Propeller Change

[Tom F.]

I have a Yamaha 70HP engine on my 26X with a 13" diameter 11 pitch propeller. I am currently running too high at 6500 rpm where the manual suggests 5800 rpm at WOT. I have tried a couple of online calculators and am getting quite a variety of options with the diameter staying relatively the same but the recommended pitches are all over the board from 15 - 23. I thought a rule of thumb was going up a pitch would lower rpm by 200 which would suggest a 15 pitch???

[Spector]

I'm running the Solas Amita 4 Blade Aluminum 11.88" dia. X 9" pitch SR on my Yamaha F60. RPM is right on the money

[Tomfoolery]

I thought a rule of thumb was going up a pitch would lower rpm by 200 which would suggest a 15 pitch???

I've always heard 400 rpm per inch of pitch, but of course, it's just a rule-of-thumb and specific engines with specific prop diameters, lower unit ratio, and desired boat speed can make a mess of any rule-of-thumb. Going from 3 to 4 blades is similarly said to be like adding an inch of pitch, but may degrade top speed a little, though our boats are slow enough that I rather doubt it holds true (our engines can power a go-fast boat quite, well, fast, so ours are more like work boats or pontoon boats in that regard).

Speaking of pontoon boats, if given the choice in a prop calculator, choose 'pontoon boat' for prop sizing. But given that you know the performance now, I think you'd be better off making adjustments based on the known starting point rather than starting from scratch. Sounds like a 13 x 13 should do it, or a 13 x 12 4-blade.

I use a 4-blade on my Honda BF50 (with 1" less pitch than the 3-blade that keeps the engine below its recommended max) mostly for the extra pull when towing a wakeboarder. Seems to pop him out of the water faster, though I haven't done controlled tests to confirm. The engine does seem to max out at the same speed, though, so the 1" less pitch with the extra blade seems to hold true for my boat and engine combo.

[Ormonddude]

I am running a 14x12 on a 70hp Tohatsu its about right

[sunshinecoasting]

Ok I will ask the question, when you say your's is about right, what makes "about right", right? I dont know what prop I have on my Nissan 50 but it easily gets to max RPM (according to manual) and sits on about 14-15 knots (GPS) at that RPM, however it seems to me that it could handle more prop and maybe get another knot or two, so how do you know what's about right without labouring your motor?

Cheers, Dennis.

[seahouse]

Dennis --It's exactly right when the rpm at wide open throttle is within the rpm range (high and low) recommended by the manufacturer for that particular engine. It might be necessary when first setting up to not move the lever all the way to wide open throttle when up to cruise speed if you are already exceeding the maximum rpm.

But it's fine for a good hole shot as long as you remember to throttle back as rpms build, and keep an eye on the tach. Just don't let anyone else at the helm unless they know this .

Outboards don't seem to be instrumented with a red line on the tach as often as airplanes, cars, sea-doo's, snowmobiles, and motorcycles etc are. Maybe they use parts-bin tachs... but who doesn't?

But you will not want to run for any length of time at WOT if the rpms you get are well below the recommended range either.

In either case you will void your warrantee (probably the easiest abuse for a manufacturer to prove), shorten the service life of the engine, and risk engine damage.

[sunshinecoasting]

Thanks Seahouse, then I guess my prop must be just about right, the manufacturer recommends 5400 rpm and the tach has a yellow line start at that point and red line at 5800rpm, at WOT I appear to be at around 5800 RPM (14-15 knots) but I always back it off to about 5200 RPM to be safe. The motor seems to be very happy at 5200RPM and feels like it would sit there all day.

cheers, Dennis.

[Tomfoolery]

Thanks Seahouse, then I guess my prop must be just about right, the manufacturer recommends 5400 rpm and the tach has a yellow line start at that point and red line at 5800rpm, at WOT I appear to be at around 5800 RPM (14-15 knots) but I always back it off to about 5200 RPM to be safe. The motor seems to be very happy at 5200RPM and feels like it would sit there all day.

That's about the speed I get with my Honda BF50, 3 men, unclean bottom (lake slime mostly), and relatively little gear (ballast out, mast up). By way of another data point. Sounds like the right prop, unless there is an automatic overspeed limiter that's preventing the engine from turning faster.

[Tom F.]

I found a local prop shop that I had previously not been aware of in the DC area. After going over my issue with the owner, he recommended a 13 1/4 X 17 prop. After the storm passes I will give it a try on Sunday.

[Tomfoolery]

I hope you can return it for a refund or another prop. You're going up 1/4" in diameter, and 6" ( ) in pitch with an engine that's only five hundred rpm over redline.

[Boblee]

Thanks Seahouse, then I guess my prop must be just about right, the manufacturer recommends 5400 rpm and the tach has a yellow line start at that point and red line at 5800rpm, at WOT I appear to be at around 5800 RPM (14-15 knots) but I always back it off to about 5200 RPM to be safe. The motor seems to be very happy at 5200RPM and feels like it would sit there all day.

cheers, Dennis.

We carry two props, one for when heavily loaded and another when not I would suggest if the manufacturer says 5400 that is WOT if you change to a better prop you will get better high speed at WOT and backed off to where it feels comfortable but we rarely go over 3000 rpm.

[Skipper]

50hp BMP Evinrude E-tec engine w/power trim
2006 Macgregor M26
Factory BMP specs: Ideal RPM = 5600-5750 (Operating Range 5500-6000)
Max speed = 13.4 mph (calm seas and wind, empty ballast, rudders & daggerboard up)
Max RPM = 4,700
14 X 11 BMP Evinrude 3-blade propeller
Engine is operated at 6325’ above sea level (Lake Tahoe, CA) & has never gotten the Mac up on plane
Two 12V marine batteries, 24 gal fuel capacity, fuel/water separator, 3/8” fuel line

We’re trying to make sure we’re using the engine most efficiently e.g. do we need a new propeller or is the current one OK if we aren’t going to operate the Mac at WOT?

We are concerned with the many warnings from Macgregor owners and outboard engine owners not to lug the engine at the WOT (Wide Operating Throttle). As you can see in the stats above our engine is not able to meet the factory ideal RPM range at our high elevation. We have been routinely cruising around 3500 RPM on multiple hour trips which provides about 9 mph and appears to be giving us around 5 mpg. We run the engine at about half that speed on short hauls where travel time isn't important. Our recent review of the Macgregor Forum says that 3500 rpm is pretty near the most inefficient speed for fuel economy but the engine hp and operating elevation above sea level weren’t specified and we are assuming that was for a Mac that was capable of of getting up on plane (ours can't right now or maybe it won't be able to at our elevation due to power loss regardless of replacing our prop).

What we haven’t found in the Forum is a recommendation for a prop size (if any is even needed in our situation) given our unusual elevation in the mountains which we’ve heard can lower the engine’s power output by around 20% compared to sea level.

So we’d appreciate learning from more experienced Mac owners how to size the prop for this engine e.g. given our elevation in the mountains which probably will not allow this hp engine to get our M26 up on plane, and the fact that we will never operate this engine very long at its WOT.

[Highlander]

Ur eng. should be protected from over revving exceedingly by the ECM that controls a rpm limiting switch my eng. rpm range I believe is 5000-6000rpm but one of my props I can go to 6300rpm,s & the ECM sets off the eng. allarm buzzing telling me I,m over revving , but still in a safe mode but if u try to go past that safe mode , the ECM will cut back ur RPM,S not allowing u go go past that safe mode , just try revving ur eng up in neutral & as soon as u try to exceed the recommended safe mode the ECM will activate the RPM limiting switch then ur eng rpm,s just surge up& down

J

[Whipsyjac]

Skipper,

Changing your prop to achieve proper RPM is done for the conditions you encounter most often. If you are always on Lake Tahoe then you should change your prop to get you into the proper zone. The elevation only becomes relevant if you take your boat somewhere else ie. the coast for a trip to the Santa Cruz Islands, at that time you might have an rpm overspeed problem.

I wasn't happy with the performance of the props that came with the boat. The PO kept things very light and used the boat on lakes often with no mast, we use it on the Ocean with a few hundred pounds extra gear at all times and a bigger family so I had to do something about the prop. Tomfoolery has already posted in this thread and he gives great advice so I followed his lead and got a prop that works better for me. Your engine performs similarly to his so you might begin by trying one of the props he often suggests. Just click on the forum link at the top of the home page and if you're logged in there will be a search box at the top right where you can search prop threads OR click on Tomfoolery's profile in this thread and then you will be able to search his posts. He's basically running 9" pitch props.

I ended up buying a new 9" pitch for my 50 and re-pitching my spare. If you can find a shop that will loan you props to test out that would be your best option. My 50 is still a little low rpm wise at WOT(but I have a 2:1 ratio) Tomfoolery has a 2.33:1 and yours is 2.67:1. Pretty much the starting place for a 50hp is 9" pitch but you might need 8. If you gain 400 rpm for every in you drop in pitch(11p - 8p = 3) you'd gain 1,200 rpm and be at 5900rpm WOT. If you only gain 200rpm per inch dropped thats a 600rpm gain and puts you at 5300rpm WOT just a tad low. The gains I've heard of are typically closer to the 200rpm per inch mark.

One thing to note is you're running about 11kts which is semi-planing even though my is different there's something that happens when you cross the 12.5-13kt(15-16mph) mark, you move into full plane(note some mac owners have felt this occurs at a slightly higher speed). At this point my engine seems to labor less and runs up an additional 150rpm or so with no adjustment to the throttle. My guess is you'll probably gain a few extra rpm if you get across this barrier.

You can also increase your rpm by reducing diameter, however smaller diameter props "slip" more. From following the prop threads on this site 13" props and larger seem to perform admirably.

So my suggestion is a 13x9...but if you can try before you buy. And to put it into perspective a few hundred spent on props is a lot less expensive than a new 70 or 90hp motor


Good luck,

Willy

[raycarlson]

No hard or single rule. three props of same size from different manufacturers will give 3 different results. You cannot hurt any modern EFI motor from over reving, they wont do it the ecu takes care of it.shoot for highest rpm range unloaded should keep you at lowest rpm range when heavily loaded. 1" pitch = 200rpm approx. 4 blades =minus 200rpm approx, stainless steel = plus 200rpm approx , double cup = minus 200rpm approx. your best friend is a generous shop that lets you try multiple props before buying, if they don't they don't really give a sh#t about your business.