Hi all,been fairly new to rc gas boats im wanting to begin experimenting with props but at the moment don,t understand how much pitch creates how much thrust so could someone tell me which pitch out of a 98 & a 114 would create more thrust?even better could you explain the math in working it out.Thanks Darren

Hi Daz

Here is my contribution but bear in mind that there are members of this community that have massively more knowledge of propellers than I have.

Hopefully I will be corrected if any information I give here is wrong:

My basic understanding of pitch is that it defines how far the prop will move forward in one complete revolution (360degrees) assuming zero slip. So a 98mm pitch prop is screwed one turn in a solid medium would bodily move forward by 98mm.

It follows that a prop with 114mm pitch (let’s assume same diameter and profile) will likely give more thrust if rotating at an identical speed to the 98mm pitch prop. In effect the 114mm pitch prop has steeper blades. But then such a prop will load the engine more so unless the engine can pull the steeper pitch performance may not improve.

On the prestwich model boat site there is a helpful prop guide which shows recommended prop sizes for various hull, engine and drive combinations. I found that very useful as a start.

It is I think handy to know how the key prop codes define their sizes.

For example with the Octura X series props X470 means that the pitch is 1.4 x the 70mm diameter i.e. 98mm. The X670 by comparison has the same diameter but pitch ratio of 1.6 x the diameter i.e. 112mm pitch. My Zen260PUM likes both of these props but is much quicker on the X670.

For example my Mocom 67/17 coded prop has the first two numbers specifying diameter i.e. 67mm and the second being the pitch ratio i.e. 1.7. So its pitch is 113.9mm. This prop gives probably the best straight line speed for me out of several I have tried for my set up. Forgive me if you already know how these codes work.

I am only scratching the surface here because there are multiple combinations of other factors that all come into play. Blade shape, blade area, rake, cupping etc.

For me personally I am focussing upon quality makes and ensuring that they are razor sharp and well balanced and simply experimenting.

I hope that my limited contribution at least is of some help.

Cheers
Craig

Hi Craig.

This was informative to me. The thing about the prop codes has allways been pure latin to me, but now I see some of the light.
But now the question is: 2 - 3 or 4 blades and what shape?
Anyway: THANKS!

Cheers!
Woodie.

Hi Woodie

I’m glad my contribution was of some use!

As to your other questions, I think it is time for someone with more expertise than I to step in.

However on the subject of 2 or 3 blades, one comparison I did this weekend was change from my X470 two blade Octura to the X470 three blade. All other settings unchanged.

I noticed that on the 3 blade prop my Zenoha didn’t rev quite as high but the hull appeared quicker (in all fairness I didn’t measure the two hull speeds so I could have been mistaken). The 3 blade did appear to load the engine more but even with less revs deliver more thrust.

We later tried the 3 blade X470 on my twin brother’s 55” Miami (same engine as my Lynx) and his boat was much quicker than on the 2 blade equivalent. Personally I am warming to the 3 blade props.

Over to the experts as I am only learning by experimentation.

Regards
Craig

Thanks Craig when you say solid media do you mean if the prop was say a screw been driven into wood?if correct it makes a little sense but because its water obviously there will be slip so really you need to take this into consideration so really its trial and error until you have tried the props,different hulls & engines etc.Will look at that guide at Prestwich seeing as this is where my props will be coming from,we are limited in the UK for suppliers really of such things don,t you think?I have looked & looked on the net trying to find others but always end up at Dave Marles of Prestwich.Thanks m8.Darren

Hi Daz

Yes the screw into wood concept was what I meant.

I agree that in th UK we are much limited to suppliers. I am a big fan of PMB because of their range of quality products and willing advice when sought from them.

However there are some alternatives also. I did try to send you a private message but the program dropped out with an error. If you want to let me have your email address I will contact you off line.

Cheers
Craig

Thanks Craig.

For my own 56" wooden mono (as usual still in build), MHZ Germany has delivered a 13,5 hp Blata engine to drive two MHZ sterndrives at appr. 27.500 rpms output on props. They have recommended me what equals to two X568 3-blade props in high grade steel.
This really seems to be the right solution compared to your experiences and advice.
(Well - we'll see some lucky day)

Cheers!
Woodie.

Woodie wrote:
Hi Woodie

If you achieve 27500 rpm on two X568 3 blade props, I can imagine your boat taking off!!!

I'd love to see some pics of your boat if you have any.

Cheers
Craig

Craig.

My mono is not at all finished. (Having trouble with getting my design of engine rails made for the right price). But it's a simple homebuilt mono - for sure totally overmotorized. But is'nt that just the exiting part?
You can look it up in my thread: "My Smokin' Cigarette".

Cheers!
Woodie.

Daz the tool bar on the top of the home page has some good reads on this subject. Go to resources~props~knowing your prop. There is lots of good info there.