Prop Question

[rhp5033]

So I just had a new to me 1993 Ocean Pro 150 put onto my 1990 Seaswirl Striper 210 WA. On my first outing, I tested two 4 blade turning point props. One was a 14x17p and the other 14x19p. the 14x19 was too much prop and killed my WOT RPMs to near 4000-4200 and top speed over ground of 26-28mph. The 14x17p was much better with wot rpm at 4800-5000, and top speed over ground of 30mph. the reason for the 4 blade props is that my old motor was terrible out of the hole with the 3 blade it had, and maneuverability was terrible. a 4 blade was suggested, and the performance gains were noticeable. unfortunately a wash-out low tide and getting stuck situation blew my motor up.

The boat weighs around 3700 I believe, and I was testing with two adults, one child and a full tank of gas (approx. 50 gallons) and no extra gear.

My next step is to try out the14.75x19 stainless steel prop that came with the outboard this weekend. but I also was curious if I should bother buying a 4 blade 15x15 to compare rpm and top speed?

What are anyones opinions on the 3 blade 14.75x19p, the current 14x17p 4 blade, or trying a 15x15p 4 blade? would a 15p 4 blade give too much slip? would top speed suffer going to the 15x15 or would the jump in rpm balance it out?

 

[Texasmark]

Shallow pitched, large diameter props with multiple blades, turning low rpms are pusher props for large, heavy, slow moving loads. Small diameter, 2 blade, high pitched, high rpm props are made for light loads going fast. The rest of the herd fit between these extremes.

Diameter is usually set by gear case clearance. Once in a range of props, like Merc outboard 4" diameter gear case uses 13-14" diameter props and the 4 ¼ gearbox runs 14-15" diameter props....... the actual prop diameter is part of the overall prop calculation. Types of props use different calculations for different reasons.

A slow moving, general purpose prop will usually be in aluminum, blades straight out, shallow pitch, with no cupping of the trailing edge of the blade.

High speed props (ocean racers excluded), run near the surface or slightly aired, will be thin bladed, usually SS to get that, high rake (blades come out from the root and folded back) which reduces the diameter for a given range, variable pitch across the blade, with cupping on the trailing edge for better grip under the circumstances.

The rest fit between these limits. As you noticed, better control is achieved with an extra blade for hole shots and slow cruising just above planing speed, or when pulling water toys. They are slightly less efficient on a side by side comparison due to the turbulence and drag of the extra blade.

Go-fast.com has a BAM prop slip calculator that you can use for getting the feel of changing parameters of a prop. It does not take into account your type of prop but indirectly you insert your "type" by the difference incalculations and results for zero slip, like you would get a screw turning through wood, and your actual prop slip turning in water developing thrust to push your boat.

Don't worry about diameter, just work the pitch. Gear ratio is just the number. Like a 2:1 gear box would be listed as 2. You plug in what you know and leave what you want to solve for open. GPS speed inputs give more accurate results as speedometers are notorious for telling lies.

 

[Sea Rider]

If the previous 3 blade prop performed lousy at hole shot seems was badly propped for that combo. Any 3 blade alum prop for that matter revving full wot rpm range for any particular OB brand as usually loaded will achieve an oustanding hole shot provided that OB sits at the sweet transom height, it's well trimmed with deck weight is evenly distributed.

Oh boy, you're considering too many props alternatives on a theoretical point of view which doesn't work, props must be water tested as usually loaded and fine tuned playing with pitches to suit your boating requirements. Good luck on your prop selection.

Happy Boating

 

[rhp5033]

Texasmark, that was an excellent explanation, thanks for it, very insightful. I don't think I am trying to many props based one theory, I mentioned this is a new outboard for me, the two 4 blade props I have trialed were ones I just bought for the old motor before it failed. I tried both on the new motor as I already had them. My theory for trying a 15p 4 blade was to move further into the rpm range from 4800/5000 up to 5200/5400 (theoretically). I am also going to trial the 19p stainless steel 3 blade that the motor had on it, as I have not yet run this prop.

How is prop slip related to pitch when pushing a heavier boat? Would it be expected to get more slip from a lower or higher pitched prop? I am not seeing huge leaps in top speed (less of a concern anyways) but more looking at would a 17p at the bottom of the rpm range be better than a 15p at the top of the rpm range? (both identical props and 4 blade)

I will probably ultimately run the 4 blade either the 17p or the 15p solely for the better hole shot and lower cruise speeds

 

[Texasmark]

Prop slip is very high (50+% for a WAG) on pusher props. Don't believe me, just look at the wake and prop wash for a given speed, then look at my avatar.

If you go with a high performance SS "ported" 3 blade you can get the best of both worlds. 3 blades of high pitch (considering the application) for high speed and ported for hole shot.

A lot of high performance SS props are ported. The Merc Enertia for one, Laser II for another.....many more of which I am not aware. The size of the ports are adjusted (rubber plugs like in the Enertia prop picture in this post) to suit you in your hole shot. The larger the hole, the faster the engine revs within reason. I liked to tweak in at about 1000 rpm increase. What that means is that with the holes blocked and a certain hole shot with nothing changed the comparison of a plot of rpms vs mph vs time, and then running the same test with the same prop ported, the rpms would be 1000 higher type thing with the time and rpm curves being steeper....shorter. Too small on the holes and no effect, too large and engine revs too high and actually reduces the hole shot thrust increasing the time.

The port functions best when you firewall the throttle in the hole shot. The exhaust gasses from the engine are somewhat blocked by the water at the rear of the prop tube, building up pressure. This pressurized exhaust gas looks for a way out. Putting holes in the barrel, just under the leading edge of each blade allows this exhaust gas to dilute the density of the water that would normally flow across the blade, thus unloading it (less resistance against the blade).

Unloading the blade allows the engine to accelerate (RPM increase) much faster. Since prop thrust is developed by engine HP and HP is (engine torque x rpm)/ 5252, the faster you get the revs up the faster you get the thrust up and come out of the hole......within limits mentioned in previous paragraph.

The only way I'd take a 4 blade of shallower pitch over a ported 3 blade is if I did a lot of Sunday afternoon cruising just over planing speed, or pulled water toys as my main usage. If I wanted a good hole shot and a fast boat it would be with a properly ported 3 blade SS prop.

If you are in the "looking" mode, a full service prop shop can help you find your sweet spot for minimum overall cost. They can advise you and supply you with test props to put on your rig and decide which is best for you. Even if that prop costs you twice what you would pay otherwise, all you would have to do is "shotgun guess" and miss once and you would be at that cost level and may not have the satisfaction you are seeking.....prop #3??????