If you raise your motor 1" you will have less drag so the 9" prop should give you the right rpm's

do they make a plane fin that fits your lower unit?  

ORIGINAL: bulldog1

Go back to your original prop and adjust your throttle stop to keep it in the rpm range.

This might be the easiest fix

ORIGINAL: rmcmillen09

do they make a plane fin that fits your lower unit?  

I can't drill holes in a new motor yet!

Use the small block planer tabs in the corners.
All they are is a wedge that goes under boat

Wanted to give you guys an update since the results were amazing!

I purchased this prop: http://boatpropellers.iboats.com/Yamaha/20_HP_(2007Newer)_8.00-Pitch_Amita_3113-093-08_Propeller/810/8512/?cart_id=681322313

When I went to install it I found the rear flange of the prop fit a little too tight in the lower motor housing. A couple minutes of sanding took care of this (Didn't even sand thru the paint).

If you recall the original 9.25x8 three blade would over-rev and hit the rev limiter at 6200 rpms. This occurred at about 19mph. I tried a 9.25x9 three blade but was only able to get about 5200 rpms and 14.5 mph. I think the reason for the huge difference was the 9 pitch prop struggled to get the boat on plane.

I tested my new prop on Wilhelm this morning. Here are my results.

5500 rpm / 17.5 mph
5800 rpm / 19 mph
6100 rpm / 21 mph (this was max throttle)

The biggest factor was this four blade prop got the boat on plane very, very fast. Planing with the three blade 8" prop was a gradual process.

ORIGINAL: JerryS

Wanted to give you guys an update since the results were amazing!

I purchased this prop: http://boatpropellers.iboats.com/Yamaha/20_HP_(2007Newer)_8.00-Pitch_Amita_3113-093-08_Propeller/810/8512/?cart_id=681322313

When I went to install it I found the rear flange of the prop fit a little too tight in the lower motor housing. A couple minutes of sanding took care of this (Didn't even sand thru the paint).

If you recall the original 9.25x8 three blade would over-rev and hit the rev limiter at 6200 rpms. This occurred at about 19mph. I tried a 9.25x9 three blade but was only able to get about 5200 rpms and 14.5 mph. I think the reason for the huge difference was the 9 pitch prop struggled to get the boat on plane.

I tested my new prop on Wilhelm this morning. Here are my results.

5500 rpm / 17.5 mph
5800 rpm / 19 mph
6100 rpm / 21 mph (this was max throttle)

The biggest factor was this four blade prop got the boat on plane very, very fast. Planing with the three blade 8" prop was a gradual process.

 
Just out of curiosity...how much load did you have in your boat in achieving this performance?  One adult? Two?  Trolling motor batteries? How many?  The reason I ask is I'm in the same boat (pun intended) as far as having a 16' Lund with a 50 hp and 9.9.  I'm thinking of doing the same thing you've done, but I'm worried about performance with a 20hp.

Bogey,

Todays test had three light people aboard,(175 lb, 120 lb and 100 lb). Since this was only a test trip, I did not have any fishing gear or trolling battery installed. I had a starting battery and 3 gallons of gas in a portable tank.

One of the biggest weight reductions I made for this transition was to empty and remove the 24 gallon built in gas tank. Not only did this lighten the boat, but it also reduced the chance of gas going stale since these 20hp 4 strokes don't use much.

As stated earlier my boat specs state a dry weight of 750 lbs. When I purchased the Yamaha 20hp at Pymy Boat Sales they stated they were averaging about 16.5 mph from their 16 footers weighing 800-900 lbs using the 9.25x8 three blade yamaha prop. I don't think they even try the after-market 4 blade props. Hopefully this info will help you determine where your Lund would fall depending on the boat's weight.

I should note my speeds were measured with my fishfinder's paddlewheel so there may be some error. Comparing apples to apples, the same paddlewheel measured my 40 hp at 31 mph(5200rpms) with same boat load and half tank of gas (12 gal).

I am tickled that I cut my horsepower in half but my speed was only cut by a third.

Jerry

Thanks for the info.

JerryS

So are you pleased with the preformance of the four blade?