Diameter questions and how meaningless it is or is it????

[Tail_Gunner]

Over the years i cannot tell you how many people have asked this question. Diameter in a prop has only one dynamaic that is is it flush with your gear hub. If it is not "flush" it will create a huge disturbance and or a loss of adhesion with the water stream resulting in huge slip results.

Below is a example that all should be aware of...and there are no exemptions to this rule...:facepalm:


OPPS...going back to school...

..Lions and tigers oh my.....diameter only counts if it mates to your hub... Its not a bicycle people...

 

[Dhadley]

Re: Diameter questions and how meaningless it is

Re: Diameter questions and how meaningless it is

The O/T (over / thru) hub design you show is fairly common on boats that use higher pitches to improve holeshot. As you show his vents are plugged so the hole is much smaller (Mercury PVS system). At low speed the exhaust vents the blades, at higher speed the exhaust goes mainly thru the hub. On some boats that use pitches in the 28 to 32" range it's not unusual to use the O/T hub and vents. On some boats we use the O/T hub and dual PVS holes.

We also use O/T type props on boats that need an extreme holeshot like flats boats. It can allow a boat to jump up in a very short time in shallow water. A lot of times we have to find a pot hole to jump up in. Or idle for a long way.

On O/T design hubs we also have the option to use an exhaust block-off ring to seal off the area and stop the exhaust from going over the hub.

The blade diameter on an O/T prop is generally smaller, in the 13 3/4" to 14 3/8" on V6 style prop, but will vary from manufacturer to manufacturer. It can be confusing when folks talk about prop diameter, sometimes hard to figure out if they mean hub size or blade diameter.

 

[Tail_Gunner]

Re: Diameter questions and how meaningless it is

Re: Diameter questions and how meaningless it is

Take's a mental note Gunner stay in I/O's

Its intresting how tech is used or can be used. I have tried more than a few undersized hubs in a effort turn a larger blade and each and every time it does aid somewhat in launch but always the price is paid in midrange and top end speed.

When you speak of 28" props i assume your talking of high horse power rigs that are cammend for high rpm or off shore rigs... That is allowing slippage at low rpm so the engine can spin up to its power band and be more effective?

That mismatch would cause a huge distrubance in water flow but are you saying that at speed itself some how corrects the disturbance...I use vent holes in all of the props that i keep and yes you can feel them hook up at a certian point but everytime i get a hub mismatch it always ends up costing perfromance..Vent holes are allowing gases to escape but at high velocity the water stream seals them


Flat boats or shallow water boats i have no ideal what makes them tick...Sounds fun.........No logs or rocks i assume...

Always learning....is there a speed at which that mismatch would be irrelavant?

 

[Bondo]

Re: Diameter questions and how meaningless it is

Re: Diameter questions and how meaningless it is

Below is a example that all should be aware of...and there are no exemptions to this rule...

Ayuh,... That's what happens when you put a prop from a mid-sized gearcase Outboard onto an outdrive...
Just because a prop will slide onto the propshaft, Don't mean it fits the drive...

So long as the barrel of a prop matches the gearcase it's installed on,..
Diameter is rather meaningless, 'n is taken care of by the prop designer, the blade design, 'n shape...
With Mercs anyways,...
The mid-sized gearcased outboards, the large gearcased outboards, 'n the single proped I/Os, generally run the same sized propshafts...
But,...
The mid-sized gearcase props won't work on the large gearcased motors/ drives...

 

[Dhadley]

Re: Diameter questions and how meaningless it is

Re: Diameter questions and how meaningless it is

We use the O/T technology any time we need to get the rpms to spool up quick. Some very popular props are made using the O/T technology like the Merc Trophy, Turbo TXP OT4, Turbo O/T series, Powertech Tro4 and so on. Obviously there are a bunch of props like a Merc chopper that don't even have a barrel. It's also helpful on most 4 strokes (non-SHO) to get then going.

The O/T technology won't work on every boat & motor combo. We're back to that old deal about matching the prop technology with the set up. Basically it's just another tool we can use to do things differently.