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Post by jimmyjet on Dec 3, 2020 21:03:20 GMT -5
I am giving some consideration to switching to 4-blade props on my Commodore from the current 3-blade 18X21 props during a scheduled haul-out this month and interested to hear from anyone who might have done the same thing on this model or something 35-37 ft. A few details I'm looking for is:
1. Did it noticeably improve acceleration and getting on-plane?
2. Did it affect or reduce your WOT cruise speed...if so by how much?
3. Did it aid in low speed maneuverability? (e.g. docking or turning with differential thrust)
4. Do you notice any change in fuel consumption?
TIA - JJ
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Post by tc33obx on Dec 4, 2020 10:13:11 GMT -5
Jimmy,
I can't comment on your specific boat with 3 blade vs 4 blade, but I did do that change on my 22 foot Bryant, which has an Alpha 1 Sterndrive on a 5.7L Merc Engine. I did this mod as I was having trouble pulling my two sons up on wakeboards at the same time with 4 to 5 people in the boat. My marina suggested this move to a 4 blade and that change made a difference in getting the boys out of the water on their wakeboards. The boat would come up on plane much quicker, even when we weren't pulling people on skis or wakeboards. The only drawback is this boat used to reach close to 55mph with the 3 blade, it will only make it to around 48mph with the 4 blade. Low speed maneuverability was improved as well.
I can't really comment about Fuel consumption since I didn't track it that much with that boat, but I would expect it to be a little better as long as you don't run at top RPMs all the time.
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Post by gofast24 on Dec 4, 2020 12:21:02 GMT -5
If at WOT and same RPM on engines the fuel consumption will be the same regardless of 3 or 4 blade prop. Agree, 4 blade will have more pull (push) when coming up on plane, but negative is WOT speed less at same WOT RPM's. Most like lhy 4 blade prop not as efficient as 3 blade?
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Post by tc33obx on Dec 4, 2020 12:52:13 GMT -5
A 4 blade should move the boat more easily at lower RPMs because it has the extra blade. At a certain RPM though, depending on the boat hull design, weight and RPM capabilites of the engines, that 4th blade will become a hinderance to rotation with the extra blade and blade tip. So when the RPMs get up there, a 4 blade will have more drag and therefore less effectiveness as the RPMs build. The question will be at what RPM do you see that and that will depend a lot on the pitch, size of the prop, and the weight and hull design. If you don't really see much drawback until an RPM higher than your normal cruising speed, then your cruise should be more efficient. However, if you need to run close to WOT, it clearly will be less efficient. At least that is the way I understand it.
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Post by jimmyjet on Dec 4, 2020 13:53:05 GMT -5
Jimmy, I can't comment on your specific boat with 3 blade vs 4 blade, but I did do that change on my 22 foot Bryant, which has an Alpha 1 Sterndrive on a 5.7L Merc Engine. I did this mod as I was having trouble pulling my two sons up on wakeboards at the same time with 4 to 5 people in the boat. My marina suggested this move to a 4 blade and that change made a difference in getting the boys out of the water on their wakeboards. The boat would come up on plane much quicker, even when we weren't pulling people on skis or wakeboards. The only drawback is this boat used to reach close to 55mph with the 3 blade, it will only make it to around 48mph with the 4 blade. Low speed maneuverability was improved as well. I can't really comment about Fuel consumption since I didn't track it that much with that boat, but I would expect it to be a little better as long as you don't run at top RPMs all the time. Thanks for the input Tom...yes, had similar results in switching for 3-blade to 4-blade prop years ago with our 16 ft bow rider with a 130 Merc I/O....couldn't get on plane towing a tube with 3-4 people in the boat unless 1-2 would sit in the bow....switching to the 4-blade prop, it didn't matter where people sat and I could "snatch" a skier up so fast it nearly pulled their arms out of the socket....but top end dropped from about 36 to 30 mph though I felt it was a reasonable trade off....am just wondering if there is an equal parallel with large twin-screw boats.... JJ |
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Post by jimmyjet on Dec 4, 2020 14:02:19 GMT -5
A 4 blade should move the boat more easily at lower RPMs because it has the extra blade. At a certain RPM though, depending on the boat hull design, weight and RPM capabilites of the engines, that 4th blade will become a hinderance to rotation with the extra blade and blade tip. So when the RPMs get up there, a 4 blade will have more drag and therefore less effectiveness as the RPMs build. The question will be at what RPM do you see that and that will depend a lot on the pitch, size of the prop, and the weight and hull design. If you don't really see much drawback until an RPM higher than your normal cruising speed, then your cruise should be more efficient. However, if you need to run close to WOT, it clearly will be less efficient. At least that is the way I understand it. That makes sense...one thing I have to continue to remember is this 360 is NOT a ski boat and I don't operate it like it is....especially when I'm paying $3.20 or more for gas at the marina....and putting in 50-100 gallons each time I pull up to the gas dock. I can "cruise" at 8-10 mph on about 12-15 gph and that's where the boat seems to be happy....which is below 2000 rpm...it takes about 2800-3000 rpm to make 18-20 mph and the fuel burn goes up significantly to about 24-25 gph....for cruising the lake, it doesn't seem worth the trade off.... JJ |
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