55.1 mph with 30 hp mercury

[zul8tr]

And I am gonna be fully honest, the motor is a 30 but its actually a detuned 40, being restricted with a small Venturi and smaller reed holes! So you put the 40 parts on and add some timing and these motors scream! And to be honest I had issues hooking up a tach so I am basically using the prop calc to estimate my rpms! The hard fact is even if it is hard to believe, my motor is that fast and right now I can even beat a few 59 ci mercurys around here!

What is a prop calc?
The speeds you are going realisticly means that the rpm exceeds about 7100 as I estimated in a previous post. You really need to get a tach. The video sound of the engine does not seem to sound like 7100+ rpm?
Why do you have it restricted with smaller venturi and reed holes?

[tyst67]

Nice! I bought one last year and I never thought to try both wires I couldn't get to read so I put it directly on the plug tip and then put the plug wire over it lol it worked for a sec but I ended up frying it lol I want another one now ! I'm an idiot for not trying both wires first

[tyst67]

What is a prop calc?
The speeds you are going realisticly means that the rpm exceeds about 7100 as I estimated in a previous post. You really need to get a tach. The video sound of the engine does not seem to sound like 7100+ rpm?
Why do you have it restricted with smaller venturi and reed holes?

So my motor came with a restricted read valve and A restricted Venturi! I changed it from that To a bigger read valves and bigger carb! And like I said I do not have a tach And I am using the prop calculator to estimate my RPMs! Did you ever think that maybe my boat is a little more efficient than 10% slip! Now I have a couple questions for you! Have you ever heard of twin Mercury like mine rev between 6500 and 7000 without the exhaust drilled out ! My exhaust comes out in the water! I race against a lot of big Johnson and Evinrude outboards And they all drill the hell out of their exhaust, They sound like they are reving 8 grand but realistically it is only the loud noise that is getting peoples attention! People say wow look at that go, "it is some fast" but actually they are slower than me but the loud noise makes them look to be flying! As well this video was taken from a cell phone and the speaker on the cell phone is probably not as good as a video camera! But I don't think that I am reving 7100 anyway I think I am running around 6700 -6800 with the less slip ! In the meantime I will try to put up better videos that demonstrates the speed and hopefully you will get to hear my motor better

[Fastjeff57]

GPS readings an trick you. If the river was running 5 or 6 knots to the right (in the video) that would appear to make the boat that much faster. Running upstream (to the left) would then yield a much lower GPS speed.

I hate those miserable pitot tube speedos, but at least they give you water speed; a GPS gives you ground speed, ignoring the effects of current.

Jeff

[zul8tr]

So my motor came with a restricted read valve and A restricted Venturi! I changed it from that To a bigger read valves and bigger carb! And like I said I do not have a tach And I am using the prop calculator to estimate my RPMs! Did you ever think that maybe my boat is a little more efficient than 10% slip! Now I have a couple questions for you! Have you ever heard of twin Mercury like mine rev between 6500 and 7000 without the exhaust drilled out ! My exhaust comes out in the water! I race against a lot of big Johnson and Evinrude outboards And they all drill the hell out of their exhaust, They sound like they are reving 8 grand but realistically it is only the loud noise that is getting peoples attention! People say wow look at that go, "it is some fast" but actually they are slower than me but the loud noise makes them look to be flying! As well this video was taken from a cell phone and the speaker on the cell phone is probably not as good as a video camera! But I don't think that I am reving 7100 anyway I think I am running around 6700 -6800 with the less slip ! In the meantime I will try to put up better videos that demonstrates the speed and hopefully you will get to hear my motor better

Prop slip is a necessary for the blades to produce propulsive force and is a natural result of the hydrodynamics of flow around the prop. It is impossible to have a blade with zero slip by virtue of the fact that the blade is screwing thru a viscus medium-water and can't advance without some slip. A pitch of 18" theoreticaly means that if there were no slip the propellor would advance 18" forward in one revolution. A 10% slip is very reasonable for your setup but in order to determine the real slip for your prop and setup you need to get very accurate time and distance speed and engine rpm and calculate the slip via the method I described above.

There is much info on propellor hydrodynamics and the slip concept from the very simple discussions to the research papers so have at it if you want to know.

[MWhite]

GPS readings an trick you. If the river was running 5 or 6 knots to the right (in the video) that would appear to make the boat that much faster. Running upstream (to the left) would then yield a much lower GPS speed.

I hate those miserable pitot tube speedos, but at least they give you water speed; a GPS gives you ground speed, ignoring the effects of current.

Jeff

What ever that gps tells u that u are going is what your going, doesnt matter if your on land are water, gps dont lie.

[MHARDEE]

Make sure you are 2to 1 gear like you stated ,if your 1.84 to one your rpm would be less but still be running said speed just at lower rpm .

[tyst67]

That's actually a good point! I'll check

[HGipson]

Those are all some pretty good numbers if they are right. We had a 2003 40 3 cyl mercury stock on a 1442 Monark flat bottom and it would only run 35-36 mph. Boat only weighed about 200 lbs dry plus driver and motor. Also have a 50 4 cyl classic ported and no rev limiter and it'd run about the same. Now I've got a 60 Etec on a 1546 Alumacraft semi v and it only runs about 41-43. So if that stock Merc runs that fast on a similar boat y'all got some world records with your name on em somewhere. Seen countless number of those motors run and never seen one hit 50 with my own eyes unless it was on a baby racing boat with a 10 yr old driving. Just my two cents though I'm sure it's possible but I've never seen it in my 20 yrs of running big tiller motors

[zul8tr]

What ever that gps tells u that u are going is what your going, doesnt matter if your on land are water, gps dont lie.

Well while what you say would seem correct you must know that there are errors in the gps recorded speed that are dependent on the the type of receiver and the way the signals are processed and a whole host of other effects. The typical gps units for public consumption usually have limited amount of signals that can be precessed in 1 sec. They also work most accuratly in a straight line. The speed is usually calculated by the difference in receipt time of the satellite signals at each distance point, then calculations of speed between the points is calculated. There is error with this approach that results in a +/- in the mph.

Here are some details if you want to wade thru it

http://en.wikipedia.org/wiki/Error_a...tioning_System

Things are not always what they seem. Best is distance and accurate time measurements to calculate avg speed over the distance.