9.9 prop help. 1000 rpm under max

[dainer]

I own a 14 foot aluminum Klamath boat. The motor is a 2001 Mercury 9.9 4 stroke, which runs like a top. However, I have never been able to get over 12/13 mph out of this combo. I recently purchased a tiny tach, which shows 4300 max rpms. The recommended maximum range is 5500-6000. The prop currently on this motor is a 9.5x9, from the factory. I am looking to gain some speed and get the motor up into the correct range. Would a new prop help? If so, does anyone have any recommendations? I don?t mind dropping some coin if it will help out. Thank you in advance.

 

[jestor68]

Re: 9.9 prop help. 1000 rpm under max

Go to the prop section, plug in your motor info and see what's available.

Each 2" of pitch will gain 300-400 rpm.

Looks like the smallest is a 7" pitch.

 

[steelespike]

Re: 9.9 prop help. 1000 rpm under max

Is the boat a bare utility with no floor or live well etc.? Are you average weight under 200 lbs?
Are you hauling a lot of gear?Do you have a whale tail or foil on the motor?Is your speed by gps?
Depending on your answers I think you have a motor issue not a prop issue.At 13 mph I have to wonder if your on one cylinder or the throttle isn't opening all the way.
In my opinion the motor should be running about 20 mph or more.
with a light load.

 

[jestor68]

Re: 9.9 prop help. 1000 rpm under max

Bear in mind that 4 strokes don't perform like 2 strokes.

According to a test shown on the Merc web site, a 9.9 4stroke on an 11ft Boston Whaler did 12.8mph at 4500rpm. At WOT 5650rpm, it did 18.5 mph. That was a brand new motor with a 9 X 8-1/2 prop; with a 2.08 gear ratio.

If you your 9.9 to do more, you'll need to put the 7" on it to get it to spin up higher rpm.

Or, since that boat is rated for up to 20 hp, swap the 9.9 for a 20. Boats perform best with their rated power.

 

[dainer]

Re: 9.9 prop help. 1000 rpm under max

Thank you very much for your responses. I did look at the prop selection, and was looking at the stainless Solas Saturn 9 ? x7. Hopefully that would get me closer to 5000 rpm?

The boat does have a removable floor. 1/2? ply with light foam underneath. We are talking maybe 30 pounds there. I usually have around 550 lbs in the boat(2 people, batteries, tackle box, rods) I do have a sea ray hydrofoil on the motor. My speed is checked by GPS.

I use this boat pretty much exclusively at a 10 hp and under lake, where everyone has 12/14ft aluminum boats. My family also has a couple mid 80?s johnson 2 strokes, and my uncle has a 2000 Evinrude 9.9 4 stroke. The norm out there seems to be 10-14mph. I have never seen anyone out there going 20, and I see 150 different boats with 9.9?s each year. If I hadn't seen so many people online talking about 18-20 mph, I would think it wasn't possible.

How would I go about checking to make sure that both cylinders are opening and that the throttle is opening all the way?
I will order the prop and see how it goes. Hopefully it will get me closer to 5000 and 15mph, which would have me blowing by people. I realize that this is a 9.9 on a little utility boat, and not a racing set up, but it?s always nice to be top dog.

Thank you for any more info you may have.

 

[steelespike]

Re: 9.9 prop help. 1000 rpm under max

In 57 we had a 14 ft fairly heavy Crestliner utility,( I can get the weight).It had a 57 Johnson 10 hp.
I can't vouch for the actual speed but with 2 of us 100 pounders on the back seat it planed and ran at least 18
With just one aboard it ran fast enough that it would clear the water over the tour boat waves.
My kids had a 12 ft steel flat bottom boat with a 5 hp Gale With a 120 lb driver and 2 40 lb kids it planed almost instantly
and ran 10 to 12 mph (a guess) The 5 is rated for about 12 mph.
Your 7" prop may help. At the least it will be peppier.Weight elimination and balanced weight is important.
Carry less gas 3 gallons will run about 3 hours at wot.
I bet at speed your dragging the foil through the water Probably costing a couple of mph.
Get rid of the foil and try to get the weight balanced so it will plane. Raise the motor about an inch just use a stick of wood to hold it up while you tighten the clamps good.