Propeller Pitch. Does it make a difference?

[gkdawson]

Hey all,

Just wondering if people have any experiences with different propellers and there pitch.

I’m currently running a 14 3/4 x 15 prop on a Honda 150hp outboard on a Merry Fisher 695 but don’t feel I’m getting enough speed on plane compared to my RPM’s. For example I ran at 15 knots on 4200 RPM. I feel I should be achieving greater speeds than this.

From doing some research I was thinking of changing the prop pitch to a 15x17 pitch propellor. I understand this would grab the water better and get give me back 400-500 RPMS which may give me better planing / cruising speeds? This was also recommended on the Honda prop selector when I entered my boat specifications (weight, length etc…)

Im not interested in achieving a high speed at WOT but more a steady 20-22 knot cruising speed at the lowest RPM’s possible. I’m aware that there are other factors which can effect speed etc, but my hull is clean, engine recently serviced with no issues etc!

Does prop pitch make that much of a difference in terms of performance? RPMS and Speed etc?

Does anyone have any experience with prop pitch change?

What prop pitch are you running?

What kind of speed Vs RPMS are you getting?


Any help / advice on this would be appreciated!

Thanks!

[thehammer]

Cant answere the question. However more pitch do make a difference. But I do not know it it improves your issue.

I do not have the same boat or motor, however have been testing alot of propellers on my 895 sport.

Sounds like you get alot of slip on 4200rpm, however would need to know the engins gear ratio to be sure.

Whats your max speed and rpm? Are you falling out of plane at that speed? Have you tried adjusting tilt/trim?

I'm waiting on new props on my boat again, but currently I have to accelarate up to 5000rpm, set the right pitch/tilt and the I can reduce the rpm back to 4000 and stil go 31kn.

If I just accelarate from 0 to 4000 i only get 20. 😉

Test the propellers from idle rpm up to max and note down the details. I take pictures and fill in a excel spread sheet after. (Gps speed, liter/gallon per hour, RPM) With this info the dealer can recommend something better.

[gkdawson]

Aug 4, 2022 19:00:06 GMT thehammer said:

Cant answere the question. However more pitch do make a difference. But I do not know it it improves your issue.

I do not have the same boat or motor, however have been testing alot of propellers on my 895 sport.

Sounds like you get alot of slip on 4200rpm, however would need to know the engins gear ratio to be sure.

Whats your max speed and rpm? Are you falling out of plane at that speed? Have you tried adjusting tilt/trim?

I'm waiting on new props on my boat again, but currently I have to accelarate up to 5000rpm, set the right pitch/tilt and the I can reduce the rpm back to 4000 and stil go 31kn.

If I just accelarate from 0 to 4000 i only get 20. 😉

Test the propellers from idle rpm up to max and note down the details. I take pictures and fill in a excel spread sheet after. (Gps speed, liter/gallon per hour, RPM) With this info the dealer can recommend something better.

Thank you for the reply!

I’ve been researching most of the day with regards to this. I’m going out next week and will do some test runs and note down some performance figures as you suggested. I will need to check if my revs are exceeding the WOT range etc! If so, I will increase my pitch and see if this makes a difference. Will put my figures up on here some time next week!

I take it on your 895 your running dual outboard engines?

From what I can remember I think I was only getting 22/23 knots at WOT with my engine trimmed. It felt like the prop just wasn’t gripping the water enough or giving me enough push. When I trimmed and reduced throttle to around 4200 RPM to achieve a more economical cruising speed, I was achieving around 15 knots. I’m sure this boat and engine can achieve around 30 knots at full RPM. This is what I’ve seen from performance tests online etc… or maybe in just expecting too much! 😅

Will test next week and keep an eye on my RPM’s to see where they are at when trimmed out after initial planing.

[lynnardm]

Gk,

Pitch, diameter, number of blades, cup, rake all make a difference. I had a 695 with an f200 but don’t remember the propeller. It had great performance. A 150 should too just not quite as quick/fast. I’d document some test results across the operating range for rpm, speed and fuel burn and use it to calculate slip and mpg. And record the 0-20 time and WOT data. Then use that data to analyze the performance and determine possible propeller changes. I can advise further if needed. A knowledgeable propeller company could also look at the data and make recommendations

Capt’n Lynn

[gkdawson]

Capt’n Lynn, thanks for the help! 

Thanks for this! I will record my results when I’m out next week! The weather looks perfect next week for where I am! So any excuse to get out on the water is a plus! I’ll update this thread with my results! Can you remember if you had a 3 or 4 blade propeller? I would ideally go for a 4 blade as I know it benefits on getting up on plane easier…

I found these results online. Same boat but with a Yamaha 150 instead of a Honda. I wonder is these performance figures are accurate? I do understand that there are many variables that can change performance tho but would I would love to achieve a nice stead cruising plane at 4000 RPM! 

[lynnardm]

Gk

I had a 3 blade on my 695. I expect the right 3 blade would be best all around for your boat and provide sufficient blade area for good acceleration but not too much blade thereby a good top speed. As discussed there are lots of variables but generally as a 4 blade requires more power to turn it you will loose top end mpg and speed. Keep in mind these boat magazine tests like you show seem to find the best possible test results. Most of the test boats don’t have bottom paint and that alone can knock a few mph off the top speed. That said, you should be able to see similar performance if your boat is clean and configured properly, your engine is healthy, and yuh have the right propeller. I could hit slightly more than 40mph with my 695 and f200. And that’s with bottom paint. Don’t forget to adjust the engine trim to get the best performance when testing. A few years ago a friend with a 23 ft aluminum fishing boat with an f200 wondered why his boat wasn’t near as fast as mine. Turns out he wasn’t trimming out the motor after getting up on plane. Water conditions wind and tide also affect speed. Pick a nice day in an area without tidal movement or run the same tests both directions if in an incoming or outgoing tide. Also be sure the boat is loaded properly without list or excess weight fore or aft. Good luck. Don’t hit a deadhead :,)

Capt’n Lynn

[gkdawson]

Aug 6, 2022 2:54:41 GMT lynnardm said:

Gk

I had a 3 blade on my 695. I expect the right 3 blade would be best all around for your boat and provide sufficient blade area for good acceleration but not too much blade thereby a good top speed. As discussed there are lots of variables but generally as a 4 blade requires more power to turn it you will loose top end mpg and speed. Keep in mind these boat magazine tests like you show seem to find the best possible test results. Most of the test boats don’t have bottom paint and that alone can knock a few mph off the top speed. That said, you should be able to see similar performance if your boat is clean and configured properly, your engine is healthy, and yuh have the right propeller. I could hit slightly more than 40mph with my 695 and f200. And that’s with bottom paint. Don’t forget to adjust the engine trim to get the best performance when testing. A few years ago a friend with a 23 ft aluminum fishing boat with an f200 wondered why his boat wasn’t near as fast as mine. Turns out he wasn’t trimming out the motor after getting up on plane. Water conditions wind and tide also affect speed. Pick a nice day in an area without tidal movement or run the same tests both directions if in an incoming or outgoing tide. Also be sure the boat is loaded properly without list or excess weight fore or aft. Good luck. Don’t hit a deadhead :,)

Capt’n Lynn

Thank you Capt! I will get out next week and so some testing! I’ve done some research on propeller pitch and when to go up or down in pitch depending on RPM’s etc… can’t wait to get out there now and start testing! I’ll post my results up on here!

[morrisond]

Aug 4, 2022 19:00:06 GMT thehammer said:

Cant answere the question. However more pitch do make a difference. But I do not know it it improves your issue.

I do not have the same boat or motor, however have been testing alot of propellers on my 895 sport.

Sounds like you get alot of slip on 4200rpm, however would need to know the engins gear ratio to be sure.

Whats your max speed and rpm? Are you falling out of plane at that speed? Have you tried adjusting tilt/trim?

I'm waiting on new props on my boat again, but currently I have to accelarate up to 5000rpm, set the right pitch/tilt and the I can reduce the rpm back to 4000 and stil go 31kn.

If I just accelarate from 0 to 4000 i only get 20. 😉

Test the propellers from idle rpm up to max and note down the details. I take pictures and fill in a excel spread sheet after. (Gps speed, liter/gallon per hour, RPM) With this info the dealer can recommend something better.

What Engines do you have on your 895 Sport and what props have you tried?

I sent you a Personal message as well.

[gkdawson]

Went out today and did some performance tests. This is what I got.

Motors trimmed down to start.

2000 RPM - 6.5 knots
3000 RPM - 8 knots
4000 RPM - 12 knots
WOT @ 5800 - 5900 RPM 21 knots

Bare in mind I’m also running a hydrofoil on my engine… I will remove and do another test without tomorrow.

I know I should be getting more out of the engine than this? I did inspect the propeller today and the locking nut is slightly loose along with the silver washer underneath that seems to rotate as it’s not tightened down enough! I know this shouldn’t be the case so will tighten that up tomorrow (not sure how much of a difference that will make tho).

I’m running a 4 blade prop 14.75 x 15” prop. Went around looking at other Merry fishers and they are all running 3 blade props. Some with the same pitch as my 4 blade, others with a 17” pitch.

I may change to a 3 blade 15 x 17” prop to see if this makes any difference. But would I get an extra 10 knots from doing so?

Could anyone tell me if those performance figures are not really what they should be in relation to the RPM’s Vs Speed?

Thanks!

[thehammer]

I can check the calculations, but need your gear ratio, or full motor name.

[thehammer]

Aug 10, 2022 19:33:35 GMT gkdawson said:

I did inspect the propeller today and the locking nut is slightly loose along with the silver washer underneath that seems to rotate as it’s not tightened down enough! I know this shouldn’t be the case so will tighten that up tomorrow (not sure how much of a difference that will make tho)

Are you saying the propeller nut is loose? If you can wiggle the propeller (in/out), or if the propeller is mounted incorrect you could get strange issues with cavitation etc. I have never seen this myself, I was just told the same from the prop manufacturer when exploring issues ourself.

The thrust-washer can spin on the shaft, however, you should not be able to see or tutch that with the propellers on? Maybe I missunderstand the "picture" your painting. 😉

[lynnardm]

GK

Yikes a loose propeller nut? Yikes that’s a concern. Not sure if that could cause damage or not but don’t use the boat like that. Pull the propeller and check the shaft and spline and large thrust washer and the propeller spline and load surfaces for wear or damage. The torque requirement is provably around 35 ft lbs if not more. You are likely set up with the wrong propeller. That performance is terrible. You should not need 4 blades and that along with other propeller characteristics like perhaps too big a diameter for 4 blades is likely the culprit with your performance. Hopefully the loose propeller is also not contributing.

Capt’n Lynn

[gkdawson]

Aug 10, 2022 20:16:14 GMT thehammer said:

I can check the calculations, but need your gear ratio, or full motor name.

The engine is a Honda 150hp VTEC - I will try and locate the gear ratio when I’m back out tomorrow!

Thanks

[gkdawson]

Appreciate the feedback all! I’m determined to sort this out! 😂

So from looking at the picture, there is a little bit of play in the propeller locking nut (left and right). The silver washer underneath the nut appears to rotate when I turn it, presume that’s because the nut isn’t tight enough. I’ll get that sorted! 



There doesn’t appear to be any play back and forth on the prop itself. I will lift it and take a closer looks at the drive shaft etc! Other that that I’m running out of ideas. If all else fails I’ll take it in to a yard and get someone to take a closer look and (or) perhaps come out and run some tests on the water with me! 

[thehammer]

Made a quick spread sheet for your outboard (assuming the gear ratio is 2,14) 

I now tried to post a picture, and realized I have never done that on this forum before. :-)



So if there is no picture here, I have failed. 

However, the numbers are as expected really bad. 

you can expect a high slip between the hull speed (6kn) and on plane speed (13-14kn?). However you have a high slip all over. 
Your propeller is spinning and using most of the energy to make bubbles and sound, and less energy on the movement. 

Looking at the theoretical speed on the prop's at 5800 rpm the boat should be moving 37 knots at 0 slip. Boat normally have between 5 and 20%, giving the top speed with this prop 30 to 36 Knots.

I do not dear on guestimating what props you should go for, but the one you have looks to have to little blade area and to high pitch.


Before continuing to examine, make sure the propellers are installed correct. 

Did you try to tilt the engines up? Do that have any effect?    

[thehammer]

Further, I looked at the Performance Bullitin from Yamaha on the 695 with 115HP. 

They have used a 13 1/2 x 14 Alu propeller, and got 32 knots. 

[gkdawson]

Aug 11, 2022 7:47:46 GMT thehammer said:

Made a quick spread sheet for your outboard (assuming the gear ratio is 2,14) 

I now tried to post a picture, and realized I have never done that on this forum before. :-)

View Attachment

So if there is no picture here, I have failed. 

However, the numbers are as expected really bad. 

you can expect a high slip between the hull speed (6kn) and on plane speed (13-14kn?). However you have a high slip all over. 
Your propeller is spinning and using most of the energy to make bubbles and sound, and less energy on the movement. 

Looking at the theoretical speed on the prop's at 5800 rpm the boat should be moving 37 knots at 0 slip. Boat normally have between 5 and 20%, giving the top speed with this prop 30 to 36 Knots.

I do not dear on guestimating what props you should go for, but the one you have looks to have to little blade area and to high pitch.


Before continuing to examine, make sure the propellers are installed correct. 

Did you try to tilt the engines up? Do that have any effect?    

Thank you so much for taking the time to do this!

I’ll take a screenshot of this and take it with me if I go into a yard and get it looked at! Unfortunately I didn’t get a chance to run anymore tests today. The prop must be slipping if it’s not giving me that power but I can’t see why! When I installed a new propeller towards the end of last year everything looked fine!

Plan at the moment is to change over to a 3 blade prop (pitch to be decided) and run another test. If I’m still seeing poor results then I will be going to my local Jeanneau dealer to have a look at the engine and try and get them to narrow down the problem. Hopefully nothing too serious! It’s only done 97 hours so far!

In regards to the prop slip it doesn’t feel it sound like the prop is slipping, but I’m not an expert on this, only from what I’ve looked up on YouTube etc… The engine runs but once I hit that 4000 RPM marker it just isn’t delivering the type of speeds that I should be getting. Just seems like the engine is giving it all and the boat is just dragging away at 12/13 knots. It’s the same at WOT, will only hit 21/22 knots at 5800 odd RPM. there isn’t any feeling or sound of the prop slipping… or maybe I don’t fully understand how prop slip works or sounds! 😅

not sure why the picture down below is to the side but that’s my RPM and speed as shown on the nav system
just to show! So frustrating! 😫

[gkdawson]

Aug 11, 2022 8:01:44 GMT thehammer said:

Further, I looked at the Performance Bullitin from Yamaha on the 695 with 115HP. 

They have used a 13 1/2 x 14 Alu propeller, and got 32 knots. 

View Attachment

This looks like a 605! But still, the RPM’s compared with the speed same to match correctly!

[lynnardm]

From the web
Propeller slip is the difference between the actual distance the prop travels forward through the water versus the distance it should theoretically travel based on the prop blades' angle of attack. The theoretical travel is determined by the prop pitch.

J


Capt’n Lynn

[thehammer]

Aug 11, 2022 21:00:30 GMT gkdawson said:

Aug 11, 2022 8:01:44 GMT thehammer said:

Further, I looked at the Performance Bullitin from Yamaha on the 695 with 115HP. 

They have used a 13 1/2 x 14 Alu propeller, and got 32 knots. 

View Attachment

This looks like a 605! But still, the RPM’s compared with the speed same to match correctly!

You are correct.. Thanks.. I dont know how I managed to screw that up. I will try to find 695 in stead.

[rene460]

Hi Capt’n Lyn,

The interesting thing about that definition of slip is that it is so universally written in all kinds of books and articles over so many years, yet has so little relevance to the physics of how a propellor works, and hence adds very little to our understanding of the issues involved.

I am sure it is of some help for a beginner to understand the concept of pitch, but I suggest it is more useful to think in terms of a thruster device rather than a screw.

The propellor produces thrust which pushes the boat at a speed where the hull residence or drag equals the thrust. It produces a thrust whether the boat is still tied to the jetty (100% slip, don’t try this with a high power engine!) or on a sail boat motoring sailing hard enough that the slip factor would conceivably be negative. The thrust at a given propellor rpm does vary with boat speed, but there is no direct relationship between boat speed and rpm as there is for a car for example, unless it is in a skid pan. Slip is just an observed correlation between pitch speed of advance an actual boat speed but not much help in prediction except for very similar boats.

So how does a propellor produce thrust? I suggest it comes back to Newtons law, you know the one about bodies moving in a straight line unless acted on by an external force? The property causing things to continue moving in that straight line is called momentum, and mathematically it can be given a value by multiplying the mass of the body by the velocity. Providing consistent rational units are used, a change of momentum per unit time is exactly equal to the force required, or in case of the propellor, the force produced. The maths is surprisingly simple. The whole principle is more neatly stated as the principle of conservation of momentum. Not as well known as conservation of energy, but equally universal (along with three other conservation laws that so far are believed absolute and universal.) Application of this law allows physicists and engineers to determine separate equations that simultaneously apply to help solve problems.

In the case of a propellor, the propellor geometry, and rotational speed combine to move water that is stationary ahead of the propellor, or if you like, approaching the propellor at the boat speed, and accelerating a mass of that water in the direction of the back of the boat at a higher velocity, also causing it to move in a spiral direction in the process, but the linear direction is the important one in this context. The mass of water accelerated by the propellor depends mostly on the diameter of the propellor and the speed of the boat, with blade pitch, profile and area, all having important effects. The pitch has to be sufficient for the water pushed out of the way to have to end up moving in the required direction.

But I detect a hint in your post that you knew all that, so back to the original problem.

So to the original poster, G K Dawson, yes, pitch and diameter do make a difference, both to the thrust produced and to the power required to turn the propellor. As always with any theory, the proof is in the testing, and there is always an efficiency factor which always means less effect than you might expect. In this case, the actual thrust at any boat speed compared with a calculated factor. The theory is simple and exact, but there is a problem determining just how much water is accelerated by the propellor, and how high the resulting velocity.

It is very difficult to even compare well documented test results. In this thread, three sources are quoted. The Yamaha test at the end for example, the lower power of the engine is not important as it was obviously adequate, but apart from the boat model, the gear ratio quoted is different from the gear ratio quoted for the 150 hp model. And the internet yields a different ratio from that quoted in the early post of the thread for a 150 hp model, so first step is to determine the right ratio. I assume there are sub models intended for different applications. It is the propellor rpm that counts, engine speed is only an adequate proxy if the gear ratio is the same.

Certainly your results certainly seem to indicate a mis-matched propellor, but the suggestion of increasing diameter and pitch worries me. That will require more power. It is interesting that the motor seems to have reached its expected full throttle operating speed, so the power requirement seems reasonably matched, but the speed result suggests either a badly trimmed hull or badly over weight, or more likely, a different diameter and pitch combination is required to get more thrust with the available power. Diameter to accelerate less water, pitch to accelerate that water to higher velocity.  Rather than try and guess, I would also suggest going to a reputable propellor manufacturer for advice and recommendations, They have special sophisticated software for predicting the performance of their propellors, and will come much closer than any of us to selecting the right one. A good one may even have a records and data for similar boats, suitable loan propellor for testing or other approach to helping you get the right prop. It is worth re-reading Capt’n Lynn’s thread on his latest propellor adventures. It is most interesting and illuminating.

There is also some indication of a totally different concept of prop slip, which refers to the mechanical integrity of the propellor.  That type of slip is normally the result of hitting something, and a one off event, not a continuing condition.

Looking forward to hearing about what you do next and how it goes,

Rene460

[gkdawson]

Thank you for your input all!

Very interesting read! If anything I’ve definitely learned a lot about all of this! Just wish I had known all this before!

Does anyone know where I could rent a propeller from as that would be more ideal?

Otherwise I will look at buying the propeller that I have seen on most other boats the same / similar to mine which appears to be a 3 blade 15 x 15” prop! This was also recommend on the prop finder tool on the Honda website! I will also buy a new thrust washer kit with locking nut etc and install!

Won’t be able to test now till the end of the month so watch this space! If all else fails, I will take the boat to a yard and have someone take a look at the engine and try and run some tests onboard with myself!

Wish me luck!

[mf755marco]

Hi,

I’ve a MF 755, that should be not so much different from MF695, below I’ll summarize my experience. I bought some years ago the second hand boat with a Mercury 150hp, the figures was:

4200rpm 17 knots

4500 rpm 20 knots

5500 rpm (WOT) 25 knots

This year I decied to change a motor from 150 to 200hp (Suzuki) and I’ve had the opportunity to test lot of propellers (I’ve also a trim tabs). Below the result (tests conditions was more or less the same, 2 people on board, fuel ¾, no wave, hull clean)

propeller (dia x pitch)	4000	4300	4500	4800	5000	MAX
15 1/4 x 17				14
15 1/4 x 19			18			23@5700
16x18,5	13	17	20	21	23	25 @5800
16x20	19	22	24	26		30 @ 5300

Of course each boat model have different behaviour, but I hope these data can help you to make some considerations for your boat

[gkdawson]

Thank you all for your feedback.

Just to update this thread. I changed my prop however did not notice any difference in performance.

The boat will be coming out this October and I have spoken to my dealership who will be sending someone out on the water with me on the day she comes out to try and source the problem! I imagine it will be straight into the shop for a full check over and performance test over the winter months!

The engine is still in warranty if there is anything mechanically wrong! Hopefully there’s no major issues! Will keep this thread updated with the final outcome which will hopefully be a positive one! 🤞🏻

[gkdawson]

Just an update for anyone who comes across this thread!

I changed out my prop to a 3 blade 15x17 prop which seems to have made all the difference! I also researched a lot more into bottom paint and re primed / re painted my hull to reduce as much growth as possible!

Had a little play around with the trim as well to see what worked best for me! All these worked a treat! I’m achieving what I should have been achieving in the first place performance wise!

Think the main factor was my propellor choice! More powerful engines would probably benefit from a 4 blade prop! That’s just my experience anyway!

Thank you to everyone who replied to my thread and helped out! 👍🏻