Round in circles I go

I have learned a lot from this forum and Dave Gerrs books. I started with a WW2 harbour defense launch as my chosen vehicle to cross the Atlantic. I have now worked up a spec for a round bilge version of a WW2 MASB to do the same job.
LOA 62'
LWL 57.5'
BWL 12.6'
Tc 1.86'
18.5 Tons.
Now I looked at how much power would be needed SL2 would be possible requiring 327Shp. The boat could have that much power but I decided to make top speed 12kts because the smaller diesel would make the boat a lot cheaper. 12Kts = 162Shp (Wyman formula). This power is for average conditions,but I can find no reference to what average calm or heavy weather actually is. I recon up to force 2 is calm, force 4 is average, and 8 + is heavy weather.

Now G. Brewer says his diesel duck needs 30Shp for 8.5kts in calm conditions. So I used the same formula to predict the power requirement for my design. Note the recommended power for DD(38foot) is 50 to 80.

From the document repowering Tortuga Calm requires 10Shp, Average requires 13.6Shp, and Heavy weather needs 25Shp for 6.5kts.
SHP in calm conditions
Kts SHP
2.......................0.23
3.......................0.77
4.......................1.87
5.......................3.8
6.......................7.1
7.....................12.3
8.....................20.3
9.....................32.1
10...................49.1
11...................72.7
12..................105.1
Installed SHP =150
13..................148.5
In a calm the boat could reach 13kts
In average conditions the boat needs 105.1 x 1.36 = 141.9Shp for design speed. Not too far from the Wyman prediction.
In heavy weather (a gale) 49.1 x 2.5 = 122.8Shp for 10kts. If the crew are up to it.
Maintaining 7Kts in a gale would require about 31Shp.
So what do you think of my calculations.

 

I don't think 30HP will get 8.5 knots in a duck. They have huge resistance. Also, in a gale, 31HP won't even give you enough speed to maintain steering. Say you get a bit better than 1 mile per gallon. You will need, depending on where you cross 1800 to 2500 gallons. Have you accounted for the weight and volume of fuel?

 

Long distance voyaging in an amphibious craft would have lots of benefits. The big one is being able to haul yourself up on a beach or even inland on islands etc, instead of being forced to anchor in tricky situations and 'commute' in tippy tenders.

 

Gonzo
Yes I don't think you would get far in a duck with 30Shp But In calm conditions the calculations say it will make 8.17kts on 31.7Shp. In the thirty Years I lived beside the river fourth in Scotland weather was seldom force 2.
Recommended minimum Shp for the DD is 50 which is 1.58 times the flat calm requirement. Maximum recommended is 80 or 2.52 times flat calm SHP. 13Shp would maintain 6.05 in heavy weather. This does seem to be a bit on the low side in a gale. But as I said before I am unsure what is considered heavy weather.
I know you can fit 750Imp/gal in an 18ton WW" MASB.

 

rwatson
I think you may be confused between a WW2 amphibious Duck and Diesel Duck. Now that's a concept I had not thought about, see what you've started.

 

I have been researching exactly average wind conditions at sea might be. I read of a cargo vessel forced to heave to in force 7. My Beaufort scale shows red triangle warning flag for a force 6 wind. So I must conclude that is the start of heavy weather conditions and force four seems about right for average conditions. Both the Diesel Duck and my prospective design will therefore need every HP installed when caught in a force 8 Gale.

 

Where in Devon are you? I used to work as a shipwright in Totnes

 

I live in a small village called Black Torrington the nearest port is Bideford. The nearest large town is Barnstable.