PDA

View Full Version : Choosing The Right Prop



TroutGod
10-23-2005, 12:42 PM
I have a Crestliner superhawk 1600. Boat is advertised at
1000 lbs. I have a 2002 50hp eft Merc outboard. Currently have a stingray hydrofoil on it with a merc black max 10 1/2 diameter prop 13 pitch. Problem is when I have 3 on board boat will barely plane out and runs about 5000 rpms. 2 people on board it runs ok about 5500 rpms. Keep in mind I do all my fishing about 5000 feet above sea level. At folsom this prop runs almost 6000 rpms with myself and 1 passanger so I'll keep it for a spare in case I fish any low elevations. I'm looking for some advice. Should I switch to a 4 blade? What size and pitch? Any advice would be awesome. Thanks - TG!

Sierra_hillbilly
10-23-2005, 01:12 PM
I have the same question TG. I heard that Mickey changed his prop for higher elevation I wonder if it has to do with the RPM's the engine will put out.

basil
10-23-2005, 07:32 PM
Your prop should make your boat hit maximum recommended rpms at top speed. I don't know what your red line is but 5000 to 5500 rpms sounds right.

Things to play with are:

(1) Find some flat water and experiment with your motor trim and find what setting your boat goes fastest.

(2) Stay with your three blade prop. Four blade props are good for high end speed but not for getting on a plane.

(3) Stainless steel props are stiffer and more efficient than aluminum. Plastic props are crap and only good for spares. But the difference between aluminum and stainless is not that great at low speeds.

(4) Make sure your prop is not all nicked and scratched up.

(5) Adjust the air screw on your motor for high altitude. It needs to be opened more to let more air in. Unless you have a computerized system, then you need to have it reprogramed by the dealer.

(6) Run with only as much fuel as you need. Weight is your enemy for speed and planning. Also, have your passengers move forward in he boat, until the boat planes. Then they can move back.

Bottom line is your boat and motor combination is performing about right. Three people is a big difference to a boat that size. Also, altitude is a killer. In fact, it sounds like your boat is over rev-ing at low altitude, 6000rpm is kinda high. That means your prop is right for altitude, but you could use more pitch at lower altitudes.

Essentially, the 13 pitch on your prop means one revolution of your prop moves your boat forward 13 inches. Of course that is in a perfect world. You know, the world your wife lives in but you can't seem to live up to!

TroutGod
10-23-2005, 07:50 PM
I would really like to be able to get the boat up around 5500-5700 rpms with myself and 2 passangers. I'm guessing with 1 passanger and myself it would probably run more like 5800-6000 and 6000 is redline so I would have a little to spare and could back off the throttle a little. From what I understand with electronic fuel injection it should adjust for the altitude changes. Any prop wizards out there HELP!

drstressor
10-23-2005, 10:49 PM
(2) Stay with your three blade prop. Four blade props are good for high end speed but not for getting on a plane.

Not true. You have it backward. :)

TroutGod
10-23-2005, 11:43 PM
With the info I provided above, what do you think Doc?? Any recommendations.

drstressor
10-24-2005, 06:55 AM
Didn't you get my PM last week?

Dennis
10-24-2005, 07:05 AM
Come on Doc aren't you going to 'share'?

TroutGod,

Mercury has a prop selector web page that will help.

http://www.mercurymarine.com/prop_selector

cheesehead
10-24-2005, 07:11 AM
I had a problem, which I thought was a motor issue, turned out to be a prop issue.

I was getting 5800rpm WOT and 55 MPH w/ a 21 pitch prop. The problem was at top end my motor was 'cutting' out. or so I thought, 4 trips to two different motor shops and several phone calls to the manufacturer. A suggestion was made to lower the pitch to 19. the idea here was optimum rpm with optimum speed.

It worked! now I have 4800 RPM at WOT and 49 MPH. My motor was working to hard and I think the term is ventilation (I'm not sure).

So perhaps a pitch change might be your strategy, but the superhawk is light in the front, try putting your anchors up front or other items that will put more weight in the nose. 50 HP might be too small too.

Good Luck

drstressor
10-24-2005, 07:25 AM
OK, the main problem is that the boat is under powered for running at high elevation. He is only putting out closer to 40 hp at 5K feet. I would bet that the boat, motor, gear, fuel, and 3 passengers weighs about 2,500 lb. You need at least a 70 hp engine to move that load at 5K ft where it would be putting out just under 60 hp.

Other than getting a larger engine, the next best thing to do is to get a prop for higher altitude. An 11" pitch prop will get the rpm up and reduce the strain on the engine. But it won't really let the boat plane any faster.

4-bladed props have less "slip" than 3 bladed props. In theory, a 13" pitch prop will move a boat forward 13" for each revolution. The difference between the theoretical and actual movement is called slip. So a 4-blade gives a better hole shot and also keep a boat on plane at lower rpm. I like them a lot for moving heavy loads. I use a 15" 4-blade on my 90 Honda when fishing with lard assed friends and at elevations of of 3,800 or above. At sea level I switch back to a 17" 3-blade, which lets the boat go a lot faster. In the ideal situation, you want a prop that will let a boat stay on plane in a chop when turning 4,200 - 4,500 rpm when loaded. That is the flat part of the torque curve for most outboards. It gives the best fuel economy and let's the engine last a lot longer. But unless you have an engine with enough hp (torque X rpm) in the first place, you can't achieve those conditions no mater whatever prop you use.

Dennis
10-24-2005, 09:41 AM
Thanks Doc! I haven't fished much above 3800' so I haven't had much desire to go out and get another prop. The one or two times at the higher elevations left a lot to be desired. I knew a prop change would help but I also realized that there's a few choices that can be made.

I'll definately head toward the four bladed props if I decide to make that purchase. I'm fishing out of a 16' Lund with a 60hp Mercury and I don't know the size of my current prop.