picklenjim

Those props are for top end only. They are harder to get on plane and need a higher speed to hold on plane. There are other props like the Mirage which give all around better performance. Easier to get on plane, run slower with out falling off plane and still have a good top end.

picklenjim

Something isn't correct with your calculations .
53MPH,10% slip,1.5 gears,19 pitch prop would take 4900 RPM.

Here's a link to a prop calculator.
http://www.marksmarineinc.com/prop_calc.aspx

rchevelle71

iTrader: (1)

Maybe that is one of those negative slip props, I have read about them a few times here on OSO

I bet he had a valve issue or head gasket problem to begin with, and as others have stated, and the extra lug of the 21 just pushed it over the edge. Dont feel bad, we dropped a valve last weekend on the cat too, they are pulling the motor in the cat tomorrow(I will stop by for a bit and lend a hand), then come home and mount my motor on the Pantera so we have at least one boat running

35thls1ss19

piclkenjim i am very confused. just the other day before i blew it up i got 3200rpm with the 21 pitch and i gps 49mph. but on that calculator it does not work? can my tach be wrong? i am so confused. whats going on here.

also this is the second tach and a brand new one that i put on due to me thinking the tach was wrong.

picklenjim

It's either your tach or your GPS, I would lean more towards it being the tach.
Is there a switch on the back of the tach to set for the number of cylinders you have ? Could it be set wrong? Some tachs also have calibrations adjustments on them.Just a thought.
Regardless of what the gauges are saying I can tell you that boat with a stock 330 and a 21P prop should run mid 50's at 4500-4800 RPMs. Mercury manual states that max RPM for a stock 330 is 4600-4800. It sounds like your there if your GPS is correct. Either way you had some kind of engine problem, well we know you had a problem but that motor should have been able to run all day at WOT with that 21P prop.

If your really want to know how accurate your tach is you can check it with a photo sensor tach.
http://www.harborfreight.com/digital...ter-66632.html

Get the head fixed,put a 21P Mirage on it and go out and have fun. Should run mid 50's all day dependably. You start building up that motor and that Alpha is going to say so-long real quick.

35thls1ss19

i have used 2 gps's and they both have read the same.

i did check the back of the tach and it is in 8 cyl mod. i noticed when i put the 21 on it took forever to get on plane. idk i am lost, guess i cant look into it any further till i get it back together.

jeffswav

You must have a 1.36 drive ratio. I have seen that combo before. I have a buddy with a Pachanga 22, it has a 330BBC with a Alpha and 1.36 ratio, 19p mirage and his boat tops out about 58MPH. If you look on the drive by the label it is most likley stamped there. After you get it together try a Mirage 19p and see what that does.

35thls1ss19

what kind of rpm does he get?

also is there a way to tell what drive ratio by spinning the motor and counting the prop spin?

jeffswav

Normal RPM, 46-4800 RPM. I have not driven his boat so I do not know for sure, raced him many times back when my boat was much slower. Normal slip numbers would be 10-12%.

35thls1ss19

i mean by turning the motor by hand and watching how many times the prop spins around to tell what gears are in the drive? thank you for the help