proper outboard and propeller mac26X

[bonati]

Hello mac friends,

Going tru the existing topics on the matter above I got no clear view for my problem.. may be somebody of you can help me.

I bought my Mac 26X equipped with Yamaha F50 4S a few months ago. Recently I put it to full throttle , without ballast ( hull was really dirty , thats true) and I could not go over 3800 rpm ( engine full power is 50 HP between 5000-6000 rpm in data sheet) Propeller is 11 1/8 - 13 -G. ( I did not measure de knots speed , just the engine rpm)

As all 26X are identical ( thus installation of engine I presume should also be identical) my questions are ; are you guys, actually getting these 5000 to 6000 rpm in your boats? Is the propeller adequate? a clean hull could make such difference on rpm ? Is it detrimental to the engine this situation ? ( Actually I currently run the engine on 2500 rpm , but in some event more power could be neccesary)

Of course it may be the case that the engine have lost compression , defective air inlet , etc ... but the sound of the engine , the fast start etc tells me it is not the likely case ..

Well if this post calls your attention I will thank you for any ideas and shared experience ...

[dlandersson]

My WOT is about 5600. I'm using a Merc 98 50 HP non-bigfoot

Hello mac friends,

Going tru the existing topics on the matter above I got no clear view for my problem.. may be somebody of you can help me.

I bought my Mac 26X equipped with Yamaha F50 4S a few months ago. Recently I put it to full throttle , without ballast ( hull was really dirty , thats true) and I could not go over 3800 rpm ( engine full power is 50 HP between 5000-6000 rpm in data sheet) Propeller is 11 1/8 - 13 -G. ( I did not measure de knots speed , just the engine rpm)

As all 26X are identical ( thus installation of engine I presume should also be identical) my questions are ; are you guys, actually getting these 5000 to 6000 rpm in your boats? Is the propeller adequate? a clean hull could make such difference on rpm ? Is it detrimental to the engine this situation ? ( Actually I currently run the engine on 2500 rpm , but in some event more power could be neccesary)

Of course it may be the case that the engine have lost compression , defective air inlet , etc ... but the sound of the engine , the fast start etc tells me it is not the likely case ..

Well if this post calls your attention I will thank you for any ideas and shared experience ...

[Tomfoolery]

Do you know the reduction ratio in the lower unit? The specs I found say 1.85:1, which makes for a fast propeller speed. I'm guessing that 13 in pitch is too much, even with an 11-1/8" prop, for such a fast turning prop (2973 rpm at 5500 rpm engine speed) on that boat.

[Flightfollowing]

You are supposed to get wot rpm within the design spec of the engine, so you should be able to attain the minimum 5000 rpm at typical boat loading conditions, I.e. Your typical # people and hull bottom conditions. 1200rpm below target seems way too low and suggests you need to reprop the engine. However, I have heard that with modern engines this is not detrimental to engine health like older engines. If it were me, I would clean the hull bottom and try again. If i still came up with too low an rpm, I would start experimenting with different props until I reached at least 5000rpm with a ballast in loading, which should result it some higher rpm but still less than 6000 rpm with ballast out.
I would not be the slightest bit surprised that your boat had a mismatched prop from installation or reinstallation, when I bought my Mac I had to reprop it to reach spec rpm wot.
However, I am no expert. You probably should try and find an outboard shop with a decent selection of spare loaner props to test out and then buy the final prop from them after sorting out the best pitch and size. You should be able to search this site and see a bunch of macx prop tests with the same or similar engine and see what other owners finally decided.

[bonati]

Thks for all the replies , they are very helpful to me
Tomfoulery: Seems that 1.85:1 is the gear ratio ;
Flightfollowing: I will do as you advise, is the wisest thing to do. By the way , and just as info, could you indicate which prop size you ended with in your own case, for Mac 26X I suppose ? ( provided same gear ratio )

[Flightfollowing]

Bonatti,
I will be no help to you, I have a Suzuki 90hp with 2.59:1 gearing on a Mac M, totally different from your Yamaha on the X. But I bet if you search you will find a discussion on installing the Yamaha on an X to include the props tested.

[Tomfoolery]

Check out this thread. It's not the only one, but it's the first I came across and read. And the sizing seems consistent with what I would expect.

"I have a 1998 Yammy 50 hp with a 12 x 3 x 9 pitch prop.
Gets me 17 - 18 mph. at wot."

http://www.macgregorsailors.com/forum/v ... 4&p=265686

I run a 11.8" x 3-blade x 10" pitch on my BF50, with a 2.08:1 ratio, for 2640 rpm prop speed at 5500 rpm engine speed. Compare that to your 2970 rpm prop speed at 5500 rpm, with 5/8" smaller prop, but 3 more inches of pitch.

I also use a 4-blade with 9" pitch, but the extra blade is more or less a wash for top speed. But I get better hole shot and stronger wakeboard pulling, which is the only reason I bother with it. Oh, and better reverse, but compared to an inboard sailboat auxiliary with folding prop (read: pitiful reverse), even the three-blade is livin' large.

[dlandersson]

I did this - but I focused on others with the same model boat AND model engine that I had.

Check out this thread. It's not the only one, but it's the first I came across and read. And the sizing seems consistent with what I would expect.

"I have a 1998 Yammy 50 hp with a 12 x 3 x 9 pitch prop.
Gets me 17 - 18 mph. at wot."

http://www.macgregorsailors.com/forum/v ... 4&p=265686

I run a 11.8" x 3-blade x 10" pitch on my BF50, with a 2.08:1 ratio, for 2640 rpm prop speed at 5500 rpm engine speed. Compare that to your 2970 rpm prop speed at 5500 rpm, with 5/8" smaller prop, but 3 more inches of pitch.

I also use a 4-blade with 9" pitch, but the extra blade is more or less a wash for top speed. But I get better hole shot and stronger wakeboard pulling, which is the only reason I bother with it. Oh, and better reverse, but compared to an inboard sailboat auxiliary with folding prop (read: pitiful reverse), even the three-blade is livin' large.

[Tomfoolery]

I did this - but I focused on others with the same model boat AND model engine that I had.

Check out this thread. It's not the only one, but it's the first I came across and read. And the sizing seems consistent with what I would expect.

"I have a 1998 Yammy 50 hp with a 12 x 3 x 9 pitch prop.
Gets me 17 - 18 mph. at wot."

I thought it was the same model boat and engine as the OP. 50 hp Yamaha, onna 26X

[dlandersson]

You do - but several others had other brands - confused me anyway.

I did this - but I focused on others with the same model boat AND model engine that I had.

Check out this thread. It's not the only one, but it's the first I came across and read. And the sizing seems consistent with what I would expect.

"I have a 1998 Yammy 50 hp with a 12 x 3 x 9 pitch prop.
Gets me 17 - 18 mph. at wot."

I thought it was the same model boat and engine as the OP. 50 hp Yamaha, onna 26X:?

[bonati]

Thks to all guys,

As summary I believe It is clear what I have to do now ( when boat is again in water) , that is to follow your advise. This to really see that the engine actually attains the 5000-6000 rpm range.

But it is also interesting the comment on that the engine would probably not be suffering from operating at 3800 rpm at wot ; if this is the case may be the old owner did purposely put this propeller to secure strength agaist hard seas to trade against speed ( if he did not care about going at 11 or 18 knots), fuel consumption could be other issue

May be anybody has a comment on this , it is just an idea

[ris]

bonati, we went through almost the same thing with a new honda 60. Went from a 13 3/4 X13 prop to 14 X 9 to get from 4300 rpm to 5250 rpm. After a lot of searching decided we needed to get over 5000rpms to keep from messing up the motor. If you always went at hull speed it might not be too harmful to the motor, but you need to get to whatever rpm the maker of the motor calls for at WOT.
At hull speed the mph is only about 2 mph different and at WOT the difference in mph is about 4 mph. Hope this helps. As others have said just drop the pitch about 2 to get a 200 to 400 rpm change in the prop. I went from a pitch of 13 to a pitch of 9 to get around a 800 rpm change in the motor.

[bonati]

bonati, we went through almost the same thing with a new honda 60. Went from a 13 3/4 X13 prop to 14 X 9 to get from 4300 rpm to 5250 rpm. After a lot of searching decided we needed to get over 5000rpms to keep from messing up the motor. If you always went at hull speed it might not be too harmful to the motor, but you need to get to whatever rpm the maker of the motor calls for at WOT.
At hull speed the mph is only about 2 mph different and at WOT the difference in mph is about 4 mph. Hope this helps. As others have said just drop the pitch about 2 to get a 200 to 400 rpm change in the prop. I went from a pitch of 13 to a pitch of 9 to get around a 800 rpm change in the motor.

Thks, Ris for your contribution . Because of the interesting points you mentioned and If you feel like it I will bother you with some further elaboration on the following concepts in your post:.
.If you always went at hull speed it might not be too harmful to the motor ... What is the hull speed of the Mac X? Seems to imply that the 4300rpm at WOT would be harmful for the engine , isn´t?
At hull speed the mph is only about 2mph different and at WOT the difference is about 4mph Understand you mean difference before and after the change of propeller

thks again

[Tomfoolery]

Remember that the power required grows exponentially (sort of) with boat speed, so running at less than hull speed, where the power requirement is very low (relatively speaking) means that the big engine is loafing. Running a high-pitch prop can bring the engine speed down for a given speed through the water, and probably save some fuel, without lugging the engine.

Like dropping down to 4th gear on a flat road (in a 4-speed car) at low speed where only a few hp is required to move at a steady speed, instead of staying in 2nd gear, with the same low road speed and the same few hp required (since ground speed hasn't changed). But pulling a hill would require a lower gear to prevent lugging the engine. Shallower prop pitch is like a lower gear in this example.

Oh, and I'm not sure about modern engines compensating for lugging conditions. I think that would depend on the engine. My carbureted Honda BF50 certainly will not compensate, but a computer controlled, common rail, EFI engine may very well do that.

[Jimmyt]

Hull speed or displacement speed is the speed at which the wavelength of the boat's bow wave (in displacement mode) is equal to the boat length. I would guess somewhere in the 6-7mph range for our boats, but don't know for sure. I would assume ris meant that you might see a 2mph different at APPROXIMATELY hull speed - since hull speed is a unique number.

WOT or wide open throttle, would be higher than displacement speed for a 26x (unless you had a small outboard on it). When the boat is planing (not in displacement mode), I would assume ris meant that you would see a larger increase in speed.

Don't have a basis to agree or disagree, just trying to address your apparent confusion.