cleaver question!

[scottyboy]

my boats a bit of a handful when at speed and always wants to push the nose down? a few people have said to me that its my prop lifting the back up? is this right about a cleaver?

 

[roofer]

cleaver gives stern lift , chopper gives bow lift, imho of course.

 

[scottyboy]

maybe i ought to get a chopper then! are' these abit snatchy when re entering the water though?

 

[Burty]

maybe i ought to get a chopper then! are' these abit snatchy when re entering the water though?

i found it fine even when surface piercing that is until you back off a bit sharpish and it tries to throw you out the boat sideways

 

[BluFin]

maybe i ought to get a chopper then! are' these abit snatchy when re entering the water though?

Work the prop & take out some of the cup. See the link http://www.mazcoprops.com/products.htm under the RE4

4 blade chopper slightly better than the 3 but it's the same effect.

see clever link also as to how it carries the boat

 

[Matt]

Typically, monohulls respond better to a bow lifting (chopper style) prop than a cleaver, since they need the bow lift to carry the bow to air the boat out. Cats typically respond to stern lifting (cleaver style) since they are already generating stacks of aerodynamic lift raising the bow.

There are exceptions of course, a very fast mono might get enough aerodynamic lift to not benefit, but that's the guideline, IMHO.

 

[Matt]

Is it correct that a 4+ blade surfacing prop is loads easier on the gearbox and steering?

 

[BluFin]

Just checked the clever link & it's been amended recently & removed the wording aft lift & used the Mazco Offshore Cleaver is the only way to fly instead, but it did state it

 

[BluFin]

Is it correct that a 4+ blade surfacing prop is loads easier on the gearbox and steering?

Yes In Theory,
"Do F ask how to prove it, coz I cant find the Book/Article at the Moment"

But in theory the fastest prop in surface applications will have two blades ??????.

All a trade off

 

[jw.]

an' a big lump of lead.



Edit: WTF happened to your 1 blade post, Matt?

 

[Cookee]

Typically, monohulls respond better to a bow lifting (chopper style) prop than a cleaver, since they need the bow lift to carry the bow to air the boat out. Cats typically respond to stern lifting (cleaver style) since they are already generating stacks of aerodynamic lift raising the bow.

There are exceptions of course, a very fast mono might get enough aerodynamic lift to not benefit, but that's the guideline, IMHO.

Batboats are also the exception (the 2 litre ones anyway), they all run sternlifting props.

And 1 bladed props are meant to be the most efficient - just got to keep them on the propshaft!

 

[BluFin]

Here we go !!!!!
Now, How about Screw Threads ( Joke)
Edit: those 1 blade fekers are a bugger to balance ?
Seriously

how about posting some high performance prop links, I know I could enjoy a good nose around those sites

 

[Matt]

Yeh, I removed it cos when I thought about it, for all effective purposes a surfacing 2 blade prop is a 1 blade - only 1 blade in the water at a time.

an' a big lump of lead.



Edit: WTF happened to your 1 blade post, Matt?

 

[Matt]

Yeh, I suppose I should have said a 'traditional' non stepped hull, but then some smartarse would have said steps are traditional because they were invented back in the 30's or something, or they would have found some non stepped hull that didn't respond to bow lift. Or noted that underpowered planing hulls don't want bow lift. Etc etc etc.

Batboats are also the exception (the 2 litre ones anyway), they all run sternlifting props.

And 1 bladed props are meant to be the most efficient - just got to keep them on the propshaft!

 

[Matt]

Mercury's prop pages are OK:
http://www.mercurymarine.com/chapter_1_-_history__development
http://www.mercurymarine.com/chapter_2_-_basic_propeller_parts2
http://www.mercurymarine.com/chapter_3_-_how_propellers_work
http://www.mercurymarine.com/chapter_4_-_propeller_technology

Wasn't turbinia one of the first high speed propeller applications.

Just did a google, here we go:

Parsons had difficulty in choosing the correct propulsion configuration and went through seven different propeller designs, including three screws on one shaft. Because the turbine turned the shaft faster than an expansion engine would, the screws were not as efficient as predicted due to cavitation. This phenomenon, the result of a vacuum forming around a screw turning at high speed, was first observed during tests on Turbinia. When the vessel failed to attain her design speed, Parsons replaced the single shaft with three separate shafts (initially, there were three screws on each shaft) and at the same time replaced the radial-flow turbine with a more efficient parallel-flow turbine. The results were spectacular, and in supervised trials Turbinia attained speeds of thirty-four knots.

Here we go !!!!!
Now, How about Screw Threads ( Joke)
Edit: those 1 blade fekers are a bugger to balance ?
Seriously

how about posting some high performance prop links, I know I could enjoy a good nose around those sites

 

[Cookee]

Actually the BananaShark has some similarities to the Batboat and hates sternlift, although we've made some changes and going to try our "batboat prop" again!

 

[Matt]

Yeah, but if you think about it, under ideal conditions a perfectly balanced stepped hull should be best with a very neutral prop. Adding bow lift (which is really stern drop) or stern left is only ever fighting against the hull, that's the whole point of it.

Although, thinking about it some more I reckon you could have 2 classifications of stepped hulls. a) Those that are designed to run on the tip of the steps, effectively with only 2 (or more depending on the design) points of contact on the water. b) Those that use the steps to aerate the water aiming to lower friction. The former would probably be the b23 style where the steps are inclined at a noticeable angle, the latter would be where the step is more of a notch in the hull.

Not sure if this makes sense or not - I'm just thinking out loud.

 

[Cookee]

Matt, the way I understand it (maybe not the right way!) is most hulls are "a" - the steps are inclined to suck air at lower speeds to stop the steps "sucking" and making it harder to get the hull to release and get on the plane quicker. The amount of aeriation (sp?) and the good it does is questionable I believe (been wrong before though!)

 

[BluFin]

Don't ya just love theroy.

But us boaters go a bugger it all up by wanting to do things
like turn, accelerate, pull skiers, calm water rough water, ect, ect,ect.

I alwyas had a two prop set up one calm & 1 rough the rough one was cut down by a 1/4 & de pitched by a 1/2", pick up was a lot better but it it flattened off I was feked.

Edit: looked at the Merc link: ok on props but there theroy of Flight F******* Sucks Fig 4.23. Not going to go off of theory of fligt as well props are enough

 

[Matt]

Why did you drop the diameter?