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

[ranger58sb]

Originally Posted by newhaul

I know where there are a couple meranised rolls merlins that are like new .

A Spitfire or a Hurricane would definitely make OP's trips go faster!



-Chris

[Kentb]

Assuming your are talking about a FULL displacement design...
(not hard chined, or having flatt(ish) surfaces under and aft of the engine bed area...
Yes with modern construction techniques and materials this can be done. One problem you will have with this type of design is roll in beam seas. 30-40' Semi-displacement/Semi-planning designs that cruise efficiently in the 8-10 knot range can also be powered up to help gain stability and steering when the wave direction is not optimal.
So your design (if not a cat) would have to have stabilizers of some sort to make the boat bearable in those conditions. At 15 knots, stabilizers would probably perform pretty nicely, but an expensive chunk of Machinery.
And I have to agree with thoughts on Diesel Vs. Gas.
A naval Architect would have to give some weight ranges for the power plant.
Depending on your desired cruising range, you likely would make it up in the required fuel capacity. (like needing 50% more gas than diesel for same range).

[Adelie]

Originally Posted by captlloyd

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

Poor comparison, they are going nowhere near hull speed let alone exceeding it.

[Adelie]

Originally Posted by oleman

My at sea experience has been on 26,000T to 27,500T and larger tax payer owned war ships. All of those ships are designed to cruise in the 28 KT range and a flank speed exceeding 30 KT. The real limitation on fossil burners is running out of fuel. On a nuc power surface ships the limitation is running out of rations for the crew, ship has to rendezvous with a supply ship. Every few weeks.

These ships are obviously displacement hulls, the REAL limitations are available power, crew comfort, and operating costs other other than fuel.



One ship I was on went from SASEBO Japan, to San Francisco, USA averaged 27+ knots without a single stop. That was the now decommissioned USS Truxtun(CGN-35). That ship was powered by 2 EA GE reactors rated at around 80,000 HP each with motion provided by twin steam turbines and screws.

Even on a large heavy ship bouncing across 25' seas at 32 kt is not a fun ride.

Not a great comparison for the OP.

Military vessels, most especially cruisers, tend to be very long compared to beam as a means of achieving their desired speeds. The current ABurke-class has a 7.5:1 Length:Beam ratio. The Truxton was 9.7:1.

Even today the ABurke boats barely exceed Hull speed.

The OP is not going to be happy with a 40' boat that is 4' wide.


That's why cats are becoming popular: speed, economy and living area.

[Adelie]

Originally Posted by jsc7

I have seen the Dickey boat. It maxes out at around 11 knots and has a 4.5 length to beam ratio. If we increase that to 5 to 1 and put in a more powerful engine could it reach 15 knots cruising speed?

Depends on how big the old and new engines are. You light make it by doubling engine size but probably not. Jumping from 4.5:1 to 5:1 is not a big change.

[newhaul]

Originally Posted by ranger42c

A Spitfire or a Hurricane would definitely make OP's trips go faster!



-Chris

Chris these are full blown ones out of the p51 mustang. Even better than a spit.

[newhaul]

Originally Posted by Adelie

Not a great comparison for the OP.

Military vessels, most especially cruisers, tend to be very long compared to beam as a means of achieving their desired speeds. The current ABurke-class has a 7.5:1 Length:Beam ratio. The Truxton was 9.7:1.

Even today the ABurke boats barely exceed Hull speed.

The OP is not going to be happy with a 40' boat that is 4' wide.


That's why cats are becoming popular: speed, economy and living area.

I was an engineer on a carrier and I have seen her well in excess of her hull speed . ( lwl 1070 weight 100,00 long ton hull speed 43 kts saw in excess of 60 )

[Cadence]

Comparing container and military ships to the OPs question?

It sounds as if he wants a semi displacement boat. He as to realize it won't make the displacement speed or economy as a full displacement hull but can get on top of the water and make the 15knots he wants.

[Siberian Sea]

My Cal-40 can exceed hull speed often surfing down waves, matter fact it has gone over 20kts

[ranger58sb]

Originally Posted by newhaul

Chris these are full blown ones out of the p51 mustang. Even better than a spit.

Woof! even faster!

-Chris

[sandy stone]

OK, I dug out my copy of Dave Gerr's book. He describes a WW2 vintage boat (the Lulworth) 40' long, 8' beam, 3.7 tons displacement, capable of 36 knots with a 275 hp motor. He says it was a "yachty cabin cruiser."

[newhaul]

Originally Posted by ranger42c

Woof! even faster!

-Chris

How about a couple Packard 5m 2500's

[ranger58sb]

Originally Posted by newhaul

How about a couple Packard 5m 2500's

Hmmm... that'd likely work, but maybe OP could buy 3 yachts for the price of one pristine Packard.

?

-Chris

[newhaul]

Originally Posted by ranger42c

Hmmm... that'd likely work, but maybe OP could buy 3 yachts for the price of one pristine Packard.

?

-Chris

They worked awesome for our PT boats in WW2.
Also I didn't see any mention of money constraint just fuel constraint.

[valhalla360]

It's a myth that you can't generate big HP out of gas engines.

Diesel took over trucking and other commercial operations due to cost efficiency. 25-30% better MPG (or equivalent measure) plus until just recently, the fuel was significantly cheaper per gallon. If you are running a trucking company buying 50,000 gallons per year, that's a big cost savings.

Plane engines were a hold out because they needed to be light weight relative to power output but once planes switched over to jet engines, the need for light weight high HP gas engines largely went away.

Large recreational boats just don't use enough engines so they get whatever is being produced for the commercial market.

The result is there has been almost no development into gas engines in excess of around 350-400hp.

But consider the Ford ecoboost V6 that puts out around 350hp. Use the same approach with a V10 and reliable HP approaching 600hp should be very doable but there just isn't a demand for that engine.