Should I change my prop? 15HP Yamaha 2 Cycle

[RonJon24]

Hello guys,

I would like to get a little more speed out of my 2003 15HP (15MSHB) Yamaha 2 cycle motor, will changing the prop help me?

Right now I am running a Yamaha 9.25X11-J Aluminum Prop

I am running the motor on a 1967 14' Sea Nymph Aluminum V-Hull. A typical load can run between 600-850 total weight including the motor and boat.

I installed a Tiny Tach and running full throttle I am getting 22-24 mph and the tach bounces from 10,800-10,900 but I am not sure if this is an accurate read? Do these numbers sound accurate? (where do you normally ground a tiny tach?)(is it really important to trim the extra line?)

 

[steelespike]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

Is this the latest Tinytach?Your readings are about double I have a feeling the tach is set wrong.
Be very careful to follow the directions carefully.My guess is the wire should be on a lower plug so it less likely to pick up stray pulses from the other plug. I'll be back.

 

[RonJon24]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

Thanks, that is what I was thinking too. I am not sure if it is because my wires are installed correctly or I need to change the setting from 360 to 180? I will work on getting a better read from my tiny tach and go from there. I inserted a picture of my wire install. How does this look to you guys? any recommendations?

thank you so much!

 

[pootnic]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

Doesn't look right to me.
Don't tie a knot in it,only 3-5 wraps(not over each other),probably better on the bottom plug and cut or tuck away the extra wire(so you don't get stray pulses).
Looks like a good ground though...

 

[1080tommy1]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

Pootnic is correct. I know the directions say 3-5 wraps but I had 5 and had the same issue. My first reading were 6800-7200 with 5 wraps then I cut it down to three and it seems to be in the ball park. I would definitely cut the extra wire and do not wrap more than 3 times because you will pick up stray signals. Good luck with her.

 

[steelespike]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

If it doesn't specify to tie a knot it should be just wrapped Pootnic may have actually done one. also it appears the wire crosses the other plug wire. I think I would take the wire outside the cover as soon as possible and use the bottom plug.

 

[RonJon24]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

Thank you guys for the help. I will make some modifications and see if I can get a better read. Honestly the directions are a little confusing. The one online is different than the one that was sent. One is wrapped once and tied, other has it wrapped 4 times with no tie. With no tie it just seems it may fall off. Would a zip tie mess with the read you think?

I don't have time to take it to the lake to test after work... would you guys recommend the muffs for water with a hose or a trash can? Sorry for the simple questions but I am new to boating.

 

[steelespike]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

You need to be able to run the motor at wide open throttle.you shouldn't run over about 2000 on the muffs.
Try it without a tie and then try a tie. I don't know why they don't have a little adapter you put on the plug and then
connect the plug wire to it.Or a clip you clip just clip on the write like with a timing light.

 

[RonJon24]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

I tried to run the motor out of a trash can and I can't get it up to full throttle without loosing too much water from the can. I haven't had time to get to the lake, but hope to tomorrow. I have reset the tiny tach and think it will work properly this time

 

[steelespike]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

You might be able to test the tach in a can but the rpm wouldn't be a correct representation of the rpm generated on a moving boat.
Looking forward to your results.

 

[RonJon24]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

Hey guys, I finally got good tach readings... At full throttle, I was getting 5300-5540 or so. I have no idea what to do with those numbers, thats where I am hoping you guys can help. Just a refresher on my other stats so you don't have to read everything above.

Quoting my first post
I would like to get a little more speed out of my 2003 15HP (15MSHB) Yamaha 2 cycle motor, will changing the prop help me?

Right now I am running a Yamaha 9.25X11-J Aluminum Prop

I am running the motor on a 1967 14' Sea Nymph Aluminum V-Hull. A typical load can run between 600-850 total weight including the motor and boat.

 

[steelespike]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

At 5540 and 24 mph your slip is about 13%Just about high normal.
A 15 is probably good for about 27 on a light reasonably fast boat.
What was your load for your test?
Is the anti vent plate(just above the prop)about even with the bottom of the transom?
Did you try different trim settings?
Did you move weight around?
11" pitch is highest available at iboats.
You could consider a stainless 11" about twice the price of aluminum.
at 5500 with 6% slip you might get 25 mph.
Usually a slight across the board improvement over aluminum.

 

[bbstacker1]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

Hi RonJon, I don't remember the exact number that Yamaha calls for (RPM's)but most small engines of that type are around 5000-6000 with 5500 generally a safe bet. Assuming the latest numbers you have for tach readings are correct, I would think you are about where you should be for propping. I am guessing on your last test you would have been alone, maybe one other person in the boat. If so and you add another person/more weight you might actually drag your engine down below the desired RPM's in which case a 10 pitch may be a better option. I have had a couple of different Johnson/Evinrudes, a Yamaha and currently have a Mercury, all 15hp 2 strokes, the average speed was around 21-23 mph max on a couple of different 14 ft semi-v bottom boats. I don't think you are going to find much more, if any more, speed with that motor/boat combo.

 

[RonJon24]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

Thank you steelespike and bbstacker.

Steelespike, I am going to include a pic to show you were the motor and prop line up with the boat's transom. I am very new to boating and do not understand some of the language you are using. Specifically slip and the percentage amount.

The load was lighter, just me and my son. Often it will include my wife, daughter, trolling motor and battery... probably and extra 300 pounds with the extra gear and all. Sometimes I do move some weight to the front and get another mph out of it. I try and keep it somewhat balanced. It actually planes out quite nice and quickly too.

I have adjusted the trim settings and I think this is best, what is the rule of thumb there?

My current aluminum prop has some damage, not a lot, but I was told that if it wasn't perfectly balanced it might mess up the foot seals. I was planning to replace the prop just for peace of mind. I am open to 4 blade, aluminum, stainless, etc. If this was your rig, what you do? I am also open to anyone's opinion that understands props and what is best for performance and the protecting the motor.

I do understand that 23/24 is really good speed for a 15hp on a 14' aluminum v-hull with 600-800 total pound load. I am happy and satisfied with the speed, and honestly don't want a lot more speed... but I figure if I am going to purchase a new prop anyway, I might as well buy the one that will perform the best... even if it is a little more money. And I would like to get closer to 30mph

I really appreciate your time and expertise. Thank you for replying. I am open any advice to what do, and any help understanding the why behind it all. I am def learning a lot. THANK YOU!

 

[bbstacker1]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

RonJon, based on your last test, you and your son alone without extra gear, I would stay with the pitch you have or maybe drop to a 10. I would be interested what RPM's you are turning with a max load, if you should drop much below 5000 RPM's with a full load that is why I would suggest a 10 pitch.

 

[steelespike]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

Here is the poop on slip. Your 11" pitch means the prop in a solid would move 11" forward in one revolution.In a liquid the actual movement is less and is called slip. Slip on a typical boat would be about 6 to 12%(lower is better) the size,weight and design of the boat could affect slip.Condition of the prop and the motor setup on the boat can affect slip. At 13% your slip is acceptable but you can see that lowering slip can improve speed.
Your motor is quite low it appears the anti vent plate is 2 to 3" below the bottom of the transom.
The less motor in the water the less drag.There may be series of holes in the mounting bracket that will allow you to raise the motor.
If no holes it is possible to just put a stick on top of the transom to hold the motor up till you tighten the clamps.
Just be sure the clamps have a good grip.
In the old days the racers would do this to get the exact height that was best.
If your handy you could make a jack plate or buy one so you could fine tune for the best height.
It is best for the anti vent plate to be just visible at speed.
A good prop will allow the most height and also may improve the slip.
In an ideal situation raising the motor could net a couple of mph.
Sometimes all you get is the satisfaction of knowing your not dragging too much through the water.
Probably would be a good idea to make a heavy test run before a prop selection.
as long as you have the picture posting solved
Maybe a picture of your prop would be good and a pic of the setup on the transom.

 

[RonJon24]

Re: Should I change my prop? 15HP Yamaha 2 Cycle



thanks for your input and the education. I am going to post some pics of my prop and transom in hopes that you can correct me where I might be going wrong. From what I understand now, it seems I would be best to try and reduce slip and fine tune the pitch I have, maybe even try for a better quality prop in the same size rather than increasing pitch and decreasing the motors RPM's by putting more work on the motor. Am I understanding everything ok?

 

[RonJon24]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

 

[RonJon24]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

 

[steelespike]

Re: Should I change my prop? 15HP Yamaha 2 Cycle

I've seen potatoes with more sophisticated blades.That is just plain primitive If you just get a new Solas amita 11" prop I think you will see a nice improvement.
Can't tell much about the height with the new pics but I did notice 4 holes that could be used to bolt the motor on.
you could add an aluminum agle iron across the transom if you end up with the bolts near to top.
Start with the plate about even with the bottom using the clamps to test.If they have a good grip.
If you lay a yardstick or other straight edge along the keel out to the antivent plate you can get an idea of how low the motor may be. The old prop may not like being raised up so wait for the new prop.I wonder if solas makes a prop for the 15.