displ vs HP vs Fuel use

I have been reading many posts regarding proper propeller selection, engine transplants so on...

I was following a thread in BoatDiesel.com that got me thinking - I just wanted to throw this out to the group here so that I my get a better understanding...It really go me thinking when a fellow was restoring a big commercial boat in SF - after he posted his fuel usage chart - it showed that at 1200 rpm and 7knts he was getting about 1gal an hour fuel burn with 2 much bigger engines - of course at 19knts its a different story- that is the same I get with a boat much smaller with much less hp.

I know that to push a boat at hull speed it takes less hp. for a given weight right? So, at 7000lbs it would take about 15hp to push my boat at hull speed. I have a bigger engine (more air and fuel) so a smaller engine (less air and fuel) would have improvments. So, if for example the same boat was all carbon and weighed half the weight, I assume I would need even less hp?

However if the boat was much lighter it would ride higher on the lines and thus have a shorter waterline??

 

If you were going to build it lighter to maximise fuel economy it would have a different shape. For a displacement hull, long, narrow and light equate to speed for a given power. Shape will change a bit depending on displacement and target speed but if you are not carrying cargo and want to go as fast as you can then the proportions of rowing sculls are most efficient.

Trimarans are based on this principle with a long slender central hull and two long slender stabilisers. So you get a practical boat that is also very efficient.

Rick W

 

thanks rick

Im beginning to get it. so, if I follow, what happens when you take a displacement hull and make it light? It will require less power, you will get better fuel use but, will you loose speed due to the lack of D/WL?

 

The reduction in waterline length is unlikely to be significant enough to reduce speed. It will take less power to do the same speed if it is lighter.

Point is if you are going to the trouble of building a lighter displacement hull you will alter the shape to get the best result.

Rick W

 

Hey, Ive been researching this very phenomenom recently, and there are various formulas that are helpful; the most overly useful rough estimator of power vs displacement vs speed vs length i have found is the following, works on displacement and semi-displacment boats, up to hull speed:

MUltiply the length in meters by the HP, and divide by displacement in tons. The square root of this number will give you the speed in knots with a 10% margin of error or so.

It helps for rough estimates, but not for the optimization of a design say the specific benefits of tradeoffs between wetted surface area vs aspect ratio and wont help you calculate max hull speed for a given aspect ratio hull... despite some suggestions Im still trying to figure that one out... cant download software from my computer access so Im just searching my time off...

 

No this formula does not work at all. It suggets, that the speed is related to the square root of power when in reality the power demand increases in power of 3-7 close to hull speed. As an example my saiing boat has a top speed of 6.4 kn with 9 HP diesel, 7.5 m LWL and 2.9 tn weight. Your formula would give only 4.8 kn.

Here is a power vs. speed for my boat based on my VPP (with probably more efficient propeller than I have):

kW vs. kn
0.5 3.7
1 4.5
2 5.4
4 6.2
8 7.0
16 7.8
32 9.2

If you compare your formula to those, you will notice a very big difference in behaviour. Notice how the spead increase from doubling the power decreases from 22% to 11% and the increases again to 18%. With your formula the increase would be 41% in each step.

Rick
Slender hulls are often not an option when space aboard is needed. Reducing displacement helps quite much without changing the hull form. As a first guess power demand at hull speed is close to linear function of displacement. I will try to have time to calculate my boat at half the displacement.

Joakim

 

The data above was for total displacement of 3400 kg (500 kg load). Here is the data for the same hull at total displacement of 2140 kg. The waterline decreased from 7.59 to 7.13 and also many other hull parameters changed.

kW kn
0.5 3.9
1 4.7
2 5.6
4 6.5
8 7.3
16 8.4
32 10.3

Thus the increase of speed was only 0.2-0.3 kn with 0.5-8 kW, but much more at higher kW. The 6.2 kn reached with 4 kW at 3400 kg could be reached with just 3 kW at 2140 kg. Thus reducing weight 37% resulted to 25% reduction of power need.

Joakim

 

Joakim
If you read my posts above you have simply put numbers to what I stated.

At displacement speeds there is an optimum underwater shape to minimise power demand for a given set of constraints such as length and displacement and a design speed. The optimum shape will change if any of the constraints change. If you were building a new boat from CF you would have a different shape to get the best result.

Rick W

 

Just to be clear: I did not change the hull shape in the example, but the inputs for the VPP, like LWL, Cp, LCB, LCF etc. change when you change displacement.

I just wanted to point out, that reducing displacement helps quite much without changing anything else. To do it well the whole boat should be redesigned for the new displacement taking into account also stability etc.

Most often you are limited to some interior volume, thus you can not design the boat just hydrodynamically.

Joakim

 

Yes; a pity really. However it is nice to see modern sailing boats being constrained more by hydrodynamic considerations rather than the distorted shapes encouraged by rules that rewarded volume back in the 60s and 70s.

I have a mechanical engineering colleague who once complained about the aircraft manufacturers putting windows in an otherwise ideal shape from a stress perspective.

Rick W

 

All this is quite challenging for someone who is math challenged - What I think I am hearing is this. just for discussion. Lets use a small full keel displacement trawler/tug - 30', 13,000 to 17,000 lbs. Now, lets just say I was rich and decided to build the same boat all from carbon and now the displacement is 6,000 lbs.

Now, what has happend? Obviously the boat will require less power, it will still go the same speed. But, it should with all the weight gone float up much higher, so less boat in water, should be less water line, and should be a little slower with different handling issues.

But is you say that it the above boat is to be that light, then a new hull form is necessary. But, if you want the same sort of ride, what do you build. A planing sort of hull that will never plane - it would most like be much shallower?

 

The other possibility is to put some of the weight in the keel , to have a far safer boat in the ocean.

If there is room more consumables (and a BIGGER tool box) also come to mind.

FF

 

ballast, of course. But, you would not want to put as much in as that is weight. However if the hull is light and you do have a good amount o ballast, you might end up with something that might have a violent motion, roll smapping comes to mind - I have resolved myself to the slow and slick philosophy. In a strange sort of way the solution to this might be a multihull...

who would have thought...