Hull speed.

I don't know the theory, but I know a man who does.
Steam launches are normally long and slender to make most efficient use of their power in the calm lake and river waters.
Mosquito, on Windermere is capable of at least 17 knots yet at 46 feet should according to the calculator do 9 knots.
She is 46' x 6' 10" beam.

http://www.steamboat.org.uk/register/html/mosq0396.htm



LakeSept30_13.jpg
 
I think the 1.1, 1.2 and 1.4 factors are very interesting and a lot better than the "mine does this or that" answers. I will work out the figure for my boat.
I suggest you start a new thread with your question as this one has fallen onto page two of the forum.
Allan
 
Sorree...got to totally disagree! /forums/images/graemlins/mad.gif Log book shows that for a period of 3.75 hours, July 2002, co.135% Long Sand Head/ Garden city. average speed: 8.3 knots......Had the benefit of of a self reefing spinnaker! /forums/images/graemlins/grin.gif....Na nicky na an! /forums/images/graemlins/laugh.gif
 
This is approaching the problem from the wrong end. The max hull speed of a displacement boat is a function of its waterline length and the 1.4 * root of W/L in feet is a good estimate. This reflects the distance between the bow and stern wave at that speed. Once you try to go faster than that the power requirement rises rapidly as you are "climbing uphill". Therefore a boat should be powered to achieve that speed at its maximum power output. The actual engine power required is then a function of weight and propeller efficiency, which in turn is affected by the maximum diameter propeller you can swing and the shaft speed. The latter is determined by the maximim revs of the engine and the reduction gear.

My Bavaria 37 with a WL of 29 feet achieves its hull speed of approx 7.6 with 29 hp with a 2.6 to 1 and a 16inch prop at max revs of 3600. Perfect. More power, for example with the 40hp option will not increase maximum speed significantly, but may allow lower revs for same speed. Weight of course has an effect, so if I loaded my boat by, say 1.3 tonnes - typical of crew of 4 and all gear for a long passage, maximum speed would fall to about 7. A bigger engine would offset that.

So, the starting point for deciding on engine power is waterline length, displacement and maximum size of propeller. Put in your chosen engine power, revs and reduction and see if it can achieve hull speed. I am ignoring at the moment the propeller as clearly a 2 blade is less efficient than 3 blade, so you have to make that trade off as well.

The different factors suggested by Nordhavn only reflect how we actually use our engines. 1.1 - economical cruise for my Bavaria is 5.9 knts which I achieve at about 2600.

If you are choosing an engine for a "motor sailor" the chances are that the boat will be heavier than a pure sailing boat of the same WL which explains why they usually have slightly larger engines. A larger engine may well also allow a larger diameter prop which will improve efficiency in heavy weather.

If you want to play with these variables yourself suggest you download the PropCalc software from Castle Marine. All will then become clear(er).
 
There was a comment a little earlier saying that as I use lower revs I am not operating efficiently. I have had petrol, diesel and turbo diesel cars and have found with all of them running at low revs uses less fuel. I know it proves I am sad but with all of them I did at least one tank running at very low revs apart from on motorways. It felt bad always being at least one gear too high all the time but for all cars it used less fuel.
Allan
 
I suggest you look at the specific fuel consumption graphs. Fuel consumed is almost directly a function of power used. So your "50hp" engine running at 1800 revs and producing roughly 24hp to power your boat at 6.4 knots will use almost exactly the same fuel as a "35hp" running at 2600 - and producing 24hp!

The anology with cars is spurious. Your fuel consumption is only lower because you are using less power at lower revs - and this only works if the engine is large enough to produce the power at lower revs. Good example would be my C Max 2 litre diesel which cruises at 80 and 2100rpm in 6th gear. Compared with a 1.8 which achieves the same speed in 5th (top) gear at about 2600 rpm. Power being used is exactly the same - but the smaller engine needs higher revs to achieve it. Fuel consumption will be similar. So why did I buy a 2 litre with the extra expense? Because it is less affected by weight and the lower revs means it is a bit quieter. However one pays if you use the higher performance potential.

Back to your boat. You gain nothing by having an engine that is too big as you do not have gearing to reduce the excess power at the lower end so you get excess speed at tickover. You will never be able to use more than 30 of your notional 50hp because that is all that is needed to achieve hull speed. The engine will never operate at its most efficient range - lowest specific fuel consumption is probably around 2600rpm.

I know your boat originally had a "42hp" engine, but IIRC it actually only produced 38 continuous - and it was overpowered. Westerly were going through a phase of overpowering boats because higher powers had started to become available at lower prices, but it did not last long. Compare 23hp in a Centaur with 20 in a Konsort for example.
 
when i removed the Perkie 4108 rated @ 40hp @ 3000RPM
i could have got away with the Nanni 29hp but chose the Nanni 37.5Hp as it almost matched the out put of the 4108.

both the 29 & 37.5 have the same fuel consumption
the 29 rated @ 3600 RPM far too high 4 me
the 37.5 rated @ 3000RPM
we can cruise @ 7Kts @ 2600RPM or 6.7 Kts @ 2000RPM
 
But you are not overpowered! You are working within the operating range of your engine. Don't understand your statement that the "fuel consumption" of the 29 and 37.5 are the "same". I expect the specific fuel consumption - that is the amount of fuel consumed to produce hp/kw is similar, but the fuel you consume is a function of the power you use, irrespective of how much the engine is capable of producing. As I explained earlier you can get the speed from a lower rated hp engine at higher revs, but the fuel consumed will be notionally the same.
 
The comment about running cars at a higher gear would be true about petrol engines. Lower RPM with wider butterfly openings would give you higher BMEP in the cylinders and greater efficiency. Not true of diesels (or probably of turbo petrol)
A
 
[ QUOTE ]
You will never be able to use more than 30 of your notional 50hp because that is all that is needed to achieve hull speed.

[/ QUOTE ]

Is this really so?

I understand that rough water knocks back progress, requiring more power to maintain a given speed than would be the case in flat water.

What about the use of a Brunton autoprop? That apparently adjusts its gearing according to conditions.

If a 33' Nauticat has a 75 hp engine installed, for example, why should a 33' Westerly, at say 6 tons rather than 8, not sensibly have a 50 hp engine?
 
I had a Laguna 2 (a very early one) when I lived in France. It had a "trip computer" which gave, among other things, a constant readout of the fuel consumption. I didnt have complete faith in it. On both a long flat road and a slight uphill, using cruise control to maintain various speeds, the figure always reduced as I changed up. Apart from boredom, the reason I did this was to prove what I thought about fuel consumpion, that is exactly as has been said above. Unfortunatly I beleive I was proved wrong. The car was turbo diesel and, I think, 2.2ltr.
Allan
 
Sorry if I was not clear. Some of the comments above say I am not using my engine efficiently. I am interested to hear how to work out the most efficient way to use it. Trebling the fuel consumption to double the power surely can't be the way?
Allan
 
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