Semi Displacement at displacement speed

[Joakim]

Originally Posted by FAST FRED

"However if it takes 30 hp to push your boat to 8 knots how much will fuel use vary between a motor with a rated hp of 30 vs a motor with a rated hp of 200?"

At the least I would guess double the fuel burn for the 200 hp producing 30 hp.

Diesels LOVE to work hard 30 hp might nor enough to even get the 200hp engine to operating temperature , and it surely would suffer from under loading , slobbering exhaust , burnished cylinder bores , oil dilution and stick piston rings.

At least for Yanmar, your guess is wrong. Yanmar 3JH5E (39 hp) delivers 30 hp (22 kW) at about 2800 rpm (on the propeller curve) and takes about 6.5 l/h (1.7 GPH). Yanmar 6BY2 (220 hp) delivers 30 hp at about 1800 rpm and takes as close to the same 6.5 l/h as I can see.
http://www.yanmarmarine.com/theme/ya...t/3JH5E_HR.pdf
http://www.yanmarmarine.com/theme/ya...t/6BY2-220.pdf

When do you expect to get all these troubles of low load? How do 200 hp car diesels survive 500 000 km (at least 10 000 hours) without any of these problems and also provide extra heat to warm up the car? Only 20-30 hp is needed for maximum legal highway speeds on normal cars.

[FAST FRED]

"When do you expect to get all these troubles of low load?

" How do 200 hp car diesels survive 500 000 km (at least 10 000 hours) without any of these problems and also provide extra heat to warm up the car? "

"Only 20-30 hp is needed for maximum legal highway speeds on normal cars."

Cars frequently are called to accelerate , few will run hundreds of miles or hours at low load.

Larger industrial sourced diesels are matched to their loads frequently 90% of rated RPM with 80% of rated load.

That car engine with 80% of its rated load might not last 300 hours.

Car engines NEED to be at 30 hp for much of their lives.

FF

Originally Posted by FAST FRED

"However if it takes 30 hp to push your boat to 8 knots how much will fuel use vary between a motor with a rated hp of 30 vs a motor with a rated hp of 200?"

At the least I would guess double the fuel burn for the 200 hp producing 30 hp.

Diesels LOVE to work hard 30 hp might nor enough to even get the 200hp engine to operating temperature , and it surely would suffer from under loading , slobbering exhaust , burnished cylinder bores , oil dilution and stick piston rings.

Twin engines on one shaft is easy as pie.

The USN (as well as other Navies) have dual engines turning one shaft on many work boats LST , that sort of boat.

These engines can work together or either one can be disconnected , under way.

Since its pure mechanical , both engines must be secured to pust the lever to re engage them. Series 1 bell housings are the norm.

FF

Please learn to quote!!!

It is a pita to read your contribution here. (apart from the fact, that it rarely is a pleasure)

Regards
Richard

[FAST FRED]

"

Is the Quote mark that is commonly understood in the English using world.

FF

[u4ea32]

Originally Posted by HowardH

How well will a semi-displacement hull work at displacement speed? What would one give up in the way of performance and efficiency at displacement speeds with a semi-displacement hull?

A boat to be primarily operated at displacement speed yet with the ability to double or more it's speed if the need arose is appealing.

At displacement speed, a semi-displacement or planing hull consume about twice the fuel as a displacement hull. Lots and lots of data support this figure.

However, there are many reasons that semi-displacement hulls are still popular, and owner's quite happy with their performance, at displacement speeds, including:

• Low fuel flow at low speeds anyway, so the cost to cruise is relatively low regardless of hull design
• fat, bouyant stern with immersed transom provides additional flotation for stuff aft, like friends in the cockpit, dinghy on davits
• Sometimes you might want to go faster than hull speed. At 1.5 times hull speed, a semi-displacement hull may use 20% as much fuel as a displacement hull, and at those speeds, lots of fuel flow, so its VERY expensive to make a displacement hull go fast.

Therefore, you may read the inaccurate but perfectly applicable remarks that semi-displacement hull forms are fine at displacement speeds, because twice as much of a little is no big deal when compared to 5 times as much of a lot.

In other words, the advantage of a displacement hull form is a small amount of money at displacement speeds. The advantage of a semi-displacement hull form at 1.5x hull speed is a LARGE amount of money.

The cross over point is hull speed: 0.4 Fn or 1.34 SL. Below that, designs with the transom clear of the water and round bottoms are twice as efficient as other designs. Between 0.4 Fn and about 1 Fn (or 1.34 SL to about 3 SL) the semi displacement hull has an advantage over planing hulls that starts at about 25-30% and diminishes to zero at about Fn of 1.

Published sea trials should of course be viewed somewhat skeptically, but tend to show that there is no real difference in semi-planing and planing hull forms.

Sounds like displacement hulls are a waste of time, right? Well, actually they are really good if you always go slower than hull speed. You may get 5 nmpg rather than 2.5 nmpg. Won't notice this on a weekend, but if you take a month or longer for a cruise, it adds up to a significant savings.

I understand the requirement for a more powerful motor but if fuel consumption is based upon horsepower produced; a larger motor, turning slower, should have minimal increase in fuel burn at displacement speed.

20 hp is 20hp and every diesel will have similar gph numbers at 20hp regardless of what peak hp is correct?

Published figures by vendors support your observation. Small diesel engines really are not all that efficient. Efficiency is improved rather dramatically by going with a turbo charger, and more with an intercooler. Small engines don't have these, larger ones do. Also, electronic controls beat the hell out of mechanical if you need the engine to run well at a wide range of speeds and loads. The old mechanical engines are efficient in surprisingly narrow range of RPM and load.

Lots of good reasons to have those old mechanical diesels, but efficiency at widely different loads ain't one of them.

[Joakim]

Originally Posted by u4ea32

At displacement speed, a semi-displacement or planing hull consume about twice the fuel as a displacement hull. Lots and lots of data support this figure.

The cross over point is hull speed: 0.4 Fn or 1.34 SL. Below that, designs with the transom clear of the water and round bottoms are twice as efficient as other designs. Between 0.4 Fn and about 1 Fn (or 1.34 SL to about 3 SL) the semi displacement hull has an advantage over planing hulls that starts at about 25-30% and diminishes to zero at about Fn of 1.

Published sea trials should of course be viewed somewhat skeptically, but tend to show that there is no real difference in semi-planing and planing hull forms.

Is there some clear definition for a semi-displacement hull? Would a modern sailboat hull be a displacement or a semi-displacement hull? They have transoms clear of water very low resistance at displacement speeds and still quite low resistance at Fn=0.6.

Planing hull form (=prismatic hulls) can have very different resistance at the semi-displacement region depending on the hull form and weight distribution. I guess you can come close to semi-displacement hulls, but then the boat will not work well at higher speeds.

[FAST FRED]

"Lots of good reasons to have those old mechanical diesels, but efficiency at widely different loads ain't one of them."

True enough , but most full displacement cruisers will match the engine and load , so except for docking or some "no wake zone" an old repairable mechanical engine will be safer and cheaper to operate .

It doesn't take many $5000 to $10,000 control boxes or sets of $1000 injectors to make up for 5 decades of "inefficient" cruising.

The ability to start after being near a thunderstorm?

PRICELESS!

FF

[DCockey]

Originally Posted by u4ea32

At displacement speed, a semi-displacement or planing hull consume about twice the fuel as a displacement hull. Lots and lots of data support this figure.

There is considerable variation in hull shape and characteristics within hulls categorized as "displacement", even if limited to round bottoms and no immersed transom. Consider a modern, light sailboat and a fishing dragger for example. Similar variation exists within "semi-displacment" and "planing" hull types.

Can you be more specific about the hull characteristics involved in the comparision you make above. By semi-displacment hull are you refering to a "trawler" style yacht or one similar to Nigel Irens efficient designs? Do you mean to imply that a semi-displacement hull uses about the same fuel as a planing hull at displacement speeds?

Also, where can I find the data?

[Pierre R]

There is no clear delineation between semi displacement and displacement hulls where one can simply provide data. You can if you want, make a distinction between semi displacement speeds and displacement speeds by choosing something like theoretical hull speed as the cut off but that still does not translate into data on hulls.

Whether a boat acts as a semi displacement hull or a displacement hull has to do with the shape of the underwater lines of the boat, the displacement and overall dimensions. There is still a bit of an artform in designing boats when it comes to underwater lines.

I think the easiest way to find out if a boat is semi or full is to simply ask the designer or read what his intent was. The worst place to find out is to ask a broker.

There are some things that can be said about either hull type. One is that you will pay a bigger penalty in overall costs trying to take a full displacement hull into the semi displacement territory than you will taking a semi displacement hull into the displacement territory. Either way, you pay a penalty and must decide which poision you will take. For most people, semi displacement is a better tradeoff.