Saving Old Seagulls

I have a 1970 approx 102 (wc 385 r6) and it has a 3 blade prop on it, can prop be changed to improve motor performance?

A prop change could increase your boats speed by only a knot or 2. Show us the prop you got now. There are 2 different 2 blade props for seagulls that I know of.

Sorry, my prop is 3 blade not 2, post edited...

Go to Help section see post titled "Prop"

Many thanks, had a read through and it looks like im after a 4 blade hydro one.

When you compute some performance numbers on seagull props, it emerges that they 'slip' badly

Increasing the drive face area lessens this loss

Basically you need more blades

What does that mean in English.

Not sure I agree Rex, let's take the example of a model 80 Century plus with a 4 blade weed free prop.

Prop diameter & pitch, 11" x 10"
Gearbox ratio, 4 : 1
Total blade area in square inches 45.52
Hull speed per 1000 engine revs with zero prop slip, 2.08 knots
Hull speed at 4000 revs 8.32 knots

Many of us have been close to that with a small light hull, what would you define as bad slip, 20% plus?

That would give you 6.6 knots.

All props must slip in order to achieve drive

Ships setup well slip at about 5%, poorer ones at about 10%. Numbers like 30% - 50% slip could use a bit of work (common seagull range)

This is where the sword cuts both ways; (Theory vs Practice)
You can reduce a prop slip by increasing the working drive face (this will be somewhat less than the actual drive face, maybe half). Simplest to increase the number of blades,
however,
The reduced slip may drop the engine rpm below an optimal power level (of say 4000rpm)
so
To get a low slip prop to work in your favour you may have to drop the gearing ratio (a practical difficulty) or reduce the prop pitch (another practical difficulty)

As a more interesting side issue, slippage (in particular excessive slip) also leads to the creation of a wasteful vortex from the tip.



You can address this with a vortex de-generator (VG). You see these in nature as the 'fingers' on hawk wing tips. Also seen on better aircraft (Noting our best aircraft are fairly lame compared to hawks)



Here you can see the dynamic with your eye. At the prop tips the sudden pressure drop in the vortex causes the helical vapour trail

Isn't slip almost directly dependant on the load?

Charles UK

Yes, there is a co-relation between load & slip.

However, (in theory at least) if you let too much slip occur,
then,
you can end up wasting alot of energy

The barge pusher designs show they are made for heavy/ugly loads
Low gear ratios & big diameter props with lots of blades

I have noticed that in the USA Outboard racing props ( in the 1950's - 60's ) had razor thin blades, and cupped. The edges being hammered forward slightly for some performance advantage. How is blade thin -ness edge sharpness , and cupping affect performance or slip? Seagull stock props are rather thick bladed. I AM certainly no expert, just interested. WB

Waterbug

Thin blades drag less & waste less energy. Water is very viscous at speed. Maybe think of it like wet clay

The cupping of the trailing edge is a good observation. Basically cupping an ordinary prop 'fools' the water into thinking the blade is longer (read greater area) than an non-cupped blade. Effectively increases the drive face a little
therefore
less slip with a cupped blade
however
The sword cuts both ways. The cupping costs you drag. So there's a cost/benefit battle that needs to be evaluated

The racing props you saw have a different dynamic again. They are not ordinary sub surface props. These were probably 'surface piercing' props. The blades actually break the surface of the water without ventilation being an issue. With surface piercing you can run the prop so high that the hub is clear of the water, which reduces drag.
however
Again the sword cuts both ways. Surface piercing is a dynamic suited to planing hulls, plus there's issues with prop walk