Need input from the Prop experts here

[1stSeaRay]

I've been keeping tabs on performance info provided by other 260 owners.

Looking specifically at some numbers that Todd recently posted. After seeing his terrific mileage on his recent trip, and comparing his cruise data (30mph @3400rpm) to mine (28mph @ 4000rpm). I'm curious why there would be such a large difference, considering both his 5.7 and my 5.0 are rated @ 260hp, with a 2.2:1 drive ratio on what I believe to be identical hulls. I know I'm pushing some additional weight (genset & AC) but not enough to account for the difference.

Sea Ray prop charts show the following:

2000 260DA 15 1/2 X 24 14 1/4 X 24 (Todd)
2004 260DA 16 X 20 CUP 14-3/8 X 20 CUP (ME)

Curious what the effect would be if I changed to the smaller props. I feel that I'm a bit over propped given how I am typically loaded. I'm currently just barely hitting 4500rpm (WOT range is 4600-5000) Anybody want to field a guess as to why they made the change in the first place?

There is a set for sale locally for a great price....I'm thinking worst case if I don't see the performance change that I'm looking for I'll have a spare set on the cheap.

Thanks for the input
Marty

 

[1stSeaRay]

Shameless bump

 

[1stSeaRay]

Running the numbers I posted above through an online prop calculator, the 24p props look to have 15% slip, while the 20p props have a 22% slip.

22% slip seems very high to me, the bottom is clean....any other reasons slip would be so high?

I can't seem to find an details on the effect of changing the diameter....can anyone help here?

 

[Lazy Daze]

You already have the smaller props (don't worry about the diameter - just look at the pitch - unless you really want to get into the nitty-gritty). You have the 20" pitch, Todd's are 24" pitch. That's why you have to turn more RPM's to get the same speed. Don't get too involved in trying to compare one boat to another. There's other factors such as: is one boat bottom painted? How much fuel and water was in each boat? How many people/how much gear were/was in each boat?

My "guess" as to why the change is because the boat got heavier as the years went on. Due to: the addition of the tower as an option and other optional things (like genny and A/C) became more popular.

 

[Asureyez]

Running the numbers I posted above through an online prop calculator, the 24p props look to have 15% slip, while the 20p props have a 22% slip.

22% slip seems very high to me, the bottom is clean....any other reasons slip would be so high?

I can't seem to find an details on the effect of changing the diameter....can anyone help here?

The basic concept is that a prop is a wheel converted to an incline plane (both invented by Archimedes and was called a water screw.) As the wheel turns each blade pushes a volume of water equal to the volume of the pitch of the blade from the vertical of the shaft. Thus a blade with a 22 degree pitch is pushing a volume of water in relation to its blade length or the near radii of the wheel (the hub pushes nothing). The more blades the longer the column of water being pushed in one revolution of the wheel

The total diameter of the wheel defines an imaginary column of water, a tube if you will. The pitch is the length of water in this imaginary tube that is pushed in one revolution of the wheel cumulatively by all the blades. Hence as the wheel turns it walks forward in a column of water which is pushed backward creating thrust forward.

Newtons Law: Force exerted in one direction creates an equal force in the opposite direction.

So if a wheel is 20 inches wide it's thrust tube is going to be 20 inches wide and the pitch creates a length of water volume to be thrust back wards or a 27 Inch pitch will push a column of the water 22" X 27" in one rev at the prop. The thrust will equal the mass of the water in that column.

Now if you widen the wheel and leave the pitch the same the column or volume of water (mass) is now greater. It takes more energy to push that water and hence consumes more HP to get the water moving. Thus, wider and higher dimensions require more HP and are slower to get up and moving. The more blades added to this consideration create longer columns and require more HP to get one revolution.

Your question of: should I go wider vs. deeper or both have to be mathematically calculated. The OEM wheel was designed to move your boat based upon the design loading of the vessel, the HP developed at the shaft at a design RPM and the all important gear reduction of your transmission.

Every single issue matters and the designers took the best guess in the build. You over load the boat, all bets are off.

Tweaking your prop should only be undertaken base upon your use variance from the design. If you generally run short hops, on empty tanks and no crew, you could prop up for speed, You run loaded in the tanks with lots of food, beer, toys and babes and you'll slog with a speed prop and need to prop down for power and lower end thrust.

Now while you are comparing two boats that seem similar, are they really.

HP at the Shaft at what RPM?
Transmission reduction?
Design weight
Actual running weight

Two identical boats having no design difference at all except the cargo will run differently and the heavy boat will require higher RPM's to get to the same speed and will burn more fuel.

There are prop calculators on the web to help you in this. but you have to know what your boats stats are. I would suspect that in the two boats being compared there is a material difference in Shaft HP and gear reductions.

 

[1stSeaRay]

Chad,

Thanks for the excellent information. I think I now have enough detail to run some numbers.

As a side note, I used the Mercury prop calculator, it recommended the 22P prop set (15.75" dia) over the 20P in 4 of 5 situations based on hull shape & weight (not current performance data)

I'll post my findings, and If I decide to switch props I'll log some data from my current set and the new set & post it here.

 

[1stSeaRay]

Having spent almost a full season with the 20" pitch props that came on my Bravo III, and finally accepting the fact that my 260 is underpowered with the 5.0L I decided to make a change.

Since the 20" pitch is the smallest offered for the Bravo III, I had the guys at my local prop shop re-pitch my wheels to 18" pitch. Picked them up yesterday, and was able to get out for the first run of the season today to check out the difference.

With the boat loaded fairly close to typical I am pretty happy with the difference in performance. Where I used to lug the engine badly around 3000 RPM trying to get on plane, I can now easily power through the entire range. WOT has improved from 4400 RPM to over 5000 RPM (I let up around 4950 but could have possibly pushed it a bit further).

I had to give up 1 or 2 mph at cruising rpm, but in this case I think it was well worth it. Ideally I'll be able to convince the admiral that I NEED to replace the engine with a 383 stroker, but until then, I'm quite happy with the change.

 

[Sundancer]

One other factor that wasn't addressed in this thread is the altitude change. Even if you HAD the exact boat, engine, props, gear ratio, etc., the altitude difference between Todd at Sea Level and where you boat would cause a difference too.

Glad the new props made a difference. You were definitely over propped not getting to 5,000 rpm and this change obviously has made a difference! Thanks for posting the new numbers!!