Speed

[warwicksail]

Hi guys does anyone know the equation for working out wot power is need to move a certain boat a certain speed ?

[warwicksail]

I know the hull speed is around 6 knots

[Cruisingdad]

Is there an equation for that? I would think it would have too many variables, including: Prop, hull design, how clean the hull is, current, wind, etc.

I have always thought a general rule of thumb is 2 HP for every 1000 lbs. The Catalina 380 weighs about 20k and has a 40hp engine. The 400 weigs about 24000 and has a 54 hp. I would err on the high side.

However, there are so many factors, it would seem impossible to calculate to me. Maybe some naval architects could give you a better rule of thumb than mine.

- CD

[Freesail99]

Ocean Navagator did a story on hull speed and horse power needed in the October 2007 issue.

[sailingdog]

There are way too many variables... engine size, gearing, prop diameter and pitch, hull design, etc. The best resource would be something like Dave Gerr's The Propellor Handbook.

[CapnHand]

Originally Posted by Cruisingdad

I have always thought a general rule of thumb is 2 HP for every 1000 lbs. The Catalina 380 weighs about 20k and has a 40hp engine. The 400 weigs about 24000 and has a 54 hp. I would err on the high side.

That seems to be about what most boats have, although I would say that 2 Hp for every 1000 lb (4 Hp per ton) is more like the upper limit. Our boat is 9600 lbs and has an 11 Hp diesel (2.3 Hp per ton). That being said, I've never heard anyone complain about being over powered.

[Cruisingdad]

Originally Posted by CapnHand

That seems to be about what most boats have, although I would say that 2 Hp for every 1000 lb (4 Hp per ton) is more like the upper limit. Our boat is 9600 lbs and has an 11 Hp diesel (2.3 Hp per ton). That being said, I've never heard anyone complain about being over powered.

I sure would not complain. THere have been many instances where a really strong current can impede if not outright stop your progress.

I have been in many blows WOT where we hardly moved at all with the wind on the nose. Other than weight, the only problem with over powering is the fuel consuption. But I wonder if a overpowered engine would hold up longer???? Not sure.

- CD

[TrueBlue]

That seems to be about what most boats have, although I would say that 2 Hp for every 1000 lb (4 Hp per ton) is more like the upper limit.

10 hp/ton would be considered WAY overpowered by that formula. But I have often been assured of a speedy journey during conditions with adverse currents and no wind, with our 90 hp iron genny pushing 18k lbs.

[Cruisingdad]

Originally Posted by TrueBlue

10 hp/ton would be considered WAY overpowered by that formula. But I have often been assured of a speedy journey during conditions with adverse currents and no wind, with our 90 hp iron genny pushing 18k lbs.

You are giving Macgregor's a run for their money!!! HAHA!

- CD

[sailingdog]

That's why your boat is considered a motorsailor...not a true sailboat...

Originally Posted by TrueBlue

10 hp/ton would be considered WAY overpowered by that formula. But I have often been assured of a speedy journey during conditions with adverse currents and no wind, with our 90 hp iron genny pushing 18k lbs.