what speed can i expect

[rpatton]

I have a 19 foot bowrider 1985 with a 3.0 stringer drive. The prop is a 3 1/4 x 17 pitch which runs at 39-4000 rpm wot trimed. I recently got the boat and have no idea what speed is normal for. Right now it runs about 23 on my gps, i think this is kinda slow but hopfuly someone will chime in. Also i should ad that this speed i posted is with only 2 adults.

 

[Scott Danforth]

Re: what speed can i expect

my '84 17' with a 3.0 stringer and 19p prop would spin 4700 RPM and run close to 41 per the GPS with two not so svelte guys in it I used the 17pitch for pulling skiiers.

move some weight to the front and see if that helps. something is wrong with your setup. are you water logged?

 

[jestor68]

Re: what speed can i expect

Not sure what a "stringer" drive is. Virtually all I/O's use props with a larger diameter than 13-1/4. That sounds like a prop for a smaller gear case outboard motor. Could be someone put the wrong prop on there.

Being in mind that you're dealing with an older motor that is probably pretty tired, you can't expect it to perform like it once did. The motor, assuming it's in proper tune, is telling you what it needs.

To get the rpm up to proper operating range(4400-4800) you'll need to put a prop on there with less pitch. Going down to a 15" pitch will increase your WOT rpm by 300-400 rpm, and get you closer to the correct operating rpm. This will provide better all around performance and could likely produce better top speed.

You need to check the prop charts for your brand/horsepower. With 4000 rpm as a starting point, figuring how much you need to reduce pitch to get your rpm up to 4400-4800. One inch of pitch equals 150-200 rpm change. A few prop makers make pitch changes in one inch increments(most do 2 inch increments) to make it easier to fine tune your choice.

Buying a used boat is like a box of chocolate; you never know what you're going to get. :facepalm:

 

[Scott Danforth]

Re: what speed can i expect

Jestor, the stringer 400 behind the 3.0 uses the same case and props from the V4 outboard. so yes, the 13 1/4" prop is correct.

however even with the larger boat, the 17 pitch should get to a respectable WOT.

I agree something may be awry with the OP's engine. I would do a compression check, check point gap, dwell and timing, and then go from there as well.

 

[steelespike]

Re: what speed can i expect

My info indicates 1.71 ratio if the motor could turn it up to 4600 I get about 39 mph.
These guys have more hands on than I do But I agree with their numbers.Somewhere around 40.
I would suspect the motor. 4000 at 23 indicates 38% slip way way high. So beside a possible weak motor the tach may not be accurate. 4000 at a more normal 10% slip gets about 33. Of course at 4000 rpm slip would tend to be high though not 38%.

 

[rpatton]

Re: what speed can i expect

Thanks for the replys . I did check the compression and all are within a few pounds of each other at 130 lb. Plufs are new wires are new but the cap still needs to be replaced and what shouls the gap on thepoints be. I have a little trouble thinking that cap and points being my problem with speed. The tach is original to the boat so i am guessing it is right . So i am realy confused now what do i do . Any help would be great.

 

[Scott Danforth]

Re: what speed can i expect

points control dwell, which controls timing, which controls combustion, which controls power

0.018" gap initially, then adjust as needed to 30 degrees dwell. then adjust timing as needed

the tach needs to be checked with a known good tach - usually an optical tach. It is the cheapest gauge the OEM could get that converts a digital pulse to a voltage to move a little needle to approximate the engine RPM. that being said, a $0.01 resistor can go bad and the tack is not reading right