seamy

hi guys; midnight express 32 with twin yanmar 6lp/315 hp,currently has bravo one x,ratio 1.50.1, spinning 28 pitch, but they are too small,any thoughts ,current speed 54 mph gps,Total weight 4200kgs,thanks.

MikeyFIN

A wild guess from here, say 30" 4 blades or 32" 3 blades

olmo40

Post your current rpms and motors max rpms for more replies

bbladesprops

Rpm level is a must. Also boat attitude will help to determine the best fit.

Brett

HabanaJoe

You don't have tall enough gears to use all the Hp you have. You can get a little better speed with props but nothing great.

If you want to go fast, the gears are too low. You can waste money with props until you spend every dollar you have and still won't go very fast!!!

Read everything I write about diesels and prop speeds etc on here and you'll see why in very simple terms.

That boat should be in the 70's, same hull 300 Cummins was 65 mph with big trans, arenson drives and 300 gal fuel and ballasted to 11,000 lbs, 4 people on board!

seamy

cheers joe,what or how would you go about rearranging the setup,simply change the reduction on the outdrive and run a smaller pitch,what are your thoughts?????

HabanaJoe

I'll explain with math.

Take your drives and put a gas engine in front of them. To get the props working right that go on those drives you need to have your engine turning 4,000 rpms - correct? That is 2,666 rpms of prop speed.

To really get those props digging in and running effecient and boat moving fast you need to 5,400 rpms - correct? That is 3,600 rpms of prop speed.

Now put your diesels in front of those drives and let's say they turn 3,500 rpms, which equals 2,333 rpms of prop speed.

Above we just concluded that props for that drive need to get to 2,666 rpm to even start to run good.

No offense and I was hard on you in my first reply but do you see my point - it's just math and it won't work.

You need taller gears to get prop speed up with a pitch that works well with a gas engine.

The prop and drive have no prejudice, they don't care where the power comes from as long as it turns fast enough!

Again didn't mean to bite off your head.

seamy

Thanks for the reply joe,what ratio would achieve optimum performance,and where would we be with regard to pitch size,How difficult is it to change the ratio on the drive?or are we talking about a transmission between the engine and drive to achieve this,Dont worry about biting my head of,have'nt been using it much lately anyway,hence all the brain picking,thanks again joe.

HabanaJoe

You can't get the ratio's you need in those drives, just don't exist and if you made them the space in the houseings themselves would not allow the gears to fit as the small gear is now the larger gear in size or equal at least.

You have to have a gear box inbetween the engine and drive, no other way. Your shifting is the drive so to put a full hydraulic trans in there is a waste of weight and size.

I wrote on here one time about we put a Lenco box inbetween the engine and drive, Lenco boxes can be run either way meaning the input and output can be turned around so you get overdrive and not reduction. I know they carry like no oil what so ever but a little 12V pump and an oil tank allows you have extra oil and install a cooler in-line. None of this is easy or cheap as every shaft flange, coupler etc all needs to be made from Scratch.

Pat from weisman (Shifter) is on this board, I think they should have something better than a Lenco to make this work and they know Merc shaft sizes and splines etc where Lenco is drag racing. The concept is still the same as the Lenco.

Say your engine truns 3,500 rpms, you need to step it up by 60% to get an output speed of 5,600 and then go through your 1.5 reduction.

Here is the problem with all this, your engine idles at say 800 rpm's and when you gear it up the output shaft idle speed will be 1,280 rpm's which will kill the shifting in the Bravo drive. You better make sure you have the engine cut outs working perfect when you change gears because you need the drop in rpm's to make the drive shift easy.

We didn't care as we shut the engine down and used them like a crash box but that is hard on a pleasure boat.

That is why the 2 speeds are better as in low gear the output rpms from the box will more closely match the orginal engine rpms.

Does this all make sense? If not tell me what parts are cloudy and I will expand of them.

seamy

cheers joe,That clears everything up for me,The bottom line is i know what you are saying is corrrect,and the speed of the boat could be great,[but with alot of effort] dont think it will happen this time around,but thanks again for all this info,there really is no substitute for experience when it comes to this sort of thing,thanks again joe.