40 foot displacement hull cruising at 15 knots -- Impossible?

[ranger58sb]

Originally Posted by jsc7

The reason for the gas engine is that it has better power performance at higher rpms. Diesel is the reverse, generating more power at low RPMs. Diesel is used in larger boats because gas engines have a hard time pushing a heavy boat at low RPM, so simply undocking a heavy boat with a gas engine is problematic.

That doesn't quite cover it all.

Gas engines have a hard time pushing heavy boats at high RPMs, too, once the overall weight and/or hull form exceed the available gas horsepower. (40' and 20K-lbs is pushing gas engine limits, for semi-displacement and planing hulls.)

So diesel is also used in larger boats because there aren't any gas engines big enough.

And also because marine diesel fuel is generally less expensive than marine gasoline... and generally offers gives more NMPGs for the amount of fuel used (all other things being equal, which rarely happens).

Docking and undocking a heavy boat with gas engines is simply a learning experience. Can be augmented by adding bow and/or stern thrusters. Generally no big deal.

-Chris

[ranger58sb]

Originally Posted by boatpoker

I cruised this Dave Martin Designed "Trendsetter 40" for six years.

40' LOA
11' 2" beam
Single 235hp Volvo
Cruise at 10 knots using 1.6imp. GPH
Top Speed 21 knots at 7.6imp GPH

Looks like a nice boat. Kinda narrow for living, maybe... but then that's a factor in favor for its speed and fuel consumption...

-Chris

[denverd0n]

Originally Posted by 44'cruisingcat

With enough power, any boat can exceed hull speed.

This ^^^^^^^

Hull speed is NOT a "limitation." It is merely the speed at which much more power has to be applied, in order for the boat to climb over its own bow wave. Too many people get way too caught up with thinking of hull speed as some sort of hard limit. It absolutely is not.

[Swanseeker]

Originally Posted by jsc7

I have read that using a "slender" design it is possible to exceed the normal hull speed limitation (https://en.wikipedia.org/wiki/Hull_speed), but there are never any specifics or formulas that I have seen that tell by how much that speed can be exceeded and under what conditions.

If I have a design goal of a 40 foot displacement hull cruising at 15 knots is that impossible? For example, if we imagine a 40-foot boat with an 8-foot beam weighing 12,000 pounds, wave-piercing prow and a 300 HP gas engine are we getting there, or is it just not going to happen?

We had a55 ft 12ft beam which raced UK to West Australia in 1979 ...we frequently exceeded speeds of 18 knots though our windward performance was inhibited by our sail design. ....a few years later they were replace and 12 knots was obtainable to windward ..the design gave immense directional stability.

[Mithril Bham]

I have exceeded hull speed many times under sail. I think it's called surfing. Very inefficient under power.

[eheffa]

My Nordic Tug 37 is almost 40' LOA & runs quite happily at 9-10 knots with ~18-20L/Hr burn. (Little beyond displacement speeds but acceptable economy)

Push the stick & it will do 17 Kn WOT but this knocks up the fuel consumption to unacceptable levels.

This is with a full keel and a semi-displacement hull & a 380 HP Cummings diesel. The beam is a little over 11 feet and is quite seaworthy even in a beam sea.

I think this represents is a very reasonable set of compromises.

- evan

[fatherchronica]

This is a picture of my chartplotter taken when my Cape George 36 was surfing down about 15 foot breaking seas. I believe that had they been, say, 35 foot, that number would be higher. I once saw a Ford Econoline fall from a helicopter and it seemed to be going very very fast when it hit the ocean.

[Brewgyver]

Originally Posted by Chuck34

Some years ago, MacGregor made a 65 footer with a beam slightly less than 12'. Never seen one but would be interesting to know what kind of speed it could achieve.

I saw one at the Newport (California) show last month. For sale (~$120k, IIRC), owner too old and sick to use it. His friend brought it down to the show from SF Bay Area, said he AVERAGED 15 kts on the trip (the California coast had some good air that week). This one was mod'ed with a sugar scoop tail that added 5 ft. To WL length. It's probably still available.

[Adelie]

Might work on a catamaran where you can make awavh ama very thin. I think you'd need to be over 10:1 get to 15kt on a displacement hull(s). For a 40' hull that means the 4' or less beam which is useless on a monohull bite reasonable on a multihull. Also you'd need to cut displacement.

[Cadence]

Originally Posted by jsc7

I have read that using a "slender" design it is possible to exceed the normal hull speed limitation (https://en.wikipedia.org/wiki/Hull_speed), but there are never any specifics or formulas that I have seen that tell by how much that speed can be exceeded and under what conditions.

If I have a design goal of a 40 foot displacement hull cruising at 15 knots is that impossible? For example, if we imagine a 40-foot boat with an 8-foot beam weighing 12,000 pounds, wave-piercing prow and a 300 HP gas engine are we getting there, or is it just not going to happen?

Crazy!

[captlloyd]

Originally Posted by denverd0n

This ^^^^^^^

Hull speed is NOT a "limitation." It is merely the speed at which much more power has to be applied, in order for the boat to climb over its own bow wave. Too many people get way too caught up with thinking of hull speed as some sort of hard limit. It absolutely is not.

Yes, think container ship. Those buggers move right along.

[barnakiel]

This 'going faster than hull speed' in a displacement hull is only half truth.

Normal displacement hulls get unstable if pushed faster. So yes one can but the boat may capsize.

Think about hydrolift acting on the hull at higher speeds. When the shape is not flat(-ish) the result is similar to Karman action.

Watch children pulling toy boats on the beach. Too bad when one gets into boating at 53.

b.

[Cadence]

Originally Posted by captlloyd

Yes, think container ship. Those buggers move right along.

And the have a WL of 1000'+-.

[Scout 30]

Originally Posted by eheffa

My Nordic Tug 37 is almost 40' LOA & runs quite happily at 9-10 knots with ~18-20L/Hr burn. (Little beyond displacement speeds but acceptable economy)

Push the stick & it will do 17 Kn WOT but this knocks up the fuel consumption to unacceptable levels.

This is with a full keel and a semi-displacement hull & a 380 HP Cummings diesel. The beam is a little over 11 feet and is quite seaworthy even in a beam sea.

I think this represents is a very reasonable set of compromises.

- evan

Too often we see trawlers with semidisplacement hulls that lack the power to exceed hull speed. Nordic Tugs are great boats that will get you there fast if you need it but will cruise well at displacement speeds. 2 miles to the gallon is respectable for a boat that size but I'll bet you can get 2 - 3 times that if you drop down to 7 - 8 knots.

[Cadence]

Originally Posted by Scout 30

Too often we see trawlers with semidisplacement hulls that lack the power to exceed hull speed. Nordic Tugs are great boats that will get you there fast if you need it but will cruise well at displacement speeds. 2 miles to the gallon is respectable for a boat that size but I'll bet you can get 2 - 3 times that if you drop down to 7 - 8 knots.

All boats and everything about them is a compromise.