posted 01-19-2007 10:34 AM ET (US)        

The last time we were out I ran the engine at wide open throttle for a few minutes to see what the max RPM might be. It turned up at 5,000-RPM and that was it. The propeller I am using is a stainless steel three-blade that I took off my Merc 175 engine before I sold it. I believe it is a 16 X 19P which is the number I took off the propeller.

Does anybody have a suggestion for a proper propeller for this boat and engine combo? Can the existing propeller be reshaped?

Top speed was 42-MPH (GPS). Load was two persons aboard with 50-gallons gas in the internal tank and two 6-gallon tanks sitting on deck. What should I be expecting with the right propeller?

Jim

posted 01-19-2007 10:51 AM ET (US)            

It would be helpful is we really knew what propeller was on there now. A 16x19P doesn't sound right at all. Mercury does not make a prop like that for that motor. 16" is a HUGE diameter for a Mercury 150. In fact, I would not expect it to be able to turn without interference.

I would not fool with modifying the exiting prop. Go out and buy or borrow a Mercury Mirage Plus, 15-1/4" x 19" prop and see what that does...or borrow some other prop in that pitch range and see what happens. Anything that will provide a baseline with a known product.

posted 01-21-2007 08:28 AM ET (US)            

The engine is mounted one hole up and I do have trim tabs. The boat comes out of the hole fast and onto a plane at about 2000RPM Rpm increases to 3000 as everything settles in. I can bring the rpm back down to 2000 and stay on a plane.

I will remove the prop tomorrow and try to read the numbers stamped on the prop. I am going to town tomorrow to look for a prop to borrow.

Jim

posted 01-21-2007 11:17 AM ET (US)            

Cookbook formulae can get most boats satisfactory propping, but if the engine height, boat loading and trim or prop design don't match the parameters of the formula the boat will not be ideally propped.

Propping a boat is like buying shoes. Several parameters can be adjusted to find the ideal fit:
* Within the engines rated operating range, what rpm do you want to reach at WOT with a typical load and trim? This is usually the top rpm in the range, but if you often go a lot lighter than typical load and trim you might want to set it at the middle of the operating range.
* What is the typical load and trim? Not just what you carry, but where. I once had to drop pitch because I had mounted a heavier bow mount troller.
* What X dimension (AV plate height relative to bottom)do you want to use? Typical X dimension is +1", but more will go faster at higher rpm at risk of prop ventilation. If the engine is mounted on a setback or jackplate add at lease 1" for each 4" of setback.
* Some prop manufacturers advertise props for "bow lift" or "stern lift". They may perform as advertised, but I will not vouch for those claims.
* Do you favor accelleration over efficiency? Four and five blade props produce more thrust at the sacrifice of drag and reduced fuel efficiency. Two blade props are more efficient, but prone to cavitation.

Bottom line? Find a dealer who will assist in on the water testing and selection with typical load and trim and preferred rigging.

The Prop Topics forum on iboats.com has several prop specialists who can offer priceless advice on prop selection and rigging for optimum performance and efficiency.

posted 01-21-2007 01:45 PM ET (US)            

A 2004 Mercury 150 has a gear ratio of 1.87:1 and a redline of 5600 RPM so 42 MPH @ 5000 RPM with a 19" propeller would suggest a slip of 12.7 percent which is perfectly normal.

To get the maximum RPM closer to the top of the WOT range, a reduction in pitch will be in order.

There is no need to visit some other web site to read boiler plate advice about propellers in general when you can get specific recommendations for your Boston Whaler right here from folks who actually own the boats, motors and propellers in question.

posted 01-21-2007 02:37 PM ET (US)            

Good advice so far but let me add something. The existing prop has the following numbers. On hub 48-16316-19, on prop side, Quicksilver, 48 16316 A5 19P.

Also, let me add that the 2 batteries are mounted below consol.

Also, originally the boat/motor had a set-back mount of 23"

posted 01-21-2007 05:00 PM ET (US)            

If you haven't been there and taken advantage of help from professional prop specialists like Walleyehed and Dale Hadley you are not qualified to make such remarks. If you had been there you wouldn't make such remarks.

If you simply object to getting help anywhere else in addition to CW I wonder if you feel threatened. You have no cause to. Boston Whaler owners who show up on iboats are regularly referred here and even to Tom Clark for specific advice and info on Boston Whaler boats.