Author Topic: Guide on selecting the correct propeller for your boat.  (Read 4734 times)

0 Members and 1 Guest are viewing this topic.

TonioMall

  • Guest
Re: Guide on selecting the correct propeller for your boat.
« Reply #10 on: 12 July, 2011, 10:04:40 AM »
To Jezzb

I read that you have 2 bravo 2  props lying around.  If they are still there would you consider selling. I had a prop strike and 2 of the blades are damaged and don't know if they can be fixed. ( The trailing edges have been shorn of )  :-[

jezzb

  • Guest
Re: Guide on selecting the correct propeller for your boat.
« Reply #9 on: 14 July, 2009, 08:17:34 AM »
Hi Skip,

Regarding the manifolds, we already have Stainless Steel manifolds which are longer than standard manifolds. They are used for through hull configurations.

Regarding you comment on the props  - I totally agree with you that it is a game. There are so many factors involved, for example a boat which runs perfectly with a prop in the US, won't run as well if used in the mediterranean. This is due to difference in water density (more salt in the mediterranean).
Same applies if a boat is used in fresh water. I, like you, had to expieriment a lot. In fact I have a bravo 2 leg at home, a Bravo 2, 26 pitch propeller and a bravo 2, 24 pitch propeller - just lying around.

Best regards
Jezzb

skip

  • Guest
Re: Guide on selecting the correct propeller for your boat.
« Reply #8 on: 14 July, 2009, 04:32:48 AM »
Wow, no kidding spinning 24P is quite something...I've only heard good things about those manifolds....are you going to do the straight through exhausts like on the video :)

The bigger engines benefit even more from breathing better and I think you really proved the point there.

Even prop suppliers call it a 'game' they get used to certain props on certain boats through experience but the majority say we need to test different props on your boat!

jezzb

  • Guest
Re: Guide on selecting the correct propeller for your boat.
« Reply #7 on: 13 July, 2009, 09:33:30 AM »
Hi Skip, sorry to hear that you had so many problems and that you had to spend so many €€€. At least you are burning less fuel - and that is a big plus considering the current petrol prices.

As you know, last year I also had a problem with my 24 pitch bravo 3 props on my Mercruiser. I bought a new 22 pitch set and at the same time I changed the manifolds from cast iron to Stainless Steel. When I launched, I tested the WOT only to find out that suddenly my engine was revving 5000 + revs (I have a rev limiter on my engine so it was cutting the engine as soon as 5000 revs were being reached. I was so frustrated. With the 24pitch I was reaching 4200 and now with the 22pitch  the enigine was revving too much.
This season I decided to try out my 24 pitch again. To my amazement, the engine was revving 4600 at WOT - this is the recommended WOT for my 7.4 MPI engine. The manifolds did all the difference and increased the engine performance. So what the manifold supplier was claiming, that you get improved torque by up to 20% was really true. I am not writing this to advertise the product but it is the truth. With the 24p I also improved the top speed. I am now seeing 43.5 knots - which is great on mt 27 foot Rinker.

Kind Regards
Jezzb




skip

  • Guest
All Props are NOT created equally!
« Reply #6 on: 12 July, 2009, 09:17:51 PM »
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  !!!

jezzb

  • Guest
Re: Guide on selecting the correct propeller for your boat.
« Reply #5 on: 19 June, 2008, 10:17:19 AM »
Hello Skip, As always, you are very informed  ;D

In my case, as I have two props, the expense will be twice as much. Will surely cost an arm to send them by courier and back.
I think I will try to mix the 22pitch and the 24 pitch, starting from the smaller propeller which is the one on the outerpart of the drive, but I guess I will do this next year as it will be difficult to replace in water.

Thanks for your advise.
Best Regards, Jezz


skip

  • Guest
Re: Guide on selecting the correct propeller for your boat.
« Reply #4 on: 18 June, 2008, 07:06:24 AM »
Dont think you can with Bravo III props. What you need to do is have your 22's cupped which will basically make them into 23's which you can't buy off the shelf and should keep you within your ideal WOT. Not aware of anyone doing it properly locally, I have sent my Bravo II S/S props to Steel Developments in the UK and they were very good. Not cheap of course due to freight associated with their weight but worth it. Might be a good winter project.

jezzb

  • Guest
Re: Guide on selecting the correct propeller for your boat.
« Reply #3 on: 16 June, 2008, 04:41:05 PM »
On the same topic - I have a Bravo 3 leg with two SS props of 15" x 22pitch. The problem is that my WOT according to the engine manufacturer should be between 4600 and 4800 revs. With these 24p props I rev 5000 revs. at full throttle. I tried 15" x 24p - as I cannot change the size - In Bravo 3 you can only change the pitch - but with the 24p I could only rev the engine to 4200. So I am a bit stuck.
I wonder if I could mix an match and have a 22p prop and a 24p prop. Wonder if this will work out - although it might be a problem to change the prop now that the boat is in the water.

What do you think?

Jezzb

skip

  • Guest
Re: Guide on selecting the correct propeller for your boat.
« Reply #2 on: 15 June, 2008, 03:27:41 PM »
It's a good idea to do the test as Jezz says with half loads, like half fuel/water, 1-2 passengers. If you set yourself up to be close to the upper range of the WOT then when you add fuel and people you should be in the sweet spot and when you're alone you should fly!!

When it comes to outboards, there's an additional variable and that's height of the engine on the transom. Some performance outboard based boats run on a bracket that gives them variable height for that very reason.

If you're experiencing degraded hole shot and having to trim your engine up alot to get to the top end of your WOT/speed consider moving the engine up an inch or so on the transom.

If you have a 4 bladed prop that prefers to run nearer the surface you might even be able to do a 2 inch lift assuming you need to. A four bladed prop will give you a better hole shot and mid-range but you might lose something from top end speed.

jezzb

  • Guest
Guide on selecting the correct propeller for your boat.
« Reply #1 on: 17 May, 2008, 12:49:04 AM »
In order for the engine to attain its optimum operating RPM, you need to know what wide-open-throttle RPM range the manufacturer recommends and a functioning tach.

As the example, we'll use the common small block WOT range of 4400-4800 rpms.

Two things you need to know about props: 1) diameter 2) pitch. The stamping in your prop hub may look like this: 14.50X21 Diameter is stated first.

Without going into the scientific equations and theories, I'll simplify...A 1" change in pitch will create about a 200 rpm change. (Going from a 21 pitch to a 23 pitch will drop the rpms by about 400 rpm...comes out of the hole slower, but has more top end speed, and vice-versa).

Changing from a three-blade to four-blade prop will decrease rpm about 50-100 rpm.

A stainless 21 prop will produce higher performance than the equivalent aluminum because stainless has thinner blades and is stronger with less flex. A bad strike with stainless will probably trash your lower gear set whereas an aluminum blade will sacrifice itself.

If you have Bravo Three props, diameter X pitch is a fixed equation and you have no options in the set. Pitch is determined by diameter and the pitch is incremental in even numbers (ie: 22, 24,26,28,30) This data applies to B3's only.

Back to our 4400-4800 example from above:

Example prop is an aluminum three-blade 14.50X21. Under good weather conditions - breeze with very light surface chop, 1/2 tank of fuel, functioning tach, and no passengers...you pull a couple of speed runs and determine that your current prop is turning at 4200 rpms. You're over-propped, and engine damage CAN occur because it's being overworked, or, "lugged".

Answer: find a wheel that's about a 18" pitch. The net change will be an increase in rpm of about 600...which means you're right near the top end...perfect.

Finding "the perfect size" may be almost impossible in some cases,and this is where a competent engineering shop can fine tune your prop by adding cup, changing the rake, or balancing the blades.

A marine engine produces its maximum shaft horsepower at wide-open throttle, and that is why the object here is to find a propeller that will allow the engine to run nearest to the top end of the WOT recommendation.

Good Luck !


 

Sitemap 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30