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Prop Size???

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Initially Posted - 02/29/2004 : 18:06:22

I'm going to replace the stock 9" x 9" prop on my 8hp 2-stroke with a lower pitch prop, but I don't know if I should go with the 9 3/4" x 6.5" or 5.5". Michigan Wheel lists both of these props for workboat/barge applications, but having never used a lower pitched prop, I don't know their particular operating characteristics.

So my questions are...

Will the 5.5" be too low and put a dent in fuel economy when cruising?

Will the 6.5" still be too high a pitch for a boat that runs at 7kts?

Does the lower pitch prop raise the motor's rpms significanly at cruising speed?

Although I haven't had any problems operationally with my current 9" by 9" prop, I think replacing it will give me better performance, but I want to be sure I select the right prop before committing the $80-90 on a new one.

Thanks.

Don Lucier

North Star SR/FK

Edited by - on

Derek Crawford
Master Marine Consultant

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USA
3323 Posts

Response Posted - 02/29/2004 : 18:34:30

Don - after speaking with the Nissan factory last year, I switched my 8 h.p.'s prop of 7.5" with a 6.5". The factory said that at cruising revs it would run about 200 rpms faster. BUT it gives me much greater control at low revs for manoevering and docking. I have no experience with a 5.5".
Derek

Edited by - on

Dave Bristle
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Djibouti
10005 Posts

Response Posted - 02/29/2004 : 19:05:41

Don: FWIW, the Honda 8 is 6.5X10 with four blades. Its HP is rated at 5000 RPM, while your 2-stroke might peak a little higher and have a little less low-end torque. That would suggest a little lower pitch. If you plan to motor-sail on cruises, you can figure about 15% higher RPMs to maintain the same speed with the 5.5. I think the 6.5 is going to give you a significant advantage over your current prop around the docks--you don't need that much more (the 5.5), and the 6.5 might make things just a little easier on your ears as well as your gas tank while cruising.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 02/29/2004 : 21:23:53

It looks like the 6.5" is the winner. I was leaning towards this prop when I asked this question, but I wanted a more learned opinion to seal the deal.

Thanks Derek and Dave.

Edited by - on

Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 02/29/2004 : 22:00:49

And I will let you know how I like my 10 x 5 on my Johnson 9.9 2 stroke sometime in April.

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cshaw
Captain

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USA
460 Posts

Response Posted - 02/29/2004 : 22:08:12

Frank,

I have had a 10x5 on my 9.9 longshaft Johnson since 1976 and love it. Plenty of power to get unstuck from the bottom when needed, and I even pull a 10ft shrimp net occasionally!

Chuck

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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 02/29/2004 : 22:26:57

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by cshaw
 Frank,

I have had a 10x5 on my 9.9 longshaft Johnson since 1976 and love it. Plenty of power to get unstuck from the bottom when needed, and I even pull a 10ft shrimp net occasionally!

Chuck

<hr height="1" noshade id="quote"></blockquote id="quote">
Thanks Chuck, that is great news. I simply ordered the lowest pitch I could, we installed it in my garage and we can hardly wait to launch.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 02/29/2004 : 22:57:51

That's 2 votes for the 6.5" and 1 and a maybe for the lower pitch 5.5".

Looks like I'm back to square one!

Thanks Chuck and Frank for the input...

Edited by - on

cshaw
Captain

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USA
460 Posts

Response Posted - 02/29/2004 : 23:26:20

Sorry to contribute to more indecision! (did not mean my comments to do that!!)

I chose the power prop because of the waters I sailed in. If you are on a lake or regularly power long distances in smooth water the "speed" prop probably has the edge. If you find yourself powering into waves or heavy chop, the power prop would improve your performance. I sailed out of Ventura, CA, and the waves and chop there prompted me to want the extra power. It also works OK for long distance powering (albeit probably at a higher fuel cost than the speed prop). I used to power all the way back up wind from Los Angeles to Ventura sometimes after races that had gone north to south. You can probably overspeed the engine with the power prop, but you have control of that with the throttle..

Bottom line is you probably cannot go bad either way (decisions that have no "wrong" answer are always the hardest, eh?)

Chuck

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dlucier
 That's 2 votes for the 6.5" and 1 and a maybe for the lower pitch 5.5".

Looks like I'm back to square one!

Thanks Chuck and Frank for the input...
<hr height="1" noshade id="quote"></blockquote id="quote">

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 03/01/2004 : 00:15:09

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by cshaw
 Sorry to contribute to more indecision! (did not mean my comments to do that!!)<hr height="1" noshade id="quote"></blockquote id="quote">

Believe me, you didn't contribute to my indecision...I was born that way!

In my sailing area in western Lake Erie, if the waves and chop are up, wind is good so the motor isn't used. It is only when the wind goes to nothing and the lake turns to glass, that the iron genny gets called upon. Additionally, I'm in a protected marina with no currents so I don't need a whole lot of low end power around the docks.

With these thoughts in mind, I've decided that I'm going with the 6.5" prop because my motor will mostly be used in rather benign conditions and I believe the 6.5" prop will give me that happy medium I'm looking for between power and speed (fuel economy).

You hit the nail on the head when you said, "Decisions that have no "wrong" answer are always the hardest, eh?"

Edited by - on

Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 03/01/2004 : 18:16:29

Changing props on a motor not designed as a high thrust version will probably offer limited success in part because of restraint imposed on increasing diameter. On motors with a two or three blade prop, that can be offset by adding an additional blade or two, that may be an option to overcoming the diameter problem.

You don't say how many blades you have and how many the option props have. Dave makes this point and I agree with him.

The other problem and hence my reason for chirping in... is that when adding another blade, some consideration to the prop reduction ratio will need to be considered. For example, the Honda Classic 8 had a three blade prop and though not a high thrust version, did very well for itself, but that motor also had a little higher gear ratio difference than many motors which means that the prop was spinning slower.

Interesting is that a historical perspective about the success of the Wright Brothers, finds that a great amount of their success was because of their efforts and successes with propellors. Its not an easy task to match a fixed pitch propellor to a load that changes with the speed. It might be great if they were variable, but that would mean more expense and opportunity for failure.

A neat comparison between props can be seen in model airplanes. Those with 2 stroke motors that run at high rpms of course are smaller than those for 4 stroke motors that turn considerably slower. One challenge of this is idle speed. While a 2 stroke idle speed is not very critical, and power is pretty well gone even if the idle is fairly fast, a fast idle on a 4 stroke with a larger prop will make slowing for a landing difficult.

Props are interesting stuff.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 03/01/2004 : 20:06:30

Today, I went to West Marine and ordered the 9 3/4" by 6.5", 3 bladed prop which was recommended by Michigan Wheel for both by engine size and application. WM sells this prop for $89.00, but I found the prop at iBoats.com for $70.55 and they matched it. It'll be here on Thursday.

I chose the 6.5" because to me it appears that this prop would give me the additional RPM's my engine needs, yet not sacrifice too much fuel economy at hull speed. I liken my current speed prop to having only a fourth gear in a car that can only go 10mph...rather dismal out of the hole and overgeared at terminal speed. I imagine the new 6.5" will probably be like having only a second gear, allowing me to take off reasonably okay and have decent RPM's at speed.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Arlyn Stewart
 Changing props on a motor not designed as a high thrust version will probably offer limited success in part because of restraint imposed on increasing diameter. On motors with a two or three blade prop, that can be offset by adding an additional blade or two, that may be an option to overcoming the diameter problem...<hr height="1" noshade id="quote"></blockquote id="quote">

Arlyn,

I don't know if I understand what you mean when you say this prop change would only offer limited success due to the restraint imposed on the increasing diameter, considering that my diameter is only increasing 3/4" while the pitch is being reduced rather significantly. On paper this change would appear, to me anyway, to have relatively dramatic results.

Currently my 8hp 2-stroke pushes my boat to hull speed at a little over half throttle. After this point any more RPM's, which actually aren't avaiable, cause the motor to labor and bog down due to the 9" speed prop that is currently on it. The lower pitch prop should allow my motor to rev closer to where it develops its power instead of stopping halfway as it does currently.

As for the 4 blade prop, I don't know is this prop would work well with a 2-stroke because of the increased bite it has at lower RPM's. I would think it would be more suited for a 4-stroke which develops more usuable power at lower RPMs.

Edited by - on

Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 03/01/2004 : 22:29:35

My bad... when you said performance... I was thinking high thrust when you were talking matching power band.

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 03/01/2004 : 22:35:13

Don... I think you're thinking is pretty much on the mark. The lower-pitch prop should allow your 2-stroke to cruise closer to its "happy speed" RPMs, and it will absolutely, certainly give you better stopping, backing, and starting power. Another advantage is prop-walk, since the "power prop" throws more water in the desired direction and has less of the paddle-wheel effect that causes prop-walk at very low boat speeds (such as starting in a slip). With about 6 knots hull speed, our boats run much slower than what a standard ("speed") prop is designed for. That design is intended for, say, a 9' inflatable that the engine can get up on plane, but which weighs only a few hundred pounds loaded. A standard prop is, by design, wrong for our 5000+ lb. displacement-hull boats.

Enjoy your "new" kicker--it's indeed getting a new life!

Edited by - Dave Bristle on 03/01/2004 22:36:56

osmepneo
Past Commodore

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USA
1420 Posts

Response Posted - 03/01/2004 : 22:35:36

Don,

Another FWIW I ordered the 6 1/2 prop for my new Nissan last w/e, I'll report on performance when the hard water goes away!

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Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 03/01/2004 : 23:14:02

<blockquote id="quote">quote:<hr height="1" noshade id="quote">certainly give you better stopping, backing,<hr height="1" noshade id="quote"></blockquote id="quote">

This was where I was headed with my comments... about not achieving a significant gain.... the change is to the power band and yes it should yield better forward characteristics but... high thrust is what is needed to increase stopping and reverse abilities. With it the foils are less asymmetrical to provide better parity between forward and reverse... to counter the loss of foil lift more surface area is added ie more blades and larger diameter.

btw, I'm not so sure that high thrust doesn't give up some forward efficiency as a trade off for the reverse abilities. The comment about the alternator output being the deal between 1.6 and 2.25 hours per gallon might be part of that... but the loss of forward efficiency might come into play as well.

I think the Honda 8 classic that didn't use reverse thrust killing thru the prop exhaust might have been the more econimical and reasonable choice for forward abilities for someone who cruises a lot where someone who is dealing with getting around traffic needs great brakes.

Having said that... the extra reverse thrust might save my skin someday in some unforseen situation...

who knows.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 03/02/2004 : 09:08:23

Thanks again guys for all your opinions...they are much appreciated!

Edited by - on

Waterboy
Navigator

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USA
204 Posts

Response Posted - 03/02/2004 : 09:12:13

I've being undergoing similar mental machinations regarding a prop to replace the somewhat dinged 8.5" X 7.5" for an 8hp Yamaha (any Yamaha prop experts here?). I was leaning towards the 5.5" but now tend towards the 6.5" too, as we'll likely be motorsailing a fair bit. Currently the boat reaches hullspeed in light winds at about 1/3 throttle, and reverse performance is absolutely abysmal - bad enough that I was seriously concerned about the lee shore off our boat ramp in moderate winds. How much does the condition (nicks and dings etc) of the prop affect performance? Can a person file these out? Of course if they're too big the prop will be unbalanced causing more obvious problems.

Incidentally an in addition to the contributions Arlyn has made here with his repies, I'd suggest reading his article on outboards posted on his website under 'theory'. I found it quite useful and informative and believe anyone with prop/thrust/hp questions will too. Thanks Arlyn.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 03/02/2004 : 11:46:17

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Waterboy
 I've being undergoing similar mental machinations regarding a prop to replace the somewhat dinged 8.5" X 7.5" for an 8hp Yamaha (any Yamaha prop experts here?). I was leaning towards the 5.5" but now tend towards the 6.5" too, as we'll likely be motorsailing a fair bit. Currently the boat reaches hullspeed in light winds at about 1/3 throttle, and reverse performance is absolutely abysmal...How much does the condition (nicks and dings etc) of the prop affect performance? Can a person file these out? Of course if they're too big the prop will be unbalanced causing more obvious problems.<hr height="1" noshade id="quote"></blockquote id="quote">

Waterboy,

While I'm no expert, the nicks and dings on your prop can contribute to cavitation which can result in degraded performance. In the past, I've smoothed over small nicks with a file, but on one of my props, I had a prop repair shop do the repair and if I remember correctly, they charged me around $25.00-$30.00.

Check out the [url="http://www.michiganwheel.com/MIWheel/pdf/obsd_03.pdf"]Catalog[/url] from [url="http://www.michiganwheel.com"]Michigan Wheel[/url]. You'll find Yamaha outboards on page 48.

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 03/03/2004 : 23:23:14

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Waterboy
 ...and reverse performance is absolutely abysmal - bad enough that I was seriously concerned about the lee shore off our boat ramp in moderate winds...
<hr height="1" noshade id="quote"></blockquote id="quote">
Arlyn mentions a key factor for braking and reverse power in general--the effect of through-the-prop exhaust. When exhaust exits from the prop hub, it's fine in forward gear, but it reduces the bite of the prop in reverse, as the prop pulls the exhaust back past the blades. Newer designs, particularly those designed for heavy boats (such as Honda and Yamaha 4-strokes) allow the exhaust to exit from ports on the lower unit above the prop when you shift into reverse. I don't know whether the prop pitch or the exhaust vent is the bigger issue in reverse, but I expect the pitch will help for stopping and backing--regardless of where the exhaust goes. So, while you can't change the exhaust design (without buying a different engine), you can change the prop.

Edited by - Dave Bristle on 03/05/2004 09:30:15

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 03/04/2004 : 21:41:22

I picked up my new prop today and compared to my old 9" prop, this new 6.5" pitch prop has bigger rounder blades that kind of look like Mickey Mouse ears!

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Sea Trac
Master Marine Consultant

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Vanuatu
1357 Posts

Response Posted - 03/04/2004 : 22:47:53

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dlucier
 ...this new 6.5" pitch prop has bigger rounder blades that kind of look like Mickey Mouse ears!<hr height="1" noshade id="quote"></blockquote id="quote">
I've been following this thread and wondering what kind of prop is on Antares...now I think I know.

Thanks, Don.

Edited by - on

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