settle a prop arguement 3 blades vs 4

my friend and i are debating this. i am currently have a 3 blade 14x18 and am looking at a 4 blade of the same diameter and pitch(due to the fact the boat has a sluggish hole shot). i contend that the boat will get on plane faster due to the increased surface area. he contends it will take longer to get on plane b/c there will be less slippage requiring more torque which the engine doesnt have as it is maxed out(throttle all the way down) during the hole shot.

All depends on the design of the prop.

This doesn't really answer your question. Instead, it sort of changes your assumptions. Anyway, from the Acme website:

Theoretically, a 3-blade propeller should always be more efficient than a 4-blade. However, in the past, two big issues have prevented the 3-blade from taking the lead in popularity. The first was a lack of manufacturing precision, which commonly caused 3-blades to vibrate more than 4-blades. When significant dimensional variation exists in a propeller, vibration is generally more noticeable in a propeller with fewer blades. The second reason for the 3-blade disfavor was an almost certain loss of blade area in the transition from four blades to three. Consequently, even though top-end gains were achieved, performance expectations on the low-end were diminished.
Through the implementation of precision manufacturing and design improvements, Acme Propellers are giving the 3-blade the reputation it deserves. Acme is consistently holding tolerances never before achieved in propeller production. This accuracy makes the Acme 3-blade incredibly smooth. In addition, the blade area of the Acme 3-blade has been expanded to equal that of the Acme 4-blade. The translation? The Acme 3-blade has raised the performance bar. From an awesome hole-shot to an unmatched top end, and all the smoothness throughout, the Acme 3-blade leaves nothing to be desired. We believe it has an edge that everyone will appreciate.

Great advice Salmon!. Never heard it described in detail the benefits of a 3-blade. I currently have a 2004 Tige 21I with a gear reduction tranny. Besides a tremendous holeshot.....it maintains a wide towing range without any compromise in top-end performance. As a boat buyer and enthusiast.....it is best to take a demo-ride on any boat to get the feel and performance of the modern towboat. Who cares if the boat can do 50 miles an hour???? The most important thing is if there is a strong holeshot with all the extra weight and passengers. Just remember....the trailer was designed for a specific max. weight.....and the distribution of that weight is critical to safety of the trailer, and the tow vehicle.

well ive read the acme info before and its seems like they are just promoting the three blades however it doesnt really settle the arguement. we did run the boat at wot today and it was 4k rpm w/ a speed of around 34.

If you have a sluggish hole shot and your WOT is only 4000 RPM you either have too much pitch or a tired motor. If it is a case of too much pitch you are working your motor too hard. In theory you gain 200 RPM for every 1" of pitch you decrease ( pitch of a prepeller is how many inches it would move forward in one complete rotation if there was no slippage). Having said this if your motor should have a WOT of 4400 RPM then you should be running a 14X16 prop.

In the past people would switch to a 4 blade "performance" prop to get increased hole shot but this would usually result in less top end due to increased drag. With today's CNC prop technology company's like Acme are changing the rules, as Salmon Tacos post shows a 3 blade can now give you the best of both worlds.

there is something severly wrong with your boat. or it is proped so wrong i cant believe it. you cant even slalom or barefoot behind a tourney boat? thats what they are for. you have the power slot tranny correct? at 4400rpm im running 45mph+/- and wot i could take it to 7000. because of the wakeboard prop im running.

NOT TRYING TO ANSWER ANY QUESTIONS...just thinking out loud.

In aircraft props, the fewer the blade count, the better. Also, the larger the diameter, the better. Fewer blades means that each blade has cleaner air to work with, and longer blade span means higher (more efficient) blade speed - more thrust with less drag (better span loading and reduced induced drag). There is some sweet spot between blade speed versus blade area versus pitch.

In planes you increase blade count to reduce diameter, which reduces tip speed (supersonic tips are very noisy and inneficient) and the landing gear can only get so tall before the prop is at risk for clipping the ground. Unfortunately, no matter how cool them 4- or 5-bladed props look, they're giving away efficiency.

In our boats, there's probably some tradeoff between making diameter large for efficiency and the shaft angle you'd have to shoot out the bottom for clearance issues. Also, I've never studied or understood the inside story on pitch versus diameter in boats.

Submarine designers got all the budget in the world and they choose like seven narrow blades. Why so high a blade count? To keep tip speed down for noise issues, while keeping blade chord narrow and angle of attack light to prevent stalls (which make bubbles and noise.) Definitely not optimized for speed.

However, if we had a stainless milled sickle-bladed prop under our boat, we'd be REEEEEAL stylie:




(Message edited by toyotafreak on October 24, 2004)

derek i think its just funny that you have pics of submarine props lying around.
anyway i think maybe the boat is proped(is that a word??) wrong. perhaps ill change to a 13x13.5. i guess i need to call a acme or oj and talk to them. for a prop recommendation. i also wanna know the answer to the question.

Alan,

The Mastercraft pro star 190 with powerslot should have the 14x18, if it does not have a powerslot (1.5:1 gear reduction) it should have a 13x13 prop. Top speed should be between 42 and 45 mph, my guess is that your speedometers are way off, peak rpm should be around 4500rpm.

If everything is truly as you say then I would definately agree with 882001, something is wrong. That something could be that somebody erroneously put a powerslot prop on your non-powerslot boat?

yeah borrow a gps, take a look at tranny make sure what you have1:1/1.5:1?, check timing, make sure your secondarys are opening, compression check.

Alan, you think that's funny, so I'll put up a modern aircraft prop too.



Ps. Here's where all good pics can be found:

All summer I ran a 4 blade Acme and last week I tried a 3 blade Acme. The 3 was smooth but not as smooth as the 4. The 4had no vibration or anything. The 3 did not vibrate but my boat had a tingle feel through it. I sent the 3 back and got another 4.

Lance, could you detect a difference in hole shot or in speed between the two?

The 3 blade had a little better hole shot but it was a 1/2 pitch lower than the 4 blade. With a 1/2 pitch lower the top speed was about the same as the 4 blade.

In theroy, a four blade prop will have more drag, and thus less efficiency, than a 3 blade prop.

If your current 3 blade prop "slips" due to excessive cavitation then a four blade prop will help. If what is limiting your hole shot is your engine torque then going to a 4 blade of the same pitch will only make things worse.

The easiest way to get a better hole shot is to use a lower pitch prop. This is equivalent to shifting down a gear in your car: better acceleration but it will limit your top speed.

Given that your WOT is only 4K and 34 MPH I would say that your top end sucks anyway and the prop you have is not ideal for anything. Unless you really like cruising at 34 MPH and don't want to impact your fuel consumption any I would definatly recommend going to a lower pitch. What is ideal is if your WOT is around 5500 RPM (or where ever the "redline" is on your engine). If you can't get to the 5000 RPM range then there are a lot of ponies inside that engine that never get out of the stable!

Rod

thanks rod. i think that answers the arguement(i was wrong) however i did talk to the acme guy today. pretty knowledgeable. that prop that is on there is for the 1.5:1 tranny. i have a 1:1 and that is where the problem seems to be. im gonna go with the 13x12 as per his recommendation and see what happens.

i just went with the 13x11.5 "wakeboard prop" it is awsome.here is a review to the prop your getting you wont be dissapointed.
http://www.correctcraftfan.com/articles/ACME_540_Prop.asp

Discount Acme. rider@boarditup.com

Karl just curious what is your price on an ACME 231.