Sleceting a stainless prop?

[jasell]

Hi,<br /><br />I'm about to replace my original aluminium prop (13 1/4x 17) by a stainless one. <br /><br />I have a Yamaha 90hp 2-stroke (2000) for my 18' bowrider.<br /><br />I do fishing, crusing and water skiing.<br /><br />Today there is no problem to pull up a person on 2 skiis, but one 1 ski its hard and often fails due to in experince riders/long time.<br /><br />My top speed is somewhere around 37 knots (5500 rpm)(haven't tested with the GPS yet).<br /><br />What I want with the new prop is:<br />-Higher top speed<br />-Better fuel economy<br />-Be abel to pull up person on 1 ski fast<br /><br />Question:<br />-There are several differnt prop designs out there what do they do?<br />-Some claims that they lift the entire boat by their design, how is that posible, isn't that the speed and the hull that lifts the boat?<br />-What do I gain/lose with 4 vs. 3 blades?<br />-What do I gain/lose with cupping, rake etc?<br />-How much can I add in rise(?) when change from Al to SS, 2"?<br />

 

[Dhadley]

Re: Sleceting a stainless prop?

To get a single prop to gain top speed and enough low end to pull a skiier is going to be a stretch. Thats why most folks have a smaller pitch prop to ski with. <br /><br />I would suggest playing with your set up to gain some rpm with your current prop. You could use some anyway. Raising the motor will reduce drag, increase top rpm and improve holeshot. However, with an aluminum prop you might not be able to increase the X dimention as much as a ss prop.

 

[Fully_Loaded1]

Re: Sleceting a stainless prop?

Also in some cases especially with smaller motors a SS prop is much heaver and you stand a chance of reducing fuel economy. Not to say if you hit a dead head or something that SS prop will not give and your lower unit probly will (I know from experience) That aluminum prop will give a blade or 2. Yes it might ruin your day but an aluminum prop is 100 bucks and a lower unit is 1000 bucks. The math is simple. I would stay with the aluminum prop, that is my opinion and experience.

 

[jasell]

Re: Sleceting a stainless prop?

But what do all this magic cupping and rake do to the performance?<br /><br />If you look in to SS props, the Solas has a traditional look and but the Ballistica has that flat blade tip, is that style only or does it realy mak a difference?<br /><br />If I go for a higher pitch, 19", I probably get a lower rpm on top, but will I get higher speed?<br />What effect will that have on fuel economy and will the motor take a beat from not reaching top rpm?<br />Is it true that 17" Al prop correspond to a 19" SS, that what I been told in the prop shop. They claim that the SS is thinner and more precize and doesn't slip as much. But if that is the case wouldn't a SS 17" actually do 17" by each turn instead of AL 17" that slips and just do 15" (just an example).<br /><br />I guess 4 blade will give me better acceleration but the extra drag will limit my top speed, correct?

 

[Dhadley]

Re: Sleceting a stainless prop?

Rake has to do with lift. Cupping has to do with bite and how high the X dimension can be. <br /><br />You can "look" at props for years and still not know how they work until you try it. Each style will act a bit different with different style hulls.<br /><br />The tips of the blade do most of the work at speed. The tips on a Ballistic are gone. Some folks refer to them as Bull-istic. Some Ballistic props work well on certian boats but only after they've been blueprinted.<br /><br />If you go for a higher pitch your rpms will be lower unless you can incresae the X dimension (raise the motor). At 5500 you cant afford to lose any rpms. And your holeshot will be worse.<br /><br />A 17" aluminum is not comparable to a 19" ss. If anything its the other way around. The ss has less blade flex.<br /><br />A 4 blade will give a better holeshot but needs to run at a higher X dimension. Once dialed in the 4 blade will give a better holeshot and at least the same top end. Maybe faster. Itll run much higher so there is less gearcase drag.

 

[jasell]

Re: Sleceting a stainless prop?

What do you mean by "X dimesnion", has it to do how much you lift the motor or the height it is mounted on the transom?<br /><br />How come the rake lift the complete hull from the water? Isn't rake the angle the blade are leaned to the housing? I just wnat to understand the theory...

 

[Dhadley]

Re: Sleceting a stainless prop?

Yes, the X dimension refers to how high the motor is up off the transom. With recreational boats it generally means the distance from the top of the transom to the AV (cavitation) plate. On race boats we talk about the propshaft centerline.<br /><br />Yes rake is that angle. The more aggressive that angle, in conjunction with other factors, the more lift the prop provides.

 

[jasell]

Re: Sleceting a stainless prop?

I have been told to lign up the AV to the keel (lowest point on the transom). Correct or is it just start poistion before fine tune?<br /><br />The theory of rake:<br />With higher rake it means the prop it pushing water in a tangential direction as well (intstead of axial which gives speed).<br />So when the blade goes down it have a large resitance, the water cannot escape easily. However when the blade is on its way up the water can just splash which give a lower resistance.<br />So the result is a lifting force up from the water.<br /><br />Is that how it works or do I totaly miss it?

 

[Triton II]

Re: Sleceting a stainless prop?

The prop on its way up will encounter water with air bubbles in it if the motor is set too high or the trim is up. As you mention, the blades on their way down encounter water with less bubbles and start to work as they are designed to. You can demonstrate this by trimming the motor up until the prop is virtually thrashing its tips anfd then try to gun the throttle. The transom will lift but nothing much else will happen apart from a roaring sound and lots of foam. So, basically... Yes, you got it. (I think!)

 

[jasell]

Re: Sleceting a stainless prop?

next thing to understand, Cupping:<br />I guess its about to trap the water inside the "working area" of the blade, so its doesn't "spill" over the tip. I that happens to much the water would just move radial to the prop and no resulting force, not up or forward, just to the center of the prop which doesn't move the boat.<br /><br />Correct?<br /><br />I heard about wrapped cup, are ther more ways to do it?

 

[Dhadley]

Re: Sleceting a stainless prop?

Theres several ways to create cup. Most everything that has to do with making a prop better (more efficient) has to do with making it "hold" the water as long as possible. Even the finish of the prop has an effect. <br /><br />The av plate being even with the keel or true bottom is a very old concept and is indeed a general rule of thumb. Even todays aluminum props can hold higher than that. <br /><br />The higher you run the gearcase, the less drag you have. The perfect scenario would be to hide the gearcase behind the transom and still have prop bite and water pressure. <br /><br />Theres 2 things to concentrate on to make any given rig more efficient at speed. Reduce motor drag and reduce the hull's wetted surface.

 

[kavika]

Re: Sleceting a stainless prop?

My SS prop came "rusted" with used boat.<br /><br />How do I polish it, and will that inprove performance?

 

[jasell]

Re: Sleceting a stainless prop?

What about progressive pitch? Is that cupping in the other direction?<br /><br />What is it supposed to do? Better accelaration?

 

[Dhadley]

Re: Sleceting a stainless prop?

Kavika, they will show surface rust. The next time you run it, it will most likely polish itself. Especially if youre anywhere near a sandy bottom. <br /><br />Will polishing it help performance? Probably not. If you were running a tunnel boat it may hurt. <br /><br />Jasell, most all props have progressive pitch. It helps overall.