still confused about RPM/Speed

[blylock]

I have a 2002 Wellcraft 21' , 5.0merc carburated, 220hp.<br />i believe my prop is a 14.5 with a 20 pitch, aluminum. i cruise alot with my buddy, he tops out around 60mph, im about 48mph@4800 rpms, which is according to my manual, is max rpms for that motor. i usually pull only tubes, or cruise around. what are my options to increase my speed? i also have an alpha one drive. any help would be appreciated. I understand the increase in pitch lowers the rpms, but does more pitch mean more speed, and can you adjust the throttle to compensate for the lower rpms with more pitch. i havent checked, but i believe when im WOT, i dont think my carb is actually wide open.<br />and i have read around alot of posts, but im still confused

 

[Don S]

Re: still confused about RPM/Speed

What kind of boat does your buddy have, and what engine and drive combination.<br />You may be comparing apples and oranges.<br />All 21 foot boats are not the same.

 

[waterone1@aol.com]

Re: still confused about RPM/Speed

When you put the control to full speed, the carb should be at full throttle. To check this, (with the boat stopped and not running) put the control to full throttle, now go to the carb, disconnect the cable and see if the carb will move any farther open. If not, you're done. If so, readjust the cable trunion to give you full throttle. After checking that it will still go all the way back to idle, put the boat back in the water and see if you gained any rpm's, if you have you will need to get a bigger prop to lower the rpm's and make the boat go faster. The most important thing is to use the prop pitch to keep the engine in the manufacturers recomended rpm range at wide open throttle. As Don said, unless your buddy has the same boat, engine and drive and the same load and weight, you cannot compare your boat to his...it's apples and oranges.

 

[blylock]

Re: still confused about RPM/Speed

ill try that. we do not have identical boats in the least, im just sick of playing catch-up on our cruises.

 

[Bondo]

Re: still confused about RPM/Speed

im just sick of playing catch-up on our cruises.

You Need a New Boat................<br /><br />The 1 You've Got,...... Is Never Going to See 60mph..............

 

[NateCCIE]

Re: still confused about RPM/Speed

My 25' cuddy with a 415 HP 502 mag doesn't even see 60. (at elevation)

 

[Don S]

Re: still confused about RPM/Speed

First of all, you are going to need a 21' boat with a lot flatter bottom than the Wellcraft. That danged ole V hull may ride nice through the waves, but it sure kills the speed. Then get rid of that sickly ole 5.0L engine with a carb.<br />Now, run down to the Volvo store and get you a 8.1/DP big block, MPI, Mega horse (420 propshaft HP) and sit down in that 21' flat bottom boat, get a couple Hi-Performance choper props for that DP and man..... You will blow his doors off. Without having to say you have a race boat or race engine...... Sweeeeeeeet

 

[rodbolt]

Re: still confused about RPM/Speed

DonS<br /> now that would be a ride. talk about waking up an old wellcraft. I repowered a 28 carolina clasic with a pair of 8.1's and it will top out at just over 50 MPH on a GPS. thats with a flybridge and the outriggers in the upright position. so far the only complaint the guy has is the throttles are to sensitive when doing dock side manuevers. there is not much throttle lag like his old carbed 7.4's.

 

[Manipulator]

Re: still confused about RPM/Speed

My 83' SeaRay 225 5.7L cuddy doesn't even see 48mph I'm lucky to hit 40mph!<br /><br />Again, what does your buddy have? Boat, Model, and setup etc.....

 

[TilliamWe]

Re: still confused about RPM/Speed

I agree with all that say your boat will never hit 60mph. But it could do a little better than the current 48 with just the right prop.<br />You state that your current prop is a 20" pitch... is it a four bladed prop? Usually three bladed props are odd numbered pitched. Unless you have a "Turbo" brand prop. If you have a four blade 20", try switching to a 21" three blade stainless, you may get a mph or 2.<br />Check your carb the way waterone1 says, make sure you are truly getting "full throttle", but I suspect that you are. Good luck

 

[blylock]

Re: still confused about RPM/Speed

my prop size is a 14.25 x 21. i was mistaken, yes a 3 blade.

 

[Doug Durako]

Re: still confused about RPM/Speed

After 4000 rpm, all your speed comes from the hull, or to be more specific, how much of the hull you can get out of the water. Most refer to this as the "pad" of the boat. Faster boats have smaller pads.<br /><br />I can gain 20 mph between 4000 and 4800 rpm, but if I try to turn the boat at 63 mph, I am dead, literally.<br /><br />Behind Cobalt, Wellcraft and Sea Ray are two of the heaviest runabouts on the water.

 

[TilliamWe]

Re: still confused about RPM/Speed

ddboater, where abouts in the beautiful Illinois River Valley are you located?

 

[Richard Petersen]

Re: still confused about RPM/Speed

2002 Chaparral 18.5' 5.0L 220hp / CLEAN bottom out of storage, 14.25" X 21"p X 3 blades Alum. does 54 @ 4900 rev limit every 2 sec. Run weight is 3600#. 2 months of LIGHT green slime is down to 48 mph. Planning pad is VERY touchy to any slime build up. Causes cavitation blow outs.