Props to try out

[Petrol Head]

Looking for a pair of 28" four blade Bravo's to try out.

Does any one have a pair I can try for a day.

Or where I might be able to get a pair for the day from?

 

[Jon Fuller]

I've a pair of 30's, bit tall for you tho', but I would have thought 28's were too tall also, what ratios are you running?

 

[Petrol Head]

I'm sure youre right on both counts and the ratios are standard.


The 27 mirages run at 4600.

I have no certain engine HP figures.

I have some s/h 26 Bravos turning up soon .

I wanted to try 28's before and after replacing the heads.

Just to see what happened.

 

[Jon Fuller]

Ratio standard......which is?, they're OMC's aren't they?

 

[Petrol Head]

Which is?, in a very good book on the boat.

 

[Petrol Head]

Ratiooooos

Went to boat and got the book

Overall Ratio 1.43:1

Consitently pulls 4600 on 27" mirages.

Boat wieght new 7000lbs dry.

Say 7500 today inlcuding 50 galls for going out for a run.

Engines.
Suppose 330 standard
plus open Exhaust
Circualting pumps removed
Cams, inlets & porting?

Steve

 

[Jon Fuller]

Would have thought it weighed in well over 7000.

My educated guess (alleged) would put that boat/setup at low 60's

to go that quick (73mph @12% slip) it's gotta be a whole lot lighter, a whole lot more power, or a searay Tacho!

All just MHO

 

[Petrol Head]

Ssearay goes negative

Ugh!

We achive better than the low sixties.

I have no experience with Props.

This is the way the boat behaves with the 27 mirages.

The Boat wont launch unless the legs are right in. Out to nuetral they will spin. The boat gathers speed to plane, it doesnt climb up and out over a hump.

Once on the plane if you trim past neutral, the revs rise but the boat doesnt go any faster.

If the boat jumps right out, sometimes when you get back in you will have to reduce the RPM of a drive right down the get the prop to grip again. this improved with the adition of nose cones.

I reach two possible conclusions.

Too high for their design.
The surface area of the blade is crap for low speed and pays off well at the top end.

You'res and anyone elses for that matter H.O. on this would be appreciated.

 

[Jon Fuller]

Mirages are a pretty good all-round prop.

The Bravo1 four blader is however, very good at gripping when getting up and out, and also gives a fair amount of bow lift once traveling at speed, these attributes make it very good for high 'X' height set-ups, tho I suspect your top speed wont improve with them, in fact I'd love to try a pair of 31" Mirages on my boat, as I reckon they'd be better all round for me, better top end, more likely to break away getting on plane, which would allow the turbos to get going sooner etc.

have you measured the drive height? stick a straight edge along the bottom and see how high your anti-ventilation plate is above this line.(directly FWD of the drive of course)

 

[Petrol Head]

London Bus

Naaah,I just bent down and looked! I shall do this thing though for it is of interest.


73! ?

 

[Jon Fuller]

4600 (rpm) divided by 1.43(ratio) X 27"(pitch in inches) divided by 1056 (magic shortcut No) = theoretical speed, then have a stab/ educated guesstimate at your slip, say 12-14% (mine is closer to 10%)

So,

4600 divided by 1.43 = 3216.78, (prop rpm) times the pitch @27" = 86853.14, divide by 1056 = 82.34 (mph theoretical), times 0.88 (12% slip) = 72.37mph

Obviously you can do this long hand, working out how many inches your prop tries to travel in one hour, then turn the inches into miles, but you'll get the exact same figure.

 

[Petrol Head]

The boat weighed 7500 new and dry so thats nearer 8000 then!

The Donzi book says that the cobra drive adds 12 inches to the draught.

The drive would accomodate a 15" dia prop.

The prop tips must be 1/2 - higher than the Skeg and the plate 1/2 " higher than the prop?

I would estimate that the plate is 4" above the base of the V.

Here is a pick showing the riser tips going out at the top of the transom timber. The hoses are right on the tip of the wood and that is about 6 inches from the hull and deck seam. Is this high?

 

[Jon Fuller]

Dunno, I'm not familiar with the Donzi transom, you really need to measure it with the straight edge, up to the anti-vent plate.


What is the beam of that boat, 8'6"?...and hull length? (actual)

 

[Petrol Head]

Book Says:

Hull 29' 11"

Beam 9'0

Dwaft Keel 27"

Draft outdrive 39"

I'll do the measurement tomorow.

 

[Jon Fuller]

9'0"


Wow, thats wide! wouldh've made a triple (nearly)

 

[Petrol Head]

Hold that thought

Dont anybody move!

At the widest point accross the cabin/cockpit bulkhead.

Looses Ireckon over a foot going to the transom.

Typical period Donzi design, sharp at the pointy bit fat in the belly, thinning down to the transom.

 

[Jon Fuller]

bit like me

 

[Jon Fuller]

That's a full 2' wider than mine albeit only midships (mine is parallel)

 

[Petrol Head]

perhaps I'll measure that too.
Flippin sales book.

 

[Jon Fuller]

No, I'm not doubting it, 8'6" is a common beam on yankee stuff, and mine is outrageously narrow for a 28, remember, it was designed for triple OB's, not stern drives