slow ?
 

have long thread going over in general tech discussion but got refered here so here i am.

have 96, 38 top gun. 2 502 mag mpi about 100 hours. motors run perfectly in all respects. props are 4 blade 24 p.

boat gets to 53 mph at 4500 and just stops there. at first i thought is was rev limited in the ecus because of how consistent it is but general consensus is no...

so... everybody says this boat w/ these motors should go 60 mph. a 22 p prop is about 8 % so add 375 revs for smaller props to go the same speed and are these extra revs going to get me into an rpm range where the boat will then accelerate thru the drag limitation and go faster ...

so.... for all you guys here that know a hell of a lot more about these boats than i do :

1) is 60 mph an unreasonable expectation for this boat

2) what would the proper prop selection be for stock 502 mag mpi be ?

3) anything else you can think of.... and yes i have already checked full throttle on the controls

and when i say 4500 i mean the tach needles split the 4500 line on both motors... not 4425 or 4562 .... just like it was programmed to stop there.

motors were 100 hour takeouts from a donzi. absolutely factory original.

Re: slow ?
 

something is not right, you should be turning them at 5000-5200 and your speeds would be right. there is a limiting factor somewhere. how is the dist and wires? with the smaller props what was your rpm?

sounds to me like there is something in the comp that is maxing them at 4500, when you hit 4500 does it die down? if so your rev limiter is too low.

Re: slow ?
 

Are you trimming the drives up and are the tabs fully retracted?

Re: slow ?
 

tabs full up but running the drives full down. absolutely nothing suggests a mechanical problem in the motors like cap/wires etc... they run clean and sharp.... other forum said rev limter supposed to be 5700 or something .... so ...since BOTH stop at 4500 are you saying that the rev limters are user serviceable ? that they can reset somehow ?

Re: slow ?
 

not sure if my last reply went thru,
I have a 95 top gun with the exact same set up.
in salt water at 5000 rpms 68mph on gps
how are you measuring speed? speedometer or gps?
we are repowering to bring our speed up
you could have a problem in one moter and that will hold the other motor back

Re: slow ?
 

What ratio are the drives?

Re: slow ?
 

I should know that, seeing how i had to replace one this year but I have a brain block
over the winter I am upgrading all the uppers to handle the new hp
nothing crazy, 502 with 500hp plus headers. going back to carbs and closed cooling

Re: slow ?
 

Steve - to see top speed you must not operate with the drives full in. I would say you should be trimmed at least to between 5 and 8 on the indicators. Only experience with the boat will let you know how much trim is too much. Drive trim makes a huge difference in top speed.

Re: slow ?
 

no idea at all what drive ratio is and taking boat out tom and will start playing w/ drive angle... nothing to lose. as i said...new to this so can and will listen to all intelligent advice.

speed noted is on my gps dash board speedometer a couple mph slow at top speed but very close at 40 and below.

don't believe for an instant notion of " one engine holding other back" all engine wiring, plumbing, fuel and ancillaries completely independent of each other. they share no systems at all. one engine could fall directly out of the boat and the other wouldn't even notice until it snagged in the prop.

will advise about drive angle dif if any. and thx for all thoughts.

Re: slow ?
 

Trim it out----------You will go faster ! ------alot faster !

LS

Re: slow ?
 

bingo Steve, therein lies your problem, as you are getting up on plane trim your drives out you should run an easy 65. keep your tabs up, start out with drives at 2 ease up on plane and trim them out...as you are going out you will pick up speed, when you stop gaining speed or it slows down, trim back in, it will take you some time to find the sweet spot, but have at it man...

Re: slow ?
 

Show some pics of the boat! :drink:

Re: slow ?
 

ding ding ding

tried two trim positions.... numbers meaningless i guess but tried some and then some more. one drive pump too weak to move the drive at speed so have to stop to make adjustments... either way, difference exactly what you predict... boat just took off . now 4900 and 62 mph easy in not great conditions... lots of flying involved.... went right past where i normally turn for home and was some miles around the island before i could focus on the gps and see where is was... oh well...

so ... 'problem" clearly identified as pilot error. will erbuild the one pump so adjustments possible at speed and report back...

and to all that offered suggestions i thank you and wish you good holidays and if you ever get to sint maarten, look me up

am sure there will be more questions coming and appreciate the opportunity to participate.

Re: slow ?
 

A little trick I learned when you have a weak pump, is to pull the trottle back on the motor with the weak pump and hit the trim button the drive will go out. Not a fix but it does make it drivable for the time being. Trim makes a big difference on these boats.
Have fun.

Mark

Re: slow ?
 

seems so.... am i to assume that by kicking the drives out a bit it picks the nose up out of the water and simly makes less hydrodynamic drag ? horsepower required is a cubed factor to overcome drag so can clearly see how reduced drag would make huge dif but would have thought that nose up aero drag just as bad.... but guess not.... at anyrate am immensly pleased and now have place to start to start dialing it all in... thx again

Re: slow ?
 

Trim out till you blowout!

Get a GPS and finetune your trim till you max out on speed then mark your trim indicators.

I had that exact engine combo on a Cafe. a gun should be close in speeds if not fater from what I heard and it ran 67 with labbed 4 blades 24" at 5000 RPM

With the 3 blade 23" it came with it ran 65

PM Otis he had close to that set up on his Gun a few years back and was close to 70.

Hope this helps.

Re: slow ?
 

The fastest I've gotten in my Cafe on GPS (502 Mags, with 0 hours) is in the upper 50's. Running 23p Mirage's and maxing it at 5200 rpm's & trimmed out. How you guys doing it?

Re: slow ?
 

dear panic...
if you read the thread complete youll see that i am certainly no expert on these matters...had to be instructed in the most basic sense... but i think your answer is self evident. prop pitch is essentially the same as rear end gearing in a car... you run less pitch and you need more revs to go the same speed if everything else remains the same. you have to remember that speed is STRICTLY a function of drag and ( as mentioned) the hp required to overcome drag is the cube of the extra drag... so .... what happens when you do less prop is you get more revs , which gets you more power ( assuming not out of power band) which gets you a by a little drag which makes you go a bit faster... in your case, if you look at your numbers, they are essentially what mine WOULD have been BEFORE the trim adjustments and if i had simply done 23 p props.

bottom line in my best engineering opinion and based on the most recent experience, asuming the motors run well ...and sounds like they do... you need to get 24's on the thing and get the drag down substantially somehow...either by trim etc ( like my issue was ) or whatever your boat needs specific to its manufacture. we KNOW that the 502 mags will pull the 24's on a 38 ft 9500 lb boat. thats a fact... so if your mags won't and they run ok then the only answer can be that there is a drag problem that you have to address... just like i had to. drag is the enemy and it never sleeps

my case CLEARLY illustrates the need to have the PACKAGE correct... no single magic bean element...

Re: slow ?
 

I owned the same boat - a 96 TG W/ 502 efi...
It would run 65 at about 5000 rpm w/ bravo 24p .
I tried 26 labbed bravos and it slowed down as the RPM was not high enough.
Everyone is right with the trim- if the motors are fine and nothing has been modified with your bravo drive height/length then trim out more .

Dave

BTW- The same boat went 78-80 once i installed procharged 500efi's- besides the top speed the boat handled much better with the needed power increase.

Re: slow ?
 

funny you should say that ... tuned in today to say that am now at 65 on the gps at 5000 ...maybe 5100 due a bit more screwing around.... not certain there is anymore but will poke at it a bit and report back.... conditions better today but still on full tanks so carrying 15 or 1600 lbs of fuel while doing this testing...

prev owner says they never got over 62 with 650 hp 454's .... which means one of two things.... ( or both) my rant about dyno numbers has another case history or those guys wern't smart enough to ask YOU guys what the problem was...

had my first official run in w/ the coastguard today.... was out testing and went by this zodiac looking thing with a couple big outboards and he started after me... THOUGHT was just a couple local guys wanted to race... only after i turned around and headed home that realized they were CHASING me... no tickets ..no fines... the locals here ( caribbean) are pretty cool once they realized there were no drugs or illegal immgrants involved...