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Old 17 August 2012, 18:00   #1
ncp
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Prop Slip - Willard 7.3m / Cummins 6BT

I posted this here instead of the North American section to get some more readers. But, it has a very specific US question too...

My prop slip numbers seem high. I heard that heavier boats tend to have higher prop slip numbers.

1. Question for US Willard owners: As far as you know, the transmission is 1:1? That's what I used in my calculations.

2. Question for all: How do these numbers look?


RPM Prop Pitch kW BHP Gal/Hr Actual mpg Estimated Calculated
Speed Speed Slip
(mph) (mph)

2600 19 stock 134 180 10 29.9 2.99 46.78 36.08%
2300 19 stock 134 179 7.9 25 3.16 41.38 39.59%
2100 19 stock 130 175 6.3 22 3.49 37.78 41.77%
1900 19 stock 124 166 4.8 19 3.96 34.19 44.42%

2600 19 cupped 157 210 11.9 31.7 2.663865546 46.78 32.24%
2400 19 cupped 152 204 9.4 27 2.872340426 43.18 37.47%
2200 19 cupped 155 207 7.4 24 3.243243243 39.58 39.37%
2000 19 cupped 149 200 5.9 22 3.728813559 35.98 38.86%

2600 21 stock 157 210 11.9 33 2.773109244 51.70 36.18%
2400 22 stock 152 204 9.4 30 3.191489362 50.00 40.00%
2200 23 stock 155 207 7.4 27.4 3.702702703 47.92 42.82%
2000 24 stock 149 200 5.9 24.7 4.186440678 45.45 45.66%

EDIT: That came in ugly...attached .xls
Attached Files
File Type: xls Prop Slip.xls (20.5 KB, 74 views)
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Old 17 August 2012, 18:35   #2
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I seriously doubt your transmission is exactly 1:1. You can come up a relative guess using 1:1 but you need the true gear ratio to get more accurate and be able to comment on "too high" slip.

Unfortuately I have no clue what it might be. Is it not printed inside on the box somewhere?
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Old 17 August 2012, 21:15   #3
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Quote:
Originally Posted by captnjack View Post
I seriously doubt your transmission is exactly 1:1. You can come up a relative guess using 1:1 but you need the true gear ratio to get more accurate and be able to comment on "too high" slip.

Unfortuately I have no clue what it might be. Is it not printed inside on the box somewhere?
Almost everything that I've read says Borg-Warner model 72C 1:1. "For Sales" and elsewhere.

There's one ad still on the web saying 2.5:1. But If you were to plug in 2.5 : 1 into the formula using 2.5 : 1, you get a theoretical speed that is less than the actual!!!! The 72c is offered in 1.00, 1.52, 1.91, 2.10, 2.57, 2.91 :1. The only other gear ratio (of all 72c gear ratios offered) that gives you an theoretical speed > actual is the 1.52. But, plugging in 1.52 tells me prop slip is 2.99%. That's too little slip!
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Old 17 August 2012, 23:40   #4
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Can't you turn the engine over by hand with the drive in gear and count the prop revolutions? I'd expect your slip to be in the region of 20+, could easily be 30 if the boat is heavy and the engine working hard, but I'm guesstimating.
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Old 05 September 2012, 19:01   #5
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Quote:
Originally Posted by captnjack View Post
I seriously doubt your transmission is exactly 1:1. You can come up a relative guess using 1:1 but you need the true gear ratio to get more accurate and be able to comment on "too high" slip.

Unfortuately I have no clue what it might be. Is it not printed inside on the box somewhere?
Figured it out: Tranmission is 1:1....outdrive is 1.43:1

Brings my prop slip numbers down to the "very low" range!
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Old 11 September 2012, 20:20   #6
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@ncp

First post. Been researching williards. Saw your spreadsheet. Which prop did you decide on for best speed vs economy? Which would you choose with a generally lightly loaded boat but one that would occasionally have to punch into high winds and steep waves?

Saw 3.8 mph at 29kts posted elsewhere but your numbers dont back that up. Did you actually test all those props?
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Old 11 September 2012, 23:16   #7
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Oops, that was 3.8 mpg not mph.
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Old 12 September 2012, 12:37   #8
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Quote:
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@ncp

First post. Been researching williards. Saw your spreadsheet. Which prop did you decide on for best speed vs economy? Which would you choose with a generally lightly loaded boat but one that would occasionally have to punch into high winds and steep waves?

Saw 3.8 mph at 29kts posted elsewhere but your numbers dont back that up. Did you actually test all those props?
1. Yes, I've tested those 3 prop variations personally on my boat: 19 inch pitch, 19 inch pitch custom cupped, and 21 inch pitch. I wanted to try a 23 inch pitch with a smaller diameter but haven't yet (the 17 1/4 x 23...it's a stock size from Powertech)

2. So far, the 18 1/2 x 21 has given me the best economy. On a rough day here in southern California on my way out to the islands, 24-25 mph at 4 mpg is plenty for me. I couldn't go much faster anyways. I know of Hurricane 733's with twin 150 two strokes getting half that gas mileage. Yes, they have a top speed of 50 mph, but they can rarely use that in the open ocean. The hole shot isn't as good as the 19 but the mid range cruising numbers show ~2-3mph hour faster at 2200/2400/2600 rpm.

As for the "3.8 mph at 29kts" that you seen quoted by Warfare Rib, he has probably gotten close to those numbers. My hull has bottom paint. He has sold some very clean boats with the original gel and if not, he's redone the bottoms so they're shiny and glossy and ready to go fast. Let's do some math: 29 knots is 34.8 mph. He usually keeps the engines at 180 hp as the Navy specs them (he only bumps them to 210hp if the injectors or pump need a rebuild or a customer requests it). At 180hp, the engine is rated "Medium Continuous" which, according to Cummins, means "This power rating is intended for continuous use in variable load applications where full power is limited to six (6) hours out of every twelve (12) hours of operation." This means you can basically run at WOT for most of your cruising unless you're doing some long runs. WOT is 2500 rpm at that burns 10 gph @2500 rpm. With the 17 1/4 x 23 inch pitch prop and 8% slip (seems to be what I am getting), the calculation show 3.5 mpg. Not 3.8 mpg, but close when you're talking 0.1 increments.... But, he's also put 25 inch pitch props on before and he has smoother hulls. I don't know and can't vouch for him. Give him a call! He's probably redone more of these boats than anyone.
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Old 13 September 2012, 17:01   #9
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Great information! Thanks so much. As you say cruising at 24-25 mph in a sea is pretty ideal and if you are getting that kind of economy, all the better.
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Old 14 September 2012, 20:51   #10
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clloyd, congratulations on your purchase!
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