Another prop and speed question

[Mike 03033]

I have a 1993 Larson all All American 190 bowrider.

Lenth 19'
Beam 7.5'
weight 2300 LBs.
engine is a Mercruiser Alpha one 4.3LX 190HP.

I just bought the boat six weeks ago. Seems to run great. When I bought it the speedo did not work. I got that fixed last weekend and checked top speed. Indicated 42 MPH. That seems low given the size power of the boat.

Rechecked with GPS. It read 40 MPH. Just me in boat, 3/4 tank gas (100LBS?). adjusted trim to get max speed. Tach reading 5200 RPM. I checked the tach reading with a dwell tach (cheapy). I did this with boat in neutral and compared with boat tach. it seemed to read 100 to 200 rpm less. Not sure which is more accurate.

Prop is 14X19 3blade, aluminum. not sure brand, looks like standard merc prop. no apparent damage.

My question and performance issue is, should this boat go faster than 40mph? it seems rather low for a 190hp motor. Don't know when last tuned, but it seems to run great. puts the boat on plane in no time and pulls great.

Based on rpm I am guessing it is under proped. I am looking for all round performance, pull my kids in tube and waterski's.

Any input is appreciated.

 

[hwsiii]

Re: Another prop and speed question

Mike, I show you have a prop slip of 21%, whih is high, and if you will buy a Solas Amita 4 blade 19" pith aluminum prop it will bring your RPM down to just about right and you should gain probably 3 MPH. It should ost about $ 125.


H

 

[Mike 03033]

Re: Another prop and speed question

Hi H;

Thanks for your reply. Can you educate me a bit on 4 vs 3 blade? Does the 4 blade reduce slip rather than just changing pitch to get rpm down? Also what do you think of a 21 pitch, some of the prop size calculators suggest 21.

Based on what you suggested I would see a top speed of 43, is that about all I can expect from this boat?

 

[hwsiii]

Re: Another prop and speed question

Four blade propellers have more blade surface area which helps cut down on prop slip at all RPM ranges, especially at the lower cruising RPM. They also increase hole shot and normally reduce cavitation and ventilation as well as let you plane at lower speeds because of the extra blade surface area.
The specific one I recommended for you is mainly a stern lifter with some cup built in as well as a some rake to keep the bow from plowing at high speeds, so it lifts the whole boat more. There are definitely 3 blade SS props that will work as well as the 4 blade, but the difference in price is a lot, and in most cases not justified. But if you want every last MPH there is in your boat we can pick a SS prop as well. This prop because of the 4th blade will redue your RPM about 300 RPM and that will bring your RPM down closer to the manufacturers recommendation. We could go to a 21" pitch but it would reduce your RPM too low.


H