this rpm sound right? took it out today..

[weider1717]

broke the pitot tube off loading it last year..

anyway the boat is a 19ft inboard/outboard ski boat. It has a mercruiser GM 350 with 260hp.

I think the prop on it is a 19p

anyway with me only in it and a full tank of gas and fairly calm water I opened it up and got 5700 rpm...does that sound about right or is there more there?

I might coulda played with the trim a bit more but it seemed pretty good at where I had it when it reached 5700 rpm.

I remember when the speedo worked this boat was a 50 something mph boat but thats going off my memory of being in it as a kid..so who knows what it is exactly and by gps but I was curious if that sounds about right for my boat.

thanks

 

[boatguya1]

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

Look on the plastic flame arrestor cover, there should be a sticker and one line should say WOT rpm range and giva a range (WOT=wide open throttle). On older units the sticker might be on a valve cover. 5700 is high for most small blocks that merc uses.
James

 

[boatguya1]

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

If the tach is correct you need more prop if you want to run wide open.

James

 

[dgopetactical]

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

what year is you 350?? 5700 seem way to high to me. Ive done alot of reasearch for my 350 and and I see them calling for 4200-4600 wot. Go up to the adult only thread at the top of this forum and download the manual for you engine your wot should be in there..
Denny

 

[weider1717]

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

the boat is a 1984 model...and I remember seeing that red sticker on the driver side valve cover so I'll look.

hope I didn't hurt anything by giving it a nice hard run yesterday! but I think it's always ran a 19p prop.

would a 21p prop be a lot better choice? would my speed increase you think? thanks

 

[bigbob_FTW]

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

if memory serves me, every size up decreases rpm 400

 

[boatguya1]

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

the boat is a 1984 model...and I remember seeing that red sticker on the driver side valve cover so I'll look.

hope I didn't hurt anything by giving it a nice hard run yesterday! but I think it's always ran a 19p prop.

would a 21p prop be a lot better choice? would my speed increase you think? thanks

See if you can borrow a 23p to test. You'll go faster but have less take off for pulling up skiers. A shop tach would be nice to verify you're getting accurate readings.
James

 

[Don S]

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

Get a shop tach and verify your tach is reading right. No way are you going to change 1000 rpm with the props. Did you by chance change the outdrive?

 

[weider1717]

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

Get a shop tach and verify your tach is reading right. No way are you going to change 1000 rpm with the props. Did you by chance change the outdrive?

how woudl a shop tach hook up? where woudl i buy one at?

and no the outdrive is the original outdrive and it is a 1.50 ratio.

 

[boatguya1]

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

Did the tach in the boat get changed? Maybe it's not set on the correct # of poles.

James

 

[Don S]

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

Depends on the type tach you buy. Most automotive store will carry different type of shop tachs.

 

[weider1717]

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

looked at my manual seems like a 19p prop is recommended as one of the prop pitches for my boat.

for quick reference what would be a normal idle rpm for my engine? and if I take it out tomorrow like I plan to I'll look and see what it idles at.

 

[dgopetactical]

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

looked at my manual seems like a 19p prop is recommended as one of the prop pitches for my boat.

for quick reference what would be a normal idle rpm for my engine? and if I take it out tomorrow like I plan to I'll look and see what it idles at.

750-800 rpm in the water in forward gear
Denny

 

[wca_tim]

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

That engine should sound like it's on it's last breath and ready to fly apart at 5700... way past the power band and the design for the valve springs, etc... unless you have a spun hub on the prop, surely the tach crapped out or the like as everyone is suggesting.

If it ran with this prop and was fine before... something's not right so repropping isn't the right thing to do.

 

[weider1717]

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

not sure man it was my grandpas boat I just happened to take a look at the rpms since the speedo pitot tube was broken off last year..and noticed it was 5700 rpm topped out...I thought to myself 5700 rpm would be a lot since it's probably what a 6000 rpm redline engine..not sure whats the redline on the tach i'll have to look.

so the prob spinning some would cause this too huh? the boat performed as normal even had my dad with me who has always maintained the boat from day one from back when my grandpa bought it new in 84.

he was like ya sounds as good as ever..then I dropped him off at the dock and went to see how fast i could get it up to and saw it reached 5700 rpm...was about 5600 rpm with me and him both in it.

not sure on this one guys...but boat runs fine? hope I didn't damage the engine by topping it out for 30seconds or so those couple of times! that is if it actually was seeing 5700 rpm.

 

[weider1717]

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

ok guys took it out today..looked at the red sticker on the valve cover says rpm range is 4200-4600..

put it in forward gear just so it putted along and it ran at about 650rpm anywhere from 650-700...

again floored it a few times saw it strongly was going to pull over the 5,000 rpm mark so backed it off...

I have no clue? is it possible there is like a 17 pitch prop on it or something? I swear I remember see "19p" written on the prop thats on it I assumed that meant 19 pitch

again the engien is a mercruiser 350 260hp engine and the outdrive is a 1.50R mercruiser MR foot.

all is original bc were the original owners..

could I just have too much prop and need to go to a 21 or 23 pitch prop?

 

[MikDee]

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

If thats the original prop thats been on the boat for years, maybe the rubber hub is dryed out (rotted) and it's slipping, letting your engine run wild, usually 4600 rpm is max allowable range for a marine small block chevy.

On another note, I would think maybe a 21" pitch prop would work best for that setup.

 

[boatguya1]

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

Maybe grandpa has it underpropped on purpose so he can pull up a 333lb slalom skier.

James

 

[weider1717]

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

Ok I will look into getting a new prop tomorrow!

Will you guys please take the info I provide below and give me some advice on prop pitch?

boat info:
make: dixie
engine: 1984 mercruiser GM 350/260hp
length: 19ft
type: I/O runabout ski boat, open bow
outdrive: 1985 mercruiser MR 1.50R ratio
current prop: 19 pitch ss

again with the current setup it turns 5650-5700 RPM...not sure on the speed as the pitube is broke.

not sure on the dry weight of the boat..a video link is below so you can see the boat it's not a heavy boat IMO It's somewhat a shallow running boat so it's not too heavy..I know a 20ft searay I used to have was A LOT HEAVIER than this 19ft boat.

with the giving info what is a good prop for me? 21 pitch or what? then you get into cupped or not cupped etc..which I have no clue about.

I have a spare like new ss 19 pitch prop maybe a boat place will take it under trade..

if I go get say a 21 pitch prop and it should obviously bring my rpm's down some but would it bring it down from 5700 to 4600? by going with a 21 pitch and it lowering my rpm's will i lose or gain speed giving my setup?

thanks for any help

http://s22.photobucket.com/albums/b347/weider1717/?action=view&current=MOV00511.flv

 

[hwsiii]

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