Yamaha 50 - new prop

[Andrew Kay]

First of all, I followed the factory instruction and had a Yamaha 4 stroke 50hp, which was a mistake since the cost and weight of a 60hp are nearly the same as a 50hp.

It was difficult to go higher than 4600 rpm with top speed of 12 knots with my 26M.
Yesterday, I changed a new propeller 14" X 11 pitch @ US$150.
I can now go up to 5600 rpm with top speed of 16 knots.

[magnetic]

Hello Andrew

I am having a similar dilemman with my , albeit at half the HP and half the kts!

My understanding is that 1 inch change in pitch should account for about 200 more rpm at WOT - would I therefore be right in thinking that you changed the pitch of your prop by about 5 inches i.e. down from a 16 inch to an 11?

Oddly enough, I hope that the answer to that question is "No" - as follows;

I haven't yet had a chance to put a tacho on my boat, but I get the strong impression that WOT is currently nowhere near the specified 5,500 rpm, largely due to the fact that, at hull speeds (which is at present can't exceed) the engine is probably not spinning at much over 3,000 rpm (at which point it just can't muster up sufficient torque to get it up on the plane).

If miraculously it could somehow get up onto the plane, then WOT might produce something closer to proper spec, but that's irrelevant, as lower down the rev range it just doesn't have the muscle to get up there. The alternative interpretation is that my 9.875 x 10.50 prop is overpitched by 2,500 revs = 12 inches, implying that the correct pitch would be negative!

I therefore suspect that my prop perhaps isn't out by much more than 500 rpm / 2 1/2 inches, but clearly, I won't know for sure until I get it out on the water again. I'd therefore be interested to know what change in pitch you required in order to get the extra speed and rpm.

Many thanks

Andrew

PS - anyone got a view on Permatrim vs Doelfins vs LZ Hydrofoil?

[Andrew Kay]

You can consider driving a manual shift gear car. The car supposes can go faster with fourth gear. But, if it is uphill, the engine is not powerful enough to push the car forward, so the engine can only go up to 4600 rpm (1000 rpm short of maximum revolution or 20% below the maximum horsepower). However, if you change to third gear, the engine can go up to its maximum specified revolution of 5600 rpm with maximum power. So you end up going faster with lower gear. The 50% increase in speed is due to the fact that once the boat is up and flat, it can go faster with less unit fuel consumption then at climbing speed. Please refer to the performance chart of your engine.

My old propeller is 13 5/8" x 13 pitch. By lower the pitch to 11 pitch, I slow down the water speed and increase the torque and allowed the engine to maximize its horsepower. Therefore, I end up going faster with a lower pitch. I am not sure my interpretation is acceptable, may be some engine expert can give you better explanation.

[aya16]

andrew absolutely right.

[gerrykite2fly]

Andrew, could you specify the exact type prop? Do you reach the 16 knots in "everyday" configuration (200-300kg of load), standing rigging, and without help of wind and currents? Or was it a one time speed record?

I could reach 18 knots with the same engine, but only once, backed by some wind and a current. Usually my 3x14x11 solas amita prop gives me 14 knots, but that even with 3 persons on board plus a wakeboarder in the back...

[Catigale]

Andrew - the critical piece of missing info is the previous prop size and pitch.

Going to a larger pitch (bigger number) will DROP your WOT rpm, unless it gets the boat on plane where the other prop couldnt...I doubt that a going up a 10 to 11, for instance, would make this difference though...unless the boat was really lightly loaded.

[Andrew Kay]

I have a Yamaha 4 stroke engine. My old propeller was 13 5/8" x 13 pitch, I tested it when it was new with new bottom paint. My boat is mooring in sea water, bottom paint is needed for protection, which is heavier and slower than those without paint. No water ballast at 12 knot with 3 persons at 4600rpm; full water ballast at 11 knot with 3 persons at 4500rpm. With my old propeller and the bottom paint was new, I could achieve 4500rpm at ~80% of gas level, but I could not go much faster even I pushed the gas level all the way. Hence, I think my old propeller has too high pitch and cannot fully utilize the horsepower of the engine.

After one and half year in sea water with some small shells at bottom, I normally got 9 knots with 4 persons with full ballast at 4100rpm.

Now I cleaned up with bottom with new bottom paint, and changed the propeller to 14" x 11 pitch, I now got 16 knots with 3 persons at 5600rpm with no water in ballast in my first trial few days ago with little current and wind. I would try again later with 4 persons and full water ballast and let you know the result. After I changed the new propeller, I could get 5600rpm now with 95% of gas level. The last 5% gas level cannot produce any faster speed.

I do not have a GPS; hence my speeds may not be very accurate. When I said 16 knots, it can be + or - 1 to 2 knots. My speed is based on my speedometer with some adjustment multiplier which I calculated with chart measurement. All I can say is I can get a much higher speed with higher rpm and allow me to use the final 15% of gas level.

Bye

[Catigale]

That sounds good - lower pitch, higher rpms...

Hence, I think my old propeller has too high pitch and cannot fully utilize the horsepower of the engine.

I think is correct..

[magnetic]

Hello Andrew - you may recall that I was having similar issues with the prop on my

I went out on the boat today with a tacho and used both the old and the new prop, which are the same diameter but 2.5 inches different in pitch.

The old 10.5 prop maxed out at WOT 3,900 and a top speed of 9.5mph / 15 kmh

The new 8.0 prop gives WOT of 5,100 and a top speed of 12.5 mph / 20 kmh

These speeds may seems slow, but a £70 prop is an awful lot cheaper than a £3,000 new outboard!

I have some further experimentation to do, but thanks to your general guidance I am now 33% faster (even of that is still quite a low number!)

Many thanks

Andrew

[beene]

Congrats Andrew.

That's great news.

I know how I would feel in your shoes if just a relatively inexpensive mod would give me a 33% increase in speed.

G

[trip01]

I have a Yamaha 60hp 4 stroke on 26M. Can get 4300 rpm with current prop size of 11 3/8" x 12" in alloy. This was set up by prev owner. Ultimately, I'd like the rev range to be between 5000 & 6000 at max speed. (ballast tank is empty)

I wonder should I try larger diameter and lesser pitch.

Kind Rgds
Dave

[bastonjock]

i have a Yammaha 50 big foot and ive repropped it recently with a 14 x 11 yammaha manufactured prop,i am getting 5000 - 5400 rpm at Wot

[Irafatsar]

I have a Yamaha 60hp 4 stroke on 26M. Can get 4300 rpm with current prop size of 11 3/8" x 12" in alloy. This was set up by prev owner. Ultimately, I'd like the rev range to be between 5000 & 6000 at max speed. (ballast tank is empty)

I wonder should I try larger diameter and lesser pitch.

Kind Rgds
Dave

im running a 14x9 with max rpm at 5400 tried the 11 but no speed.

[trip01]

Sounds like the larger diameter provides speed. I reckon I'd like to try a 14" x 9" as well. Replacing the existing 11 3/8" x 12". These are all alloy I take it. Mine is 13 tooth spline.
Thanks for your replies.

Kind Rgds
Dave
Mac 26M / Landcruiser HDFT

[DaveB]

There is a lot more involved than just Diamiter and Pitch. Blades surface area, cuped for traction and the cutting thickness for same type motor.
When fellow sailers mention their prop. they should also include manifactures name,type and model Number so others can get same prop.
Dave

Sounds like the larger diameter provides speed. I reckon I'd like to try a 14" x 9" as well. Replacing the existing 11 3/8" x 12". These are all alloy I take it. Mine is 13 tooth spline.
Thanks for your replies.

Kind Rgds
Dave
Mac 26M / Landcruiser HDFT