The other prop question

[Krazymaan]

Reading about props doesn't always tell you what you want to know so I'll just ask, its much easier.
I'm only speaking of outboard motors here.
I know the prop questions have been beat to death so I'm going a different route with my questions.

This first question is the one I am most interested in: Ok, whats determining why you would want a 13 to 16" diameter prop and all the half digits between and I am not talking about because it will hit your cavitation plate if you go too big. Is it the torque that your engine can produce like maybe a 2.4 200 fishing motor cannot produce the torque of a 300X so therefore you would never put a 15" diameter prop on such a low torque engine? Or is it the boats weight? I do understand the physics between different diameters, mass and momentum and what it takes to spin them, but I need to hear it from the experienced.

So, I see boats like the F1's, SST,s and such seem to only be using props that are under 21ish pitch. I never read about a guy that runs his SST120 with a 34 pitch. Is this all because they need that get up and go after a hard corner and can spin 11k which makes up for the lack of running a 34 pitch. Or something like that?

But on the other hand, I see heavy boats with twin outboard running a pair of 34's. Can I assume they don't really have much take off power but just have a lot of top end and thats if the motors can handle the high pitch props without the motors not being able to achieve the proper rpm range? How come I don't see twin 300's sporting 21 pitch props on big heavy go fast boats, they always seem to be up in the 30 plus pitch.

Lastly for example. something maybe i can relate too. Being these are my only two engines I've ever had. Lets take my carbureted 2.4 chrome bore oval port mod vp motor with a 1.87 CLE on my STV Pro Comp. It came with an already set up for my boat some kind of yamaha chopper cupped ear prop that sticks to the water like glue with my 2.4 mod vp. will take the boat to 100+ very quickly with plenty left over. So when I go crazy and put a 350hp 2.5 heavily modified motor with a 280 offshore mid and a 1.87 CLE lower unit on it. If I use the same prop do you think I can expect the same results? We'll find out this summer, just asking if the prop set up is mostly for what kind of boat you have or maybe both boat and motor

I realize a prop is like a rear differential in a car, you can't have 4.56 gears and go 188mph and on the other hand you cant have a car with 2.79's and win a drag race. But the two stroke, rpm, power band is a completely different animal than a car so I need to know.

Eventually in the future I'll be asking things about how do i know if I need bow or stern lifting props, why a clever works excellent on one boat but a chopper works excellent on a different boat. But thats for later. I can't handle too much information overload at once.

Thanks for your help

[us1]

It's all about boat weight versus prop blade surface area versus hydrodynamic drag versus gear ratio. All of these things have to be carefully balanced to get the most efficient propeller for acceleration and top speed. Then you have to start calculating cup and rake into your prop selection. A light boat is going to take off like a rocket with a large diameter prop on it but you do not 20 or 30 miles an hour off the top end.

[90 5.0]

Tunnel boats also run totally different gear rations than drag or performance lake boats normally run that's why they typically run 17-21p props and run in the teens

[us1]

Exactly, our F1 boats run gear ratio of about 1:1 depending on which lower unit you are using. That's why the props are 9 to 10 inches in diameter. When the motor turning 10 grand so is the prop.

[90 5.0]

I know basically nothing about tunnel boat racing but a popular gear ratio is 14:15 correct? Very close to 1:1 that's the one you are mentioning?

[us1]

Yes that's a
#4 speed master the #6 is1:1

I know basically nothing about tunnel boat racing but a popular gear ratio is 14:15 correct? Very close to 1:1 that's the one you are mentioning?

[90 5.0]

What ratio is a #6?

[Krazymaan]

Being prop choosing is very complicated. Let me ask this. How do I know if i am buying a prop that has bow or stern lift. Is there such a thing as neutral. choppers, clever, raker, besides spending $600 a pop, I'd rather listen to some experienced folks on here to help steer me into the right decision. How do I know if i need bow lift? or stern lift?

[olboatman]

Scribed....been thinkin some of the same. Gary

[90 5.0]

Put a prop on , any prop and see how it acts. Does the bow stay flat? Need more bow lift, does the nose pop up too easy and try to blow over, need more stern.

Generally speaking a chopper style will provide more now lift and a cleaver less, v bottoms run choppers and tunnels run cleavers generally speaking but there will be both styles on both hull styles depending on set up and goals.

Be prepared to buy a lot of props. Buy used at first buy run test sell, buy run test sell. Repeat for the next 3-4 years steady lol.

[us1]

Choppers are for bow lift cleavers or for tail lift. More rake is for bow lift less rake is for tail lift. The best way to go is to buy the most common prop that other people use on your boat motor combo and then I make adjustments from their.

Being prop choosing is very complicated. Let me ask this. How do I know if i am buying a prop that has bow or stern lift. Is there such a thing as neutral. choppers, clever, raker, besides spending $600 a pop, I'd rather listen to some experienced folks on here to help steer me into the right decision. How do I know if i need bow lift? or stern lift?

[Krazymaan]

Choppers are for bow lift cleavers or for tail lift. More rake is for bow lift less rake is for tail lift. The best way to go is to buy the most common prop that other people use on your boat motor combo and then I make adjustments from their.

Thank you, I can understand this one.

[patchesII]

What hull do you have

[Krazymaan]

1986 STV Pro Comp Sprint Bottom

[Krazymaan]

The prop i have on it now is fantastic. It was already worked over by DAH for Mark Tabara Racing. It grabs, hooks up, pulls like a freight train, doesn't slip or have blowout and it will take me to 100 mph plus is a few seconds it seems. I'm just trying to learn prop stuff for if i get another motor or on info for my other boats.

Its some Yamaha clever chopper cupped concoction thing they made for him. I don't even know what the pitch is. I just know it hooks and books.