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Prop Size Revisited

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Initially Posted - 08/12/2004 : 16:10:38

Yesterday, in anticipation of an upcoming rather long trip, I removed the lower pitched 9 3/4" x 6.5" prop that I installed last April and put on the original 9" x 9" prop.

The main reason for reinstalling the old prop was that my fuel economy with the new lower pitch prop was about half of what I got with the original prop due to the increased rpm's that this higher thrust prop allows my motor to rev up to. In the past 4 months, I think I've used more fuel than I did in the last two seasons with the old prop.

Additionally, the performance I thought I would gain with the high thrust/lower pitched prop was never realized. With the low pitch prop, my top speed was lowered and to reach this lowered speed, my motor had to rev to a much, much higher rpm as compared to the old prop which hit a higher speed at half throttle. Aside from this, the difference in performance on open water in all kinds of conditions (wind, waves, currents,...etc) was undetectable. Also I thought I would gain some performance around the docks, but again, since I manuever so slowly in and around the docks, I never realized any performance change with the high thrust/low pitch prop.

So now that I'm back to my old prop I'll have...

<ul><li>Better Fuel Economy </li><li>Higher Top Speed </li><li>Less RPM's which means less noise </li></ul>

This prop experiment has me rethinking the whole "high thrust" idea as applied to our sailboats with me leaning towards the notion that it simply isn't necessary for our hulls. The reason I believe this, is due to the fact that our relatively slippery hulls don't need the power of a tug boat to move. This is evident when I'm at hull speed, put my motor in nuetral, and pretty much glide on forever. An application that I can think of where high thrust would be needed is on the 10' wide, blunt front utility barge up at my father's cottage. When you put a motor in nuetral on that slug at speed, it's hydrodynamically challenged hull stops it rather quickly. In this case, since you are pushing a lot of water, a high thrust outboard would be ideal, but does our slippery hulls need this?

Don Lucier

North Star SR/FK

Edited by - on

jedperkins
Deckhand

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USA
8 Posts

Response Posted - 08/12/2004 : 16:34:48

I have a Honda 4 stroke 15 HP OB on my Catalina 250. I sail on Lake Granby in Colorado, at 8280'. I just got my boat this season, brand new motor, and had a lot of trouble with it stalling out when I was starting the boat from standing still, or switching from ofrward to reverse. After we switched the injector to lean out the mixture for higher altitude I was still having problems. It turns out that the culprit was the stock propeller, which had too high pitch. I switched to a lower pitch propeller and it hasn't stalled since. So, here is at least one application where a low pitch propeller is required. Of course most of you aren't sail more than 1 1/2 miles above sea level!

Edited by - on

matsche
Captain

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USA
280 Posts

Response Posted - 08/12/2004 : 20:29:58

A high thrust prop on a sail boat never made any sense to me either, unless you like to come into the slip at hull speed! My little 6hp Tohatsu with a 7.8" prop gives me more than enough thrust to maneuver to the slip even in high winds. I think your observation about speed and fuel economy speaks right to the point of having an "auxillary" motor in the first place. To get you in when the sails are no longer an option, and through the marina without banging up somebody's boat.

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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 08/12/2004 : 23:18:30

As Don probably remembers, I got an even lower pitched prop at the same time he got his. I believe mine is 10"x5. I have lost all my cruising speed and do not see any real difference around the docks. Maybe there would be at very low RPM, (a difference) but I am afraid to rum my 1982 motor at too low an RPM, it will die. My old prop is less than straight but I may still pt it back on.

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Mark Maxwell
Captain

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USA
329 Posts

Response Posted - 08/13/2004 : 01:02:24

I am considering a high thrust prop for these reasons, to reduce speed at idle and there by adding control. Tell me if this is wrong.

I have the new 9.8 Nissan 4s. It runs great, even at 7000'. However, at idle, I am dong 2knts. I'd like to slow this down without having to slip the gear shift in and out of gear. I see how the lower pitch/high thrust prop will require more rpm's to achive the same speeds as the higher pitch/oringinal prop, but isn't that the point? Kind of like shifting into a lower gear? Starts and stops are quicker but top end is less.

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 08/13/2004 : 11:00:16

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Mark Maxwell
 I am considering a high thrust prop for these reasons, to reduce speed at idle and there by adding control. Tell me if this is wrong.

I have the new 9.8 Nissan 4s. It runs great, even at 7000'. However, at idle, I am dong 2knts. I'd like to slow this down without having to slip the gear shift in and out of gear.<hr height="1" noshade id="quote"></blockquote id="quote">
Mark,

A high thrust/low pitch prop may or may not lower your speed at idle, and even if it does, I can't imagine it doing so significantly. Even with my high thrust prop I still had to shift to neutral to bleed off speed in the marina.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Mark Maxwell
 I see how the lower pitch/high thrust prop will require more rpm's to achieve the same speeds as the higher pitch/original prop, but isn't that the point? Kind of like shifting into a lower gear? Starts and stops are quicker but top end is less.<hr height="1" noshade id="quote"></blockquote id="quote">

Yes, that pretty much sums it up, the low pitch/high thrust prop is like running in first gear. The only problem with this is, running in first gear when motoring in open water might not get you to top speed even though the engine is running at maximum rpm's. As for quicker starts and stops, I didn't notice any difference in this area between the two props, but then again, I rarely have a need to stop and start so quickly that I need a special prop. When coming into my slip, I'm in neutral the last 100' feet or so and do not use my engine to stop. When backing out of my slip, I usually push off to get myself moving, then engage the reverse at it's slowest rpm so I never need to gun the engine in gear.

With my rather long upcoming trip, I plan on doing a considerable amount of motoring, and whether in a boat or car, travelling long distances in first gear is not that appealing on so many levels.

Edited by - dlucier on 08/13/2004 11:03:34

Ed Montague
Captain

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USA
499 Posts

Response Posted - 08/13/2004 : 15:43:56

It shouldn't be surprising that the lower pitched prop needs higher rpm to go the same speed as the higher pitched prop. For every revolution you theroreticly move forward the distance of the pitch. Changing the prop and without also changing the gearing in the lower unit to match was pointless for me also. I can tell however, that my Yamaha 9.9, high thrust motor absolutely out performs my old Evinrude 15 hands down. There is more to this high thrust discussion then just the prop.

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 08/14/2004 : 08:52:13

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Ed Montague
 It shouldn't be surprising that the lower pitched prop needs higher rpm to go the same speed as the higher pitched prop...<hr height="1" noshade id="quote"></blockquote id="quote">

The increased rpm's with a lower pitch prop was not what surprised me. What surprised me was the realization that my boat didn't need high thrust propulsion!

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RichardG
Admiral

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USA
990 Posts

Response Posted - 08/14/2004 : 10:29:30

<blockquote id="quote">quote:<hr height="1" noshade id="quote">my Yamaha 9.9, high thrust motor absolutely out performs <hr height="1" noshade id="quote"></blockquote id="quote">

Without being too technical, the working surface area of the prop on that motor just by casual observation appears to be much larger (maybe double?) than that on the Merc/Nissan/Tohatsu. I think the term "high thrust" is relative.

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ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 08/14/2004 : 11:16:50

I think Don is correct in saying that our boats don't need much power to move at hull speed. Most of us report hull speed at about 1/2 to 2/3 throttle with 8 hp engines. Probably 4 to 5 hp will do to keep things moving.

Where the high-thrust engine will really come into it's own is under those less than perfect conditions... bucking seas and headwinds, boat heavily laden, making emergency manouvers around a dock.

My good 'ol Honda BF-8 does pretty good with it's stock prop. However, there are some times (as noted above) when I wish I had a high-thrust engine. I'll be in situations where I'm running fairly hard with a lot of prop slippage... sometimes around the dock I wish I had more thrust for quicker response. But those moments are the exceptions rather than the rule.

All that said, with the price of a new outboard approaching 1/3 the value of the boat, I'm gonna take good care of what I've got and keep her running as long as I can.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 08/15/2004 : 09:36:09

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by ClamBeach
 ...However, there are some times (as noted above) when I wish I had a high-thrust engine. I'll be in situations where I'm running fairly hard with a lot of prop slippage... sometimes around the dock I wish I had more thrust for quicker response...<hr height="1" noshade id="quote"></blockquote id="quote">

I, too, wished that I would get more utility when I switched to the low pitch/higher thrust prop, but that just wasn't the case.

The reason I made the prop switch was because, in the past couple seasons I've been caught in some serious stuff (50mph winds, large breaking waves,...etc) that reduced my speed to around 1kt even though the motor was reving as high as it could. I thought that with the low pitch/higher thrust prop, I would have better performance in less than optimum conditions, but on a couple of occasions this year even with the higher thrust, the boat slowed just as it did with the original prop. In these situations, I don't think a 15hp high thrust motor would've made a difference.

As for quicker response around the docks, I didn't notice any difference.

Edited by - on

Brooke Willson
Admiral

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USA
983 Posts

Response Posted - 08/15/2004 : 11:19:30

Folks, something about this whole discussion seemed backwards to me, so I checked a number of sources about the relationship between pitch and thrust, and I'm pretty sure I'm right: lower pitch means LOWER thrust, not higher. Thrust is the propulsive force generated by the screw (prop). A lower pitched prop generates less force at a given rpm. Higher torque/more powerful engines are able to drive a higher pitched screw at a given rpm.

When airplanes take off, they have the props at low pitch because the engine can operate at maximum revolutions/higher horsepower. That's the best power/thrust relationship at low speed. However, once the plane is in the air and moving at speed, the pilot increases the pitch because the apparent angle of attack of the prop is reduced by forward speed (like apparent wind for us sailors). A low pitch prop for us might allow max revs at low speed and theoretically higher thrust in that situation, but be a distinct disadvantage at or near hull speed. Just as we trim in our sails on the wind (higher pitch) because boat speed moves the apparent wind forward, once the boat is moving we need a higher pitch prop to be efficient.

If you are revving too high, you need a higher pitch prop because you have LOW thrust.

Brooke

Edited by - Brooke Willson on 08/15/2004 12:21:35

ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 08/15/2004 : 11:21:50

I'm suspecting there's a performance difference between having a low pitch prop installed on a 'regular' engine... and a having purpose-built 'high-thrust' engine.

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 08/15/2004 : 17:36:36

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Brooke Willson
 Folks, something about this whole discussion seemed backwards to me, so I checked a number of sources about the relationship between pitch and thrust, and I'm pretty sure I'm right: lower pitch means LOWER thrust, not higher. Thrust is the propulsive force generated by the screw (prop). A lower pitched prop generates less force at a given rpm...<hr height="1" noshade id="quote"></blockquote id="quote">

Brooke,

The thrust we are referring to (at least me) is the power (thrust) initially put to pavement or, in our case, the water.

Prop pitch is sometimes compared to gears in a transmission with low pitch props equating to low gears and high pitch props to high gears. Like in the case of a drag car at the line, more power (higher thrust) can initally be put to pavement (water) at take off in first gear (low pitch) than in fourth gear (high pitch). Conversely, if that same car took off from the line in fourth gear, the car would bog down and stumble with very little power (or thrust) being put to pavement.

So for our relatively small outboards and slow displacement hulls, theoretically, a lower pitched prop will develop more thrust to get that skier up than will a higher pitched prop which may just bog down and leave the skier in the water.

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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 08/15/2004 : 18:37:46

If you check the descriptions the low pitch props are specifically referred to as sail boat auxiliary props. I checked my top end last night and it was 5.4 knots.

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Ed Montague
Captain

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USA
499 Posts

Response Posted - 08/16/2004 : 14:17:05

Everybody has made good points as most of the comments concern forward motion. I have found the biggest improvement with the high thrust motor/prop is in reverse. My old Evinrude needed high revs to pull me out of my slip and was a poor brake. My newer Yamaha is a great brake and pulls me out of the slip at very low rpm's.

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Dave Bristle
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Djibouti
10005 Posts

Response Posted - 08/16/2004 : 20:26:17

What we might be seeing here is the difference between a low-pitch prop retrofitted onto an engine geared for a higher one, and low-pitch props on "high thrust" engines designed and geared for them. Some of Ed's reverse thrust is due to the exhaust not venting through the prop in reverse, and therefore not diminishing its bite.

I assumed a normal 2-stroke would benefit from a lower pitch because the standard pitch is generally designed to drive a fraction of the weight at several times the speed through the water. I based that on my own observations with the Power Thrust Honda 8 compared to an older, standard pitch Honda 8. The power around my slip, even at low RPMs, is phenomenal. The RPMs at cruising speed are probably slightly higher, although I don't know how much. Arlyn Stewart has complained about the increase in fuel consumption between the same two engines--maybe this is why. But he relies on his more for long distance cruising than I do. I especially need mine for precision maneuvering in the slightly unusual slip situation I have (8-10" between boats, cross winds, and traffic going by to and from a nearby ramp).

Don, I'm sorry if I contributed to your failed experiment--you've added some data for those who would follow.

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 08/17/2004 : 09:30:06

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle
 ...Don, I'm sorry if I contributed to your failed experiment--you've added some data for those who would follow.<hr height="1" noshade id="quote"></blockquote id="quote">

Dave, or anyone else,

In no way did anyone contribute to my experiment, it was all my doing.

I've been thinking of making this prop switch for a while now based primarily on the props I see hanging off the backs of most of the sailboats in my marina, which are mostly low pitch props.

Where I made my mistake was when I assumed that I needed a low pitch prop because everyone else had one, when I should have followed the wisdom of....

If it ain't broke, don't FIX it!

Anyone need a practically new Michigan Wheel 9 3/4" x 6.5 prop that will fit 1986 and newer Mercury 6-9.9hp 2-strokes.

Edited by - dlucier on 08/17/2004 15:14:50

oldsalt
Admiral

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USA
578 Posts

Response Posted - 08/17/2004 : 18:39:06

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by RichardG
 <blockquote id="quote">quote:<hr height="1" noshade id="quote">my Yamaha 9.9, high thrust motor absolutely out performs <hr height="1" noshade id="quote"></blockquote id="quote">

Without being too technical, the working surface area of the prop on that motor just by casual observation appears to be much larger (maybe double?) than that on the Merc/Nissan/Tohatsu. I think the term "high thrust" is relative.
<hr height="1" noshade id="quote"></blockquote id="quote">

Bingo.....you got it.

I was lucky enough to spend the day sailing and took the opportunity to take a good look at the "high thrust" prop that came with my '96 9.9HP Johnson Sailmaster. The prop has a huge surface area compared with the one that came with my "spare" outboard; an '83 Evinrude 9.9 Yachtwin which is essentially the same 2 cycle engine.

The prop for the Evinrude was described as a sail prop in the manual that came with the engine and it delivered good performance.

The high thrust prop on my new Johnson Sailmaster looks totally different and has at least double the blade area of my other one.

The shape of the prop blades can best be described as "Mickey Mouse ears". They're not tapered towards the tips and have a very blunt tip radius, in fact they can also be described as paddles rather than prop blades.

They really dig in and deliver thrust when I need it and provide very good speed and fuel economy as well. My two outboards are basically the same but the newer Sailmaster performs far better than the Evinrude ever did. I'm absolutely certain it's due to the redesigned prop.

I believe that Richard has hit the nail on the head with his observation.

Edited by - oldsalt on 08/17/2004 18:45:13

Frank Hopper
Past Commodore

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Pitcairn Island
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Response Posted - 08/17/2004 : 18:54:11

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by oldsalt
 <blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by RichardG
 <blockquote id="quote">quote:<hr height="1" noshade id="quote">my Yamaha 9.9, high thrust motor absolutely out performs <hr height="1" noshade id="quote"></blockquote id="quote">

Without being too technical, the working surface area of the prop on that motor just by casual observation appears to be much larger (maybe double?) than that on the Merc/Nissan/Tohatsu. I think the term "high thrust" is relative.
<hr height="1" noshade id="quote"></blockquote id="quote">

Bingo.....you got it.

<hr height="1" noshade id="quote"></blockquote id="quote">

Mark, did you get my email?

Edited by - on

oldsalt
Admiral

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USA
578 Posts

Response Posted - 08/17/2004 : 20:00:02

Thanks again Frank, I switched the URL for the the picture. And it only took me two tries this time.

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 08/17/2004 : 23:17:25

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by oldsalt
 I was lucky enough to spend the day sailing and took the opportunity to take a good look at the "high thrust" prop that came with my '96 9.9HP Johnson Sailmaster. The prop has a huge surface area...looks totally different and has at least double the blade area of my other one.

The shape of the prop blades can best be described as "Mickey Mouse ears". They're not tapered towards the tips and have a very blunt tip radius, in fact they can also be described as paddles rather than prop blades.<hr height="1" noshade id="quote"></blockquote id="quote">

You just described the prop I removed from my outboard. The blades are huge and almost circular in shape and I, too, described them as looking like "Mickey Mouse ears".

Edited by - on

oldsalt
Admiral

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USA
578 Posts

Response Posted - 08/18/2004 : 18:39:13

Don, Considering that your "high thrust" prop looks just like the one that came as original equipment with my Sailmaster, I have must been mistaken in my belief that my 1983 9.9 Evinrude Yachtwin and 1996 9.9 Johnson Sailmaster are virtually identical engines.

The high thrust prop on the Johnson has provided excellent performance including top end speed, and low speed thrust and backing ability as well as better fuel economy so perhaps the engines are geared differently or have different power curves.

Or something like that.

But I'm fairly certain that the engine/prop combination were designed to work properly with each other, whereas retro fitting that kind of prop on an outboard that wasn't designed for it won't work well.

Edited by - oldsalt on 08/18/2004 18:41:47

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 08/18/2004 : 22:48:59

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by oldsalt
 ...I'm fairly certain that the engine/prop combination were designed to work properly with each other, whereas retro fitting that kind of prop on an outboard that wasn't designed for it won't work well.<hr height="1" noshade id="quote"></blockquote id="quote">

I believe most outboards are designed for a range of props depending on application. The prop that I tried was not a "retrofit that was not designed" for my outboard, but one of a range of props recommended by the manufacturer, Mercury, for my outboard.

Edited by - on

Gary B.
Admiral

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USA
969 Posts

Response Posted - 08/22/2004 : 22:35:10

I have the Yamaha 9.9 four stroke high thrust. I know that I have to back down on the throttle a LOT to allow my friend Terry Annis (with a nearly identical boat) to keep up with me. He has a Merc 8 hp two stroke (Sailboat model).

Also, I did an ocean race this spring on an Ericson 27. The owner had a Merc 8 that would push his boat at about 4.5 knots wide open. I thought it was mightily underpowered since he had to rev way high and we waited forever to back out of his slip. On my suggestion, we put my Yamaha on his boat and motored at just under 8 knots (I know...the KM needed calibration, but the difference is very clear). Obviously, the Ericson is much heavier than a C25, but the owner has since bought a Honda high thrust four stroke....

I know...I have probably too much engine on my boat for many situations, but for currents on the Columbia River....close quarter manuvering in current, and long passages with no wind in the current ridden San Juans....I am NOT changing engines...and I am happy with what I have, although the newer Yammy 8 hp high thrusts might be just as good. I will have a pulling contest with anyone who wants to challenge me....(downside?... I changed the engine myself this summer and nearly got a hernia!)

Different "strokes" and props for different folks.....

Gary B.
Vice Commodore
s/v Encore! #685 SK/SR

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