Advice on prop diameter

[charleso]

I have a 10 1/4 X 14 SS prop on a 2008 Yamaha F60 TLR paired with a 17'7" War Eagle stick steer. The wot RPM's run low~4500 or less. The boat does not plane out well. Looking at several calculators, they show lower pitch as well as some options with increased diameter. I understand that lowering the pitch will increase the RPMs. What effect if any will changes in prop diameter have?

 

[tpenfield]

Increasing the diameter will also lower RPM, but it is not well understood by how much; at least not as well as pitch changes are understood.

The diameter of a propeller determines how much water the prop will try to move, and the pitch determines how far the prop will try to move it. The combination of those 2 factors yields the total volume of water that the prop will try to move, which requires power. Increasing (or decreasing either factor) changes the power required, thus affecting the maximum RPM that an engine can achieve.

 

[charleso]

Thanks for the info. Looking at a Michigan Wheel Vortex with virtually identical diameter and 3" less pitch, I feel with the increased RPMs from that and going to Aluminum vs SS should get me in the recommended range.

 

[Sea Rider]

At 4500 wot rpm OB is 1 K under max 5500 wot rpm range, right at the bad lugging side. Will need to go for a less pitch prop, how much less, will depend entirely if combo is light, medium light or heavy as this is not exact rocket science.

Happy Boating

 

[charleso]

The calculator on Michigan Wheel recommended a 10 3/8 X 11. Shouldn't this increase the RPM's and therefore improve the planing performance.?

 

[charleso]

One more question, anybody have a preference as far as manufacturers? I know and have owned Michigan Wheel, but how does Solas or Hustler stack up?

 

[Sea Rider]

How heavy is that boat ? Going less pitch will increase wot rpm, will plane combo faster loosing slight top end speed. If going low on pitch OB will overrev. Any prop you choose muest be water tested on a wot spin as usually loaded.

Start with what prop calculator indicates and see how it goes. Brand props should perform same if pitch, diam & cupping are maintained same.

Happy Boating

 

[charleso]

I am trying to get a hull weight. I would consider the boat heavy. It is a War Eagle 754vs. Then add on extras like me, the wife, two batteries, fuel, etc, it is definitely loaded.

 

[charleso]

I don't have an option to be able to test another prop to check performance, so I am hoping to improve if not nail it on the head on the first shot. I would like to be close enough that it would at least be an acceptable backup. I do boat in a lake/river system with a better than average chance of a prop strike.

 

[Sea Rider]

If you buy a X prop pitch that achieves close to max wot rpm for that OB, you can re pitch -0.5-1 sizes at a prop shop to pull wot revs a bit up if in need. It's very rare to dial top prop for any application at first try, if so, you?re a lucky boater.

Happy Boating

 

[Texasmark]

One more question, anybody have a preference as far as manufacturers? I know and have owned Michigan Wheel, but how does Solas or Hustler stack up?

First off I have a Turning Point Hustler (so I can talk about them) and several SS Ballistics. Hustler advertises all the amenities of SS props except for price and thicker blades due to aluminum vs SS strength variance. They are a good $100 contender and if in rocky water, or low budget, for my money they can't be beat. That means it is (most are) ported which acts like a shallower pitched prop when you need to get the rpms up under the hole shot load and disappears when at speed and you don't have the low rpm load. That translates to your being able to run a higher pitched prop for a higher top end while getting the hole shot associated with a lower pitch.

I am one of the guys that preach that the top end of the recommended range is where you run when in your normal running configuration at WOT. Per your data supplied, you are WAY overloaded (over pitched) when you consider 150-200 rpms per inch of pitch change is what folks use in calculations and if your recommended range top is 5500 that's 5" too much pitch. However for that size boat (not knowing anything else about it) a 14 P with that HP and a low gear ratio is way too much pitch and 5 may be what it takes.
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Once you get into a certain hub diameter to fit the lower unit housing diameter (10" prop diam. give or take in your case) of the engine upon which the selected prop will run, the diameter varies as a function of the pitch.....all part of prop engineering design. Think of the blades folding back as the pitch increases, especially on SS props that usually carry a goodly Rake (bending of the blades back from the root for bow lift which produces less drag and higher speed and rpms. So don't get hung up on diameter. Pitch is what you chase.
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www.go-fast.com has a prop slip calculator that is very useful in determining the answer to your question.

You need your gear ratio....like your 10" dia prop may be 1.65:1....put 1.65 in the gear ratio box.

The rpm box is your manufacturer's recommended WOT maximum rpm for your engine.....most engines fall in the range of 5000-6000max rpm rating and that is for your regular boating configuration, not your heaviest, nor lightest load. So if it's 6000 put that in the box. For a number a 1994 Mariner 3 cyl looped 60 hp has a gear ratio of 1.64:1 and a recommended max rpm operating range of 5000 to 5500. Assume you had that engine running at your 4500, you have 1000 rpms that you need to go and find somewhere......and the sweet thing is that you have to back out pitch to get there and in doing that the hole shot just gets better and better.

Next is prop pitch (forget the diameter). If it's 14, put that in the box.

Solve for the next box which is absolute max you could run if the prop were a screw turning in wood.....no slippage. Let's say the number comes out to 30 mph.

Now you have 2 ways to go. You can play the guessing game and guess your prop slip (difference between prop turning in wood and water which allows the prop move forward less per rev than it would in wood. On light fast boats, 10 is a good number. On tubs 25 or so would be the number. Put that in and solve for actual.

Or if you have a gps mph number (speedometers are notorious for errors...my last one read 2 mph high at 50 mph) leave the slip blank and put your actual speed in and check for your slip. If you are getting 25 for a light fast boat, you have something dreadfully wrong. Could be instrumentation, could be setup, could be boat design, something wrong like a hook in the hull, engine sitting too low causing too much drag etc, etc, etc.

If you want to play with numbers, forget the last 2 blocks, work backward from the screw in wood speed number....like from the 30 I used above for max speed. Erase your rpms, and decrease your pitch and solve for rpms. That will give you a pretty good idea as to where you need to be in terms of pitch......but it's not for free. This calculator assumes that nothing changes but what you change. If you change prop manufacturers, or different prop types from a give mfgr, or your boat performs differently as a result of the difference in the prop and rpm number, you just threw a monkey wrench in the works and the result will not be as accurate as you would like.

HTH,
Mark

 

[charleso]

Thanks for the insights. I will look into the Hustler propellers. My final gear ratio is 1.85:1. The Hustler calculator called for a 14p. Does the hole make that big a difference in RPM? I understand that it lowers the resistance at low RPM to allow the engine to turn up faster. Is that correct?

 

[Texasmark]

Thanks for the insights. I will look into the Hustler propellers. My final gear ratio is 1.85:1. The Hustler calculator called for a 14p. Does the hole make that big a difference in RPM? I understand that it lowers the resistance at low RPM to allow the engine to turn up faster. Is that correct?

The Port (hole) sits just below and just behind the front of the blade root; one per blade. At low rpms there is little "slip stream" water flowing by the prop so exhaust gasses can exit where they find a hole. With the blade turning the exhaust gasses are pulled across the face of the following blade lowering the density of the water across it. With a lower density, the resistance against which the prop is attempting to spin up to higher rpms (the load on the engine), is reduced allowing the engine HP to develop faster. Since HP = (torque x rpm)/5252 the HP is developed faster as the rpms increase faster spinning the prop faster and getting your available thrust built up faster. This results in a faster hole shot, especially boats with heavy sterns and props pitched for the WOT max speed (bass boats for one) and is the reason for the ports. You basically get 2 different pitches in one prop with an automatic pitch change.

Once up on plane the slip stream water pressure flowing past the holes and the suction created by the props tube passing through the water will cause the path of least resistance to be out the tube and the holes are sealed off, out of the picture. Prop responds to the geometry of the prop as if no holes were present. You can tell when they seal off as you get an instantaneous drop in engine rpms and lurch forward in speed....on my boats anyway. They work good on water skiing with a higher pitched prop than would normally be useful. The ports act like a lower pitched prop on engine loading for ski pop ups.

They can reenter in areas where external turbulence occurs like skipping on wave crests at moderately high to high speeds with high trim angles, and tight turns with high trim angles. One has to decide what is more important and capitalize on what you like about ports and minimize what you don't through how you handle your boat. In both situations cited, usually dropping the trim a few degrees, or in the case of tight turns, doing that and dropping the throttle a tad will stop any blow out caused by the ports ventilating. Course, about ? of the way through the turn, you smack the throttle and trim her back out and she screams at you and then settles down in the straightaway. Great sport....for me anyway.

"Size matters" and the port holes determine the hole shot firewalled max engine rpm and the time it takes to recover rpms once on plane. Props like the Laser II have inserts of different sizes that are made for just that purpose, allowing you to "tune" your prop for your application.