26x props, Honda BF50A, 3 different altitudes

May 4, 2019
7
MacGregor 26x Colorado
I have a (new to me) 1997 Mac26x with a Honda BF50A.

Specs for speed test...... (to give an idea of weight)
10 gallons of fresh water
3 people (total 400lbs)
2 batteries and solar panels
ballast tank empty
60lb refrigerator just behind forward V-berth
10 gallons of fuel.
minimal other supplies onboard

I cant seem to get past 8mph with my current prop while in Denver at 5000ft altitude.
At lake McConaughy in Nebraska (3600ft) I could only reach 8mph.


And I will also be running this boat in the mountains at appx 8500ft.
And I plan on going to lake powell at about 3700ft.

The prop is a 11.25 x 13. I have no idea of the brand. It is constructed of Aluminum.

I would love to have a higher top speed. It seems I pretty much hit max speed at about 3500 rpm. Going to WOT gives me just shy of 6000rpm with very little addtional speed. Just seems like a lot more air bubbles coming from the prop.

Any help would be greatly appreciated.

Thanks,
Gregg
 
May 4, 2019
7
MacGregor 26x Colorado
I have a 2nd prop that gives similar results. Its unlikely I have 2 spun props.
Given the weight (of the loaded boat) and the altitude (5000 ft) what speed should I be able to obtain with the proper propeller?
 
May 25, 2019
7
Macgregor 26X Chesapeake Bay
I can't give you the information you want, (I was frustrated when the marine mechanic explained propping a boat to me years ago. The process is not as mathematical as I expected and more touchy feely than I think it should be.) What I took away from that conversation was that speed is not the criteria that should be used as the goal while propping the goal is to achieve the rated WOT of your given engine in the case of the BF50 5500-6000rpm. Altitude reduces power of a carbureted engine 3 to 4 % per 1000 ft so you have lost 15-20% of your horsepower in Denver assuming the carbs have been retuned. Your BF50 is effectively now a bf40. But I digress you say you are reaching the rated WOT therefore the prop pitch is in the neighborhood as you feel performance is lacking you need to look at other factors as you feel you have eliminated hub slip and your comment about more bubbles the next place to look is prop slip / cavitation. Move weight aft, trim motor down and check to see if the motor is mounted correctly cavitation plate needs to be at or preferably below the bottom of the hull (under water at speed). Once the boat is on plane then you Can commence trimming the engine to get that last little bit of speed or a more comfortable ride. Guess I should add to increase WOT rpm decrease prop pitch, to decrease rpm increase pitch.
 
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May 4, 2019
7
MacGregor 26x Colorado
I already heard about propping a boat to find your ideal rpm. And I know my 26x is achieving that at 6000RPM. My frustration is in such a slow top speed. 8mph is rather slow.......I would be thrilled with 14mph.

when I am cruising at about 3000rpm the boat moves at a decent pace, its not too loud and I get decent fuel economy.

I would just like to be able to pull my son around on a tube at a bit faster speed once in a while.

I marked my props today with a punch to test for spun hubs. I have to remove the prop to check for a spun hub because the washer covers the hub and the punch mark.

The cavitation plate is just at the bottom of the hull. I looked at the motor (honda bf50a) and transom.......I dont believe it can be mounted any lower. The next thing I can try is to move all the gear, anchor, etc......to the berth under the cockpit to see if I can get more weight aft. Perhaps that will resolve the possible cavitation issue. (assuming that is an issue)

I did pull the carbs when I first got the boat about 2 months ago. The carbs were gunked up.....after a good cleaning the motor is running like a top. It purrs. This boat has been in Colorado its entire life. I have no idea if it has been tuned for altitude........I am not sure it can be tuned for altitude.
 
May 4, 2019
7
MacGregor 26x Colorado
I did some more testing and in some respects had to start over. I was using the speedometer on my hummingbird fish finder......turns out that is very inaccurate. Now I am using a GPS speedometer......appears much more consistent in reporting speed.

I installed a new prop. 10.5 x 13 Turning Point Hustler Prop.
The good news is that I achieved slightly higher speeds and it just felt overall better and faster getting up to speed. I hit a max speed of 13.6MPH with minimum extra weight (just some basic supplies, 2 anchors, a heavy refrigerator and 3 people). I adjusted the trim while at WOT and really could not find any difference except when I trimmed it too high and it started making more bubbles. :)

The bad news is that 13 pitch is too high. I only achieved 4400 RPM at WOT. I think I am going to try again with a new prop from the same company. They have a 12.5x8 prop that I suspect will put me in the right range of having just shy of 6000 RPM at WOT. (keep in mind I am doing these tests in Denver at an altitude of 5000ft.......my Honda BF50A will have less power due to the thinner air)

I am open to suggestions......I want to try and get the correct prop/pitch this time. Getting new props can get pricey.

Thanks,
Gregg
 
Jun 3, 2004
1,863
Macgregor 25 So. Cal.

captnron
Joined Jan 6, 2010
1,520 posts, 70 likes
Catalina 30 Mark II
US John's Pass Florida
A,

Do this simple test for troubleshooting your prop/speed problem. This has worked for me & others over the years. You can do this test while tied at the dock & confirm your findings testing while underway.

Keep it simple & not fret: The proper propeller size for your boat and engine combination is based in part on the wide open throttle (WOT) operating range for your particular engine. You can find this in your operator's manual, expressed in terms of a certain horsepower at a certain r.p.m.
The goal in propeller selection is to determine what style and size will maximize your boat's performance, while allowing your engine to operate in the recommended r.p.m. range. The correct propeller will prevent the engine from over-revving, yet allow it to reach the minimum r.p.m. where the maximum horsepower is produced, with ideal engine loading.

Using your existing propeller, determine your maximum obtainable r.p.m.. If during this test, you begin to exceed the maximum rated r.p.m. of the engine, reduce the throttle setting. If the engine over-revs beyond the maximum recommended r.p.m., you may need to increase the pitch of the propeller. Increasing the pitch increment by 2" will result in approximately a 200-400 r.p.m. drop. Also, switching from an uncupped to a cupped propeller will reduce your r.p.m (see "Cupping" paragraph below). The cupped propeller of the same pitch and diameter will typically reduce your r.p.m. by approximately 200. If you cannot reach maximum r.p.m., then pitch should be decreased. These recommendations apply to single engine installations only. For most twin engine installations it is necessary to increase pitch by 4".

Once your WOT r.p.m. falls within the recommended range of the engine manufacturer, you have a propeller that is suited to your boat with respect to r.p.m. However, you may not be satisfied with your boat's skiing performance or trolling speed. It may be advisable in these circumstances to have multiple propellers, each to accommodate different boating activities. In all likelihood, more than one propeller will be suitable for your boat and motor combination, depending on your usage. Ski boats need more top end speed, and should choose a prop with a higher pitch. Cruisers and houseboats need more performance at displacement speeds, and should use a prop with a lower pitch to acheive low-end power.

It is imperative, however, that the WOT r.p.m. fall within the range specified by your engine manufacturer. If your engine is not able to reach this r.p.m. range, it's operating under an extremely loaded condition and premature failure is highly likely. Our Manager of Technical Services Bob Adriance would like to remind you that the wrong prop can wreck an engine. "I've talked to mechanics that think using the wrong prop is the single greatest cause of premature engine failure," says Bob.

While most of these comments are geared to outboard engines, some also pertain to inboards. There are other factors that can adversely affect the performance of your propeller. One of these is dings on one or more blades. Another would be having the blades out of alignment, as would occur if you hit something. Either of these could cause vibration or undue stress and ultimately damage your transmission, cutlass bearing and other components. Usually the best way to deal with a propeller issue with an outboard is to get a new prop if it’s damaged or, if you’re not sure you nave the correct prop, try on different ones, with the recommendation of a qualified dealer, until you’ve got it right. But with inboards typically you’ll need to have the propeller(s) pulled and sent to a good prop shop so that they can work their “magic” on your existing prop to repair it or determine that you need another.

This includes things such as “swinging it” to determine balance, checking for alignment of the blades, and actually working on blades, such as adding or removing cup and many other adjustments. You would give them all the information about your boat that they’d request and describe fully all the issues you’re experiencing.
Propeller Size

The size of a propeller is defined with two sets of numbers, diameter and pitch, with pitch always following the diameter.
Diameter

The diameter is two times the distance from the center of the hub to the tip of the blade. It can also be looked at as the distance across the circle that the propeller would make when rotating.
Pitch

Pitch, the second number listed in the propeller description, is defined as the theoretical forward movement of a propeller during one revolution. Since there is almost always a small amount of "slip" between the propeller and the water, the actual distance travelled is slightly less.
Cupping

Many of today's propellers incorporate a cup at the trailing edge of the propeller blade. This curved lip on the propeller allows it to get a better "bite" on the water, resulting in reduced ventilation and slipping, and allows for quicker acceleration, or "hole shot," in many cases. A cupped propeller also works well in applications where the motor can be trimmed so that the propeller is near the surface of the water. The cup will also typically result in a higher top end speed.
Some Problems to Avoid

Ventilation

Ventilation occurs when surface air or exhaust gasses are drawn into the propeller blades. When this happens, boat speed is lost and engine r.p.m. climbs rapidly. This can result from excessively tight cornering, a motor that is mounted very high on the transom, or by over-trimming the engine.
Cavitation

Cavitation (which is often confused with ventilation) is a phenomenon of water vaporizing or "boiling" due to the extreme reduction of pressure on the back of the propeller blade. Many propellers partially cavitate during normal operation, but excessive cavitation can result in physical damage to the propeller's blade surface due to the collapse of microscopic bubbles on the blade. There may be many causes of cavitation, such as incorrect matching of propeller style to application, incorrect pitch, physical damage to the blade edges, water flow obstruction caused by parts of the boat’s hull or running gear to close to t the propeller and others.
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CR

Taken from: http://www.boatus.com/boattech/articles/propellers.asp