# The mystery of a proper prop and terrible performance.

Discussion in 'Props' started by missinginaction, Jan 25, 2020.

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### missinginactionSenior Member

As many of you know I've been using my restored 1973 Silverton sedan "Pearl" for the last 6 years. I never exceed about 8 knots, even though this is technically a planing boat. I knew that this boat was unable to plane when I decided to do the restoration. That's OK. But I've always wondered why she wouldn't go. I've been studying Dave Gerr's book Propeller Handbook for the last few weeks and have worked out the Formulas for Crouches Planing formula and the Bp - (delta) method. I don't need anyone to do the math for me. Just for fun I'll list out the parameters below and post a picture of the boat at the end of the post.

LOA - 25.5'
Displacement - 7,000 lbs. +/-
LWL - 21'
BWL - 8.33'
Draft (hull only to keel) - 1' 3"
Rated Horsepower - 220 (The 302 Ford is sound and making rated HP or close to it)
Rated RPM - 4,400
Gear reduction 1.48 : 1
Shaft max RPM - 3,000
Shaft horsepower - 209 (reduced rated HP by 5% for v-drive and shaft bearing)
Desired cruising speed (75% max HP) - At least 20 knots

After we launched her in 2014 and worked out the minor shakedown details I wanted to open her up just to see. At 3,000 RPM (2,020 shaft RPM) this little boat just churns up water and makes a lot of noise. At 2000 RPM she's making about 9 knots (over hull speed but not planing). I could throttle up past 3,000 with little effect and really never even bothered. She wasn't going anywhere and certainly not even close to planing.
So, while most prop calculators ask for speed at full throttle this isn't practical in this case because there isn't any appreciable speed at full throttle.

Math to the rescue:

Since I couldn't get any good real world data I decided to do some math and see what the theoretical prop diameter and pitch would be. My current prop is stamped 16 diameter and 14 pitch. Took time to measure it on the work bench and guess what? It's really 15 diameter and 14 pitch. The original owner must have had it shaved down sometime over the past 47 years. Guess I'm not the first one to grapple with this issue.

So she's running a 15 X 14 3 bladed prop with developed blade area (Ad) of 98 square inches. This tells me the E.A.R. (expanded area ratio) of .56 which checks out. It's just a plain Jane 3 blader.

Crouches planing speed formula says she should run at just about 26 knot's at full power.
I ran all of the formula's in Gerr's book and checked my math twice.

The math tells me that the 15 X 14 prop is correct.

Then I checked the blade loading. There is some pretty sophisticated math involved here. Working it all out the allowable blade loading for the prop I have is 9.7 PSI or 9.7 pounds per square inch.

The installation parameters for the prop are interesting. The blade clearance between top of blade and bottom of hull is 3 inches, that meets the allowable parameter as does the space between the prop and rudder and the keel. So the install is right but the blade area is way, way off.

Since the blade to hull clearance is 3" I'm limited as to what diameter prop I can get in there. a 15" prop is ideal. I could squeeze a 16 inch in there but a 17" is cutting it pretty close. Pitch would have to come down to about 12" on a 16" prop and about 10" or 11" on a 17". Going to 4 blades is a must the way I see it and maybe even 5 but I can't find a 5 bladed prop small enough for this installation..

The way I see it, ideally I'd need a 16 X 12 (or 13) inch 5 bladed prop with an expanded blade area of just about 1.00.

Maybe a custom prop? I'll have to make some phone calls next week.

In the meantime if anyone has any input or knows of a prop shop that they'd recommend I'm all ears.

Moral of the story: There's a lot more to propeller selection than diameter and pitch.

MIA

Last edited: Jan 25, 2020
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### Mr EfficiencySenior Member

You think it was cavitating ? It would have gone beyond 3000 RPM.
Surely worth seeing what happens ?

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### missinginactionSenior Member

If you'd have been there you'd have realized that it was a futile effort. She's smooth as silk down around 2,000 but up at 3K it's just a lot of churning and some light vibration. While I understand your point I've been around planing boats for 40 years. This one wasn't going anywhere.

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### 7228sedanSenior Member

While not exactly apples to apples, my 72 Luhrs 28 with a fresh 454 (340 HP) 1.5:1 responded tremendously to a medium cup. The original prop was a 17X16 3 blade. When I sent it to the shop it was found to be worn down to 16X16 over the years. They added a cup to it which more or less brought the thrust up to where the 17x16 was when new. With tabs deployed, she'd plane at 2,500 at 10-11 knots, cruise at 17-18 knots at 3,000 WOT 26 knots @ 4,250. Didn't see if you had mentioned trim tabs. Mine had tabs probably 30 inches wide. They were very helpful in getting her to plane off and run the most efficiently.

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### 7228sedanSenior Member

Also, I see your in NY. as I'm in NJ not sure how far you want to drive, but I HIGHLY recommend Ocean Propeller in Toms River. His hours aren't the best anymore (Mon-Fri 8-4) but they know props.

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You dont' state whether 1 or 2 engines? Im not familiar with the boat and a quickie search shows it comes with 1 or 2 engines.

However, assuming 1 engine - for now.

The numbers don't stack up. With that size of boat and displacement and the given rated power, the max you'll get is around 18 knots. This suggests that the boat is too heavy.
Also with that rated power and gearbox ratio and speeds etc...you're really looking at a prop of around 14 x 10.

This suggests you need more BAR and a slower turning prop.

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### missinginactionSenior Member

No trim tabs. As for the power aspect of this I ran Crouches planing speed formula. Knots = C / (displacement/Shaft horsepower) to the 0.5 power or the square root of the displacement/SHP figure. "C" is a constant from 150 to 230 based on the type of planing hull you're working with. Lower for cruisers average runabouts and higher for higher speed boats. I use a C of 150, the lowest. SO....

Knots = 150 / the square root of (7,000 lbs/209 SHP)
Knots = 150 / the square root of (33.5)
Knots = 150 / 5.79
Knots = 25.9

Based on my understanding of this math 25.9 kts is the approximate planing speed that this boat should be able to attain with this horsepower. I'm not disagreeing with you ad hoc but this is what the math tells me. There is much more math involved than just Crouches formula but that is where I started.

MIA

Last edited: Jan 25, 2020
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### 7228sedanSenior Member

It seems like the calculation doesn't take prop efficiency into account correct? Any time I tired to calculate theoretical prop specs via the math, I always came out at 45% slippage at some RPMs. How's your transmission, are you sure you're not getting any slippage under the planing load?

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### Mr EfficiencySenior Member

15 x 14 prop with a 1.48:1 box and a 302 V8 are not far from about right, you really must floor this thing before doing anything, 3000 RPM should just about have planed it, what is the deadrise angle ?

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Firstly, still no idea if 1 or 2 engines?

It is a simple empirical generalised formula only and must be taken with a pinch of salt. Those empirical formulas are based upon vessels of that 'type' of that time. Things have moved on considerably since then hence the caveats. Also one would not quote to a decimal place for a "rough and ready" calculation! A vessel with an L/B ratio of circa 2.5 is very low indeed to be using such a formula.

Additional, it is not just the maths, but the reasoning behind such. The hydrodynamics.
As you can see, given your weight and length, you are in the very high hump resistance realm (below 5) as your boat is heavy for its length, and small differences make a huge impact on performance.

You're climbing up the curve and the prop cannot deliver the thrust it is given.

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11. Joined: Oct 2010
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### Mr EfficiencySenior Member

Looking at a few pics, the brand appears to be a moderate vee, a deep vee Bertram 25 would plane with a 302 V8, and it is a heavier boat.

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### YellowjacketSenior Member

You should be able to plane 7000 pounds with 200 hp easily provides you have sufficient planning area (which you do). We're planing 3000 lbs with 55 hp on an inflatable, .

You've already answered your question. A 15 inch 3 blade prop is going to be able to absorb about 100 hp before it goes over the hill. You're trying to put 200 hp through that much blade area and as you noted the loading is twice what it should be.

While your loading and speed callc's are looking at the high end, these all assume that you can get over the hump, which is really the present issue.

Good luck, it does look like you've got a handle on what needs to be done. That boat should easily plane with that much power and the right prop.

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### Mr EfficiencySenior Member

Seriously, it is hard to believe he did not give it full throttle. That gives you some clues.

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### missinginactionSenior Member

BTW, I physically measured the diameter and pitch on a workbench. Diameter is pretty easy. For pitch I used the 45 degree method.

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### Mr EfficiencySenior Member

So what is the deadrise angle ? It is a significant factor, 15 degrees will plane a lot easier than 25 degrees.

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