Power predition formula

I bought the Molland book after Alik previously recommended it. I also recommend it to anyone with an engineering or similar background.

 

Thanks my engineering background is in electrical/electronics. But I can handle what most of my friends impossibly difficult mathematics.

 

The recommended book is a bit expensive so I will try to get a library copy first. I have discovered that the graphs in Voyaging Under Power give reasonable results. The problem is salesmen lie or at least bend the truth. Even the truth can be made to look like a lie.

I used the graphs to calculate the diesel duck needs 78hp to reach 9kts @ 32600lbs. The recommended engine is 80hp. 9kts =SL 1.486 so the duck will probably not go over 8.11kts requiring 51hp in calm conditions and 64hp when it's a bit rough. If 32,000lbs is half load condition it could weigh more than 17 tons ready for a long voyage. This would require 62hp in calm conditions and 78hp when it is a bit rough. 9kts top speed comes from VUP, 8.11kts is the top speed published by the NA, so nobody lied. My guess is the NA new 80hp would guarantee his customers would not be disappointed with his boats performance in any weather they dared to travel in.

 

When sea-trialing the Krogen Express 49 hull #1 (now the Krogen Express 52) I compared the graphs in Voyaging Under Power to results from a CAT Slide Rule (given out by Caterpillar, the diesel engine manufacturer). I found them to be identical, so I suspect that's how the graphs were calculated or that they have a common source. They accurately predicted our sea-trial result once an accurate as-tested displacement was utilized.

Does anyone have an original source for Wyman's formula? Is it better than other simple formulae for semi-displacement boats at semi-displacement speeds? Also, I see that M.Raven's post here was never answered: de Groot method https://www.boatdesign.net/threads/de-groot-method.53410/

For an excellent discussion of formulae inferred from model tests see https://www.boatdesign.net/attachments/van-oossanen-resistance-prediction-of-small-high-speed-displacement-vessels-state-of-the-art-pdf.140505/

 

According to https://www.ericwsponberg.com/wp-content/uploads/2021/09/THE-DESIGN-RATIOS.pdf

There is a discussion of Wyman's formula beginning on p. 52

The paper references this thread: center of flotation calculation and implications? https://www.boatdesign.net/threads/center-of-flotation-calculation-and-implications.30857/