HOW FAST SHOULD MY BOAT GO

SHALYN · September 24, 2017

Hi everybody, I bought a 2006 Monterey 234FS boat last august with a 100 hours on it - was in great shape. Boat came with the 5.0L gxi engine and would top out at 40 MPH. Boat has a Volvo penta sx drive... This winter a bought a brand new 5.7L fuel injection engine from Marine Power - engine came with everything, just drop in and run... HP listed around 320 HP... I run the boat in the Chesapeake bay in Md.. The best speed I can get from this boat is 42 - 45 MPH tops with 1/2 tank gas, smooth water, 15-17 stainless steel prop ( I tried all sizes and types of props) between 4800-5100 mph.. Is this the best I am going to get ??? I don't know if adding boat trim tabs will help ??? Do I need to replace outdrive with a better unit..... Thanks, Glenn

Rip5 · September 25, 2017

I think that is about all you can expect out of the boat. RPM's sound correct. Tabs will help the ride and

to level the load, no effect on top speed. The SX drive has more drag than an Alpha One, but changing

that would be of little use as well. You will get better maneuverability and some efficiency with a duo-prop

drive, but negligible increase in top speed, at the cost of thousand's of dollars. Enjoy what you have, and

go boating.

wirth315 · January 31, 2018

My 2007 234FS with GXI motor will reach 50+ mph on GPS with 2-3 passengers and half a tank. In the pic below I'm just cruising at 4000 RPM and running 46 mph with 5 passengers and over 1/2 tank fuel. The GXI is rated for 4800-5200 RPM so your safe propping boat to achieve most power / speed while keeping RPM in this range. Too low rpm and your over propped = to large a pitch. Max RPM or above 5200 then your under propped = to low a pitch. ETC...

I have the Volvo dual prop DPS-A drive with 1.78 gear ratio and F4 props front / rear. It's a very efficient set up as ALL cruising speeds are more MPH than RPM.

Check your outdrive gear ratio. Could very well be it's the wrong gearing for the much more powerful motor. Your GXI should be matched to the 1.78 gear ratio which is what is printed on my outdrive. Discuss with a Volvo certified mechanic what gear ratio is available for your single prop outdrive.

Lastly, if its a salt water boat with bottom paint, barnacle build up etc, then boat performance will be reduced.

Good Luck

wirth315 · August 16, 2018

UPDATE: August 2018 and I replaced my F4 Props with F3 props. Why? With the F4 Props, I could not achieve target RPM the motor is designed to hit. With a light load, the most RPM I could hit was 4500 rpm. Loaded with 4-6 passengers and over 1/2 tank of fuel, I could only hit 4000 RPM. My 5.7 GXI 320 hp motor is capable of 4800-5200 target RPM.

After installing the F3 Props, I'm AMAZED at the improvement in hole shot, mid range pep/torque and lastly TOP SPEED! Everything improved! Now I know I have 320 HP!! Just 1/2 to 3/4 throttle gets my boat on plane fast. 0-30 mph times are much quicker by about 2-3 seconds. Mid range cruise is still faster MPH than RPM, and Top Speed is 5000 RPM at 54 MPH with 4 adults / gear, 3/4 tank fuel and my Wakeboard tower bimini deployed. The bimini usually knocked 3 mph off my top speed. I have not yet ran the boat with a light load and under 1/2 tank with the bimini down, but expect to be able to reach 5200 Max RPM of the motor.

Pulling my 245 lbs up on a slalom ski took everything the boat had and I would barely get up after dragging 16-18 seconds with the old F4 props that came OEM on the boat. With the New F3 props, my wife has to roll onto the throttle over 2-3 seconds or the rope will just jerk out of my hands! I can pop up on slalom in half the time loaded with 3 passengers and drinks, lunch etc in the boat. Major improvement for sure.

I'm very pleased with the F3 prop upgrade and it was worth every penny $1050 I spent on the props. Thanks to Great Lakes Skipper for having the lowest price on the planet for new Volvo Penta props. I couldn't put shiny new props on without painting the lower unit... so I spent an hour to sand and tape off the lower unit before applying 3 coats of new paint. The ideal time to do this is when the props are removed, but I didn't have the paint in hand. :-) End result is a great looking lower unit to compliment the new F3 props.

Luckily, I had the volvo penta tool to remove the inside prop... It was still a bear, as the manual says torque to 45 ft lbs... It felt more like 120 ft lbs! But luckily, the shafts were well greased so both props just slid right off.

There is very little info. on volvo penta prop selections, pitch size, engine RPM etc. NOTE: IF you CANNOT achieve within 400-500 RPM of the max RPM the motor is rated for... your lugging the motor and leaving a lot of performance on the table. In my case, I was 1200 RPM down at 4000 RPM when loaded with my usual load as target RPM for my GXI motor is 4800-5200 RPM. Usually, by dropping one size on a prop nets 400 RPM increase on average... But not so with the Volvo Penta, as I increased 1000 RPM when loaded from 4000 to 5000, and at least 500 RPM unloaded as I went from 4500 to Undetermined as I haven't driven the boat with a light load yet, but expect to hit 5100-5200 for a 600-700 RPM gain.

Note: I will not run the boat that hard very often, and when I do it will be for very short periods of 5-15 seconds just to separate the men from the boys in the 50 MPH Club! :-) I'm happy to cut the throttle back to save gas and wear / tear on the motor. 4200 RPM is now my 45 MPH fast cruise and at 45 MPH you get where your going in a hurry. My favorite cruise speed is 3200 RPM and 35 MPH as the motor sips fuel and the boat rides well in the chop.

Hope this evaluation of F4 and F3 Props helps others. Had I read a post like this I would have re-propped to the F3 props day 1. :-) Happy boating Monterey fans.