this rpm sound right? took it out today..

[weider1717]

Re: this rpm sound right? took it out today..

Weider, dont buy anything yet. Something is wrong here, I show you ought to be using an 18" prop. Here is what I show for your boat, if it is wrong please tell me. I show the 1984 Dixie 19' weighing in at 2,700 pounds, I also show the mercruiser normally came from the factory with an alpha drive at 1.32 gear ratio. Although I am using your 1.5 gear ratio for my calculations. I show the total weight you are running with at about 3,700 this includes 40 gallons of gas 3 people and various normal items. At that gear ratio and weight I show the boat ought to be doing 62 MPH at 5,700 RPM with 10 % slip. I show that with an 18" Pitch prop you ought to be doing about 47 MPH at 4,600 RPM, and my formulas are usually right at what reality is, unless something is very different than what is stated. Look at the Capacity tag on the boat it should show the weight of the boat as well as what total weight you can put in the boat.
You say you have a spare 19" stainless spare prop, try that before you purchase any new prop. Also have that tach checked out. Something doesn't add up.
In theory if you are actually doing 5,700 RPM with a 19" prop you should be doing over 62 MPH at 10% slip. With a change to a 21" Pitch it would bring your RPM down toi about 5,200 RPM and still doing 62 MPH.
Just borrow a handheld GPS for speed or borrow someones phone that has GPS software.

H

boat is docked at my uncles so can't view the tag but 2700ish lbs soudns about right..

this might help tho, the gas tank is a 25 gallon tank and only I was in the boat when I saw it on 5700 rpm.

but the rpm range for my boat says 4200-4600 rpm so wouldn't running it at 5700 rpm be bad on the engine?

 

[QC]

Re: this rpm sound right? took it out today..

Also have that tach checked out. Something doesn't add up.

Don't do ANYTHING until you confirm the tach . . . Garbage in, garbage out, you sir, have bad data

 

[krisnowicki]

Re: this rpm sound right? took it out today..

also when you confirm your tach . Take down your tach and speed ratings, so like 3000 rpms 29 mph and 3500 35mph. You can use this info to see if you have alot of slip or a bad hub and pin point the problem. But, the tach must be checked first. Use a gps to check your speed the speedo on the boat is worthless.

 

[weider1717]

Re: this rpm sound right? took it out today..

I agree guys..but 2 questions which are probably stupid but...

how does one of these shop tachs hook up? how do you install it to look at your rpms?

also is there a video showing how to replace a prop? never done it.

 

[MikDee]

Re: this rpm sound right? took it out today..

I agree guys..but 2 questions which are probably stupid but...

how does one of these shop tachs hook up? how do you install it to look at your rpms?

also is there a video showing how to replace a prop? never done it.

A shop tach usually you just clip one wire on the small (negative -) terminal on the coil, (The one going into the distributor). You might have a Tach with 2 more wires, a red, & a black, to power it up, off the respective terminals on the battery.

To take your prop off, you simply pry away the flat lockwasher tabs from the base, then while holding the prop with gloves, take off the big nut with the proper size wrench (lefty loosey, righty tighty), slip the prop off then, and make note of any spacer washers, above or below it. Put the washers back on, where they belong, and then slip the prop back on, then tighten the nut as
tight as you can get it, while still holding back on the prop by hand, this will be just right, not too tight, or too loose, then line up the slots in the lock washer to it's base, and bend over a few tabs. Thats it!

 

[weider1717]

Re: this rpm sound right? took it out today..

not sure what the issue was but later in the summer i opened it up a couple more times this time with my megellan car gps hooked up and the tach read pretty much dead on 5,000 rpm each time and i topped out on the gps at 51mph each time.

maybe something needed exercising such as the rubber swelled back up on the prop or the tact indicator...bc these numbers seem about right..

anyone know where a place online is to get a decent deal on a 21 pitch?

 

[marlboro180]

Re: this rpm sound right? took it out today..

"anyone know where a place online is to get a decent deal on a 21 pitch?"

Scroll right up on top of the page of "iboats.com"

 

[QC]

Re: this rpm sound right? took it out today..

That's more like possible/reasonable, thanks for the update. How quickly does it plane ? Based on what you are telling us a 21 shoudl ve faster and the current setup should jump onto plane . . .

 

[Rumblefish]

Re: this rpm sound right? took it out today..

I just recently went through a similar situation where my WOT RPM was all of a sudden running about 1000 RPM over the usual 4600 RPM Max. I suspected a possible spun hub and brought the prop in to the prop shop and sure enough it turned out that the rubber hub was slipping on the original 14 year old stainless steel 3 blade prop

They rehubbed the prop and I am now back in business with a WOT 4600 RPM max


...Glenn

 

[Maclin]

Re: this rpm sound right? took it out today..

Welcome aboard Rumblefish! Check your PM's here....

 

[weider1717]

Re: this rpm sound right? took it out today..

looking for a 21pitch prop now.

question is there is one on ebay thats 4 or 5 blades but it is 21 pitch would this bring my rpm's down and give me a couple mph like a 21 pitch 3 blade would? and have better hole shot too?

or no and just stay with a 3 blade.

 

[intrepidvoyager]

Re: this rpm sound right? took it out today..

1 ....... if you really where running over 5500 RPM on a non bluprint 350 CID you would have blown the engine

2 ....... you cannot tell the right prop for your boat without having an accurate tach and speedo or a marked mile range.

3 ....... once you get your tach and speedo fixed go to a prop shop ...they will be happy to sell you the right prop and will give you loaners so you can get the right pitch so you are at 4400 RPM at WOT.

4 ...... the amount of RPMs you change per inch of pitch depends on a lot of factors ...... prop diameter, engine torque curve, hull design, etc ....so best to trial and error the solution .......start with 15 X 21 and work down. BUT get the tach working first....... you can also use a test tach ......around $20 at an auto parts store.

RE WOT RPMs on 350s ........you may find info that you can run these engines at a higher RPM ....... but you are starting to push the envelope, particularly on older engines with a lot of hours. If you need more on this or want a high performance marine blueprint engine call Jack at Prospeed in Vancouer, B.C. He is the top performance marine engine builder on the west coast.......

 

[weider1717]

Re: this rpm sound right? took it out today..

1 ....... if you really where running over 5500 RPM on a non bluprint 350 CID you would have blown the engine

2 ....... you cannot tell the right prop for your boat without having an accurate tach and speedo or a marked mile range.

3 ....... once you get your tach and speedo fixed go to a prop shop ...they will be happy to sell you the right prop and will give you loaners so you can get the right pitch so you are at 4400 RPM at WOT.

4 ...... the amount of RPMs you change per inch of pitch depends on a lot of factors ...... prop diameter, engine torque curve, hull design, etc ....so best to trial and error the solution .......start with 15 X 21 and work down. BUT get the tach working first....... you can also use a test tach ......around $20 at an auto parts store.

RE WOT RPMs on 350s ........you may find info that you can run these engines at a higher RPM ....... but you are starting to push the envelope, particularly on older engines with a lot of hours. If you need more on this or want a high performance marine blueprint engine call Jack at Prospeed in Vancouer, B.C. He is the top performance marine engine builder on the west coast.......

I think you missed this post i made later in the thread

"not sure what the issue was but later in the summer i opened it up a couple more times this time with my megellan car gps hooked up and the tach read pretty much dead on 5,000 rpm each time and i topped out on the gps at 51mph each time.

maybe something needed exercising such as the rubber swelled back up on the prop or the tact indicator...bc these numbers seem about right..

anyone know where a place online is to get a decent deal on a 21 pitch?"

 

[intrepidvoyager]

Re: this rpm sound right? took it out today..

OK ....got it ...... here is a boat test comparison .....

Caravelle?s Interceptor 192 measures in at 19 feet 6 inches overall, with a beam width of 8 feet. She weighs in at about 2,900 pounds and has a fuel capacity of 30 gallons. I tested this model on Lake Blackshear in southern Georgia with a MerCruiser 260-hp sterndrive. She was quick to plane in just 3 seconds and was at 30 miles per hour in just 8 seconds. I found her cruise speed to be around 33.5mph which will take you about 137 miles on a full tank of fuel. Wide-open-throttle performance was very good at 54.2mph.

Don't know the pitch but you can get from the manu.

HOWEVER , I think your tach is off a bit ......

you may get a better deal on line but watch the shipping charges and if you have a problem with the prop ...what then ??

 

[weider1717]

Re: this rpm sound right? took it out today..

Yet another update on this thread.

I bought a mercury veageance 21p prop off ebay and a flo torq II hub kit.

When the warm weather comes I'll get that mounted up and report back on the rpm and mph readings.