All Props are NOT created equally!

[skip]

Some of you might be saying what a bizarre title or oh yeah I already knew that but for the past few weeks I've been buying (ouch) and experimenting with different props to try and find the right one.

The saga started like this, Factory Tohatsu 13.8 x 15P 3 blade Aluminium (made by Mercury)

Max revs trimmed around 5300 (right at the bottom of the WOT), add more people and you're stressing the engine.

Okay I say so I need around 400pm to make at least 5700rpm (WOT is 5250 - 5850), I'll drop the pitch down to a 13P and whilst I'm at it I will try a four blader as they are supposed to give you a better mid-range and bite lower down, acceleration (hole shot) etc.

So I bought the Solas Alcup 4 13P and installed it on the boat.....Whilst I gained my rpm's the reduction in pitch (bite) meant that I remained where I was in terms of top speed and maybe gained 0.5knots at the mid range cruise. Pretty disappointing really.....later I found out the prop was 13.25 x 13P so not only had I reduced pitch but also diameter.

In hindsight perhaps I could have tried a 13.25x15P 4 blader, keep the pitch drop the diameter making it easier to spin.

Not quite sure I said.......Let me get a 13P re-worked to a 14P as I was convinced that was my ideal pitch, but props only tend to be made in 2 inch increments, 13 - 15 - 17 - 19 etc.....

So I stuck with the same brand and model Alcup 4 and bought the prop from a specialist in the UK who re-worked it up to 14P! I reduced RPM a bit but it was too similar to the 4 blade 13P I had. I think I should have def gone for a 15P but without any to test locally it's turning into an expensive experiment.

Solas also make a 13.75 diameter series but this was out of stock so I couldn't try this. Instead I chose to go to local Solas Agents Ronnie's Marine and buy a 13.75x13P 3 blade Amita 3 reasoning that this is nearly identical in diameter to the factory 13.75x15P prop so I should gain my 400rpm and be spot on.....

You think!!! Oh no....now I'm bouncing off the rev limited getting that ever so polite beeeeeep warning as you hit 6,000rpm, back off straight away. I was running light only 40litres of out 100 and 2 big guys on board so I decided to leave further testing to a full fuel load which is how i like to go out.

Today i filled up, 100L and did a test but still got the limiter although I did gain some speed (more weight in the back seemed to help the bite). I met up with my cousin and added him and his gf (+130kgs) convinced that now I would definately see the rpm's drop....NOOOOO!! Still on the limiter.

Bollocks......!! Don't get me wrong, boat flies out of the water, has some crazy low fuel burn numbers like 13.8L per hour at 18 knots versus the 17L per hour before.....BUT top end speed is down due to a reduction in pitch and so I think also the mid end cruise.

Going back to the point of all of this, well two identically sized props in terms of diameter seem to be a world apart....dropping to 13P should have meant going from 5300 to 5700 per all the rules you find online (1 inch = approx 200rpm) instead I've gone 6000+ coz there's still quite alot left on the throttle.

Which NOW means ideally I stick to the Solas Amita 3 range and try their 15P or maybe (doubt it) 17P prop if I can convince Ronnie's to let me....because I ain't buying ANOTHER prop and I find it disappointing that there are no propeller specialists in Malta.

So if you're finding you're not hitting your ideal rev range, before following most online advice and dropping pitch, you might want to consider another brand/model and sticking with the same pitch. If you are going to drop pitch make sure its the same brand and model range otherwise you won't be getting a fair comparison.

Most engine suppliers will let you test before you take the correct prop with the new engine or engine+boat you bought from then, and in which case you are likely to be testing the same brand/model of prop.

But if you try something aftermarket, bear this all in mind....when you email the manufacturer of props you get all sorts of cryptic answers as it's impossible for them to offer you any kind of guarantees. These online calculators are mostly just a bit of fun because EVERYONE will tell you, try in the water.

In closing in case anyone needs to try out some props, I have a 13.25x13P 4 blader for 4 1/4 inch gearcase and a 13.25x14P (13P cupped) 4 blader for 4 1/4 inch gearcase. Probably in the near future I will also have a 13.75 x 13P 3 blader......I might be buying the domain skipsprops.com  !!!

[Moonwalker]

Interesting read skip. I need to test a new prop too since my top speed is lower that expected but first I need to install the trim tabs which seems vanished on their way to Malta.

[skip]

To add to this the Mercury Prop advisor seemed to be the closest http://appcenter.mercurymarine.com/propSelector/home.do

You need to know your current WOT rpm, current pitch and current top speed but it's pretty accurate at least based on what I've been encountering.

Also play around a bit with what you want to achieve, mainly All round or mid range cruise.

For my application Mercury's calculator is recommending:

Alumiunium
Best Mid Range Cruise: BlackMax Alu 13.75 x 15P
Best Overall Prop: BlackMax 13.875 x 14P

When you switch to stainless it seems one reduces the diameter:

Stainless
Best Mid Range Cruise: Vengance Stainless 13.25 x 15P
Overall and Top Speed: Vengance Stainless 13.375 x 14P

[fishfinder]

@ Moonwalker

if it happens to have some props for test please let me know if I can borrow them cause I need to test mine too and until now we didn't saw Karmenu to borrow his props.

[kurtfalcon]

Skip hope this might help.
   
    *  Adding 1 inch of propeller pitch will reduce full-throttle (WOT) RPM by 150 to 200.
   
    * Subtracting 1 inch of propeller pitch will increase full-throttle RPM by 150 to 200.
   
    * Moving from a 3-blade to a 4-blade propeller, a 4-blade generally turns 50 to 100    RPM less than a 3-blade propeller with the same pitch.
   
    * Switching from an uncupped to a cupped propeller will also reduce your RPM by 200.

Since you changed the 3 blade prop to a 4 blade the rpm change is not the usual 200 rpm per 1".

+ Rake and Cupping need also to be considered since these have drastic effects on rpm change.

I found you this web site which explains thoroughly all propeller characteristics that have to be taken.

http://sites.mercurymarine.com/portal/page?_pageid=126,48572,126_49283&_dad=portal&_schema=PORTAL

Hope this helped 

[Moonwalker]

Sure fishfinder. But first need to get those trimtabs becuse supplier is not replying to emails now either

[skip]

Skip hope this might help.
   
   *  Adding 1 inch of propeller pitch will reduce full-throttle (WOT) RPM by 150 to 200.
   
   * Subtracting 1 inch of propeller pitch will increase full-throttle RPM by 150 to 200.
 
   * Moving from a 3-blade to a 4-blade propeller, a 4-blade generally turns 50 to 100    RPM less than a 3-blade propeller with the same pitch.
 
   * Switching from an uncupped to a cupped propeller will also reduce your RPM by 200.

Since you changed the 3 blade prop to a 4 blade the rpm change is not the usual 200 rpm per 1".

+ Rake and Cupping need also to be considered since these have drastic effects on rpm change.

I found you this web site which explains thoroughly all propeller characteristics that have to be taken.

http://sites.mercurymarine.com/portal/page?_pageid=126,48572,126_49283&_dad=portal&_schema=PORTAL

Hope this helped  

Thanks that's the theory but it doesn't hold much water unless you are sticking with the same make/model of prop and type ie. comparing 3 bladers.

This is because manufacturers do not give you specific technical data about their props, like exact amount of rake, cup etc it's all generic talk like high rake or medium cup etc etc.

Everyone I spoke to abroad says you need to test different props in the water with a specialist, which of course we can't do here because no one offers this service.

[shanook]

eh moonwalker, fishfinder told u about 'Magic marine Uk' no. They are excellent suppliers and quick to reply to any query.
Hope u get this settled as soon as possible. Tell the supplier that u will post him in the negative on the forum if he doesnt answer u.

[Moonwalker]

It was another supplier shanook as I said in PM.

@skip - If you find a professional on propellers and outboard tuning pls let us know.