Propelling Dr Jekill and Mr Hyde.

How do I propel a vessel with two extreme states of use?
In unloaded condition it weights only 4400 kg but in loaded condition weight ticks of at a maximum 32000kg.
Two engines of approximately 35HP each are going to be used.
So, I'm a bit confused as I have two different hull-speeds to consider.
First calculation suggest a loaded hull speed of 5 knots – and an unloaded hull speed of 13 knots.
Must admit that these calculated speeds are from spreadsheets found.
It's idiot to change the propellers , the engines can't run nearly 3 times as fast...Is the only solution a gearbox?

 

Depends also on the operational envelope, ie how frequent and how long are the periods the vessel will operate in the extreme conditions. To put it in other terms: which loading is earning money? What are the main dimensions?

One further reflection on your hull: in fully loaded position you have a very large wetted area, not contributing much to carrying capacity, but adding a huge amount of friction drag. On top of that, the resulting bow shape will push oceans forward, creating increased wave drag in a speed region, where wave drag normally is of small concern. Taking those influences into consideration, your drag estimates are probably a bit overenthusiastic.

 

You need to learn how a prop works and what prop slip is. You can nearly get both performances with the same prop at the same rpm. Suppose in the loaded condition you are doing five knots but the prop has an absolute slip of around 22 ft/sec. That's a total advance of about 31 ft/sec, which equates to a 10" pitch prop at 5400 rpm with 2.42 gearing (leaving out half a dozen fudge factors). When lightship, you wont need nearly the thrust, maybe 1/6 as much at same speed. Slip might reduce from 22 to 9 ft/sec. That's 18 ft/sec advance. That's 3135 rpm. So you have a lot of throttle left, but not enough. even at 6000 rpm, you are only at 9.5 knots. So you could choose to go to an even higher slip prop in the loaded case- smaller diameter and more pitch delivering same thrust at 5 knots. Or you could go to a bigger motor, say a 50hp, and run a higher pitch at slower rpm and pretty much the same diameter. The first method has several problems regarding fuel efficiency and cavitation. the second method costs more up front. Both allow a higher lightship speed. The 35HPx2 is not realistic in my opinion. If a computer is telling you that is an appropriate installed HP, get the computer to drive the thing.

 

-baeckmo-
'Depends also on the operational envelope, i.e. how frequent and how long are the periods the vessel will operate in the extreme conditions. To put it in other terms: which loading is earning money? What are the main dimensions?'

A full loaded 20' container has a weight of 25,000kg. Add the weight of the boat and it will be close to 32,000 kg. I expect an average total container load of less than 8000 kg but it needs to accept an occasionally load of 25,000 kg as well in case someone else like to get his 20' container shipped.
The drawing shows form and displacement of the hull @8000 kg load.

'Taking those influences into consideration, your drag estimates are probably a bit overenthusiastic.'

Maybe I don't need the speed figures in full loaded condition. I try to recalculate my speed envelope in unloaded and average load conditions before implementing conclusions into the design.

-philSweet -

I have read and re-read your post a few times and started finally to convert all figures in metric units. You know our brains are smaller, (that's why we came up with the metric units in the first place) so forgive me. Up until now I have only worked with air-plane propellers and I see that you can't compare both. Is it wise to play with the various prop-calculators I find on the internet?
A prop ain't cheap to I like to avoid a wrong purchase.
Is there a max blade angle that can't be past? I expect that from a certain angle you just mix water with no significant thrust.

'Or you could go to a bigger motor, say a 50hp, and run a higher pitch at slower rpm and pretty much the same diameter. The first method has several problems regarding fuel efficiency and cavitation. the second method costs more up front.'
Then I definitely choose the second option and change type of fuel too, diesel instead of gasoline.
Diesel allows me to run an engine at low power settings without guzzling fuel.

'The 35HPx2 is not realistic in my opinion.'
I will recalculate with average loaded container.

'If a computer is telling you that is an appropriate installed HP, get the computer to drive the thing.'
The program I use is OK, it's just the one who operates the keyboard who is missing experience.

Johan

 

Here is a new approach on my hull form and used engines. I choose to work with the average weight of a loaded container so with 10,000 kg of container weight + 7,000 kg for the boat l + 2,500 kg reserve, the vessel displacement becomes 19,500 litre (or less than 19,000 litre sea water).
For engines, I was considering gasoline engines but both ends of the speed envelope are too far apart. Diesel is more appropriate if I need only a small part of the available power in full loaded condition. Two engines of 90HP each are going to be used, 180 HP total power.

Question is now, how can I find or calculate the speed of my boat in unloaded condition (7,000 kg) at full throttle and how fast would it go in loaded condition (19,500 kg) at minimum fuel burn a mile. Are formulas I find on the net, thrust worthy?