new engine/new gearbox/new propeller questions

[luis]

I like to buy new engines made in Italy , the FNM HPE 250 Hp with ZF 45A gearboxes. The engine is a 5 cyl in-line with a max out at propeller of 241Hp/4200 rpm. My question is if I should keep the reduction gear at 1.5:1 and what propeller size and pitch should I put. I also am making the shafts in Aquamet with 35mm.
The correct gear reduction and the type of propeller are the only things that make me stop on ordering those engines.
These engines are nice on price and the common opinion is that they develop very well. I always prefeer Cummins but in Europe those are forbiden due to the price and maintenance costs. This is my better option to new engines.
Some advise on this matter is very welcome. Tks

[CaptPatrick]

Luis,

You'll need a 2:1 ratio to take avantage of the available torque & 20" X 22" four blade or 21" X 23" three blade props would be a good starting point.

Br,

Patrick

BTW, the PP transfer cleared... Many thanks!

[wmachovina]

not to hijack the thread but the 250 cummins I bought are coming w 2:1 220a gears. Good idea?? what size prop estimate--Bill

[In Memory of Vicroy]

Bill - the 2:1 should be okay with the 4BTA 250 since it turns up more than the 6BTAs...as I remember its rated at 3,000 RPM? Same prop setup as Patrick says above should work.

Luis - I agree with Patrick that 2:1 with those porp set ups should work out fine with those engines, or even a little higher ratio, say 2.5 to 1 to slow down the prop shaft some and get the most out of the larger diameter props.

UV

[wmachovina]

Vic, thanks for the reply but the engines are 6BTA s. Just old 250 fresh wateraftercooled, dont think they turn all that high-think the same engine you use??

[In Memory of Vicroy]

Sorry, Bill, don't know where I made up the 4B deal. Yeah, those are the same as mine. Everyone I know that runs the 6BTA 250s in the B31 uses 1.5 gears and are able to get the props pretty square, like 21x22 three blade like I use, or 20x23 4 blade like Andre. 21 is about the largest prop that can be turned under the 31 and with the 2:1 gears you would have to pitch up to probably 25 to get it right. The 6BTA 250s are rated at 2600 rpm WOT under load but need to be propped to reach 2650 under full load to keep from overloading them and make them last 125 years......I've heard there are a number of places (advertise in B&H) that will swap out gears of different ratios for short money if you decide to drop back to 1.5.

UV

[Tony Meola]

Bill

I just put the 270 recons in mine. Basically the same engine as yours. We put on 20x23 and right now its a little light so we will put some cup in it over the winter. I am turning 2750. Should be 2600.

[wmachovina]

Tony, what gears you running--1.5:1 or 2 Thx Bill

[Tony Meola]

The tansmissions are 1.5/1.

4 blade Prop. Real smoot no vibration at all. With the big blocks and 3 blades, I had a slight vibration that drove me nuts. The hatches on the deck used to rattle. When we put the boat in the water I didn't even have the deck fully screwed in and when we ran it not a rattle.

Maybe we got lucky on the alignment or the guys that set the struts/ shafts and engines in just did a great job. I am thinking great job.