Propeller for small runabout

[DavidS]

Hi,

I'm just about set to launch my Rascal runabout.

Engine is a 50 HP four stroke outboard. Wondering what is the proper propeller pitch for this type of light weight, high performance, type of hull?

Hull + engine + gear is approx. 850 lbs. w/o people.

Max diameter of the prop is 11 inches.

Thanks,
Dave

[mmd]

What is the max. engine rpm and final gear ratio?

[DavidS]

Max rpm range = 5500-6000

gear ratio = 2.08:1

HP rating at prop shaft: 50 HP at 5750 rpm


Thanks,
Dave

[mmd]

My "quick-and-dirty" prop calculator indicates that a 12D x 20P with a DAR (disc area ratio) of around 0.21 is required to achieve about 35 knots with 50 BHP. The DAR is a bit low - most 4-blade props have a DAR of between 0.50 and .65, but this is a place to start. See your engine dealer or a reputable prop shop that deals in speedboat propellers and they will be happy to help you make your selection. If you are really lucky, they will hook you up with someone who has a spare prop of the dimensions that you need so you can do a trial without having to purchase the prop first. Also, when you get your prop, ask the price of having it fine-tuned after initial trials - they may tell you that a minor re-pitch will be included in the cost of the prop.

[DavidS]

The largest diameter that can fit on the 50HP engine is 11".

The following is from a website dedicated to the original Rascal runabout (from 1990 or so):
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When pushed by a stock 60-hp Mercury ELPT engine, this is one fast boat. Bassett puts some numbers to the prototype's performance: "After the proper running-in period. Rascal ran 54 mph in a 6" chop. But this speed (with a 10 x 17 aluminum propeller) came at 6,200 rpm — definitely beyond the manufacturer's warranty specifications' A 10x 19 prop dropped the maximum rpm to 5,500, but the speed fell to less than 50 mph and acceleration became sluggish. So, we had a propeller shop form a 'super cup' in the prop's blades The engine now tops out at 5,800 rpm, giving a speed of 52 mph. Acceleration is brisk (25 to 35 mph in 2.7 seconds), and the power holds well in turns.
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Wondering how this translates when dropping from 60HP to 50HP ?

Thanks.

[mmd]

Well, I did say it was a "quick & dirty" calculation...

I missed the 11" restriction; sorry. Also, the calculation suite I use doesn't recognize the semi-exotic prop blade configurations such as cupping, high skew, and semi-cavitating, all of which wring every last bit of thrust out of the applied horsepower.

However, I must ask the obvious: If you already had detailed information on the size & type of propeller used on a similar boat, why did you come here to ask the question?

[mmd]

Ok, back to this one for a moment:

I've had to make a few assumptions, the primary ones being that the boat listed in your trials information above weighs the same as yours, the 60-hp motor is 25 lbs heavier than your 50-hp one, and that in both cases there will be only one 175-lb person in the boat. All things being equal, if the trial boat will do 52 mph (why doesn't everybody agree that boatspeed is measured in knots and spare me the problem of having to convert to make my 'pooter formulae work?) with 60 horsepower, by extrapolating Crouch's Formula and using these figures, it appears that with 50 horsepower and a prop that is as efficient as the one on the trial boat, you should expect to see about 48 mph (41.7 kt) with a 50-hp motor.

However, before you set yourself up for disappointment, please be aware that planing hull speed is wonderfully capricious. The weather, the tune of the engine, the condition & shape of the bottom, how crisp the trailing edge of the hull is, and - most importantly - how well tuned the prop is will add or subtract five knots or more to the realized boatspeed. The simpler the object, the more the devil is in the details, and a planing hull is a deceptively simple object. I won a bet one time by saying I could add at least two knots to the top speed of an acquaintance's ('glass) speedboat with a $5.00 can of body putty. All I did was do a quick scrub of the bottom with a ScotchBrite pad and mold the intersection between transom and hull bottom from the 1/4" round edge from the mold to a crisp sharp edge. It didn't last the day before it got all knocked off by the water pressure, but in the first two runs on the river is went an average of 4 knots faster. I greatly enjoyed my 40-oz bottle of Lamb's Navy!