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### Prop for a Yamaha F70 on a 26M?

Posted: **Wed Apr 05, 2017 2:37 am**

by **schoeni**

Hi to all,

I am going to order a Yamaha F70.

Which prop size would be fine for a 26M?

Boat is most of the time fully loaded.

Thanks for your reply,

Rainer

### Re: Prop for a Yamaha F70 on a 26M?

Posted: **Mon May 15, 2017 5:35 am**

by **kadet**

Looking at doing this soon myself, get the high thrust 14x11

### Re: Prop for a Yamaha F70 on a 26M?

Posted: **Mon May 15, 2017 9:08 pm**

by **1st Sail**

I can't speak for the Yamy 70 however, I'm running an Etec 50, 14 x 11 prop, 12gal fuel, 2 anchors, 2 batteries, lightly loaded, 2 adults, 18.2mph (no ballast), 5,900rpm. Etech 50 rated 6,000 rpm. When I run my back up prop 13.75 x 13 with ballast in I can't pull enough rpm and bogs down to 5,400 rpm which is to low for wot.

I suspect 14 x 11 on a Yamaha 70 would be seriously under propped at WOT.

### Re: Prop for a Yamaha F70 on a 26M?

Posted: **Tue May 16, 2017 1:00 pm**

by **ris**

We run a Honda 60 with a 14X9 and can only do 4500 RPM loaded, 12.5 mph. Range is 5000 to 6000. Just two people, 24 gal of gas and no dinghy motor, food, or water aboard and she will do 5400 rpm about 20 mph.

### Re: Prop for a Yamaha F70 on a 26M?

Posted: **Tue May 16, 2017 2:29 pm**

by **kurz**

I have on my merc60hp 14/10.

So maybe 14/11 for 70hp will work fine...

### Re: Prop for a Yamaha F70 on a 26M?

Posted: **Tue May 16, 2017 5:57 pm**

by **kadet**

1st Sail wrote:I can't speak for the Yamy 70 however, I'm running an Etec 50, 14 x 11 prop, 12gal fuel, 2 anchors, 2 batteries, lightly loaded, 2 adults, 18.2mph (no ballast), 5,900rpm. Etech 50 rated 6,000 rpm. When I run my back up prop 13.75 x 13 with ballast in I can't pull enough rpm and bogs down to 5,400 rpm which is to low for wot.

I suspect 14 x 11 on a Yamaha 70 would be seriously under propped at WOT.

The Yammy F70 revs 300rpm more than an E-tec at WOT and has a different Gear ratio 2.33 vs 2.67 so the 14x11 should be fine.

I have been researching this since the EMM shat itself on my 60hp E-tec (rant post coming soon

). Yamaha specifically makes a K-series pontoon high thrust prop that is ideal for our Mac's and it comes in only one flavour 14x11.

I had the same 2 props as above for my 60 E-tec with almost the same results. HP only affects top end speed, and then only if your prop's not slipping like crazy, it's the Torque that matters with our barge a$$ beasties so you can swing a larger prop to lift that a$$

.

The F70 is just the High Thrust T60 with 4 valves per cylinder instead of 2 to give the 10 extra ponies.

The motors I am considering for my repower are;

- Yamaha T60 High Thrust or F70

Mercury 60 Command Thrust basically the T60 but made by Merc in China instead of Yammy in Japan.

Suzuki DF60 Volume Suzk's equivalent of a high thrust.

### Re: Prop for a Yamaha F70 on a 26M?

Posted: **Wed Nov 01, 2017 6:08 pm**

by **Terry**

I re-powered my 26m from the 50 Honda to the Yamaha F70 in 2015. I also got the K series 3x14x11 prop which spins right up to 6300 rpm. Range is, 5300-6300 so I get top of the range. Seems like a nice match and I can reach 19-20 mph wot. So I decided to carry a spare prop and purchased a 3x 13 5/8 x 13 prop thinking I would only lose 400 rpm. Well I dropped 1000 down to 5300 and could barely hold 5400 rpm. That is a huge drop so I took it off and stay with the trusty 14x11. Cannot explain the huge drop, maybe someone else can. Oh at 5400 I got 22 mph! Love the engine!

### Re: Prop for a Yamaha F70 on a 26M?

Posted: **Wed Nov 01, 2017 7:53 pm**

by **kadet**

Cannot explain the huge drop, maybe someone else can.

The maximum torque of the F70 is produced at 4500rpm maximum HP at about 6300rpm.

Torque is the ability to do work, HP is how fast the work can be done. So simply put our

s have too much drag and weight to maximize the HP (go fast) we have to have the torque (strength) to move the weight. By changing the pitch you reduced your strength so you lost power rpm.

This explanation is way oversimplified but is the best I could come up with

A mechanical engineer might explain it better

### Re: Prop for a Yamaha F70 on a 26M?

Posted: **Thu Nov 02, 2017 11:54 am**

by **Seapup**

Range is, 5300-6300... 6300 rpm 19-20 mph ... 5400 I got 22 mph!

If 5300-6300 is the range, why use the 6300 RPM prop that goes 20 while the other is in the recommended range and goes 22?

### Re: Prop for a Yamaha F70 on a 26M?

Posted: **Thu Nov 02, 2017 12:09 pm**

by **Tomfoolery**

Seapup wrote:Range is, 5300-6300... 6300 rpm 19-20 mph ... 5400 I got 22 mph!

If 5300-6300 is the range, why use the 6300 RPM prop that goes 20 while the other is in the recommended range and goes 22?

It may also be governed for 6300 max, with the prop not putting enough load on the engine to bring the speed below that.

A prop shop could put a little more pitch or cup into that prop to load the engine a little harder at speed. And the second prop, that tops out at 5300 rpm, is probably better at low, sub-hull speed work, like trawler service. Important if/when fuel mileage is important. Or a shop could take a little pitch out of it to speed the engine a little more.

But as Seapup implied, if it works, why mess with it?

### Re: Prop for a Yamaha F70 on a 26M?

Posted: **Fri Nov 03, 2017 7:31 pm**

by **Terry**

kadet wrote:1st Sail wrote:I can't speak for the Yamy 70 however, I'm running an Etec 50, 14 x 11 prop, 12gal fuel, 2 anchors, 2 batteries, lightly loaded, 2 adults, 18.2mph (no ballast), 5,900rpm. Etech 50 rated 6,000 rpm. When I run my back up prop 13.75 x 13 with ballast in I can't pull enough rpm and bogs down to 5,400 rpm which is to low for wot.

I suspect 14 x 11 on a Yamaha 70 would be seriously under propped at WOT.

The Yammy F70 revs 300rpm more than an E-tec at WOT and has a different Gear ratio 2.33 vs 2.67 so the 14x11 should be fine.

I have been researching this since the EMM shat itself on my 60hp E-tec (rant post coming soon

). Yamaha specifically makes a K-series pontoon high thrust prop that is ideal for our Mac's and it comes in only one flavour 14x11.

I had the same 2 props as above for my 60 E-tec with almost the same results. HP only affects top end speed, and then only if your prop's not slipping like crazy, it's the Torque that matters with our barge a$$ beasties so you can swing a larger prop to lift that a$$

.

The F70 is just the High Thrust T60 with 4 valves per cylinder instead of 2 to give the 10 extra ponies.

The motors I am considering for my repower are;

- Yamaha T60 High Thrust or F70

Mercury 60 Command Thrust basically the T60 but made by Merc in China instead of Yammy in Japan.

Suzuki DF60 Volume Suzk's equivalent of a high thrust.

Well, I checked both my props and it appears they are both K series, the K is stamped on both. So theoretically they are the same design but different sizes. I do understand the basic drop in rpm for more pitch but it is usually 200 rpm drop for every inch of increase in pitch. So if my 14 x 11-K gets 6300rpm then my 13 5/8 X 13-K should only drop me 400 rpm, also it is 3/8 smaller in diameter so that should help reduce my loss. I would speculate that my loss should be no more than 500rpm which is high given the similar props. I should come in at about 5800rpm at WOT which may even give me the advertised speed of 24mph. That is why I cannot explain the drop to 5300 rpm for only two inches of increase in pitch, that is twice what I expected. Can anyone else explain?

Also, some time ago someone posted the math equation for calculating slip, I remember running it on my calculator several times for each of my two props to see how efficient they worked and I think I was around 30 percent slip or somewhere there about. Anyway I cannot seem to fully remember the full equation or the sequence of calculations, but I think it started with rpm devided by gear reduction 2.33 for my Yamaha then multiplied by 60 for rpm per hour then devided/multiplied by pitch somewhere, then converted to miles per hour and then in the final calc I would subtract varying percentage of slip to finally match my wot of 19 mph or 22 mph to determine my slippage on each prop. I think I was coming in at the high 20,s foggy memory but it was not that bad. I also remember doing that equation for varying amounts of rpm that I expected on my higher pitch prop (5800 To be precise) and figured my speed at wot should be higher than only 22 mph. I am positive I am being cheated by some engineering quirk and would love to know what it is.

Can anyone reproduce that equation again here?

### Re: Prop for a Yamaha F70 on a 26M?

Posted: **Fri Nov 03, 2017 9:10 pm**

by **kadet**

but it is usually 200 rpm drop for every inch of increase in pitch.

K series is the hub design for the gearbox nothing to do with how the blades are designed Yamaha makes 6 different props in the K series and who knows how many aftermarket types are out there, your 13" maybe a Pontoon model cupped and have a larger than normal surface area creating lots of drag, your 11" might be a regular Aluminium prop and not a Dual Thrust so have smaller sized blades so way less drag etc..

200 rpm per inch of pitch is not an arbitrary value it is an often quoted average your peak power is at 4500, so at 5300 your torque has dropped to a point where the motor can no longer produce enough power to turn the bigger pitched prop any faster so you have reached the WOT value for that prop. You have to compare apples with apples not apples and oranges

There are plenty of online slip calculators e.g.

http://bblades.com/propeller-slip-calculator/

### Re: Prop for a Yamaha F70 on a 26M?

Posted: **Tue Jun 18, 2019 4:05 am**

by **tigr1869**

Hello, friends! Greetings from Karelia.

I've got Yamaha F70 on a 26M, and I have a question. What is the manufacturer of prop 14x11 you are using? Is it Yamaha 14 x 11 K ?

Thank you very much!

### Re: Prop for a Yamaha F70 on a 26M?

Posted: **Thu Jun 20, 2019 3:50 am**

by **kurz**

well, cannot help in calculating props. The only I know is: try different props.

All theorie does not help when in real live it is different.

The rule I know is you should reach max rpm from the motor, then you get max HP out of it. I got this with my merc60 with an 14/10.

But maybe also one could say wanna have lower rpms, go with bigger prop and will not have possibilty to ride the motor more than 4900rps...

### Re: Prop for a Yamaha F70 on a 26M?

Posted: **Sun Jul 14, 2019 2:54 pm**

by **beene**

On my Mercaha 75, as people call it, I've tried a

14x11

14x13

13.75x15

Heavily loaded, like for a trip, the 14x13 is best

The 14x11 was great for water skiing

The hole shot was very impressive, if you weren't sitting down, you soon would be

G