very short time before failing. Rich

Grady Smith · Post by **Grady Smith** » Mon Apr 23, 2007 3:09 am

Thanks Rich, I will take it up with my engine builder.....I have a pair of ZF 220A that I was going to use with the diesels......til I decided to drop in the gas motors......what trannies should I use? Thanks, Grady

Post by **dougl33** » Mon Apr 23, 2007 4:51 am

Go with the ZF's. Regards, Doug L. 1986 33 Bertram FBC Queen Elizabeth Marblehead, MA [img]http://www.bertram33.com/photogallery/p ... sabeth.jpg[/img]

Grady Smith · Post by **Grady Smith** » Mon Apr 23, 2007 9:31 am

Doug, How about the ratio?........ Grady

Post by **dougl33** » Mon Apr 23, 2007 9:53 am

Well if you want to use the same set-up you have now (i.e. shafts and props) then I'd go with a 2:1. The problem is that if did decide to swith to diesels then you couldn't use the same tranmission as the prop for the diesel running 2:1 trans ratio would have to be at least 27-28 inches in diameter (which is waaayy too big to fit on these boats). The bottom line is I don't think you can get a transmission ratio that can use for both diesel and gas. For instance, for my diesel conversion, I went with ZF220a's with a 1.5:1 ratio. I will be swinging 21x22 inch props which is essentially the stock diameter prop for my boat which is an 86. Buzz Kistler has the same engines but has 1.75:1 ratios. He's swinging 23x25 inch props which was the stock diamter for his boat which is an 88. Regards, Doug L. 1986 33 Bertram FBC Queen Elizabeth Marblehead, MA [img]http://www.bertram33.com/photogallery/p ... sabeth.jpg[/img]