midnight express/diesel/props
 

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.

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

Post your current rpms and motors max rpms for more replies

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

Brett

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!

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?????

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.

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.:ernaehrung004:

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.

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.

Your welcome, do your self a favor just for ha ha's.

Get ahold of Yanmar and find out how much boost those engines make at max Hp?

Put a boost gauge on yours and see how much boost they make at WOT. You can get an idea of how much Hp your making and my guess is about 240-250 hp that's all.

Personally, if you could get 1.33 or something close (I know they make a taller gear) you will get into 60's with ease and have a 50 plus cruise boat.

Truth is your body and passengers can't take too long of a cruise at 50 mph!!!

The test 34 that I reference on here like I said was run 8 hours a day at between 50 - 65 mph and that was work, no one could do it back to back days for long.

If I could cruise 55 mph I would take that in a heart beat and let all the others zip by, with their fuel and rest stops and all I'll be 50 miles further down the coast at days end!!!

Good luck!

There is a company in the UK that can bring the rpm,s up by a few hunderd more (I thought 4000 rpm),and believe they claim 380 hp (crank possebly) Do a search on : D max 380 Yanmar
that can help you with some more prop speed ,and with the help of a 1,26 :1 ratio in an set XR,s will prop you to almost 3200 prop rpm,s .
I will try to find out the company that do the work on the Yanmars.
Was that the blue white Midnight that was for sale in the UK a couple of months ago ?
I believe that this boat had already diesels (Sabre ?) and strange surface drives ?possebly homemade ?

d-max
 

I contacted the d-max company 9 months,maybe a year ago but the only want to sell complete engine packages not wanted to share or sale the information and R&D on there package .

That correct ajay,same boat,was fitted with surface drives,lancing marine,cant remember the engines that were fitted, but i do know they only produced 165 hp each at 2400 rpm barely enough to put it on plane,did you ever get sorted those yanmars fitted in your boat any updates,someone mentioned that you were going to change to cummins???

You never actually told us the rpm's you were running initially.

Just a FYI

30P Bravo 1's tunning 3500 through 1.24 Bravo drive gears with a 15% slip will reap 66 mph.

I don't know that you could pull them with the tall gears but the numbers are there.

Hello Seamy

Ok so were talking the same boat here ,I like those midnights a lot ,was this a good solid hull ? not soaked ?
I stopped with working on the Baja /Yanmars ,several months ago to finish my Scarab first ,so I would have a boat ready for the summer ,I started working again on the Baja last week ,there was a guy that was intrested in my Yanmars ,so thats why I was looking around for a set Cummins 480 ,s , if I could sell the Yanmars....

Can you post a few pics from the Midnight /engines /drives etc ?

A.J

Hi, ajay ,yes the boat is in really good shape,hull is spotless,few soft spots in the floor which i will replace with new floor soon, and perhaps sell on to fund the 38 scarab project,original plan was to remove the 6ly yanmars and outdrives for the scarab and sell the midnight express hull on but it seems a shame to split it up,so some more thinking to do before i decide which route to take,im currently running around in the express at the moment,it runs really well with lots of torque and handles great, we will have to see how it goes,how are things your end,any pics i can drool over??? will send some pics when im down at the boat next,cheers take care.:ernaehrung004:

any pictures yet of the 32 midnight?

the yanmars wot is 3800 they can be propped for 4000 with a light load and perfect conditions but can't be run continuously at that rpm.