Prop Selection for Increased Speed and/or Holeshot

[Splash]

I can reach a max. of 23knts (or around 42km/hr) on this rig with the following specs:

Boat details:
- 4.1m polycraft
- Tiller steer
- 280kg bare hull weight (probably 520kg in total weight with 2 people on board, fuel, etc)
- F60 2012 Yami outboard
- Prop 11 3/8 diameter x 12 pitch (3 blades)
- Link for hull : http://www.polycraft.com.au/model-ra...nger/open-boat (http://www.polycraft.com.au/model-range/410-challenger/open-boat)

Max speed reached - 22.5knts (41 km/hr)
No jack plate used.


From this list of propellors below, what would be a better suited prop for quicker holeshot and/or top end speed?

http://yamahaoutboards.com/sites/default/files/pdfs/Four-Stroke-F60-T25.pdf


Perhaps, props from Solas?


Splash

[Moonlighter]

The key information you haven't stated is what the current max rpm you have at full speed Splash. And, at full speed, is the engine hitting the rev limiter?

To be very clear, what is your rpm at 41km/hr?

[Splash]

Thanks Moonlighter.

Back to the dwg board - i will set up the taco and advise later.

SPlash

[Dan5]

Splasher just as guide when I had my 4.5 Bluefin with 60hp Yamaha I would get around 62kph WOT.....

My current little barra boat a Trailcraft 435 is running a 50hp Yamaha which is similar to your motor with a restrictor i'm getting 31.5kts WOT or around 58kph.

60hp was running a Yamaha alloy 13P and 50 is running a 12G,,,,,,,I think you should look at the 13P for yours you should be able to pick one up for around $150 new.

41kph seems way under what you should achieve with that motor but saying that I've had nothing to do with Polycraft hulls.As Moonlighter has said somehow you need to see your wot max revs maybe and interface for your HDS would be the way to go..........Unable to tell you what my max revs are as I have no tacho facility either but it feels like i'm getting what I should so i'm happy to travel blind in this case.

Dan

[Moonlighter]

The Poly is a relatively wide and heavy boat for its length, and, the fact is, that they are not a fast hull. They will never be a "sports car" of a hull.

So that means they can't really be compared with other hulls from a prop selection point of view.

Having said that, I tend to agree that Splash should be able to get some more speed out of her. And better hole shot. And probably better economy too.

But it the only way to know if you have got the optimum prop for any boat for sure, is to select a prop that will allow the engine to achieve something close to its maximum recommended rpm. That choice will also get you close to the best hole shot and fuel economy too.

An an engine that has a max recommended RPM of say 6000rpm will not perform to its potential with a prop that will only let it rev to 4800. It will be lugging. Bad for the engine, too. Change to a prop that allows that engine to reach 58-5900 at WOT and you will see a marked improvement to speed, cruise speed, hole shot and economy.

Once you get to that point, yes you can then play around with other props of a similar size or different number of blades, and see if you can get the final 3% that you are after.

The other factor at play here is engine height. It is argued by many that the first task, before you play with props, is to get the engine height set correctly. And the only way to do that is to set it, and test it on the water. Generally, the aim is to get the anti-vent plate (the large plate on the leg directly above the prop) to be running on the surface of the water when the boat is at a fast cruise speed with the engine trimmed at its usual optimum level for that speed.

If that plate is running under the water, it causes excess drag and slows the boat down and often affects its handling too.

So you set the engine height, go and test to see where that plate is running, come back, adjust engine height if necessary, test again until you get it right, and then, once it is right, you start playing with props.

So Splash, when you go and check the max rpm you can get at full throttle, at the same time have a look and see where that big SE sport foil is that is bolted to your anti-vent plate!

If it is running all the way under water, then you have a problem and likely need to lift the engine. I think we have been down the path of that discussion with you before, have we not? If the engine is as high as you can get it and the foil is still under water, then I guess you have to live with that and work on the prop selection alone. But it will be a definite limiting factor in that case. It might mean selecting a prop with proven stern lifting characteristics and maybe removing the foil to see how that works.

[Splash]

Thanks Dan and Moonlighter!

I luv this forum. You guys are great. Great help and now given me a lot to go on with....

I will need to rig up the electrics from my outboard to the hDS to see the rpm (so i will need to spend more money and get this stuff).

I do not have any big SE sport foil bolted to my anti-vent plate...

Dan - just out of interest - total weight of your 4.5 Bluefin loaded up without the motor?

SPlash

[Dan5]

Splasho i'd say round the 650-700kg mark mate......hull 325kg....fuel 65ltr +25ltr portable....2 people 200kg esky fishing gear beer etc etc....then the 120kg of motor

The 60 was a good match for 2 up with 3 it felt it though but was doable but not prefered...........

Check on the HDS cable/wire should not be a big expense from memory the one I bought for the 140 suzy to Garmin was around $110???.......The Yamaha genuine tacho is about $260 (already priced one) but then there is the inconvenience of fitting a tacho somewhere which will mean more money.


Post up some motor height photos Splasho then we will know where your motor height is at.......sounds like a combination of a few things.......Your WOT speed now should be your slow cruise speed IMO.

[Moonlighter]

Splash. Do a google for "tiny tach" this would do the job for the current issue.

[Splash]

thanks guys - WOW Dan your hull (with loading) is heavier than mine. I feel ripped off with the WOT speed and motivates me more to want more as it should out of this donk.

How was your holeshot with the 13P prop?

is the 10 3/8 or 11 1/8 diameter preferred for this 13P prop.?


Moonlighter - will checkout that tiny tach.

[Dan5]

I ran an 11 1/8 but you could try either IMO.....hole shot from memory was more than acepetable ....You have that big ass sounder slowing you down bro!

I'd go the interface cable personally then you can use your fuel burned vs revs vs speed to work out your best economy and fuel remaining etc blaa blaa.....

Tiny tach is ok but you have the equivalent to a 50'' plasma tv right there in front of you begging to be utilised.

We need those pictures Splasher to get some idea where your at with motor height........good ones showing the motor trimmed down with the hull level and from a few angles and include the bolt mounting position so we can tell if it can go up a hole.
Dan

[Splash]

ok - will take some later today after work.

what interface cables do i need for the taco, psi, volts, etc for this motor? Recommended Vendor to acquire this from?

SPlash

[Dan5]

You should only need one NMEA2OOO Yamaha to Lowrance interface cable Splash,,,NFI on where to buy in Melbourne.

Send a PM to Chocolatemoose.........I'm sure he will know exactly where to get it sorted.

Dan

[Splash]

hi dan.

photos of my transom and outboard leg/prop

[Dan5]

Yeah Splash that motor looks low mate.....You have a couple of spare holes there so crank it up one and go for a run mate.

Dan

[Splash]

Thanks Dan.

Best way to prop/mount the outboard up safely?

What is my measure of improvement when i holeshot and WOT next?

What is the impact of having a too low leg like this one?


Splash

[Dan5]

Your speed Splash....you should get better speed/performance due to your leg drag.This is where it all starts bud get the motor at the correct height then play with props.If it's not at the right height no prop will work to it's potential.......

Dan

[Fed]

Any cavitation burn marks?
Are you trimming it out Splash?

[McKnight]

Best way to prop/mount the outboard up safely?

Splash

Block and tackle to a strong support in the shed roof, or an engine crane.

That would be how I would raise mine, I don't know how others have done it in the past.

Or something along these lines (stolen from another forum)


First: Crank your jockey wheel all the way down to raise the rear of the boat. block your wheels and the jockey wheel so the trailer cannot move. attach your tie downs to the trailer.

Second: block the motor under the skeg (or anti-ventilation plate on each side), making sure the skeg is perpendicular to the ground and sits flat on your blocking.

Third: Remove the upper mounting bolts from the transom....clean remaining silicone from threads and washers (http://viglink.pgpartner.com/rd.php?r=814&m=1568059960&q=n&rdgt=1424622897&it=1425054897&et=1425227697&priceret=19.96&pg=~~3&k=6f7dbf7a192944735cc170f4997ec98d&source=feed&url=http%3A%2F%2Fc%2Eaffil%2Ewalmart%2Ecom%2Ft%2Fc sepg01%3Fl%3Dhttp%253A%252F%252Fwww%2Ewalmart%2Eco m%252Fip%252FTekSupply%2DFAME54%2DHot%2DDipped%2DG alvanized%2DFlat%2DWashers%252F31282485%253Fwmlspa rtner%253Dpricegrabber%2Ecom%2526affcmpid%253D2181 432090%2526tmode%253D0000%2526veh%253Dcse&st=feed&mt=~~~~~~~~n~~~).

Fourth: Loosen the lower bolts to the point the motor is no longer tight to the back of the transom.

Fifth: Crank jockey wheel up thus lowering the rear of the boat. Due to the motor being blocked, it will be able to move up on the transom. It may take a bit to get it started since the silicone might be holding the motor in place a bit.

Sixth: When the upper mounting bolts holes line up, put bolts back through, being sure to put silicone around the washer before you push it through.

Seventh: Tighten down the upper mounting bolts, being sure to put silicone around the bolt hole opening so it seals out water. (some clean-up may be needed here too)

Eighth: Reseal and re-tighten lower mounting bolts.

PDF on setting up outboard, including benefits of position of anti-vent plate to hull. (http://media.channelblade.com/boat_graphics/templatesites/dts/dealerlibraries/8441/copetipstricks.pdf)

[aussiebasser]

You're already running 20% over the maximum horsepower. Be very careful lifting your motor too high. That hull was never intended as a high performance boat, the bottom sheets flex as you hit waves, and the hull shape in not conducive to high speed. If you compare your propeller to a diff, a lower pitched prop works like a lower speed diff. It will give you better acceleration but lower top speed. A higher pitched prop will give you higher speed only if the motor has enough horsepower and the boat has the correct design and the set-up of the two is correct to allow that prop to work. 4.1 Polys don't accept trimming out the motor as well as most rigid hulls. They are a soft riding, stable hull that wasn't designed to go fast, it varies between having a rocker and a hooked hull depending on the water conditions.

[scottar]

The method McKnight has copied and posted is the easiest. Have used it multiple times on up to a 200 Hp engine. Only loosen the bottom bolts after the top ones are removed and only loosen them just enough for the motor to move.

[Fed]

It is a PorpoiseCraft but can you get it to porpoise?

Mercury testing advised that for every inch of lifting the motor resulted in a 1 MPH speed increase... but get this, that's at 80 MPH.
Raise an inch and go from 80 MPH to 81 MPH?

Sort of makes me wonder if raising motors to gain speed is really the silver bullet.

How many tinnies are there out there running around with the AV plates below the hull but above the external keel I wonder?

Here http://www.ausfish.com.au/vforum/showthread.php/201334-Barcrusher-bow-heavy SHOOTER increased his speed by lowering his motor.
Increased his leg drag by ~1-2% but reduced his wetted surface by who knows what? ~10-20%?

A straight edge parallel to and off the bottom of the hull with the AV plate trimmed parallel to the hull would give a much clearer picture.

[Splash]

thanks guys.

very interesting feedback.


fed - i will see what photos i can drum up ...

what /where are cavitation burn marks?

[Fed]

Take some proper pictures Splash, has to be easier than moving the motor if you don't need to.
The pics you put up have some interesting marks on the prop what does the transom side of the blades look like?

Are you trimming it out Splash?

[Splash]

what is the AV plate?

[juggernaut]

This:

http://continuouswave.com/ubb/Forum4/HTML/006491.html

[Moonlighter]

what is the AV plate?
Then flat (horizontal) plate on the outboard leg immediately above the prop.

Often referred to as the cavitation plate - incorrectly, if you want to get technical.

Flash, leave the motor height where it is for now. Go and take the boat for a run on a nice smooth river etc and get up to a good med-fast cruise speed, trim the engine out to the optimum position, then have a look over the back and see if you can see that plate. Ideally it will be just skimming the surface or splashing along about level with the water surface.

It can help to see it easier if you put a red sticky dot on each side of it on the trailing edge. Take a pic if you can or get a crew member to do so. But either way, come back and tell us if you can see the plate, or if it is well under water, buried deep, so it can't be seen at all.

If if it is buried deep, it is pretty likely that you can lift the engine at least 1 hole higher.

Then you go and test it again.

Your boat is very flat V at the stern, which means it may tend to slide around and the prop can lose grip in turns. The higher the engine is set, the more likely it is to lose grip. So it is a balance between getting the engine high enough for best performance without the prop constantly losing grip. And the only way to find out is to give it a try. Often, flat hulls like that need the engine set a bit deeper that other boats with a deeper V hull.

i have also used the method described earlier by one of the guys to lift the engine up to a higher position using the trailer jockey wheel. It's easy, especially with a 60hp. Get a couple of mates to help you to keep things steady and it will be even easier.

Keep it simple. Do one thing at a time before worrying about things further down the track. Setting the engine height at optimum for your boat is step #1.

In my experience an engine set at the right height helps improve the boats general handling characteristics and also improves responsiveness to trim changes. So the benefits are not just about speed.