A quest for speed...

[Mischief Managed]

Re: A quest for speed...

. Plus the low end tq would make it "feel" faster. Tq is a good thing.

I seriously doubt you'll have any increase in torque, since it's reallya function of engine displacement if you keep the same cam. A prop change to take advantage of the new found HP will likely result in a slower holeshot too...

 

[SS194]

Re: A quest for speed...

Did you guys read the links posted to people who have actually swapped to the 4bbl? Unless everyone who has done this swap is full of it, I'm going to believe them...

Steve

 

[45Auto]

Re: A quest for speed...

Take what people post on the internet with a grain of salt. Be happy with what you get in your pursuit of greater performance and enjoy your boat. Don't be disappointed if your boat doesn't do what others CLAIM theirs will. There's some posters (wca_tim and myleslem (sp?) come to mind) who make legitimate claims to their boat's performance increases, but it's obvious from the history of the contents of their posts that they have some experience and performance background and are NOT just throwing on a few bolt-ons and hoping for a miracle.

Did you look at any of the Boattest.com test results?

Length - Brand ---- HP -- Top Speed
17 ----- Larson --- 190 -- 50
18 ----- Caravelle - 190 -- 51
18 ----- Sea Ray -- 190 -- 48
19 ----- Bayliner -- 220 -- 53
19 ----- Glastron -- 220 -- 53
19 ----- Caravelle -- 260 -- 54

Your boat should currently have the 190 HP V6. Figure you'll probably be around 220 after your mods. Be happy if you get 55 MPH (make sure you're running downwind and with the current).

 

[indy440]

Re: A quest for speed...

Rule of thumb I was told:

"50% increase in power can equal a 15% increase in top speed."

Of course there are exceptions to every rule, but I take this as a pretty good starting point.

 

[superpop]

Re: A quest for speed...

a 4V carb will give you better hole shot but that is about it. The biggest impact you can have on top speed is the prop. I had a 1996 Wellcraft Eclipse 20 footer with a Vortec V6 and Holley 4V, it was a pretty heavy boat but with a 23P prop on it I could do 56MPH GPS, but my hole shot was really bad, so I went to a 18P prop and had great holeshot and top speed of only around 46MPH which is still plenty fast for a boat. Top speed is dictated by torque more than HP since it takes a lot of torque to spin a bigger prop faster. Trimming the boat up out of the water helps too but is only good for maybe a mile or two in speed.

 

[_chris_]

Re: A quest for speed...

a 4V carb will give you better hole shot but that is about it. The biggest impact you can have on top speed is the prop. I had a 1996 Wellcraft Eclipse 20 footer with a Vortec V6 and Holley 4V, it was a pretty heavy boat but with a 23P prop on it I could do 56MPH GPS, but my hole shot was really bad, so I went to a 18P prop and had great holeshot and top speed of only around 46MPH which is still plenty fast for a boat. Top speed is dictated by torque more than HP since it takes a lot of torque to spin a bigger prop faster. Trimming the boat up out of the water helps too but is only good for maybe a mile or two in speed.

When I am at WOT I am doing 42 MPH in my Four Winns and if I trim up I can get up to 48 MPH which is a 6 MPH gain from just trimming up. My Glastron could only gain about 2 MPH so I think it just depends on the set up. I would do the carb because it is known to help and then do the prop and thats it.

 

[QC]

Re: A quest for speed...

Top speed is dictated by torque more than HP

This is false . . .

it takes a lot of torque to spin a bigger prop faster.

and this is kinda true. But I would say it takes more torque to actually get a bigger prop turning.

HP is what determines top speed. If all else is equal, higher low end torque improves hole shot with the same prop, or allows OK hole shot when going to a higher pitch. But top speed of any motorized vehicle is dictated by horsepower.

I always post on this because it is so misunderstood. HP = Torque x RPM / 5252 Which also proves that more torque at a given RPM results in more horsepower as long as you maintain that same RPM. But torque is useless without RPM because then you've got no horsepower which is what you actually pay for, both with your initial purchase and fuel.

 

[xxturbowesxx]

Re: A quest for speed...

I'm not exactly saying I want to go 100 MPH with this boat.

I like this boat, got it for a great deal. I also am impressed with the engine.

Some simple mods to free up a little power isn't worth selling the boat over and getting one that's made only for going fast. If I had the money, I'd have two boats.

Steve

I am just saying that speed is addictive once you gain 3-4 mph you will want more. once you get to around 55 figure about 1k per mph gained. If speed is what your after(even 55-60mph) then a hull designed for speed would be the most efficient way to do so. I started out with a 50mph boat as well. Ive gone from a 50mph boat to 85 then back to 60.

Dont get me wrong make improvements like smart tabs and so on but I wouldn't modify for speed.
I would take a long look at your boat smile then plan fer some boating.

 

[werthert]

Re: A quest for speed...

So.... say a thousand bucks for *maybe* 5 more HP.... that NOBODY but you will notice. Plus the permanent increase in fuel costs every outing... Huh?

Sorry, someone has to play devil's advocate. That said, it's your money and your boat.... if you got the funds and that's what makes you happy, all the best.

 

[SS194]

Re: A quest for speed...

Some of you guys talk like owning a boat is actually practical. Probably just as practical as my motorcycle. It's a hobby, and I get a lot of enjoyment out of tweaking things, making them my own. Small improvements that make it my own. If someone asks me what I've done to my motorcycle, I'd have a lot to tell you, but most people would have no idea. I don't do these things to show off or to impress other people.

Anyway, doesn't matter. Like I said, it's kind of a hobby.

I have dreams of dropping a built turbo diesel engine into a speed boat. 1000 ft lbs of tq and 400 hp would be a lot of fun! All the while sipping fuel.

I'm done. I realized I have a 23 pitch prop on the boat. Beat up al. 23 prop. Next week I'm going to get a working tach and see what she's doing.

Steve

 

[marlboro180]

Re: A quest for speed...

Spend the dough and have fun!!!!

 

[dg512055]

Re: A quest for speed...

My used Wellcraft Excel 19 SSX will do 52 on GPS, no wind or current with 3 adults and nearly full fuel. It has the 4.3LX with Alpha1. I am running a Mirage 21P SS prop. Trimming up is a must to get any speed out of this boat. It has the stock 2bbl.

 

[Larson1]

Re: A quest for speed...

Great thread guys! I want to add what my boat does and it looks like it will not be the norm.

2001 Larson 190LXi 4.3L Volvo Penta 110hrs

I just purchased this boat about a month ago. Have owned and run boats for 25 years....mainly outboards.

Top speed on the boat just touches 60 mph with the RPM's at around 5,600 rpms. (Garmin GPS verified)

I was very impressed that it could do this. After researching other boats this size and power it looks like it performs better than most. Not sure why but I would like to find out. I have seen that most 4.3's have a top RPM of 4800 (mine is 800rpms higher). Most have a top speed of 48-52 mph (mine is right at 60). Trimming up a little did help it (maybe 4 mph)

Should I be concerned that the RPM's are so high? Should I get a higher pitch prop to bring down the rpm?

 

[Larson1]

Re: A quest for speed...

Also will the high rpms hurt the engine?

 

[45Auto]

Re: A quest for speed...

Top speed on the boat just touches 60 mph with the RPM's at around 5,600 rpms. (Garmin GPS verified)

I was very impressed that it could do this.

You may want to verify the calibration of your tach and/or GPS. Of course it?s always possible that a previous owner installed the ?extra 100 HP spark arrester?, but this isn?t real common in my experience.

Horsepower peak of your engine is right at 4800 RPM. From the GM marine engine site:

If you?re really spinning 5600 RPM, you are well past the HP peak (about 10% down on HP) and are putting less HP into the prop than you would be at lower RPM. Propping the engine so it is spinning at it?s HP peak will give you the maximum speed possible in your boat, so a higher pitch prop will make your boat even faster.

You?ll usually find that if you are running the exact same combination as everyone else, and you think your junk is 10 ? 15 MPH faster than everyone else, then something is wrong with your readings ??..

Also will the high rpms hurt the engine?

Mercruiser recommended peak RPM for your motor is 4400 ? 4800 RPM, per Mercruiser Manual 25, 1998-Up V6:

Spinning your motor past the recommended RPM is costing you power and speed, and adding wear on your motor as well as risking engine damage from floating the valves by running at higher RPM?s than the valvetrain is designed for. Your outdrive is also not designed to spin over 5000 RPM, they have a reputation for coming apart at higher RPMs.

 

[Larson1]

Re: A quest for speed...

You may want to verify the calibration of your tach and/or GPS. Of course it?s always possible that a previous owner installed the ?extra 100 HP spark arrester?, but this isn?t real common in my experience.

Horsepower peak of your engine is right at 4800 RPM. From the GM marine engine site:

If you?re really spinning 5600 RPM, you are well past the HP peak (about 10% down on HP) and are putting less HP into the prop than you would be at lower RPM. Propping the engine so it is spinning at it?s HP peak will give you the maximum speed possible in your boat, so a higher pitch prop will make your boat even faster.

You?ll usually find that if you are running the exact same combination as everyone else, and you think your junk is 10 ? 15 MPH faster than everyone else, then something is wrong with your readings ??..



Mercruiser recommended peak RPM for your motor is 4400 ? 4800 RPM, per Mercruiser Manual 25, 1998-Up V6:

Spinning your motor past the recommended RPM is costing you power and speed, and adding wear on your motor as well as risking engine damage from floating the valves by running at higher RPM?s than the valvetrain is designed for. Your outdrive is also not designed to spin over 5000 RPM, they have a reputation for coming apart at higher RPMs.

Thanks for the great info! Does it make a difference if it is a Volvo Penta? This is not a Mercruiser engine and outdrive.

The same GPS is within 1 mph of my trucks readings so I know it works. I need to get another tach and verify the rpms.

 

[DavidW2009]

Re: A quest for speed...

I regularly pass a boat parked in a fenced side yard that is incredible looking.

I don't know what brand boat it is, but it looks fast and expensive.

It's about 26' - 28' long, has two distinct hulls joined by a helm, 4-place passenger compartment, like a motorised catamaran, powered by (2) 300-HP O/B Mercs, one on each hull. All on a multi-axle trailer.

The whole rig looks almost new. It looks fast. I'm thiking 60-70 mph(??).

 

[Snobike Mike]

Re: A quest for speed...

Of all of the things you've considered doing the ONLY thing I would do is put a SS prop on it. That will give you a gain if you pick the correct prop and then you leave your engine stock and nice and reliable.

You know what the say: Fast, cheap, reliable. Pick any 2.

Have fun!

 

[wizbang 13]

Re: A quest for speed...

I got a 20% increase in speed for FREE by changing my gps to mph from knots!

 

[indy440]

Re: A quest for speed...

I regularly pass a boat parked in a fenced side yard that is incredible looking.

I don't know what brand boat it is, but it looks fast and expensive.

It's about 26' - 28' long, has two distinct hulls joined by a helm, 4-place passenger compartment, like a motorised catamaran, powered by (2) 300-HP O/B Mercs, one on each hull. All on a multi-axle trailer.

The whole rig looks almost new. It looks fast. I'm thiking 60-70 mph(??).

Of course I have no idea what it is, but a catamaran with two 300Xs is probably faster than that... more like 110+ I'll bet...