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I'm wondering what advantages I would realize if I change from the 3 blade original props on my 31' Express w/327Q's to 4 blade props. My planned use for the boat would be slow putting around, cruising on plane, and very seldom I'll crack the throttles forward to the stops.
It seems like I could get a faster plane, and better mid-range throttle response. I would hope to get better economy at cruise, and better slow speed handling (docking).
Brass? Bronze? Nibral? Stainless? With my speedboats, I was always told stainless props performed better than aluminium props as they don't flex / bend.
All input / advice is appreciated.
Thanks in advance.
'06 CC Launch 22
'69 31' Commander Express
ALL inboard props are either bronze or nibral. A 4 blade MAY help a little, but the cost of replacing those expensive props will probably never be recouped. The first step is to determine how your existing props perform.
You will gain more benefit for the money by making sure your engines are in top notch condition and dialed in properly..carbs are calibrated properly, and distributors are advancing correctly.
Kevin, I swapped out a pair of Michigan Dyna jet 3 blade for a pair of Michigan DQX 4 blade wheels. The first thing I noticed was the change of harmonics. The boat ran so much quieter, and smoother at speed. I did not loose any top end, and I gained a little mid range efficiency. These nibral wheels are not cheep. I keep all my wheels tuned to the highest standard, so the most noticeble difference for me between a tuned 3 blade to a tuned 4 blade was the lower harmonic noise level with the 4 blade wheels. I am running 22 X 24 nibral 4 blade DQX tuned by Acutec propeller of North Hampton N. H. The boat is a 1977 30 Hunt Sportsman, set up for off shore fishing, with a pair of 300 Cummins diesels. I run the boat off shore 20 to 60 miles so the lower noise is worth the cost for me.
Hmmm. I hadn't really given thought to noise / vibration advantages. Interesting. Less noise / vibrations in my mind equates to less wear to the entire drive train, or for that matter, the entire boat.
From what I've researched about the 31' Express, it seems that a properly tuned boat will cruise around ~ 20mph (+/-) and I'm looking to maximize my fuel economy.
The cost of fuel this past summer just about put me off buying this boat. While crude oil has come way down from the summer prices, it will surely head back up again. A previous post makes a good point regarding the recouping of the cost for new props, but in the long run, (long run meaning years of use) with expensive gasoline, improving economy, even slightly, is a goal I hope to achieve.
I made the switch from 3 to 4 blade on my 38 Express, and unfortunately I didn't do my homework very well. I went from a 23 x 25 3-blade to a 23 x 24 4-blade, when I should have gone to a 23 x 23 4-blade. As a result I'm overpropped, and I plan on having them repitched next time the boat is out.
The photos above show the props when installed, and they sure look good! They actually perform well, but they will need to go back for a repitch someday in the near future.
(ABOVE): TRADITION at speed, Cumberland River, November 29, 2008, photo by Reece Ewton, aboard FANDANGO.
What I noticed, however, is the boat will get onto a plane at lower rpm, probably a function of overpropping more than the number of blades. This is really a non issue, however, as nobody in their right mind is doing "hole shots" in a cruiser, waaaaaaaaaay too much stress on the equipment. I use my boat for moderate cruising and many times we're down in the 1200 to 1500 rpm range for cocktail cruising. At 2900 RPM the 427 is developing max torque so they really don't mind all that much, but I am not able to get the RPM up to the rated 4000, again another signal I'm overpropped. The rule is to drop two inches of pitch if you go from 3 to 4, and even then, better check with your prop manufacturer about your particular boat and prop style. And yes, they ARE expensive. Top end "normally" will suffer from a good 3-blade to moving to a 4-blade, simply because there is more prop surface and therefore more friction to rob power. There are other factors, but seldom if ever do you see higher speeds with more blades. Thats why most all the racing hydros use a two blade.
I think they run smoother and they pull better at docking speeds. That's not to say you can't dock like a pro with 3-blades!
1966 38 Express
Original big dog power
Thanks Paul. FWIW, what I mean by hole-shot is powering up and getting on plane in a reasonable, but shortest (efficient) amount of time. I've got a speedy runabout for going fast in a hurry if need be.
I frankly don't have any idea how good or bad the bottom gear is on this boat. I've talked to the previous owner, but he's pretty chill-axed and cocktail cruising at ~1400 is mostly all he ever did. I've also talked to the mechanic he's had working on the boat the past 5 years, as well as the yacht service outfit that lifted the boat and did the bottom job. All say the old boat is real solid, etc. We'll see.
I like the idea of running smoother, especially on older equipment like this. I imagine that when it's time for it's next bottom job, I'll go through the expense of making sure all of the shafts / props / bearings / etc. are dress-right-dress and in proper condition.
'06 CC Launch 22
'69 31' Commander Express
I would take your existing props to a prop shop that has computer diognostics. It is normally more expensive but they can make sure that the props are perfect in pitch, flare, and blade matching. I recentyl sent mine out and they confidently boast that i will gain 2 nots in speed and fuel efficenty. 4 blade props from what I know ( i have them) they run smoother, but i have not been able to notice any difference in efficency at the pump or in performance.
I had rock harbor send them off for me, I am sure that they made some money off of the deal.
Johnny's Propeller Service
17307 East Warren Avenue
Is the company that did the repair.
They actually tested them to ISO Class I props, so they are better then new. It knocked out a lot of vibration and it seems to run a LOT better. 1900 RPM is a great cruis speed with no vibration and noise.
I checked into opening a prop scan business, but spending 60k on software / training seemd a little high. Still doing the math, but I think I could develop a program that does the measuring a lot cheaper.
Wow. I sent my props in and had them computer scanned. The guy told me on a scale of 1 to 10 with 10 the worst, mine are a 9. ( excluding colision props) Last prop job was a mess. There was a 3/4 inch differnce in pitch between the two props. The blade lengths were varried up to 1 inch on one prop. He thinks some one modified the props and did a terrible job. Any way. He said it is the source of my vibration. He is going to match the props ( so make sure you have both done) and adjust the pitch. He is also going to add a #3 cup. He feels the tuning of the props will give me more rpms. but i hit my wot. The cup will give better cruse and take 50 rpm off wot. The place is call Atlantis prop in NJ. It came highly recommended. Expensive though. two 4 blade props 825 total. He is guarenteeing 3knot increase at cruse. That means i should be able to run 18 knot at 1500 rpm. or 24knots at 2000. big difference. check in with me in the spring to see how they perform.