Propeller Selection for 26M w/ 90 HP Honda Outboard

[larryd97]

Hi Everyone. I know there's been a lot of prop talk on this forum, though I'm struggling to find any experience / recommendations on a repower to 90HP engines.

My original outboard was a Honda BF50 (carbureted) on my . The power and speed was lackluster when trying to get somewhere far away. When that engine died, I was looking to go with at least a 70 HP (the Yamaha looks nice) though I found a great newly rebuilt used 2011 Honda BF90 from a local mechanic so this became a no-brainer.

It used to be on a pontoon boat. The prop is 13.75" x 13" 3-blade and is too aggressive for the . At WOT it tops out at 4500 rpm with an empty ballast tank, 4200 with full ballast tank. Target RPM for this motor is 5500-5800.

For reference, the turning point propellers website (via West Marine) suggested 14" x 11" 3-blade.

Any suggestions? Do prop shops accept returns in case I guess wrong?

Thank you in advance.

Larry

[Starscream]

Hi Everyone. I know there's been a lot of prop talk on this forum, though I'm struggling to find any experience / recommendations on a repower to 90HP engines.

My original outboard was a Honda BF50 (carbureted) on my . The power and speed was lackluster when trying to get somewhere far away. When that engine died, I was looking to go with at least a 70 HP (the Yamaha looks nice) though I found a great newly rebuilt used 2011 Honda BF90 from a local mechanic so this became a no-brainer.

It used to be on a pontoon boat. The prop is 13.75" x 13" 3-blade and is too aggressive for the . At WOT it tops out at 4500 rpm with an empty ballast tank, 4200 with full ballast tank. Target RPM for this motor is 5500-5800.

For reference, the turning point propellers website (via West Marine) suggested 14" x 11" 3-blade.

Any suggestions? Do prop shops accept returns in case I guess wrong?

Thank you in advance.

Larry

Hi Larry,

Here are some data points on my 26X:

Etec 90 (rated 4500 - 5500 max RPM) with 2.0 : 1 gear ratio, 14x11 3 blade Evinrude prop was in the RPM range with ballast in and out, so the 14x11 prop was turning about 2500 RPM. Top speed without ballast was about 24mph.

Suzuki DF90A (rated 5500 - 6500 max RPM) with 2.6 : 1 gear ratio, Suzuki 3-blade 13.875 X 15 prop is in the RPM range with ballast in and out, so the prop is turning at about 2300 RPM. Top speed without ballast touches 30mph.

I don't know what your Honda 90 gear ratio is, but I would never have guessed that a 14x11 prop is correct for that motor, because that's the prop I had on my Etec90. The numbers that you got with your original 13.375" prop sure show it's overpropped, but a 14x11 3 prop should only increase the RPM by around 300 by my guess. That would mean that the 14x11 is still over-propped??? Something's odd, here.

[larryd97]

Hi Larry,

Here are some data points on my 26X:

Etec 90 (rated 4500 - 5500 max RPM) with 2.0 : 1 gear ratio, 14x11 3 blade Evinrude prop was in the RPM range with ballast in and out, so the 14x11 prop was turning about 2500 RPM. Top speed without ballast was about 24mph.

Suzuki DF90A (rated 5500 - 6500 max RPM) with 2.6 : 1 gear ratio, Suzuki 3-blade 13.875 X 15 prop is in the RPM range with ballast in and out, so the prop is turning at about 2300 RPM. Top speed without ballast touches 30mph.

I don't know what your Honda 90 gear ratio is, but I would never have guessed that a 14x11 prop is correct for that motor, because that's the prop I had on my Etec90. The numbers that you got with your original 13.375" prop sure show it's overpropped, but a 14x11 3 prop should only increase the RPM by around 300 by my guess. That would mean that the 14x11 is still over-propped??? Something's odd, here.

Hi Starscream,

Thank you for the data points.

The Honda BF90D gear ration is 2.33 : 1. So I guess that means at target RPM we're looking at around 2400 RPM.

I agree 14x11 suggestion from the online tool seems to be not much different than the 13.75" x 13" I already have. I suspect the online prop selector tools cannot take into account the uncommon attributes of the .

This is my first time selecting a prop for a boat. Is it literally trial and error? Do prop shops have loaners or return policies on stock prop so customers can test them out, or is it $100-$300 per experiment (aka 1-3 "boat bucks")?

Thanks,

Larry

[Starscream]

Most engine dealers will have a few loaner props that you can try, and if you buy a new engine they usually will let you try a new one for a trial run.

$300 is only 30% of a standard boat buck .

A 90HP 26M is known to be slightly slower than a 90HP 26X. It's said to be slightly faster under sail, but only the white ones.

A 2.33 gear ratio on a 90HP engine should be relatively well matched with your existing prop so I'm puzzled. One possible idea that occurred to me is related to the time that I tried a 13" Piranha prop with "strakes" on the blade, with my Etec90 because I was trying to get another 100 rpm above my 14x11. The Etec only turned that one at under 4000 rpm, even though on paper it shoulda been much higher.

Trial and error it is. Gotta find a shop with loaner props.

[C Buchs]

I have an with an Etec 90. I run the Solas 9411-141-11 Rubex 3 High-Thrust Aluminum Propeller with 14.1" Diameter x 11" Pitch. This "High-Thrust" version was designed for pontoon boats. My boat is slower than Starscream's, but my RPM range is similar. I don't know if this will fit your "new" motor, but they are currently selling for $123 on Amazon https://www.amazon.com/gp/product/B07BS ... UTF8&psc=1.

Jeff

[Be Free]

You can find the right prop using two different methods. The first requires lots of measurements and and very complicated math. When you are done you will have spent a lot of money and time and you might even have the right prop. If your measurements are correct and if you applied the very complicated math correctly you will definitely be close but you may still need to tweak it a little.

The second way is to make a good first approximation for our boats by using a manufacturer's app and telling it you are putting it on a pontoon boat. That is going to get you in the ballpark. If you are lucky it might even give you a prop that meets your goals.

If the engine won't get up to the target RPM you need to lower the pitch (or the diameter) of the prop. If it hits the target RPM before you are at full throttle you need to raise the pitch (or diameter).

A good rule of thumb is that for every inch of change in diameter the RPM will change by about 500 RPM. Every inch of pitch will change the RPM around 150 to 200 RPM. Whether you change the pitch or diameter will depend on what your goals are.

If you mostly run at hull speed and want to maximize fuel economy, go for a larger diameter prop that will get you there at a lower RPM. You still want to be able to hit the manufacturer's recommend RPM target range but you will probably be closer to the low end if it.

If you are all about top speed you may need a smaller prop to get to the top of your RPM target with enough pitch to get your prop slip into a reasonable range.

[larryd97]

Thanks to everyone that responded!

My goal is maximizing "cruising speed" under power, not top speed at WOT. I define that as 75%-80% of WOT RPM. Both cruising or top speed will not be anywhere near hull speed. When powering I am primarily concerned about getting to a destination too far away to sail in a the time I have for the outing.

I spoke with a local prop shop and they had never dealt with a or heard of it. Their software came back with only one option (entered as if it were a cuddy cabin - before I saw you recommended calling it a pontoon) - 4-blade 14" x 9". Unfortunately, that configuration is only available in stainless steel which is 3-4x the cost. Not worth it for my use case.

I'm leaning toward trying an aluminum 3-blade 14" x 9" and see how it goes. I'd rather bias toward under-prop than over-prop for the initial experiment , given that the latter will put undue strain on the engine and potentially reduce the engine life.

[Be Free]

Whatever you end up with you should be able to get at or at least near the low end of the manufacturer's recommend RPM range. If you can't get there then you are correct that your engine will be working too hard and it will effect its long term health. Think of it like starting in 10th gear on a bicycle. It puts a strain on the gear train and the main bearings in the top end.

[kurz]

Thanks to everyone that responded!

My goal is maximizing "cruising speed" under power, not top speed at WOT. I define that as 75%-80% of WOT RPM. Both cruising or top speed will not be anywhere near hull speed. When powering I am primarily concerned about getting to a destination too far away to sail in a the time I have for the outing.

I spoke with a local prop shop and they had never dealt with a or heard of it. Their software came back with only one option (entered as if it were a cuddy cabin - before I saw you recommended calling it a pontoon) - 4-blade 14" x 9". Unfortunately, that configuration is only available in stainless steel which is 3-4x the cost. Not worth it for my use case.

I'm leaning toward trying an aluminum 3-blade 14" x 9" and see how it goes. I'd rather bias toward under-prop than over-prop for the initial experiment , given that the latter will put undue strain on the engine and potentially reduce the engine life.

14*9 is far to small. I have an mercury 60hp and use 14*10. So I geht at least 5500rpm. So your 90hp MUST be much stronger!

[larryd97]

@kurz I would not have expected a 14 x 9 would be right either. However, "conventional wisdom" on props claims that 1" of diameter changes RPM by ~500, and that 2" of prop pitch changes by ~200 from what I've read.

I figure it's a crap shoot - 14 x 9 vs 14 x 11. 50% chance I'll be right the first time...

[kurz]

sorry no, 14x9 is definitely to small für your 90HP, you cannot get out the HP with a pro as small lik this.

[Starscream]

@kurz I would not have expected a 14 x 9 would be right either. However, "conventional wisdom" on props claims that 1" of diameter changes RPM by ~500, and that 2" of prop pitch changes by ~200 from what I've read.

I figure it's a crap shoot - 14 x 9 vs 14 x 11. 50% chance I'll be right the first time...

Larry, if you do a search on this forum for the term "14x9" you'll find a bunch of hits and lots of data points. 14x9 props are installed on many 60HP and 50HP motors, and even one on a 70HP (but with 4-blades). As Kurz is saying, a 14x9 3-blade prop on a 90HP doesn't make any sense to us. Is it possible that there's another issue with the motor? I'm not an expert here, but could something like restricted fuel flow or throttle cable adjustment prevent the motor from reaching its full-throttle position?

Here's an example thread from the search: Ris has a 14x9 on his Honda 60 and it was able to turn it at 5250 RPM (I know, he has an X, but it's not that different). viewtopic.php?p=327971&hilit=14x9#p327971

Here's another one: a 26M with a Yamaha70 14x11 prop turning at 6200 RPM with a 2.33:1 gear ratio. The contributors also suggest a 14x9 for a Honda 60HP, not a 90HP. viewtopic.php?p=306713&hilit=14x9#p306713

Your performance data suggests a problem other than the prop.

[larryd97]

Your performance data suggests a problem other than the prop.

That is fantastic feedback! Thank you!

I will check the throttle cable. The seller is also a local marine mechanic and a highly respected one, so i didn't even think to doubt his work. I'll take the cowling off and look check it out this afternoon.

[larryd97]

I will check the throttle cable. The seller is also a local marine mechanic and a highly respected one, so i didn't even think to doubt his work. I'll take the cowling off and look check it out this afternoon.

Throttle arm had a small mount of play in it. Without tweaking it underway I have know way of knowing if it will move the needle.

Another potential variable - we reused my old RPM gauge for the new motor. It *should* work though could it be off? How does one find out?

[Starscream]

Throttle arm had a small mount of play in it. Without tweaking it underway I have know way of knowing if it will move the needle.

Another potential variable - we reused my old RPM gauge for the new motor. It *should* work though could it be off? How does one find out?

Unlikely to be the wiring of the tach: I just looked up the BF50 and BF90 meter wiring diagrams and they have the same 5 wires, with the same color coding. Would be odd for a pro shop to make that mistake.

We can possibly eliminate the RPM gauge as a problem if we consider the speeds you were attaining at the 4200RPM/ballasted and 4500RPM/unballasted conditions. If you were in the teens then it's unlikely to be the RPM gauge, if you were in the 20s then the RPM gauge could be the issue.