Prop question anyone?

[jasoncsc]

Hi guys!

I would like to pick your brains here.

I am a newbie. I notice that everytime when I put the boat (A surface drive Microburst 21, X440/3 prop) onto the water, the boat would accelerate very fast, but this "boost" soon died out after a straight of 30 feet, and the speed of the boat, even though its fast, but is not as fast as like when I dropped it on the water. I thought about this for a while, and think that if I could keep this "boost" on all the way then my boat would be much faster.... Can anyone enlighten me that this "boost" effect, maybe is due to that when I drop the boat onto the water the prop is fully submerged, and when the boat reached a certain speed the prop would become 'surfaced', and if this reasoning of mine is true is it that I am using a prop too small and I should use a bigger prop I could ?

Also, what's the performance difference in running a 2 bladed and 3 bladed prop?

Thanks!

Regards,

Jason

 

[MikeP]

I have one idea,

The motor gets to a good operating temp after you start it and get it to the water. Then 30 feet out it starts getting water flow to the motor and cools down.

-MikeP

 

[Chris Cohea]

Jason,

The 440/3 should be good on the 3.5 mono's

That's the prop my MicroBurst liked.

Sounds like your on the LEAN side a bit.

Try and richen the fuel mixture on the carb some and try this before changing too much to the boat like props and such.

Make sure you give the boat a good toss at the launch,you don't just want to drop it in. ;D

What engine are you using?What's pipe length?

Two-versus Three Blade....

Well I'm not a prop tech guys....I'm still learning the props....

But the three blades seem to work good on the mono's as a rule of thumb.Two blades work good to I run two blade props on my A-mono right now with good results.

Kind of a try it out deal. Props are funny that way..That's why you end up with soo many in your box. ;D

Others should key in here and be of some help to you on this.

Good luck

Chris

 

[Propjockey]

Does the engine speed up as the boat slows down? If so, the prop is cavitating. If this is the case, try lowering the prop 1/16th to 1/8th inch at a time. See what happens.

If an engine swings a three blade screw at the same RPM as the same size two blader, then the boat would go faster. 3 blades = 33.333(etc)% more prop in the water. Unfortunately, it ain't that easy.

More prop surface = more load = less RPMs. Also, different props will affect handling differently. Chris is right...best way to find out is experiment.

 

[ClayGlover]

pj,

actually, 3 blades would be 50% more blade area. and the same rpm should give you slightly more speed, but only because the same 3 blade prop is a little more efficient with less slippage, not more travel (pitch).

 

[Propjockey]

actually, 3 blades would be 50% more blade area.

True...IF the prop is fully submerged.
So I guess if the prop is surfaced at the centerline, it's only 25% more prop in the water??? I hate math!

Now let's see...in a perfect world ( : ), if a 3 bladed prop were 50% submerged, you would have 25% more thrust than a two bladed???

Propjockey Racing.....Pie R square? No, pie are round...cornbread are square. ;D

 

[ClayGlover]

hmmm, not sure about thrust. but i do know that one revolution of a two blade prop submerged would give X amount of travel in inches forward. the same prop with three blades will go the same X amount of travel forward per revolution.

RPM x Pitch = Speed

in a perfect world with zero slippage, a 2, 3, 4, 5, or any # of blade prop with the same blades pitch and diameter will go the same speed at the same RPM.

 

[Ian Inverarity]

A prop like an X series Octura has fairly wide blades. Look at the prop from behind, if the water is thru the centre of the prop then you should see that at times there will be just one blade in the water, but at other times both blade will be in the water. On a 3 blade X series at times sections of all three blades will be in the water!

The number of blades is a factor in blade area, and more blade area is needed to push higher drag hulls. There are a lot of other factors involved too.

But the whole concept of slippage may be flawed. Often the speed of the boat is equal to the leading edge pitch x rpm. Our props do not have a constant pitch over the blade, they start at low pitch at the leading edge and finish at higher pitch at the trailing edge. Higher drag boats need more pitch progression from front to back. But blade area and pitch progression are also related, so if there is little blade area then more pitch progression is needed and visa-versa.

Nitrocrazed racing: This stuff is too complicated....

 

[paul heuston]

Hi guys!

I would like to pick your brains here.

I am a newbie. I notice that everytime when I put the boat (A surface drive Microburst 21, X440/3 prop) onto the water, the boat would accelerate very fast, but this "boost" soon died out after a straight of 30 feet, and the speed of the boat, even though its fast, but is not as fast as like when I dropped it on the water. I thought about this for a while, and think that if I could keep this "boost" on all the way then my boat would be much faster.... Can anyone enlighten me that this "boost" effect, maybe is due to that when I drop the boat onto the water the prop is fully submerged, and when the boat reached a certain speed the prop would become 'surfaced', and if this reasoning of mine is true is it that I am using a prop too small and I should use a bigger prop I could ?

Also, what's the performance difference in running a 2 bladed and 3 bladed prop?

Thanks!

Regards,

Jason

ok the boost that you are describing sounds lik the engine is on the pipe and than falls off the pipe after a distance.

try changing the pipe lenght but donot cut it just pull or push it to change the length if this is no help than you may whant to change the height of the prop in the water and or change up or down 1-2 prop sizes..

just remember to practice for a while and than change something and practice for a while again til you get it right..

paul