1994 Yamaha 40 hp

[Duffy.yyz]

What prop do I need to be running on this outboard?

[keakar]

it all depends on boat size, hull type, and weight load

[Duffy.yyz]

1548, semi-v and a heavy boat. Need to keep the front up.

[Duffy.yyz]

I have done 0 adjustments to this motor since I have put it on my boat. Threw it on the jackplate and ran it. Running 32 now, but it needs to come down, and it has a 4 blade prop.

[Lil Silver]

What RPM are you turning is your starting point!

[swamp snorkler]

32 sounds pretty good with a 4 blade and a heavy load, you may be able to improve a little.

Bring a pen and pad with you and record, the rpms and mph at every 500 rpm from 1,000 to redline and bring it to a prop shop or post it here. Someone will help you.

[MattMigues]

I am running a 13p 3 blade on my 50hp, 15/52 custom semi v 36-37mph so I'd say you're right there. I've read every 10hp you should get 5-7mph more on same hull.
I bought a Tinytach for 2 stroke engine I'm running light load 5700 and heavy 5550. Also keep in mind if you drop motor you will lose a little speed.

[Duffy.yyz]

Originally Posted by MattMigues

I am running a 13p 3 blade on my 50hp, 15/52 custom semi v 36-37mph so I'd say you're right there. I've read every 10hp you should get 5-7mph more on same hull.
I bought a Tinytach for 2 stroke engine I'm running light load 5700 and heavy 5550. Also keep in mind if you drop motor you will lose a little speed.

I dropped it an inch and got another 2 mph.

[Andy C]

Originally Posted by Duffy.yyz

I dropped it an inch and got another 2 mph.

The prop has to be in the water to make it go!! If you just want a "good spary" you got it and you can run around looking cool" ?? For some reason that's the new yeti,Costa balls hanging from the truck that's never been hooked up to a trailer fad!!!

[CajunChristian]

The realistic formula is 5hp=1mph.

[Andy C]

Originally Posted by CajunChristian

The realistic formula is 5hp=1mph.

Very true

[B-Stealth]

Originally Posted by Andy C

The prop has to be in the water to make it go!! If you just want a "good spary" you got it and you can run around looking cool" ?? For some reason that's the new yeti,Costa balls hanging from the truck that's never been hooked up to a trailer fad!!!

Lol, dude I just Goodled "spary" thinking it was some kind of new Nautical term I was unaware of. And when nothing came up from the search...

Spray.....

It looks like your engine will run pretty high, judging by your spary.

[Duffy.yyz]

I definetley do not need a rooster tail, so the motor is going to come down a little. It will get there. mid to upper 30s and I will be satisfied. I think that is a realistic goal for my setup once it is set right and I have the right prop. It is ported with 60hp carbs. Its got the power, but the bottom of the boat is not in the best shape, as it is a 1984. But it was running 42 on a tunnel hull a foot longer than mine when I bought it. Reliable source too, I have seen it.

[BassYakR]

I think You will end up gettin 40 out of it man... just take a lil time to see what she likes.

[MattMigues]

Originally Posted by Duffy.yyz

I dropped it an inch and got another 2 mph.

way to high then... I started mine low and went up and gained until it would blow out dropped 3/8" and handles great. I'm no guru but Cajunchristian is the man with the plan! and PS put kill switch on

[CajunChristian]

You have to go about this somewhat scientifically, with heavy emphasis on SOMEWHAT..
ALL work requires horsepower.. You are working with a given amount of horsepower, let's say 40.Throwing water up into the air is work. Pushing a boat forward is work.
If you are working with 40hp, how many will you use to push the boat forward and how many to carry the bow, how many to throw water in the air? The more you use to carry the bow, the less you have left to push the boat forward. If you can make raising the bow a function of the propeller, it uses less horsepower from the motor.
Rake=bow lift
The best possible speed you will push any boat is with the outboard trimmed up 3 degrees positive. And yes, there is a tool to measure the trim angle at WOT, I have one.
Most 40hp motors, 2 or 4 stroke, 2 or 20 cylinders, will push the average aluminum boat about 34 mph. That is average.
First you need a tach. Without a tach, it is all a guess. That motor, if stock, will make the most horsepower and torque right at 6000 rpm. The port mapping in that motor does not lend itself to high rpm.
You are throwing so much spray because your motor is so high on the transom. If you want to run your outboard that high, you need a large diameter prop. The PowerTech SRD is a 11.5" diameter prop, most others are 10.25" diameter. The SRD has 20 degrees of rake, which gives ample bow lift.
My guess is that the prop you have now has cup added, that's why you can run it so high.. Cup holds water on the blades longer. The longer you hold water on the blade, the less efficient the propeller. That works great for running the engine high, but not so much for speed.
If you could give me more specific data, such as WOT rpm, prop pitch, gear ratio, I can tell you how much slip you have. The secret to going as fast as that engine will push that boat is to get the slip to a manageable number.
IF you can turn a 13 pitch prop 6000rpm with 8% slip, you should run right at 37mph and still have a good hole shot. Once you determine which prop design works best on your rig, then you need to play with different pitches to determine the best compromise for YOU between hole shot and top end. There is no magic prop that does both well.

[Andy C]

Originally Posted by B-Stealth

Lol, dude I just Goodled "spary" thinking it was some kind of new Nautical term I was unaware of. And when nothing came up from the search...

Spray.....

It looks like your engine will run pretty high, judging by your spary.

Glad I made your day trying to look it up!!! Lol lol
it was put that way for a reason, cause it's going to be the big fad!! Just wait an see.

Back to op, cc advice is spot on, no prop will give you great hole shot and speed, depending on what/where you plan on using it the most is your prop you need. Again rpm would be nice to know while your trying to figure that out..