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

[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?

[Training Wheels]

I'm no engineer or naval architect, but I think you'd need a l/b of 8/1, maybe even 10/1. And that's waterline measurements. Note, this is a WAG. Think you'd have to get the displacement down, too.

[TJ D]

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?

Well, a narrow waterline beam does a lot to reduce the amount of power needed to overcome the bow wave.
So, I think that a boat with your dimensions and weight would probably be able to exceed hull speed under optimum conditions, even under sail.
However, there's been a pretty marked tendency to go to more wide, flat designs that lend themselves to planing.

There have not been very many extremely narrow designs which have been competitive in quite a long time. I think that if you want to go for speed, you're better off with a planing hull, frankly.

Also, an 8' beam on a 40' boat would make a pretty terrible boat to try to live on, even for a short time, eh?

As far as the formulas go, that's out of my depth, but certainly, going narrow can help with exceeding hull speed.

TJ

[sandy stone]

Suggest you get a copy of Dave Gerr's 'The Nature of Boats'. He talks about this kind of stuff (as well as a lot of other things). If I get a chance I'll dig out my copy.

[44'cruisingcat]

With enough power, any boat can exceed hull speed. With a fat hull you basically need enough power to push the boat up it's own bow wave.

Slender hulls effectively don't have a hull speed. If they're slender enough, 8:1 or higher, they'll only generate very small bow waves.

[Lepke]

The gas engine is a bad idea above about 30 feet. If money is the issue, there are many good used diesels around that would be better than any gas engine. Probably best economy with an older big diesel with a big prop. 36" or more. I have a narrow boat 83x17 and is cheap to run at 10 kts. I had a commercial fishing boat 55x12 that was built as a rum runner and also cheap to operate @ 7kts. 15 kts is probably unobtainable on 300 gas hp and would use 6-10 gallons an hour. Narrow boats really limit your cabin design and comfort. Easy to tip over, too. I did 3 90° rolls in the fishing boat. And I do mean 90°.

[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.

[jsc7]

Originally Posted by Lepke

The gas engine is a bad idea above about 30 feet. If money is the issue, there are many good used diesels around that would be better than any gas engine. Probably best economy with an older big diesel with a big prop. 36" or more. I have a narrow boat 83x17 and is cheap to run at 10 kts. I had a commercial fishing boat 55x12 that was built as a rum runner and also cheap to operate @ 7kts. 15 kts is probably unobtainable on 300 gas hp and would use 6-10 gallons an hour. Narrow boats really limit your cabin design and comfort. Easy to tip over, too. I did 3 90° rolls in the fishing boat. And I do mean 90°.

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.

With a light boat gas engines make sense, at least that was my thinking. The length does not really matter, it is the weight of the boat that is significant.

[Scout 30]

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.

With a light boat gas engines make sense, at least that was my thinking. The length does not really matter, it is the weight of the boat that is significant.

Your thinking of the old technology diesels. These days marine diesels typically redline at 3,400 rpms plus and don't have anywhere near the torque that they used to have. Catamarans are a perfect example of slender hulls exceeding hull speed. However, there's no free lunch here. To achieve high speeds you have to keep the weight down. I think a downeaster (fine entry, full keel, soft chine, flattening out at the stern) hull type would be your best option if you want to cruise at 15 knots without using a lot of fuel.

[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

These are my actual numbers, This yachting magazine had slightly different numbers and different tankage.

[Be Free]

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.

According to MagGregor Yachts, a 65 “averaged over 10.5 knots for 1150 miles, in generally upwind conditions, and hit top speeds of over 25 knots” in the Los Angeles to Puerto Vallarta race.*

http://www.jordanyachts.com/archives/2739

[SV Siren]

Bob Perry designed and had built what he called his Sliver Project, "Francis Lee". A 65' double ender, with a VERY narrow beam. I have not found a top speed posted in a quick internet search, but from what I have read that thing is lightning fast.
Bob Perry-designed double-ender Sliver

[barnakiel]

You build it either long and narrow and it will scream then. Or else build it flat and it will plane.

You must be from Texas if you want to push water about.

Cheers,
b.

[Scout 30]

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

These are my actual numbers, This yachting magazine had slightly different numbers and different tankage.

That's an interesting hull design. It looks like a lobster boat in the stern with it's soft chines & it's flat planing surface which will help greatly with fuel efficiency but forward it has hard chines which will reduce roll & spray. I prefer running the keel all the way back with a shoe under the prop just to save on prop repairs & keep her tracking straight at displacement speeds or in a following sea but you can't argue with these fuel burn numbers. Pretty impressive & indicative of what you can expect with a downeaster type of hull. Very nice boat.

[valhalla360]

All boats operating in displacement mode have a hull speed. The confusion is the old formula is a simplification.

The issue is the 1.3-1.4 at the beginning of the equation assumes a length to width ratio of around 3-4 to 1, which is typical for monohulls, so it works well when talking about typical monohull designs. As the ratio increases, the factor rises very quickly and thus you have theoretical hull speeds increase very quickly. Eventually, you reach a point where skin friction exceeds wave making as the primary drag. Hull speed is still present but it becomes much less of an issue.

A 3' beam 40' boat with a really deep keel should go like the dickens in a good breeze but as a cruising boat it's not of much use. Instead, do two 3' beam 40' hulls and make a catamaran and you have a different ballgame. This is why you see high performance cruising cats with cruising speeds in the teens.

Note: Once you start talking about surfing and planning it's a totally different discussion.