michigan props - The Hull Truth

lonewolf5347 · 08-06-2003, 06:54 PM

Was wondering how close to trim tab is o.k.just installed a new prop on my 90 hp 2003 yamaha and what I found with the new prop is I am only about a 1/4 inch from the top of the prop to the trim tab itself. My old prop was 13 1/4 x 17 k and the body lenght is 6 1/4 the new michigan match is a 13 1/2x15 and the body lenght is 6 1/2 inches so being longer it brings the blades closer to the trim tab. The prop is correct for the year and model would there be a great amount of flex in the blades at WOT to move a 1/4 of an inch?

MarkA · 08-06-2003, 07:05 PM

Lonewolf,

I don't think you have anything to worry about as long as you don't prang a blade and then have the blade move 1/4 towards the tab.

Still, the tab is a sacrificial anode, so it is soft zinc anyway.

You should have play measurable in the hundredths of an inch if the prop is correctly fitted with the right thrust washer and nut.

Think of it this way, look at the tolerances of the prop hub into the foot flange. Now that's small!

Mark

The first coozie was a brown paper bag.

f4phantomii · 08-06-2003, 07:11 PM

A quarter of an inch seems like a lot of deflection to me, either for an aluminum or a stainless steel prop.

I can run a quick calculation to check at work tomorrow for you....I need a bit more info...

How many blades?
How much does your boat weigh fully loaded?
What does the blade measure from the outside of the hub to the tip of the blade?

It will be a fairly conservative calculation....meaning I'm assuming the blade will be less stiff than it actually is. I'll still be shocked if it flexes more than .1 inches or so.

Also, in forward gear, remember that the high pressure is behind the prop, and would want to deflect the blades forward and away from the trim tab, not towards it. And I'd be surprised if your prop can develop enough thrust in reverse to deflect the blades at all.

You should be fine.

-Michael

lonewolf5347 · 08-06-2003, 08:02 PM

let me ask you guy's somethng it seems with the new prop on it is a little stiffer when you try to spin it in netural do you think because it new,the old one spins very easy and a little smoother or the new one has to be broken in?

MarkA · 08-06-2003, 08:06 PM

that's gotta be the clearance between foot and prop hub.

I suspect it will wear off when you run it... probably just the thickness of the powder coating...

The first coozie was a brown paper bag.

lonewolf5347 · 08-06-2003, 08:31 PM

thanks again guy's will see how she runs now.The reason I change the prop could not get the rpm up to 5500 rpm with the 13 1/4x17 k prop top rpm's were 5000 at wot so I went to the lower pitch 13 1/2x 15 should do the trick

MarkA · 08-06-2003, 09:30 PM

I bet it does the trick.

That's close to the prop I run on my 90 Yamaha (it's actually a Nissan OEM 13 3/4 x 15). Still, my OEM Nissan prop is a long way from the tab.

Good luck, that pitch prop suits my boat and motor perfectly!

Mark

The first coozie was a brown paper bag.