13 pitch for a Bigfoot or lugging your engine to death

[gerrykite2fly]

My brand new M is equipped with a Yamaha High thrust 50HP. I tried the boat with a 13 and 11 pitch prop and found that the 13 gives a max speed of 17 knots (20mph) @ 4900rpm whereas the 11 pitch gives a top speed of only 15 knots (17mph) @ 5600. All measured with 2 people on board, light load, unbalasted and flat water.

I like the lower rpms anyway - assume that its eating less gas also. Wakeboarding was fine with it too - we reached 14 knots with a 85kg rider.
What I don´t know is, if the engine dislikes a top rpm of under 5000 on the long run (more wear).
My plan is to convert the engine to a 60HP which should be easy as I´ve the carburated version and then get the 13p into higher rpm ranges...
Anyone experiences with that setup ?
Aloha Gerry

[Catigale]

What I don´t know is, if the engine dislikes a top rpm of under 5000 on the long run (more wear).

Short answer - yes - this reduced rpm at WOT means the engine is 'lugging' by analogy to a car engine - climbing the hill in top gear.

If you use it in this mode for 1% of use, not a problem.

50% of use - problem

In between, you make the call.

Im running a 50 HP Mercury BF with a 14x11x3 prop which makes 5000 rpm or so - overpropped by Merc specs..but I seldom use WOT.

[gerrykite2fly]

It´s really tricky, because in my case the engine obviously gives more speed in any setup (balast empty AND full, with wakeboarder...) with the higher pitched propeller.
It makes me think that the max. torque is reached at just this RPMs (4900).
When wakeboarding I realize even better acceleration than with the 11 pitch.
I also noticed that with the 11 pitch prop the engine is spinning at whatever rpms i put it no matter what the conditions are, whereas the 13 pitch makes it "swing" 200 rpms up and down eg, when the water gets choppier or when the wakeboarder pulls hard into a turn. So the "water drag" issue is bigger with the smaller prop either.
My Yamaha Dealer here suggests that we try mounting the motor higher to the transom which will get revs up a bit.

I understand that lugging is an issue with the 13, but would like the higher speed @ lower revs and most probably less fuel consumption at the same time.

So it comes down to the question, in which way does this setup affect the engine negatively on the long run if I keep going like that let´s say 30-40% at WOT.

[Catigale]

Im not fatalistic but 30-40% use would pose a high risk of major engine damage imho. I would listen for pinging at WOT and if you can hear this, cease and desist immediately.

Sounds like you are in the market for more HP!!!

[Chip Hindes]

Assuming the same diameter (you never said) the jump between 11 and 13" pitch is quite large.

What is the factory recommended RPM range? Is it likely the motor is hitting the rev limiter with the 11? If so that would be a plain indication of too little prop.

If you can you should try something in between; say a 12" pitch same diameter, or larger diameter 11 pitch if it will fit.

Almost every prop choice involves some degree of compromise. There is not necessarily a "perfect" stock prop for every motor and boat combination. If you're really into it, a custom prop shop will repitch a standard prop to whatever pitch you want. As you might expect, not cheap.

I keep hearing about people who think they can get 10HP additional with some simple change, and I can only say I'm extremely skeptical. If it were easy and cheap, why would anyone buy the smaller motor to start with?

[Robert]

gerrykitefly wrote:

I also noticed that with the 11 pitch prop the engine is spinning at whatever rpms i put it no matter what the conditions are, whereas the 13 pitch makes it "swing" 200 rpms up and down eg, when the water gets choppier or when the wakeboarder pulls hard into a turn.

..
I suggest that your 11 pitch prop is not a high thrust model. That it holds constant RPM with widely varying load means it does not get a good "grip" on the water. Are the blades of your 11 pitch larger than your 13 pitch?
..
While experimenting, you might try a 4 blade prop in 11 pitch. The extra blade in 11 pitch will act more like a 12 pitch for RPM purposes. A four blade 10 pitch might be just right.
..
If your lower pitch prop has a better grip on the water and your engine runs higher in its allowable RPM range, you should be getting more thrust and higher boat speed, if you are getting more power you will spend more fuel.
..
On my nine prop tests the lowest pitch props were also the fastest, except for one prop which had scary little low end thrust but had excellent top speed performance beating the low pitch props by less than 1mph.

[gerrykite2fly]

Sorry, i forgot the diameters. the 13" is 13 5/8 and it is a yamaha propeller that came with the engine. The other one is a michigan 14x11"
The blades look almost identical, the Michigan ones are maybe an idea bigger.
As I said, I like the 13" better in every respect, but as I need to run that baby fast every now and then, the WOT rpms are just not ok. So whats the point, I can try only a 12" or my engine tuning project...
My dealer mounted the engine relatively low, the cavitation plate is a good inch below the bottom of the boat, so my dealer recons that the revs will go up by placing the engine right.

Chip, for the tuning issue: it is the 60 powerhead AND it is carburated, which makes tuning a lot easier compared to an EFI. As my fuel consumption with both propellers is totally in line with the EFIs - if not better- it makes me think that the manufacturers went EFI so their customers are virtually unable to play around with those things any more.

Robert, wow, 9 propellers, that´s what I call a private testlab. If the higher HP guys say, a bigger engine gives peace of mind, I guess, they´re right. But that 4 blade idea sounds like it´s worth trying, although I learned that physically a 2 blade prop has the highest efficiency! Interesting, all the Thai fishermen here use 2 blades for their modified truck engines on their longtail boats . In another forum some propeller experts recommended the solas SS props. One guy stated, they would give higher revs due to the special cut compared to other props.

Thanks for your input, I will let you know about my future changes and results!

[Terry]

Gerry,
If it is that simple to squeeze an extra 10 HP out of your power head I would get right on it yesterday, it may improve the efficiency of the Michigan 14X11 prop. As for lugging your engine I would address that too.
When towing people behind your boat I have read that it is advisable to lower the pitch and increase the diameter, not sure why this is reverse in your situation.
As for the 2 blade efficiency, there is less drag therefore higher top end speed, same with 3 blade vs 4 blade, the more surface area the more drag. On the flip side though, more surface area equals less slippage and it takes a larger surface area to move these big boats efficiently. You may wish to have a selection of props for your various needs.
I have two props for my Honda, the original is a 3 X 10.75 X 13 and it gets me into the 5700- 5900 rpm range which is fine but the top speed is not there due to the amount of slippage the 10.75 diameter encounters. I also have a Solas 4 X 11.4 X 9.75 aluminum which I had re-pitched a couple times to get the sweet spot. (only $45.00 for repitch-second time free because he didn't get it right the first time) The Solas company lays claim to a 5-10% HP gain due to their larger thru-hub exhaust, even though it is clearly larger than my 3 blade Hustler I am skeptical. Funny thing is my 4 blade is just as fast as my 3 blade which indicates to me the three blade slips too much. The 4 blade gets you up on plane faster, has a higher mid-range speed and offers better slow speed control especially in reverse. It may also pull wakeboarders more efficiently. If you want it in SS prepare to pay more, they are expensive but have thinner blades so spin more efficiently, are stronger and may put more load on your lower unit because they do not flex as easy as aluminum. There has been a lot of discussion on props, you should visit the archives.

[gerrykite2fly]

I wanted to close this thread by adding a few informations that I could obtain since we talked last time.

The upgradeable parts from a 50 to a 60 HP Yamaha Txx are the cdi unit and the carburetors. Total cost for the upgrade: around 1000,- Euros excluding work, probably a bit more compared to the new versions - and maybe still worth doing.

Finally a word about the prop issues we talked about. I´m now operating a solas amita 3x14x11 (amita). This Prop is giving almost identical results as the stock yamaha 3x13.75x13!! Which shows how complex that prop issue is.

The solas planes the boat at less that 11 knots and gives 14 knots topspeed even fully loaded with 6 persons and lots of stuff. Alone I´m reaching 16 knots, just as with the yamaha before.
I am still getting only 4.900 RPMs maximum and I am still fine with that as there are no (hearable or viewable) signs of wear. As I said earlier, it seems that the engine is delivering its max torque at under 5000rpm. However from time to time I put on the Michigan 14x11 and zoom along for half an hour at 5500 to clean her out a bit.

[Divecoz]

While experimenting, you might try a 4 blade prop in 11 pitch. The extra blade in 11 pitch will act more like a 12 pitch for RPM purposes. A four blade 10 pitch might be just right.

Robert where might I ask are you finding 4 blade 13 1/2 =14" 11,12 OR 13 pitch props.

I have looked a lot and can not find one. Pitch I find is 21. We seldom, as in only twice for less than 5 minutes each time, ever use our motor at WOT. But I have good reason to suspect that a 4 blade even close to 11 pitch would work wonders when I need it most. Those times are, Maneuvering and Pushing thru Big Waves against Big Wind at lower speeds. So if you would , point me in the right direction.

Also if anyone has good info for the right pitch of a 4 blade 14 " for a 50 HP Merc. Big Foot . Please come forth.

[Ivan Awfulitch]

Robert where might I ask are you finding 4 blade 13 1/2 =14" 11,12 OR 13 pitch props. ...

You might want to try A1discountprop. (LINK)



ModEdit, adding link ~fc

[Catigale]

One more post Ivan

Chip, for the tuning issue: it is the 60 powerhead AND it is carburated, which makes tuning a lot easier compared to an EFI.

[Greg]

I have a 50hp Yamaha. What exactly do I need to do to get 10 more ponies?

[Divecoz]

Ivan JSYK I searched your site and several others and I find no 14 " x 11=14 pitch - 4 blade props. 18 is as low as they go and then not for an outboard. I will continue every so often to. do a search If I ever find one I will post it in Big Red Letters .
Till then its my 14"- 11, 3 blade .