Need more rpm's
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Posted on Sunday May 13, 2007 at 5:14 PM  
This may be a dumb question but, I bought a 165 Alumacraft Navigator with 90 horse Evinrude E-tec last year. My top speed is in the mid 30's but I'm running less than 4000 rpm. I was reading in the owners manual about top end speed, and they said for best performance run in the 5000 5500 range. I feel I could get a lot more out of it. Do I need to get a different prop? If so what kind or any details? Thanks for any info.
Posted on Sunday May 13, 2007 at 5:47 PM  
Yup different prop pitch what pitch you have right now and is it steel or aluminum
Posted on Sunday May 13, 2007 at 8:15 PM  
Its a steel one, I have no idea on the pitch, can't find anything that says what it is.
Posted on Sunday May 13, 2007 at 9:45 PM  
What year is the motor I might have a prop you can use to give you a starting point. Tell me the year of your boat also so I can get the weight and I assume these rpms are at wot with the motor properly trimmed. Also if it is an older motor might be cheaper to replace the rectifier for $20.00 to verify the proper reading on your tack or if you have access to a timing light with an RPMs on it. You need to consider the age or the motor and gauges before you drop 500 on a prop. I can give you a test for the rectifier if you would like just need a good meter to test it.
Posted on Monday May 14, 2007 at 1:28 PM  
The motor is a 06, the boat is an 04 165cs Alumacraft Navigator. Waht kind of performance do you think I would gain with a different prop. Also, would I lose my low end speed, I troll quite a bit.
Posted on Monday May 14, 2007 at 3:55 PM  
I want you to know up front that I am not good at guessing prop selection but I will help you eliminate the others like proper motor height and jack plate use along with those whale fin thingys. You said you troll a lot you need to decarb your motor every year is that is so. I do know lugging your motor like that will lead to a lot of problems which there are to many to list. These outboards like to get to 5000 - 5500 and it will make a difference in the life of the motor if you allow them to.

Jack Plate -- The jack plate will most likely give you additional performance enhancements beyond just getting the motor height up. By having the lower-unit further from the hull, it runs in less turbulent water, you should get lots more lift which reduces drag.

Raising the Motor -- From a geometric standpoint, raising the motor on the transom would be like shortening a lever. You'd get less bowlift.

Motor Heigh -- Every boat is different but this is a basic rule. If you draw an imaginary line past the bottom of the keel,(as if extending the bottom) anti ventilation plate should be in line with this line and 1 - 2 inches above.

I think Ol Slick could help out here he seems to have a good grasp on props and pitch.


Here is something for you to read I did not write this but here

"The Basics of Selecting A Propellor"This is a "general" rule guideline and in no way will indicate the "perfect propellor", as all applications are different in many ways.There are 3 basic items to consider for propellor selection:#1 The engine manufacturer's recommended wide-open throttle RPM.#2 The primary use of the vessel.#3 The amount of money your budget will allow. Propellors are manufactured out of several different materials, with 3 being the most common, they are:#1 Aluminum.#2 Composites.#3 Stainless steel. The most popular by far, is Aluminum, and here are some advantages of this material:#1 Largest selection of sizes.#2 initial cost and ease of repair and/or alteration.#3 Durable and yet forgiving of hard objects as the blade will bend or break usually before major damage can occur. Disadvantages of Aluminum:#1 Thick blades, lowering efficiency.#2 Some blade flex. Advantages of composites:#1 Cheap to purchase.#2 Very forgiving of hard objects as the blades will break free.#3 Very light-weight, easy to store in boat as a spare. Disadvantages of composites:#1 Blades break quite easily.#2 1-piece molded props cannot be repaired with the exception of the replacable blade models.#3 Blade flex under load. Advantages of Stainless Steel:#1 Highest performance props, with many custom built to suit your application.#2 Not easily damaged.#3 Thinner blades creating much higher efficiency. Disadvantages of Stainless Steel:#1 Cost is considerably higher than all other materials.#2 Much harder to repair, and cost more to do so.#3 Less forgiving on hard-object impact. The first rule of thumb is to know and understand your engine manufacturer's recommended wide open throttle (WOT) RPM. If you don't know the Max WOT RPM for your engine, contact your dealer or ask the experts right here on iboats under the Forum listing your engine manufacturer. Select a prop based on the Type of boat you have and how the current prop is/was performing. If you do not have a Tachometer, understand this is the most important instrument you can have. The amount of work your engine is doing is directly related to RPM, not boat speed. In selecting a prop, consider a normal or average load that you will generally have on board. Select a prop that puts you "AT", or "AS CLOSE TO" Max recommended WOT RPM. Some manufacturers give an RPM "Range". It has been found best for engine longevity, to prop for the "TOP" of that range.There are several different "STYLES" of props.Your boat type, size and weight, with consideration of available Horse power, will determine the type of prop best suited to your needs as well.Prop styles include options such as number of blades, generally 2-5, with 3 blades being the most common. These props come in variations of "Flat-pitch", "progressive pitch", "cupped", "un-cupped", "Raked", etc. Progressive pitch, cupped props are the most common in use today. The progressive pitch allows the blades to pull the same load the full length of the blade. The cupping adds a certain amount of "Bite" to decrease ventilation to a minumum. High-Rake, progressive pitch props are used mostly in Stainless Steel props. The best explanation of "Rake", is the amount of angle of the blade to the rear. High-Rake props are common on most high-performance hulls such as Bass Boats, select Flats-style Boats, and some Racing configurations. Generally, the higher the Rake, the more Bow-lift, which is important on heavy, or nose-heavy type boats Pitch Recommendations:As a general rule, 2 inches in pitch change will result in a 350-400RPM change. EXAMPLE: A 15 inch pitch prop turns 5600 at WOT. A 17 inch pitch on the same engine/boat would turn about 5200RPM at WOT. NOTE: Not all manufacturer's props listed as the same pitch will deliver the same RPM. Diameter is another factor that plays a role in prop performance. In most cases, as a particular prop style increases in pitch, the diameter decreases. EXAMPLE: 13-3/4X15, 13-1/4X17, 13X19, 12-3/4X21. A larger diameter prop in one particular pitch will have more "Blade area" and tends to be a better heavy-load prop, but often times suffers "slightly" on top-end due to the increased drag of the larger blade area. Smaller diameter props in a given pitch tend to be "slightly" faster for the same reason..LESS drag from less blade area. The smaller diameter props are usually better suited to "Lighter" boats. An exception to this is in High-performance applications where the prop blades break the water's surface and a larger diameter prop is preferred for better performance.Choosing a prop with fixed-hub or consumer replacable type hub. Three hub types are available in the propellor industry, and I will describe all types briefely. #1, The Manufacturer installed rubber hub system, which has been the most common type for years. This hub type is designed to absorb "shock" in several forms including shifting and prop strikes. The hub in this style is "Intended" to slip free before damage occurs to the lower unit and it's components.This type of hub is found in both Aluminum and Stainless Steel props. #2, The consumer replacable hub system under such names as "Flo-torq", "XHS", etc., are hubs that are sold as kits to make many different manufacturers prop casings fit most any application. This system allows easy replacement of the outer prop assembly at a lower cost than replacing a "spun hub" in a "fixed hub" type prop.It also allows the consumer to purchase propellor casings at a much lower price, and therefore makes it more affordable to carry different pitches. If a hub or prop is damaged, a simple hub change or casing replacement allows "new prop" installation at almost half the cost of fixed hub props. #3, The "solid hub" type is a prop with the splines cast or machined directly into the material the prop is made from, and is actually part of the prop.This type is used mostly in high-horsepower applications where the torque is too high for a flexible type hub such as those listed above, with the exception of Composite props, which also fall under the "solid hub" list, although the composite material will "give" before damage is done to splines or other lower unit parts. V-drives and standard thru-hull inboards use this type of prop as well, although in these applications, the prop material usually consists of some aluminum, brass and nickle to allow a "softer" material in the case of a prop-strike in the V-drives and Inboards. This is not a science, only a guidline to follow. The best set-ups have come from countless trials with different props and many test-runs. It is up to the owner to fine-tune any application, and I hope this helps.
Posted on Tuesday May 15, 2007 at 12:30 PM  
I'd think a 90 horse on a 16 and a half ft. aluminum boat would certainly be enough motor, and there's several things as mentioned that would help the performance, BUT...sure sounds like it's got the wrong prop on it to me unless that boat has some kind of a load on it.

Could be wrong length shaft for the boat. I wouldn't think the tilt/trim would be that big a factor on aluminum, where's the prop in relation to the bottom of the boat, and is the motor in good running condition?
Posted on Wednesday May 16, 2007 at 5:59 PM  
The motor is in good running condition, brand new last year. I did notice the dealership put the motor on the lowest brackets when they rigged my boat up. My rpm reading is when I have my motor trimmed up as far as I can go without causing the front end to hop to terribly bad. Boat was new last year, so everything on it is factory. It is a very wide boat though, don't know how that would effect it. I've got max hp on the boat.
Posted on Wednesday May 16, 2007 at 7:09 PM  
OK, the motor is in good running condition. Are you running a three or a four bladed prop, a three blade will give you more RPM. How much rooster tail are you throwing....If it's more than 4-5 ft. the motor is too high...if you don't see a foot of roostertail over the cowling of the motor the motor is to low.

Now, the horizontal plate on the lower unit above the prop...proper terminology escapes me...should be within an inch or so of being on plane with the bottom of the hull at the transom...that would have the top of the prop just another inch or so below said plane. Your motor sounds like it might be too low.

A smaller pitch would increase your RPM around 150-200 per inch decrease in pitch, but I would make sure the motor is positioned on the boat properly before I would change props.

Hope this makes sense, and I hope it helps....I'd suggest taking it back to the dealer, sounds like the motor was placed on the boat too low, maybe they would help you out some.

That motor should be able to run to within a hundred RPM or so of it's max.
Posted on Wednesday May 16, 2007 at 7:22 PM  
The pitch on most props are stamped on the hub of the prop. Take the prop off and it should be stamped on the inside...sure sounds like you got a prop with too high a pitch. I tired a slighter higher pitch prop on my boat hoping to get a few more miles a hour out of it and just lost too many rpm's and lost my holeshot.
Posted on Thursday May 17, 2007 at 9:55 PM  
Bigmike raise the motor a little and let us know what that did for ya. It is the cheapest just get a few buddies to help if you dont have a cherry picker or a come-along and a good tree and chain. If you wanna drive to Claremore or Broken Arrow just call me and I will set it up for you, for a hand shake just call me.
Posted on Friday May 18, 2007 at 11:28 AM  
I'm going to try to raise the motor. I need to bring my boat to the shop anyways. I appreciate all the ideas and help you have given me. I'll let you know how how it all turns out.
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