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I have just installed to my new 26M, Yamaha FT50 CEDL four stroke engine, which has a maximum 5500 to 6000 RPM. The proppellar size is 11 x 13.
I was very desparate to note the maximum speed which only 25 KM/Hour on 6000 RPM, with Ballast tank empty and with only two persons on board.
How could I increase the speed? Should I change the size of the proppellar ?
I shall be very greatfull to receive some suggestions.

shehzad,

Welcome to Heath's forum. Sorry to pull a switch on you, but I have removed your question to your own thread, since it was unrelated to the 90 hp topic, and your question deserves its own topic.

Don't know anything about your motor, but that's a very respectable speed for a 50 hp on a 26M, especially with a 13-pitch prop. Most 50 hp outboards on a Mac would be using 9 to 11 pitch. It's surprising to me that you got as far as the top of your Yamaha's max rpm range.

I don't know anything about your motor either, but the fact that you're hitting max motor RPM at only 25kph indicates to me that your motor is hitting a computer-limited top speed, rather than a mechanical power limit as it should be. Speed is not dependent solely on HP and pitch. Gear ratio and prop diameter are just as important. They all have to be optimized for maximum top speed.

Others with Ms and this motor can correct me, but I believe empty, unballasted, unloaded you should be seeing something between 30 and 35 kph. It seems from the information you've given that your prop diameter may be too small.

Robert or somebody who's done quite a lot of work in this area should be able to help you.

Well Frank, I am very new to this sports and this is my first boat. I only came to know through this forum that people are getting easily 18 to 22 miles per hour with 50 HP motor. That is the reason I was worried for getting the speed of 25 KM per hour. By the way, my Yamaha engine is also brand new and just completed its first 10 hours. may be thats the reason, it is touching 6000 RPM.
Should I go for 11 x 14 proppeller.
Also please inform, for a safer and longer life of engine, how long could I continuesly keep the throtle to 5000 to 6000 RPM.

Now I understand that I am only 5 to 10 KM per hour shortter in speed with others.

The point still remains, if I could increase this with a bigger proppeller.

Six years ago the 26X was my first boat too - still is my first boat!

If my comment was a little obscure, it was because I'm not familiar with your motor. As Chip mentioned, important factors are the gear ratio and the max prop diameter. Especially because it's brandy new, I don't want to give you half-baked advice and cause a problem for you. I think your best bet with a brand new motor is to take your performance results back to the outboard dealer and ask him to provide you with a swap. You should take him at least three benchmarks, speed at 2000, at 4000 and 6000 - top speed.

Also, I'd say Yes, it's too early to be pushing a new outboard at WOT (wide-open throttle) for more than one minute duration - if that. Your Yamaha owner guide must surely advise on this? Anyway ... what about a phone call to the dealer?

The place I am living is Karachi, Pakistan. There is a dealer for Yamaha, but his knowledge is very limited. Therefore, it is very little cooperation from that side. I must rely on my own.
Any thing I have to buy, like different size proppeller, it has to be imported from Japan, therefore it is not simple to make any trial.
Therefore I always have to be 100% sure before making any order.

shehzad, Your FT50 number I think the T is for high thrust. IF this is true, then your lower unit is the type with the 2.33 gear ratio and the ability to run a 14 inch prop. The non high thrust would be an F50.
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Do a search on this discussion about the 14 x 11 pitch and 14 x 10 pitch props available for the Yamaha High Thrust and the Mercury BigFoot type 50hp outboards.
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If you are getting 6000 RPM with a 13 pitch prop, you might have a 12 inch diameter prop meant for the non high thrust outboard. But the 12 inch diameter prop should not fit?
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How many months of the year are good for sailing in Pakistan?

You are right. T stands for High thrust. Do you really think that I would be able to get better speed with 14 x 11 ?

In Karachi, We can sail all around the year except in the monsoon season, from May till Middle of September, when the sea is very rough and we are not allowed to go into the open sea. In this period, we then sail into the channels which are ambushed by jungle of mangroves.

shehzad,

Robert's correct, and the USA Yamaha spec page does show a 2.33 final drive ratio. He's also correct to ask, "How can you get a 11-inch prop with a hub that fits?" It was a somewhat strange prop size that confused me. Are you sure of your prop size at 11-inch (diameter) x13-inch pitch?

Anyway, you should take Robert's advice and search for any prop advice on Mercury Bigfoot - same motor - which usually is best with a 14x11 or 14x10 prop, depending on how heavy is the loading in the boat.

As an example, enter the following words in the search field:
bigfoot prop
Next, click to indicate a search "for all terms"
Then click the search button.

You'll find dozens of previous discussions. After reviewing them you may have other questions - if so, feel free to post again, and watch for Robert's answers !

It appears we all agree the prop is way too small in diameter for that motor. I knew Robert would come through on this one.

I wonder what possible use there is for an 11 x 13 prop for this motor? Is it possible this lower unit and prop is also used on a much smaller motor, say a 25HP?

shehzad, the 14 inch diameter prop with its large blades will push more water than a smaller diameter prop. The bigger push can also be thought of as less slip or a better grip. You can also think of the larger diameter prop like larger tires (a very rough analagy since tires don't constantly slip and props do constantly slip), you don't have to turn it as fast to go fast.
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If you can turn a 13 pitch prop at 6000 RPM with a 50hp motor and you are not going very fast then you are experiencing a lot of slip. The larger diameter prop will reduce the slip and allow you to go faster at a lower RPM, assuming that your outboard cannot deliver all its power because it reached maximum RPM with the small prop without achieving the rated torque. The larger prop will allow the outboard to use all available torque and still operate in the required wide open throttle RPM range and with that achieve maximum power and maximum speed.
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The 50hp high thust has been discussed here many times and the correct prop identified and tested and results posted.

Robert, I understand very well that if I would use 14 inches dia proppeller, the speed would definately improve.
What would be your advise for the pitch, should it be 10 or 11 ?

there are some pads which are advisable to attach beneath the engine, to minimise the lift of the boat.
Are these helpfull to increase the speed

You can buy foils that attach to the outboard to adjust the hull's attitude, usually to bring the bow down. You can also buy a ring that surrounds the propeller, improving power & slow-speed control.

A search on dolefin reveals this thread, and the following picture
of a "clamp-on" fin called Turbo-Lift (not Dolefin) mounted on a Yamaha-60.


But this boat is probably going twice the top speed of a Mac 26.


A Macgregor powersailer remains at the slow end of speed range compared with most other 50 hp boats, much more like a pontoon boat. Therefore, all devices that attach to the hull or outboard mainly increase drag rather than adding performance. It's much better to first choose the optimum prop for your engine, and for the way your boat is loaded - before trying any performance gimmicks.

I'm sure that Robert can explain the compromises involved with your choice of a 14-inch prop with either a 10" or 11" pitch.