New Prop For Yamaha 70hp

[spongy]

Hi fellow raiders,

I think I need a new prop for my outboard.

I have a Yamaha 2stroke 70hp on the back of a Savage 4.6m fibreglass cuddy cabin.

With a full load its difficult to get on the plane.

Currently top speed is around 33knots at 3200rpm, using a 13 1/4 pitch aluminium prop.

What are your recommendations for a new prop specs?

Im thinking I need a smaller pitch prop to get my rpm up. If so what size pitch should I be looking at?

Also another thing, any comments on using those foil/wing attachments on the leg of the outboard?

Is it going to improve performance/efficiency?

cheers,

[Flightmanager]

Hi Spongy ,

I think you have only given us half the information , the 13 1/4 figure would be the prop diameter. The Pitch would be more like 15 or 17 inches. You should be able to rev to approx 5500 RPM at WOT. If you are only able to get 3300 RPM ,I would suggest looking at a 15" pitched prop.

Ross

[pelican]

Lots you can try first if you haven't already but first things first.

Hopefully someone here will have experience or own a similar boat to compare details with or comment on that boat with a foil. You are right to show concern as your motor is under stress be it from too much load and will not last unless you fix the issue.

Has this always been the case or is the motor tired or out of tune?

Stainless or ally prop what exact diameter , pitch and brand?

Speed at which boat falls off the plane?

Does the boat have any other bad habits at speed?

Is that speed in Knots? On gps or water pitot?

Just how much weight do you have in it?

Get motor tuned compression check etc and in peak condition then again do some tests and confirm your Wide open throttle revs (WOT). Your current WOT revs seem very low meaning something is not right with the motor or as you suggest the prop is dammaged / wrong. Running a motor Check those WOT against what the manufacturer recomends. You may then decide what to do.

Weight distribution

If there heavy moveable items like fuel tanks you can move to better balance the boat so it planes quicker. That goes for helmsman and passengers as well.

Foils / wings- can work on some boats but majority are put on by people trying to solve a symptom not the problem or cause. Try and solve the cause first and then a foil may help if a particular sympton remains. Different designs are better or worse at different areas. They can work but can also create many problems some of which are very dangerous.

Props.

Props can be remanufactured and pitch , cupping adjusted to some extent and this may be cheaper than going and getting a new prop. Different props are designed or different uses on different boats. All props are a compromise to some extent between speed , power, bite lift etc. The best compromise is the one that looks after the motor and makes comfortable boating.

Motor height

Motors too low in the water can cause significant drag and kill performance

Motor Trim

Trimming in the motor will help holeshot onto the plane and then trimming out will help comfort , control and speed allowing better WOT revs.

Bad fuel

Bad fuel saps horsepower and can damage motors. Make sure you are running fresh fuel and claen filters.

I'm sure others will be able to add more and some may have dealing with a sisterboat.

Edited November 12, 2007 by pelican

[achjimmy]

Hi fellow raiders,

I think I need a new prop for my outboard.

I have a Yamaha 2stroke 70hp on the back of a Savage 4.6m fibreglass cuddy cabin.

With a full load its difficult to get on the plane.

Currently top speed is around 33knots at 3200rpm, using a 13 1/4 pitch aluminium prop.

What are your recommendations for a new prop specs?

Im thinking I need a smaller pitch prop to get my rpm up. If so what size pitch should I be looking at?

Also another thing, any comments on using those foil/wing attachments on the leg of the outboard?

Is it going to improve performance/efficiency?

cheers,

Hi Spongy

Bit more info here http://fishraider.com.au/Invision/index.php?showtopic=24900

Pending how much you want to play around i would be discussing situation with Huey @ Huett marine and talking to him about a new prop before chasing genuine props or looking at foils.

[Red]

"Currently top speed is around 33knots at 3200rpm"

Mate, i'm not sure if your tacho is reading properly. 33knots at 3200rpm sounds a bit over the top for a 70hp motor on a glass rig?

Cheers

Red

[a boat]

Hi Spongy, I agree with Red, there is something strange going on because I doubt a boat with only 70HP will be able to do 33 knots at 3200RPM. Your Tacho could be faulty and as Ross has mentioned you have given us the diameter, while Pitch is the important thing we need to know in relation to WOT RPM. With that diameter I would be thinking you might have a 19 or 21 inch when if everything is working as it should on your outboard you would need a 15 or 17 inch.

Let us know what you find,

Cheers,

Huey.

[spongy]

Thanks everyone for their response.

Yes come to think about, the engine sure sounds like its spinning faster than 3200rpm. Will need to check if I have a faulty meter.

I will get the actual pitch of the prop and let you know.

Aluminium prop about 6 years old (came with the outboard new). It is slightly damaged but not too bad.

This "problem" has been around ever since Ive had the boat (3 years).

I didnt think I had a problem until I discovered my WOT is only 3300rpm.

I had Blacktown boats have a look at the outboard a year ago and they suggested to lift the outboard up 1 notch. I think it has improved my top end speed very slightly, but at the cost of some cavitation on some sharp turns.

I will get more info and possibly some pictures of the prop.

Again, thanks for this forum and all your help.

Jeff

[spongy]

I have checked the prop on my yamaha 70.

It is a 13 1/4 X 17-K.

[a boat]

Hi Spongy, now we just have to confirm that the tacho is working correctly. If it is and you are only seeing the RPM you are talking about we need to make sure the engine is running on 3 cylinders and if it is than you are needing to go down to about a 13 inch.

As mentioned check and make sure there is nothing wrong with the outboard like no spark etc on one cylinder and then we can sort out the correct prop.

Cheers,

Huey.

[netic]

Huey....quick question mate about a yamaha outboard....

When i place my boat into gear it starts moving at about 2 knots.....if i wanted to reduce this speed would i decrease or increase the pitch........Reason i ask is that i want to be able to slow down the speed i downrigg at......which downrigging as done literally with the boat just sitting in gear.....

[pelican]

Trim it up so it isn't as effecient at pushing water backwards is the easiest solution or tow a bucket on a lanyard. Going to a lower pitch prop will also affect your top speed and probably allow your motor to over rev at WOT.

[a boat]

Hi Oz, what Pelican said. Going down in pitch will slow you down slightly at idle, I doubt you would notice it though and you run the risk of over revving. Best to run a sea anchor or the like, or better yet just try what Pel said.

Cheers,

Huey.

[netic]

DOnt like to run any buckets or sea anchors in the water as they can become an obstacle whilst fighting a king and lifting the trim up creates way too much splashing.........

[a boat]

Oz, what is your WOT RPM then, You might be able to go down in pitch size and still stay under about 5500RPm at WOT.

Cheers,

Huey.

[spongy]

Hi Spongy, now we just have to confirm that the tacho is working correctly. If it is and you are only seeing the RPM you are talking about we need to make sure the engine is running on 3 cylinders and if it is than you are needing to go down to about a 13 inch.

As mentioned check and make sure there is nothing wrong with the outboard like no spark etc on one cylinder and then we can sort out the correct prop.

Cheers,

Huey.

Thanks Huey,

I will check the tacho and let you know.

[johnno]

Sorry to hijack thread

Netic or another option is get a used prop/2nd hand and get it repitched

but this won,t stop to over rev situation.Also this will hurt max speed and fuel eco if you are doing long runs on the prop.

Or i could come fishing with you it will help slow the boat down.

[gone_wishin]

I don't know if this helps or not but I remember on one of my older rigs something about the tacho having 2 settings depending on the size of the outboard.

From what spongy has said is the tacho set to read the RPM off a larger outboard therefore he see's half the actual RPM's?

Huey, can you confirm if some older tacho's have a switch or different connections for different outboard sizes or am I dreaming?

[a boat]

Hi Guys, all Tachos have a pulse setting. I am honestly not sure what the settings needs to be on a Yamaha, Spongy have a look behind the tacho and try the different settings in the water.

Good pick-up wishin

Huey.

[wildfish]

johnno your a funny man!

[netic]

guys, whereabouts on the prop is its size and pitch info...tryin to find out what im running

[pelican]

guys, whereabouts on the prop is its size and pitch info...tryin to find out what im running

The prop diameter and pitch info or part number is normally cast into the center of the prop hub written around your prop nut where your exhaust comes out.

In regards to a drogue to slow trolling it doesn't have to get in the way of your fishing. We have used a strong industrial bucket with 4 inch hole in the bottom and a divers weight tied to the inside so it dosn't float and attach it to the boat with 18 inches of lanyard clipped to low point on transon like ladder. It sticks out no further than your motor. I didn't suggest this but the large PVC traffiic cones cut short and with a divers weight work as well. Just have to remember to unclip it before motoring off. Amazing how much resistance a bucket produces- just try and hold one in the water as you idle along and you will understand. Your rod will clear it no worries. There is a product called Troll-o-matic (I Think) that attaches to your cav plate that flips down a flap to dissrupt part of the prop wash to slow trolling but I have never seen one used . Huey could probably order one for you. A lot of fish don't seem worried at all by the prop splash but if you trim up lots you have to watch you still have water cooling the motor both when you are standing at transom and at the helm.

Pel

Edited November 14, 2007 by pelican

[spongy]

Hi Guys, all Tachos have a pulse setting. I am honestly not sure what the settings needs to be on a Yamaha, Spongy have a look behind the tacho and try the different settings in the water.

Good pick-up wishin

Huey.

OK, not sure what Im looking for, but will look anyway.

Also, I have an automotive induction tacho, but I think its only good for 4,6 or 8 cylinder engines, not 3 cylinders. So I was thinking I could still use it on the 6 cylinder setting, but just have to remember to double the reading.