Prop diameter on SSM props
 

Can one of you prop experts give me the 10 cent story on how different diameters of props work. For instance, what would the results/differences be on the same boat with a 16.5 X 33 and a 17.5 X 33 props on a #6 drive?...or a 16.5 X 29 vs. a 17.5 X 29.

Is it planning ability? attitude? speed?

I'm just wondering...and obviously bored out of my skull at work!! :D :drink: :evilb: :evilb:

Re: Prop diameter on SSM props
 

get back to work...

Re: Prop diameter on SSM props
 

Got off of work at 2:30 ;) :D

Thanks for your riveting input Sean!! :evilb: :evilb:

Re: Prop diameter on SSM props
 

Wow that's a loaded question.

Larger Diameter propellers in general will produce more stern lift and of like pitch will bring the RPM down of course. And beyond that I need to know what hull and power package to speak on this topic.

Julie

Re: Prop diameter on SSM props
 

Fair enough, I'll call you tomorrow.

Re: Prop diameter on SSM props
 

No Don't call do here on this thread...this is a great topic.

Let's say the boat is a 35 fountain executioner....

Re: Prop diameter on SSM props
 

With #6's???

I asked the question because I am re-powering my Top Gun with a pair of lower HP engines. It was running 16.5 X 33 four blades @6000rpm's. I've got a pair of 16.5 X 29's to try when I get the boat back. I no idea if they'll be right or not...but I'll be finding out in a few weeks :cool:

Re: Prop diameter on SSM props
 

[QUOTE=OldSchool]With #6's???

I asked the question because I am re-powering my Top Gun with a pair of lower HP engines. It was running 16.5 X 33 four blades @6000rpm's. I've got a pair of 16.5 X 29's to try when I get the boat back. I no idea if they'll be right or not...but I'll be finding out in a few weeks :cool:[/QUOTE

If the boat handles well don't change the diameter. I ve had
three sets of the same props and they all responded differently.Seems like when I increased diameter 1" equaled
about 2" of pitch pitch. That was with 1:1 gears and 16.5X31s
Just my 2

Re: Prop diameter on SSM props
 

[QUOTE=HotPursuit]Craig
I can tell you what my experience was with our Outerlimits. Our slip numbers were terrible at WOT. It was about 16-20%. We were running 16x34 4 blade Mercury cleavers. In an attempt to reduce slip we went with 17x33 4 blades. The slip went down the MPH went up. The down side was that it gave us much more lift on the stern, and magnified the chine walk that was already their. Very bad. The 34's were probably worked a couple of times and I should have had them checked out before buying new ones. I think you mentioned your boat was rock solid at speed. Thats something you dont want to sacrifice.

Re: Prop diameter on SSM props
 

The key is to get the drive height, prop diameter, # of blades, and prop pitch just right. Changing any one of these variables will change many aspects of the boat's characteristics.

The best way to do this is by trying different props, then put in a spacer depending on the results with the first series of tests, then test again. It's very time consuming and few manufacturers spend the time, have the props to do this, or the knowledge. When you get into a high performance boat with a unique engine or drive package it may take a loot of time to get the boat dialed in just right.

Matt

Re: Prop diameter on SSM props
 

Thanks Matt,
I'll gather as much data as I can when I run it with the new power and go from there.

Randy,
You are correct about the boat being very stable in the 90-109 mph range with the 16 x 33's. I really didn't get enough seat time in it to fully "figure out" how to run it...or dock it. :crazy: Those props spinning in were tricky around the docks!! There is nothing like running a boat with twin 1200's, dry exhaust and a flashy paint job until everyone is watching and you look like a 12 year old trying to park for the first time!! :drink: :evilb:

With the new engines, I'll be turning the props out...but still running the dry Stellings :D :D

Re: Prop diameter on SSM props
 

No, I have IIIs with 800SC... :p

Re: Prop diameter on SSM props
 

Good advice i was given for docking....Dock it like a single engine & use the inboard {dockside} engine...seems to work pretty well

Re: Prop diameter on SSM props
 

Old School,

I know I'm getting into this conversation late. I was out of town for the Holidays. I love working with these types of situations.

Working with #6 drive propellers can be tricky but the options you have are also largely diverse. As Matt stated, there are many different issues to consider when working with #6's.

You kind of answered your own question when you mentioned the fact that the boat handled great with the 33's. If you are only decreasing HP, keep the same diameter and decrease the pitch! The boat will be the same old boat but a little slower. Now that being said, it depends on the history of the props in question. By that I mean do they match on a basic geometry level? We would need to measure that.

OK, now we are talking about changing rotation. Everything changes from here as far as our original handling base line is concerned. Yes, your docking will improve. Every other characteristic will also change. From planing to trim characteristics will be different.

Do what you said, test, take notes, retest and we can talk some more.

To answer the original question.................................
changing diameter changes "many" things...................
depending on the boat, too many to list........possibly.

Best advice, use as much diameter as the boat will "allow" you to use!

If you would like, call us anytime and we can go deeper into the subject.

Use as much diameter as the boat can handle? Can you expand on this?

here's a old one brought back to life.

My diameter is more blades area. This means better efficiancy, cruise and top speed due to more water being moved.

However, adding diameter decreases rpm and increases stern lift. If your boat becomes negatively affected by these changes, reducing diameter is required.

Therefore, use as much diameter as the boat allows you to.