Propeller Blade Area

[petrel]

I have a 56' DMR w/ twin 671 TI's. Gears are 2.5 to 1, but the shafts are only 2 inch. The boat has always had 28x34 4 blade wheels, according to the previous owner, who bought it from Dwight. I figured there were a couple of reasons f/ being so over-square. One being marginal shaft size and the other being speed because of the reduction (would seem to be more ideal around 2 to 1 w/ a 28 square?) My prop guys says the blade area on the wheels I have is slightly less than 70% and I would benefit from going to a wheel w/ about 80% blade area if I understood him correctly. He says there will be some top end loss of of speed, but it should be better efficiency for cruise speeds. Current top end is 19 or 20 knots at 2300, cruising 13 to 15 knots at 1700 to 2000 depending on conditions, load, etc. The new wheels we are talking about are the ZF's. I'm curious to hear from others who might have made a similar change w/ blade area. Theoretically, it should save a little fuel, possibly enough to pay for a set over the season?

 

[jerseysportfisher]

First thing i would look into is figuring out what your current prop slip is, then it will be easier to determine if its worth the investment in new props

 

[petrel]

How do you figure that?

 

[Downeaster]

While we're waiting for jerseysportfisher to explain this to us, let me share my thoughts.

First, your 2" shafts are right on the money for a 28" wheel. Secondly, the 70% figure is arrived at by comparing the blade area against the area were the blades replaced with a solid disc of the same diameter; 70% is about right for a planing hull.

Its likely that your prop guy has found some evidence of cavitation but he's stymied because a 28" wheel presents as large a diameter as the hull will allow hence his suggestion for new wheels with more blade area (the pitch of your existing wheels is more evidence of this).

Now I'll wander into the deep end of the pool ... My guess is that any issues would be more evident at higher speeds, not cruising speeds. That said, I have serious doubts as to 'pay back' with the new wheels. I would suggest that you think about the points mentioned and then ask your wheel guy for his thoughts about wheel replacement cost vs. fuel savings.

Remember, Internet opinions are worth every penny you paid for them.

 

[petrel]

There's room for a larger wheel, but I've been told not to put a 30" wheel on a 2" shaft by others and the prop guy seemed to agree w/ that. He said trading some pitch f/ more blade area would be beneficial in terms of economy at cruising speed, so I was just wondering if anyone had any practical experience w/ changing wheel style to this type of four blade, and how performance changed as a result. I was thinking there was a good chance that someone on here had done it given the fact that that so many downeast hulls limit the size wheel that one can swing. So in this case it's not the matter of no more room for diameter, but not wanting to give up too much pitch and not wanting to push the envelope w/ a 30" wheel. Anyhow, while a DMR does get up and run on the water, it's not a hardcore planing hull that doesn't like slower speeds.

 

[captainlarry84]

Slippage

I can figure your slippage no problem. Once done we will then re-visit new wheels $$$

I need:

Reduction gear
WOT RPMs
Current WOT speed this must be very accurate
Current pitch of wheel

For your cruise speed
Reduction gear
Cruise RPMs
Current cruise speed this must be very accurate
Current pitch of wheel

Keep in mind that blade area is not free. Once you increase the blade area you will need to reduce pitch, which puts you back where you started from.
A 28 X 34 wheels is not that far out so square for a twin screw boat with wheels in clear water. Square wheel theory is more accurate with single screw New England boats

 

[F/V First Team]

So much drag with twin screws that efficiency models and formulas don't always pan out.

If boat X has 400 hp and does 20 kts, that doesn't mean if we add another 400 hp engine and running gear that we'll be running 40 kts. Chances are you'll only pick up like 5 kts. More weight means the boat is lower in the water column, therefore more wetted surface - aka drag. Additional running gear is additional drag as well. Plus when doing propellers if you have two props that are supposed to do 20 kts at 100% efficiency (0% slip) then you're still trying to get just 20 out of it. Over square the wheels for the best effect.

My racing propeller used to give me 125% efficiency, mathematically I shouldn't have been able to spin it let alone get the performance out of it. Wish I still had it.

 

[jerseysportfisher]

While we're waiting for jerseysportfisher to explain this to us, let me share my thoughts.

First, your 2" shafts are right on the money for a 28" wheel. Secondly, the 70% figure is arrived at by comparing the blade area against the area were the blades replaced with a solid disc of the same diameter; 70% is about right for a planing hull.

Its likely that your prop guy has found some evidence of cavitation but he's stymied because a 28" wheel presents as large a diameter as the hull will allow hence his suggestion for new wheels with more blade area (the pitch of your existing wheels is more evidence of this).

Now I'll wander into the deep end of the pool ... My guess is that any issues would be more evident at higher speeds, not cruising speeds. That said, I have serious doubts as to 'pay back' with the new wheels. I would suggest that you think about the points mentioned and then ask your wheel guy for his thoughts about wheel replacement cost vs. fuel savings.

Remember, Internet opinions are worth every penny you paid for them.

one you can google prop slip calc and have a computer do it for you or you can do it in longhand. Either way means nothing if you numbers are not accurate

pitch* rpm
________ = theoretical speed

gear * 1056


then take

theoretical speed - actual speed
___________________________ = slip

theoretical speed


Like larry said, blade area comes with its own problems. You can blaze a trail buying new wheels and working them for no return and you might find out the wheels on the boat now work best for what you have.

 

[jerseysportfisher]

Also, I have found that George Crouch's planning hull theoretical speed calculations to be right on the money. At least for the planning hulls we tried them against, not sure how it would fare on a semi displacment.

 

[petrel]

I have not done an exhaustive check on the RPMs w/ a photo-tach since changing my tachs, but I think I'm around 2300 WOT, w/ 2.5 to 1 reduction, 34 inches of pitch. Top speed is probably about 19.5. Looking at 14.5 around 1900 w/ the same variables. Boat is on the hill now. I've always figured those wheels were a pretty damn good choice, but lately guys down here have been going to the ZF's w/ more blade area around here on the planing hulls. I know I will have to give up some pitch to make up f/ more blade. My deal is I that don't want to lose anything at cruise, I don't really care about wide open speed. I'm going to get some new wheels at some point, but I don't want to buy something that's going to have me going a knot or two slower at cruise. Boat is 40,000 pounds, over 50 feet on the waterline. I know a lot of twin screw planing hulls here have wheels more over square than what I've got, but I had attributed that to clearance, shaft size, and or reduction. Good to know there's not really anything wrong w/ the current set up. I had always figured it was sort of a bastard combination that resulted from Dwight Raymond getting a good deal on some running take-outs w/ 2.5 to 1 gears when he built the boat. The transmissions actually got changed since then, but one at a time, so the ratio remains the same. I always figured 2 to 1 was the best ratio for a boat like this. Still don't understand the slip thing. I know it changes depending on speed.

 

[captainlarry84]

effciency #s

Your efficiency %s are as follows:

2300 RRMs with a speed of 19 – 20 knots

2300 WOT @ 100% = 25.76 knots

2300 WOT @ 80 % = 20.61 knots

2300 WOT @ 74% = 19.06 knots

Therefore @ 19.06 knots WOT you are at 74%
@ 20,61 knots WOT you are at 80 %

Cruise RPMs @ 100 % only as your cruise speeds are to vague:

1700 RPM @ 100% = 19 knots
@ 80% = 15.20 knots

1800 RPMS @ 100 % = 20.16 knots
@ 80 % = 16.13 knots

2000 RPMs @ 100 % = 22.4 knots
@ 80% = 17.92 knots


In reviewing your WOT speeds 74% or 80 % is very good. Therefore it is my opinion that if you change wheel so more blade area you will gain nothing. Although the blade area increase will increase wheel efficiency to anywhere from 85 % to maybe 90% you will have to reduce the pitch & pitch is what moves you forward. Lastly if you went to a 2: 0 transmission and squared out the wheels based on where currently you would gain little or nothing.

 

[petrel]

Thanks, Larry. I wish I had more to go on, but it looks like my actual cruise speeds are in mid to high 60'%'s in terms of efficiency, based on your numbers. Do you think I would lose anything in there (1800-1900) w/ more blade area and less pitch?

 

[captainlarry84]

Nibral

That is a very expense guess. You cannot rob from Peter to pay Paul. If you increase that blade area pitch must be lost. You WOT #s are good. I think you need to make sure on the cruised speed numbers and also you load on a given day. Trim tabs may help you gave some cruise speed. You may want to just rework your current wheels and scan the to class I specs. If you have never had that done you should try it. It is a lot cheaper that new wheels. Lastly 34 “ of pitch is a lot so I assume that you are turning Nibral wheels. If not that maybe a new set down the road. Nibral will give you a knot!

 

[F/V First Team]

Explain to me the reasoning behind "Nibral will give you a knot"...

Does it work on the same principal as "chrome valve covers give you 20 hp"?

 

[captainlarry84]

Funny

Chrome valve covers….LOL. It is more that that. Nibral wheels are a little lighter in weight & because the metal is much stronger the blades do not flex under load. The blades are also slightly thinner, which also makes them easier to turn. Nibral dones not always improve your speed. It did on my boat and some others. Once wheels have pitch greater that diameter the blades if bronze will flex in some cases under load causing a slight loss in pitch. Nibral will not. I bet a chrome Nibral may work even better…..LOL Very good on the valve cover theory.

 

[CaptDave]

Larry,

I ordered my new four blade nibrel prop 22 X 23" from Accutech Propeller in New Hampshire. Larry is a gentleman to work with and with his numbers Relentless would have a cruising speed of 15 knots with full load of customers and gear and I believe it was 19 Knots WOT. Picking up a week from Monday as the shaft had to be bored from 1.75" to 2". We will see what the new numbers do for us.

Dave

 

[jerseysportfisher]

Petrel,

is 40000 lbs an approximation ? also whats the hp ratings on the 671 ?

 

[petrel]

40,000 pounds is from the travel lift. 671's are TI's w/ M15 injectors, not exactly sure about the horsepower.

 

[jerseysportfisher]

According to crouch's formula, 40000 pound vessel with 800hp wide open will go 23.3 mph (20 knots).(sounds right for the 671's (390-410) Just to get you to 23knots in that would require 1000 hp total. Now remeber this is all approximations as you can't vouch for the travel lift, you dont know your hp, you really dont know your props untill you have them scanned. But real world to gain 3knots in something that heavy you would have to add 100-130 hp a side.

 

[petrel]

Not looking to gain 3 knots, just want to be sure that if I went to 80% blade area that I would not be losing at cruise RPMs b/c of the diminished pitch compared to 70% blade area, which is apparently more typical of 4 blade wheels. The current props are indeed Nibral, which also sounds like it adds to the efficiency. So it could be that I am already at the pinnacle of efficiency w/ the current power in the boat.