Is it just me or is the boat to slow

[lilb]

Re: Is it just me or is the boat to slow

I am not really sure what the max prm is for this motor but I have got it to 4800 mabye 5000 with triming it up. This is really confusing me why it wont go faster than than 44. I will be taking the boat out in an hr or so i will get back and let you guys no if I can get a faster speed out of it.

I would imagine with the stock 260 hp that it would go at least 50 rite?

 

[QC]

Re: Is it just me or is the boat to slow

No guarantee on that ^^^^ And please note that a "stock 260" Is 20 - 30 bhp shy of what they rate a 5.0 MPI 260 today. It's a big deal considering whether we are talking horsepower at the flywheel or horsepower at the prop. Independent engine estimates are always at the flywheel as opposed to modern ratings by Mercruiser and Volvo. Annnnd, I am still not convinced you're even making 320 at the flywheel. Not arguing, or beating up your engine, just need more info to be confident.

 

[Philster]

Re: Is it just me or is the boat to slow

The only way the additional engine revs will return more MPH is if it moves the power peak closer to the drag curve. If you're overpropped (engine can't reach max power RPM) a smaller prop will make you faster. If you're underpropped (like you're example, the engine revs past max power RPM) then a bigger prop will make you faster.

It's all taught in freshman statics and dynamics. I've been using it for 25 years to optimize performance.

I've been using it for over 26, Junior.

 

[QC]

Re: Is it just me or is the boat to slow

I just did some checking. A new 232 Cobalt (probably lighter as there was nothing on board) with a 280 Volvo ( I honestly believe more horsepower than you have) only does 46 MPH in a test. I think you are right where you are supposed to be with two exceptions.

1) Is it 4800 or 5000 RPM? We need to know exactly what it is, GPS with you trimmed right for max performance. No kinda 4800 or kinda 5000 stuff, and

2) I still am getting over 30% slip. If it's 5000, then even worse. So I suspect there is a bad number somewhere. My guess is either pitch is wrong or the gear ratio is wrong as reported. I don't think the speed is

 

[Thad]

Re: Is it just me or is the boat to slow

I've been using it for over 26, Junior.

You two are killin' me.

Just say "bigger is not always faster and smaller is not always slower"

Sound about right?

I have a 25p that moves my boat 6mph slower than my 23p. And my 21p is 2mph slower than that. So, my 21 runs 48mph, my 23runs 56mph and the 25 only runs at 50mph. Sounds a lot like what you two are trying to say only not so "easily".

 

[Fireman431]

Re: Is it just me or is the boat to slow

OK..I have to chime back in here...mainly to say..

45Auto...you suck! That is WWAAYYYYY too much math to put into a forum! Holy crap! I thought I left that behind in college calc! I got half way through and now I have a headache! You owe me 2 aspirin!

 

[Philster]

Re: Is it just me or is the boat to slow

You two are killin' me.

Just say "bigger is not always faster and smaller is not always slower"

Sound about right?

I have a 25p that moves my boat 6mph slower than my 23p. And my 21p is 2mph slower than that. So, my 21 runs 48mph, my 23runs 56mph and the 25 only runs at 50mph. Sounds a lot like what you two are trying to say only not so "easily".

You got us all wrong. Props: Smaller pitch can be faster or slower. Bigger can be faster or slower.

Adding HP can also make you slower. No, not kidding.

Salt water can make a boat faster. No, not kidding.

GPS speeds can be terribly (terribly) deceiving. Could be way more innacurrate (as for speed over water) than a stinking pitot type speedo when not used properly.

Let me know when you need an Excedrin or two.

 

[lilb]

Re: Is it just me or is the boat to slow

Well got the boat out yesterday for some more testing and 46 mph with proper trim and 4900 rpm. There was hardley any waves and two people (320 lbs combined) and about 30 galons of fuel. (less feul than other test runs but not by much mabye a couple of gallons) Some other things on the motor are 9.5 to 1 comp, roller tip rockers, double roller timing chain and ballanced and blueprinted.

QC you dont think with the combo I have Im not at all close to 320 hp? That number is supposed to be at the flywheel but didn't put on a dyno or anything. I person that built the engine has been doing so for 30 yrs. and said it should be rite around there if not more.

 

[Philster]

Re: Is it just me or is the boat to slow

It's the dreaded "I added HP and am going slower or not faster" dilemma.

Most guys that build motors know how to get 'em to accelerate faster.

Trouble is... They don't get to build motors, run 'em up to max RPM, hold them there for a few minutes and then re-tune the motor over and over around the need for more HP right up at the peak. Can she pull faster through the given gears/prop? I will say that is likely. You can throw a bunch of money into hop-ups and get through to a certain speed .30 seconds quicker. Takes a crapload of smarts and experience to add just the right pieces to return HP at 4900-5000 RPM. Darn, if you could add just the right stuff that would take the 300 HP at that RPM and jump it to say 330 HP at just that RPM (right where you hit your wall right now), you might crack 50.

You'd need enough new peak power right at that RPM range to push through that wall.

 

[45Auto]

Re: Is it just me or is the boat to slow

I've been using it for over 26, Junior.

Wow, and you STILL don't understand it?? Where do they teach that 300 HP boats are faster than 320 HP boats??

So, my 21 runs 48mph, my 23runs 56mph and the 25 only runs at 50mph.

Perfect example. Your slower 21 is underpropped - it's revving past the peak power of your engine, similar to Philsters example, so it's slower on top speed, but will have better acceleration - similar to 1st gear on a car.

Your 23 is letting the engine rev very close to it's horsepower peak, so it's delivering the maximum power into the water for top speed. More speed means more drag, the max power you can deliver will overcome the maximum amount of drag thus delivering the highest top speed. You can see it in the equation in post #16, more drag means more HP is required to balance the equation.

The slower 25 means that the HP peak is on the far side of the drag curve. The engine runs out of power to accelerate when the drag force exceeds the power available from the motor, so it can't accelerate to it's peak power.

 

[Philster]

Re: Is it just me or is the boat to slow

Wow, and you STILL don't understand it?? Where do they teach that 300 HP boats are faster than 320 HP boats??

.

Any 320 HP boat with a screwed up power curve will get beat by an equivalent 300 HP boat with a great power curve.

We didn't even talk torque!

 

[NYBo]

Re: Is it just me or is the boat to slow

45, it doesn't matter where the power peak is per se, it's whether the engine can reach the maximum recommended RPMs with a given prop. Doesn't matter if HP has peaked at that RPM or not as long as it can spin that fast.

At least I think that's how it works.:redface:

 

[45Auto]

Re: Is it just me or is the boat to slow

Any 320 HP boat with a screwed up power curve will get beat by an equivalent 300 HP boat with a great power curve.

Totally wrong if you're talking top speed. Correctly geared and propped, more power will ALWAYS increase speed.

If you're talking acceleration, the amount of AVAILABLE POWER is what determines acceleration. AVAILABLE POWER will depend on the amount of AVAILABLE TORQUE at any given RPM.

We didn't even talk torque!

I shudder to imagine how badly you would bolix that up ......

45, it doesn't matter where the power peak is per se, it's whether the engine can reach the maximum recommended RPMs with a given prop. Doesn't matter if HP has peaked at that RPM or not as long as it can spin that fast.

Have you ever looked at any of the dyno curves at GM Marine engines? There's a reason EVERY single one of them develop MAX POWER right in the middle of the recommended RPM range, and it stays within 5 or 10 HP of the peak for a couple of hundred RPM above and below. That's so if you prop your boat within the recommended RPM range, you will be garuanteed to be within a couple of MPH of maximum performance from your boat.

http://www.gm.com/vehicles/innovati...engines/specialized/marine/marine_engines.jsp

 

[QC]

Re: Is it just me or is the boat to slow

OMG, this thread has turned into a flippin' I don't understand anything about horsepower disaster. Please, please, please:

a) trust 45Auto, he knows of what he speaks.

b) read this thread:
http://forums.iboats.com/showthread.php?t=225803

c) If anybody brings up torque and doesn't calc to horsepower I'm gonna puke

 

[Summer Fun]

Re: Is it just me or is the boat to slow

I can't take this thread anymore !!.

 

[Philster]

Re: Is it just me or is the boat to slow

45 knows his stuff. I politely disagree with some of the nuances of paper vs real-world results regarding peak HP at given RPM and its affect on top speed.

_end of input for this thread.

 

[Matt89GT]

Re: Is it just me or is the boat to slow

you can have the best of the best top dollar parts and brands however it all comes down to the right combination if its not the right combo its not gonna run the best, its all gotta work together like the spokes in a wheel

 

[lilb]

Re: Is it just me or is the boat to slow

Well thanks to all of you who actucally had some input for me I am done wit this thread as it turned out to be like two old ladies bickering back and forth wit no help