Seeking Prop Advice Please

[mustang5]

Thanks Garry for the offer mate. Luckily I have sourced a couple more options from a mate here in Gladstone.

[gofishin]

I looked at the cav plate whilst planing and I could see it on the surface. Unsure if thats a good way to check though...

yep, that's the way, & if you can see it that's great.

...I believe it to be the factory setting and there is one more bolt hole I can go up....

...As I am planing, I can tilt the motor VERY FAR up before break-out.. If thats anything of relevance...

Yes, it is very relevant! With such good grip with a 3 blade, means you can probably come up another hole, especially if you find a suitable 4 blade. Due to the physics involved, coming up more means less of a bow-up attitude, not to mention other obvious benefits. (Does the prop look like it has had extra cup added?)

...This weekend with 3POB and an empty tank (And empty esky ) In flat it got up to 5400rpm and 34 knots but 5100rpm at 31knts against wind/chop..

. At those figures your current prop is ~4" too big. However remember that 'pitch' is not an 'exact science', & sometimes there is a lot of trial & error! Remember too it is the combination of pitch & diameter which causes 'a prop' to work well on one boat, and not so well on another similar boat.

Check the hull truth for prop comparisons of your rig. Also put a REV 4 on your hit list. A 15"p will probably be too big, but can't remember if the make a 13". SOLAS also make a 15 1/4 diam 4 blade, (maybe called 'Overdrive'), but ~3 yrs ago they did not come smaller than 18"p. maybe they do now?
Cheers

[gofishin]

Mustang,

Re your question about prop diameter vs pitch, I’m back home now & thought these graphs might give you a bit of an appreciation as to how a certain boat/motor combination reacts to different propellers with different pitch & diameters (and blades) – in this case my boat. However, please note that it is smaller, lighter, has less windage, has a lot less fuel capacity (read consumable load) and (sounds like) less ‘general’ load than you do regularly for your big trips; but, I have more HP and my donk has VVT (F250A), which supposedly translates to greater low speed torque.

There are many attributes to a ‘propeller design’, but most important are diameter and pitch. The ‘size’ of a propeller is a combination of its diameter and pitch. Designers also use a term called ‘pitch ratio’, which is the pitch of a prop divided by its diameter. Different types of craft require different pitch ratios, and each has a ‘range’ of pitch ratios which ‘should’ provide an appropriate propeller depending on the speed, fitted power, load, and the number of blades etc. Now I am certainly no expert, so won’t try and explain the ‘ins and outs’of prop design (as I don’t know enough), but form what I have learnt and experienced over the years I can hopefully help a little…

The bigger the boat and the heavier the load, the more important it is to‘maximise’ diameter. By maximise I don’t mean fitting the largest diameter prop the outboard can handle either, I mean in suitable prop size – i.e. the combination of diameter and pitch (and No. of blades if relevant) which works for ‘a particular boat’.

The larger the diameter, the more load it can take, however the more torque (hence HP)required to push the boat – especially at low to moderate speeds (i.e. getting on to, and staying on the plane). For any particular craft 'in general'; a propeller sized for high speed has a smaller diameter and maximum pitch. A propeller sized for thrust (or pushing power) has a large diameter and small pitch. You can’t have a prop that can do both very well– as always in boating, everything’s a compromise! However, you will probably also find instances where your results go against theory at times – as I have found.

The constraint in your case is the 225HP, and to ‘possibly’ a lesser extent, the large variations in load you carry etc.

The attached graphs are compiled from a few of the prop tests I have done over the last ~3.5 years on my 685 Outsider, from a mix of 3 and 4 blade props, and combine average SOG and economy (runs each direction). The first graph shows what happens when the prop diameter is a little too large (see the solid black line) for the low speed torque available from the motor between 3 to 4,500 revs – which is the rev range I am most interested in. This prop went straight back to the supplier, as the boat wallowed and did not perform in this range, however, it actually posted the fastest speed at WOT of the 3 props I tested that day!

Note how that above 4.5k, all props have near identical speed (and very similar economy), but they are quite different props (1 x 3 blade, 2 x 4 blade – see graph legend). Below 4.5k they have completely different profiles/performance). These were the first prop tests I did, and I kept the smaller 4 blade and have run it over varying loads and conditions since. With this prop, when the boat is heavily loaded the fuel economy drops off substantially – I believe because of its ‘relatively’ small diameter. FYI, a SOLAS 4 x 14 ¼ x 17” offers marginally less economy and speed at light ships duty, but is better when heavily loaded (such is the difference a1/8” in diameter and ‘theoretically’ 1” in pitch makes) – but tests on my and other boats show that this prop was actually closer to 18” pitch, as in one case we were getting <1% slip – which is not physically possible, hence the real pitch is bigger than 17”, and probably closer to 18"!

The second graph shows another mix of prop tests (1 x 3 bld, 3 x 4 bld), again with varying diameters and pitches. Again, some quite different profiles in both speed and economy, especially from the three 17”p 4 blades! The big diam Powertech has a sweet spot at a lot lower revs (3.5 vs 4.2), and considerably more speed b/w 3 & 4k. It also performs very well under high loads, but on my boat suffers from ventilation quite easily. However, this same prop on a 625 with L4 Verado, with the donk one hole higher than mine, stuck like ‘you-know-what’ and just wouldn't let go!

The REV 4 on the other hand had unbelievable grip in comparison – however was a tad small (eased back at 6100rpm lightly loaded to avoid hitting the limiter). With this prop I could trim out to 8 bars without any ventilation/breakout! It also performed very well under heavyloads. I have since tried a 19” REV 4,however on my boat it dropped the WOT b/w 5-600rpm. All the theorists predict that 2” increase in pitch (@ same diameter) should only drop the WOT by between 3 – 400 rpm!

So what does all this mean? Well, it’s alot easier if you can find someone else who has tested props on an identical rig to yours . Otherwise, it is trial & error, as a prop that works well on one boat may not work so well on another (but similar) boat.

Cheers & happy prop hunting!
Brendon

[Chang Jiang]

Mustang, you are playing the game of magic and science, I have just spent the last 6 days balancing ( propping) three different RIBS 7M 8M and a 11M normally I only play with Merc motors so I am unsure of what the wide open throttle RPM is for your engine as i beieve the yamaha depends on year?

I have gone over the thread and your are at the delima stage, the vessel in heavy configeration is taking a hell of a lot of power to get onto a plane, the prop does not appear to have much slip but is actually bogging due to size and pitch, and is still not achieving a full theoretcal plane as the strern is still being sucked down even at 30 knots

are the trim tabs working ?? I see you say that you have played with the wait distribution and this is not helping the boat move from the displacement phase to the planing phase, or is it just not practical many times it is the later because it means that you have to move some wait aft later due to sea conditions and bow down.
I have played with specs of a sea swirl 2601 from the web and your prop you have on it is dam close to the theorectical prop for a dry boat plus 650 kg The data shows that if you drop diameter you will get prop slip even if you add pitch which you already know ( your boat will play at the 10-12 knot burn zone) and atleast 1 piece of software works. Also the software shows that if you maintain diameter, drop pitch by 3 inches and go to 4 blade you should have a truer all round prop or drop pitch by 4 inches and use a 3blade you should still achieve a decent result

I have an Armacraft 30ft open consul much lighter boat and just finished trialling last week, played with 7 props and different weights, firstly i am a true believer in making sure the boat achievs Manufactures recomended WOT ( is the motor working to its full capacity) with standard(most common load) weight, then I will play with both 3 and 4 blade props to see effect of rpm and speed and handling at low RPM ( 6 KNOTS) crusie RPM ( 4500) and WOT as i use the boat normally for reef trips from townsville . It has been great and quite supprising the difference especially at the 3500-4900 rpm range when using this method with the heavier RIBS it has been more beneficial in cruising speed selection and initial planing.
If I can assit please just ask

cheers terry

[mustang5]

Ok guys, sorry to revive an old thread but I got an email back from the Prop Gods which is pretty much on par with what you guys have all said, so I thank you.

Ken the man has suggested a Powertech OFS4 15 pitch with a little extra cup.

First question, does anyone sell these in Australia?

Garry, I may take you up on that offer if it is still available?

I am testing boat tonight with the motor raised and will report my findings (Have a foil on also which I will try with and without).

Cheers

[DaveNkelly]

mustang, I'm very interested to hear how you go, I have also emailed the prop god and he said he thinks I have other issues like changing what hole I have it mounted on..

I'll be watching this space..
Dave

[DaveNkelly]

Mustang,

When you say your are going to try with the engine raised! does your outboard have the "hook" on the top of the mounting that is meant to sit on top of the transom I know the honda's do I'm just wondering as I assumed that it had to sit at that height for engine support?? I'm probably wrong just asking?

Dave

[mustang5]

I'm not sure on that one mate.

Took for testing this afternoon and found the following!!

Installed a foil and get this, picked up about 10km/hr but at lower rpm????

Looked back and I could clearly see the motor is about 2 inches too low as all I could see was spray from the foil.

So will raise and further test tomorrow afternoon..

Currently having a fuel issue though where it won't run with fuel filter installed.... But that's another topic...

[mustang5]

ImageUploadedByTapatalk1366204086.251618.jpg

There is a foil under there somewhere

[gofishin]

...does your outboard have the "hook" on the top of the mounting that is meant to sit on top of the transom I know the honda's do I'm just wondering as I assumed that it had to sit at that height for engine support?? I'm probably wrong just asking?

Dave

Dave, nothing really to do with the 'hook' as you describe it! More to do with the physical height of the transom vs where the donk should be in relation to the A/V plate and the flowstream coming off the hull when at a clear planing speed. Mostly, outboards are installed at a conservative height, i.e. a little low, but even then you will find the hook above the tramsom lip.

Not a very clear pic, but here is mine.
Cheers

[mustang5]

Some more pics.

Raising 2 holes this afternoon and going for another test run. Hopefully sort out fuel filter issue at the same time!

ImageUploadedByTapatalk1366204294.576070.jpg
ImageUploadedByTapatalk1366067077.819765.jpg
ImageUploadedByTapatalk1366067097.126417.jpg
ImageUploadedByTapatalk1366204276.645040.jpg

[DaveNkelly]

mustang, are you raising two holes straight up? what made you decide to step up 2 instead of one at a time? Just wondering as I think I'll have the same issue but I have to wait a good two weeks to get home from work to find out.

It appears that you're getting better numbers light load then I am with an extra 3 foot of boat and the same size motor! so I have a fair way to go. In saying that I think Mine maybe be user error in regards to trim to add to the issue
did ken recommend a four blade prop? He seems to recommend all murk props but does know his S#*T no doubt.
garry recommended the powertech 4 blade which cost a lot less and he has tested them and is happy with its performance and from what he told me will make a massive difference to just about all aspects..
First things first I guess see if my motor is also to low.. bloody work always get in the way

Dave

[mustang5]

mustang, are you raising two holes straight up? what made you decide to step up 2 instead of one at a time? Just wondering as I think I'll have the same issue but I have to wait a good two weeks to get home from work to find out.

It appears that you're getting better numbers light load then I am with an extra 3 foot of boat and the same size motor! so I have a fair way to go. In saying that I think Mine maybe be user error in regards to trim to add to the issue
did ken recommend a four blade prop? He seems to recommend all murk props but does know his S#*T no doubt.
garry recommended the powertech 4 blade which cost a lot less and he has tested them and is happy with its performance and from what he told me will make a massive difference to just about all aspects..
First things first I guess see if my motor is also to low.. bloody work always get in the way

Dave

My main issue is not so much speed and numbers for me, it is mainly the positioning of the boat on the plane. The Bow sits too high and gives a bit too much slap for my liking, which is not right.

I asked Ken to suggest what will give me more stern lift, and bring the nose down. He reccommended a Power Tech OFS4 with a little extra Cup.

Have to be careful as there are some props with Stern Lifting characteristics and some with Bow lifting characteristics. So the OFS4 obviously will lift the stern.

From this point I thought, before spending the $800 on a new prop I might just throw the Hydrofoil on for shtis and giggles, as it was on special at BCF for $99.

Well with the Foil, it has helped outline my height issue as the foil is buried significantly! It should be riding on the top.

The most confusing part is that the AV plate is above the keel line at rest, but obviously these days you may as well skip that test from the get go and simply base it on water testing.

Why 2 holes?? Well, I dont want to go just the 1 hole and then wonder, WHAT IF?... Id rather go to the extreme and then knock it back to where it works (Which Im guessing will be 1 hole at this stage).

If I get an extra 300rpm with a raise, that will put me around the 5700rpm mark with a 17p 3 blade.. with a 15p 4 blade I should then see close to 6100rpm for probably roughly 43mph or so, which is pretty damn good for a 26ft boat with a 225hp Yamaha (Which is really only 208HP rated).

I will be happy with that until I repower later this year with a zuk 300hp.

[mustang5]

Ended up going 2 holes and its a different boat....

Not sure if its where I want it as now I think the bow may be a little too low??

But gained about 200rpm and feels much more steady.

36knts at 5500rpm.

Did full speed turns and was still getting good trim..

Put the trim tabs down and it dug the nose in a lot more than I expected. But will be more effective than they were with the motor lower.

Hopefully test it in swell on this weekends trip.

Cheers all

[mustang5]

ImageUploadedByTapatalk1366285755.034555.jpg
much better