23 septre i.o.prop size info

[joehat70]

bought 73 septre i.o.with 350-260 hp engine swinging a15.17 alum prop .with 280 volvo drive engine only turns 3850 rpm any body with simaler set up please help.need more rpm.

[1bayouboy]

Is that puppy running on all 8??? I'm not sure what the WOT range is on that engine, but if I had to guess I'd say
4400-4800 so 3850 is really low. I would also thing a 17 pitch prop would be close to correct for that boat. I have a heavier boat with a 300HP mercruiser turning a 15 pitch Rev 4 up to 5150RPMs. It's a great prop to keep you on plane at lower speed.

[BigLew]

IT is funny this topic should come up. If I've got the same question and the same configuration. I've held off a asking the question because I plan to repower soon, and will likely try and put in a 350 CID Vortex putting out 310-325 horsepower.

One thing not mentioned previously is that some of these boats have tanks holding as much as 145 gallons (mine for example). That is nearly 900 pounds of fuel. Also, in my case, the aluminum prop is cupped. However, I believe mine has reached 4000-4100 RPM. The above mentioned target WOT RPM range is accurate though.

As I said, I will be a REAL interested in the results of this.

[BigLew]

Fr. Frank,

Where are you when we need you? Please bless us with your encyclopedic knowledge of all things having to do with marine power.

Bless you brother!

[1bayouboy]

My sentiments exactly of Fr Frank....

I can tell you his usual is max should be max at max....

Meaning your max rpm should hit the max end of the range when you boat is max loaded out. I propped mine accordingly and could over rev it when lightly loaded if I tried or come very close. The max rpm on the 350MAG Horizon is an insane 5200rpms. The only time I come close is a brief WOT run every other time out.

[Fr. Frank]

Two possibilities: Either the motor is not developing full power, or you need a different prop. First make sure your motor is running well.

As to your prop, I can't get you close without knowing your gear ratio. According to my book, the Volvo 280 series single prop outdrive was available with six different gear ratios over the years, 1.5:1, 1.61:1. 1.65:1, 1.73:1, 1.88:1 and 2.08:1.

Here's the formula:

. SPEED X RATIO X C
PITCH = ( -------------------------- )
. RPM X [1 - (SLIP/100)]

If we assume the speed is 36 statute miles per hour, the C in the above equation is equal to 1056. We will also assume an average propeller slip of 15%, and a max RPM of 4800. Now the equation looks like this:

. 36 X RATIO X 1056
PITCH = ( -------------------------- )
. 4800 X [1 - (15/100)]

So give me your drive ratio, and assuming a clean hull bottom, I'll give you a style, diameter and pitch prop to put you in the sweet spot for rpms. Deal?

Of course, knowing the present pitch, speed, rpm, and using 15 as an approximate slip ratio can give you a pretty close estimate on your gear ratio if you can't find it anywhere else.

. RPM X PITCH X [ 1 - (SLIP/100) ]
RATIO = ( ---------------------------------- )
. SPEED x C