4 Blade Props fo 30 HP

[leachy]

Hi All

I was just wondering if anyone has ever put a 4 blade prop on a smaller 2 stroke. I have a 3 cylinder 30HP Yamaha and need to get a larger prop for it. I am hitting max RPM at 60-70% throttle. I was thinking of going for a 4 blade over a 3 due to it being better balanced and more efficient. I have read that you loose top speed but if it was the right size and there was enough torque the boat should do the same speed but at a lower RPM?

Thanks Leachy

[leachy]

Well just an update, did as much research as possible and have ordered a Solas alumin. 4 blade prop. 10 x 13, the standard is a 9 7/8 x 12 3 blade. Everything points to this being slower, and maybe it might not go as fast as it possibly could, but it will be interesting to see what happens. It is replacing a stainless Solas 9 7/8 x 12 3 blade which hits top RPM at 60% throttle.

Will let you know the outcome in a few weeks.

[Danmaso]

I have always been told that if you go 4 blade then you should drop a pitch to the equivelent 3 blade, what are you trying to achieve with the prop? higher revs/top speed or hole shot?

[Brad Y]

4 blade props dont have the slippage that a 3 blade will. They are more efficient. Saying that yes generally if your running a 14 3blade then look at a 13p 4 blade. If your 4 blade has some decent cupping to it you might lose some more revs as well. But if you know for sure (tacho) that your maxxing out revs at 70% throttle then you are probably going to be fine. With a two stroke you want to be at or as close to the max rpm for that motor. Thats how they work. 2 strokes dont like being run at low revs. Another thing is you are going from a stainless prop which will have little flex, to ally which will have more. Under load it might actually be more like a 12.5p And your diameter is smaller too. Will be interesting to see but dont be surprised if your holeshot is great, fuel economy at low revs is great, but your overall revs are a fair bit lower and top speed is down.

Years ago I was running a 15p 3 blade ally on a 50 merc and I wanted more out of the boat. I changed to a 13p 4 blade with aggressive cupping and a slightly smaller diameter. The result was higher revs, faster speed, improved cornering, no cavitation, easy to get on the plane fully loaded with livewell and co angler and all round awesome performance. I even was able to raise the outboard up a hole on the transom which helped too.

Propping is a big suck it and see process. What often looks good on paper proves to be pretty average, and what looks impossible on paper is sometimes pretty darn awesome.

[leachy]

Hi Dan

I was trying to get more out of the motor, by that I mean make it a little smoother (the reason I went for 4 blades), and slightly higher top speed. At the moment top is 26-27 knots (GPS) and it gets there at 60% throttle, maximum RPM. So that tells me it got a lot more spare torque to be able to run a larger pitch. The prop on it at the moment is a Solas SS 9 7/8" x 12", same as the standard Yamaha but with more cupping.

So I figured this 4 blade would be like running a 14" pitch 3 blade as you suggest. Hole shot is not an issue there is more than enough power as it on a 395 AllyCraft, same as the Qunitex Explorer with a casting deck up front and electric bow mount.

Regards
Leachy

[leachy]

Hi Brad,

I am a little unsure and there is nothing really around on the smaller engines running 4 blades. I agree the high RPM will still need to be maintained, but this 3 cylinder engine seems to have so much more torque than the 2 cylinder versions. The reason I know I am hitting the rev limiter is my mate has a 25hp (2 cylinder) on a small boat. We have the same size prop the motors and boats are very different but they are the same speed, he hits max revs at about 75% throttle and my a little less, but the props are turning at pretty much the same speed so the boats go down the river at the same speed. I tried a tinny tac but it caused and issue with arcing through the high tension lead it was wrapped around.

One thing I did seem to notice is the stainless is so much heavier than an aluminum prop and was wondering if that would have much of an issue in accelerating a prop as you have to spin a heaver object from start. The rotational torque would need to be greater to accelerate it at the same speed.

I will let you know how it works out good or bad, at worst case wasted $100 dollars.

[boris]

At least with the solas ones you can prop swap the get the right prop for your boat!

[leachy]

Yeah but its from the USA my 4 blade test prop. My existing Solas SS is a few years old originally purchased for a 2 cylinder 25HP, at great expense.

[Danmaso]

when you are quoting throttle percentage, is this of the lever? or the max revs the engine is running at? you need to work out your revs in order to work out the correct prop pitch, ie if the engine is supposed to max at 5500 and its doing 5700 then you need to up a pitch on a 3 blade or keep pitch and run a 4 blade, to reduce by 200rpm, without tacho readings its just a guessing game... my old boat is a 2 cyl 25hp merc that i converted to 30hp, what i found is that it reved to hard and didnt seem to have as much go as it should, so i used a tacho to get the top revs and it was slightly over the factory specs, so i went up 2 pitchs 11' to 13' and a little bit of diameter 9.8 to 10, so that the revs would come back to an acceptable level and the prop would get more drive, the result was better fuel econ and heaps higher top speed, i stayed with a 3 blade because its easier to work with, 4 blades are better for heavier boats and holeshot, which you shouldnt have an issue with in yours. i think the whole 4 blade thing could be complicating the issue...

if your looking to smooth out your engine, then have a look at a set of boysen reeds, have then in mine and they are much smoother running than the factory stainless steel ones...

if you think the engine has plenty of spare torque, you wont have an issue running a s/s prop, when i changed mine i didnt notice anything except its more responsive

sounds like your control box needs an adjustment, so that the lever @100% matches the engines to settings

[Brad Y]

Very hard to get an accurate gauge of revs too without a tacho. And you wont necessarily know if you are indeed in your motors operating range. How boat shops choose the best prop for a boat without one installed is a little worrying. My etec actually has a little more travel in the throttle after it reaches max revs.

[leachy]

Hi Guys

I had the tinny tac on long enough to see that its getting up into the hi 5700RPM and although I don't have it on now I can tell its revving to high, or the same amount. When I mentioned 60% its when the butterflies are open that amount. I checked the percentage on the tiller throttle to their position and it pretty right even that second stage advance comes in.

If I was staying with a three blade I would have gone for the 14". I only went for aluminum as its a whole lot cheaper if it doesn't give me the results. I have also heard interesting stories that if you do hit something your liable to cause less damage to the internals with an ally prop and seeing most of my boating is in the back waters of the Hawkesbury there's lots of things to hit.

I will let you know how it goes hopefully on the weekend, ordered it from the USA yesterday and already been notified of delivery for Friday.

[leachy]

I will have a look into the Boyesen Reeds, hadn't thought of that and wasnt aware you could get them.

Thanks

[breambucket]

Originally Posted by Danmaso

when you are quoting throttle percentage, is this of the lever? or the max revs the engine is running at? you need to work out your revs in order to work out the correct prop pitch, ie if the engine is supposed to max at 5500 and its doing 5700 then you need to up a pitch on a 3 blade or keep pitch and run a 4 blade, to reduce by 200rpm, without tacho readings its just a guessing game... my old boat is a 2 cyl 25hp merc that i converted to 30hp, what i found is that it reved to hard and didnt seem to have as much go as it should, so i used a tacho to get the top revs and it was slightly over the factory specs, so i went up 2 pitchs 11' to 13' and a little bit of diameter 9.8 to 10, so that the revs would come back to an acceptable level and the prop would get more drive, the result was better fuel econ and heaps higher top speed, i stayed with a 3 blade because its easier to work with, 4 blades are better for heavier boats and holeshot, which you shouldnt have an issue with in yours. i think the whole 4 blade thing could be complicating the issue...

if your looking to smooth out your engine, then have a look at a set of boysen reeds, have then in mine and they are much smoother running than the factory stainless steel ones...

if you think the engine has plenty of spare torque, you wont have an issue running a s/s prop, when i changed mine i didnt notice anything except its more responsive

sounds like your control box needs an adjustment, so that the lever @100% matches the engines to settings

hey mate just slightly off topic but i have been looking into the boysen reeds just curious if hey increased topend and mid range as well, looking to refine my 3cyl yamaha a little

Cheers

Pete

[Danmaso]

Originally Posted by breambucket

hey mate just slightly off topic but i have been looking into the boysen reeds just curious if hey increased topend and mid range as well, looking to refine my 3cyl yamaha a little

Cheers

Pete

I wouldn't say they increase anything as such, although your motor will run smoother and a lot more responsive, much smoother at idle and start easier... for the money they are a good upgrade, however don't throw away your factory reeds, keep as spares, its an easy install if your hand with a spanner, I bought direct from boysen.com quick shipping and best price

[breambucket]

Originally Posted by Danmaso

I wouldn't say they increase anything as such, although your motor will run smoother and a lot more responsive, much smoother at idle and start easier... for the money they are a good upgrade, however don't throw away your factory reeds, keep as spares, its an easy install if your hand with a spanner, I bought direct from boysen.com quick shipping and best price

cheers bud thanks heaps for the info