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Levity



Joined: 02 Nov 2003
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City/Region: Shippensburg
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C-Dory Model: 22 Cruiser
Vessel Name: Levity
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PostPosted: Tue May 27, 2008 5:57 am    Post subject: Four Blade Results Reply with quote

Allow me to share performance results for an aluminum Solas 15X3 four blade prop on my 22 with a 90 hp Honda 4 stroke. The three blade it was compared to was a 15x 3.5 aluminum. Based on speed log comparisons the 4 blade drove the boat the same speeds at the same RPM (within .1 nmh or so more or less)through the entire displacement range. The 4 blade required more RPM to climb over the bow wave and more RPM to match speeds with the three blade throughout the remainder of the test. The three blade at 4200 RPM will drive the boat at 18 knots. The four blade at 4200 RPM drove the boat at 14 knots.The 4 knot speed difference was consistent throuout planning speeds. Is the .5 difference in pitch responsible for the performance differences? What other performance parameters can be evaluated to determine which prop would serve best?
Mike 'Levity'
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jlastofka



Joined: 10 Jun 2007
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PostPosted: Tue May 27, 2008 6:21 am    Post subject: Reply with quote

18 times 3 divided by 3.5 is about 15.5, so your 14 kt result is slow and it sounds like the 4 blade prop is a less efficient design or one of the props is pitched inaccurately. In general, fewer blades are always more efficient. I've read some books on props. Two blades are best. You make them bigger when you have more power. At some point they get too large and you start adding blades. You try to match the pitch to the engine RPM and boat speed. You want the diameter small to not waste power, but big enough to not have too much slippage, which is the difference between the blade pitch and the actual amount it moves forward in the water. If you have a lot of power and not much room you start adding blades.

Jeff
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thataway



Joined: 02 Nov 2003
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PostPosted: Tue May 27, 2008 11:53 am    Post subject: Reply with quote

Other factors in the prop are rake of the blades, the amount of cupping of the blades, the actual size of the prop blades. Some 4 blade props are designed to give more stern lift than 3 blade props. Props are complex, and there are not just one or two simple factors (like pitch and diameter) as we would like to think. Another question is the amount of "slip" (basically the effeciency of the prop driving the boat thru the water--ideally for 12" pitch, one rotation would drive the boat a foot. ) If it only drives the boat 6" there is 50% slip. 9" 25% slip. I find in my CD 25, with a 4 blade Honda Prop, that as I load the boat, the slip increases.

Also materials vary. The SS prop is stiffer, aluminum and plastic deform as the thrust is applied. Generally the SS prop is better, but in many cases the two props are not equal in other factors.

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Thataway
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flapbreaker



Joined: 26 Jan 2005
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PostPosted: Tue May 27, 2008 1:30 pm    Post subject: Re: Four Blade Results Reply with quote

Levity wrote:
Allow me to share performance results for an aluminum Solas 15X3 four blade prop on my 22 with a 90 hp Honda 4 stroke. The three blade it was compared to was a 15x 3.5 aluminum. Based on speed log comparisons the 4 blade drove the boat the same speeds at the same RPM (within .1 nmh or so more or less)through the entire displacement range. The 4 blade required more RPM to climb over the bow wave and more RPM to match speeds with the three blade throughout the remainder of the test. The three blade at 4200 RPM will drive the boat at 18 knots. The four blade at 4200 RPM drove the boat at 14 knots.The 4 knot speed difference was consistent throuout planning speeds. Is the .5 difference in pitch responsible for the performance differences? What other performance parameters can be evaluated to determine which prop would serve best?
Mike 'Levity'


Wow, this is interesting to me. If I'm reading this right the 3 blade is way more effecient than the 4 blade? I assume you didn't have any fuel flow numbers but if the boat will do 4 more knots at the same RPM's it would be safe to say that you would burn less fuel with the 3 blade setup. I'm all for saving fuel. I might have to look into a 3 blade setup.

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flapbreaker



Joined: 26 Jan 2005
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PostPosted: Tue May 27, 2008 1:53 pm    Post subject: Reply with quote

Is the 15 the pitch? What diameter prop's are these?
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starcrafttom



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PostPosted: Tue May 27, 2008 3:43 pm    Post subject: Reply with quote

Quote:
15X3 four blade prop


yeah is that a 13 x 15 pitch??? a 15x3x4 is a what?? most props , if I'm right, are blade x diameter x pitch. I run a 4x13x15 so thats a 4 blade by 13 diameter by 15 pitch right? with this prop I get 27 mph at 5400 rpm. and at 4400 I get 20. all else being equal a three blade would have a pitch of what? 17? I was thinking of going to a 3x13x15 to get the rpm up a little. or a 4x13x13 to do the same thing.

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Capital Sea



Joined: 16 Dec 2007
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PostPosted: Wed May 28, 2008 2:13 pm    Post subject: Reply with quote

Thomas, I really do not have this whole prop thing down but will say this.
The Capital Sea is powered by the new Honda vtec90 programed fuel injection and the factory matched this and boat with a three blade aluminum 13.5 X 15" prop.
So far I would say the fit is a good one with the boat loaded 9.9 kicker and so on. I have made 34 mph (sog) on glass slack with two aboard. We almost made Friday Harbor from Edmonds on one tank running between 4500 and 5000rpm the whole way with four and gear aboard.

Still, I intend to replace the prop with a stainless and use the aluminum as a back up. At this point I will go with the same 3 blades, pitch and size but am looking for advice.

If I had a cool fish box I would store the spare prop in it. Confused

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starcrafttom



Joined: 07 Nov 2003
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PostPosted: Wed May 28, 2008 2:40 pm    Post subject: Reply with quote

Steve what you have to keep in mind is that the new honda 90 is 105 to 110 hp not 90 hp so you will always get more speed then me. I am going to reprop and keep the one I have for a spare, but have not decided if I want a 4x13x13 or a 3x13x15. maybe a custom 3x13x14 is in order. Which brings up a question. Can a pulse prop, which is adjustable , be set to 14 or are the adjustments 12,13,15,17????
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20dauntless



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PostPosted: Wed May 28, 2008 8:37 pm    Post subject: Reply with quote

Steve, what kind of performance are you getting with that combo at 4000, 4500, and 5000 rpm?
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tpbrady



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PostPosted: Wed May 28, 2008 8:41 pm    Post subject: Reply with quote

Steve,

I have a 3 blade 15 pitch Powertech stainless prop I will sell you. It is my #3 choice after my 15 pitch stock aluminum prop, and my #1 choice a 14 pitch Quicksilver 3 blade prop. That one is especially good for heavy loads and seems to provide pretty good economy at 4100 or less rpm.

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Wefings
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Joined: 29 Nov 2005
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PostPosted: Wed May 28, 2008 10:00 pm    Post subject: Reply with quote

Here is a simple point of view from someone who is an amateur propologist . The only apparent benefits I see in my day to day dealings with four blade props on outboards are; #1 a boat with a tunnel that creates "dirty water", #2 a boat with a Jackplate used to get on plane quick in shallow water with prop near the surface, and high performance boats [none of which apply to C- Dorys] More important factors are the blade shape [Bob said rake and cup] which is what I lump as blade shape , and the one we all are most familiar with , pitch and diameter. Never will a plastic prop be the best choice, sometimes aluminum is just fine and you can buy two for the price of one stainless, especially with lower horsepower and lower speed preference, and you can spend a fortune to save a couple drops of gas tweaking and record data all day instead of enjoying your boating experience . There is no new mousetrap in the world of props and the available ones are there for a reason . Get one to turn [with a load] fairly close to where your motor's RPM range should be and you have a winner .
Marc

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starcrafttom



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PostPosted: Wed May 28, 2008 10:39 pm    Post subject: Reply with quote

steve, you have to remember that your new injected 90 hp is really making 105 to 110 hp so comparing it to a older 90hp is not apples to apples. and its lighter.
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@ C



Joined: 15 Mar 2006
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PostPosted: Wed May 28, 2008 11:33 pm    Post subject: Reply with quote

Marc:
Good post.
Thanks,
Bill

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cemiii



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PostPosted: Wed Jun 04, 2008 1:32 pm    Post subject: Reply with quote

you'd think brp and prop specialists would know something about 4 vs 3?

http://www.evinrude-parts.com/boat_propeller_selection_article.html

Chris
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breausaw



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PostPosted: Mon Jan 26, 2009 7:19 pm    Post subject: Reply with quote

starcrafttom wrote:
steve, you have to remember that your new injected 90 hp is really making 105 to 110 hp so comparing it to a older 90hp is not apples to apples. and its lighter.


Would someone explain this to me?

BF90, WAT 5000-6000 rpm rated 90hp @ 5500 rpm
90EFI, WAT 5300-6300 rpm rated 90hp @ 5800 rpm

I see no difference in HP here, where’s that extra 15 to 20 hp coming from?

Above taken directly from manufactures website.

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