The perfect 10 knot boat/Lowest Cost per Mile?

[RHough]

I've read many posts here and not found a thread on the total cost per mile for motorsailers compares to either pure power boats or pure sailing boats.

At the large end of the scale there is Mirabella V ... 200ft and 795 tonnes. No doubt a great sailboat 18 knots under sail and 14 knots at S/L = 1. She also has 2 1,000 HP engines.

She makes her passages under power ... because sailing her is more expensive!

It seems to me that a good design would return the lowest cost per ton/mile.

Why add sails to an efficient power boat if the net running costs are not lower?

Pure sailing vessels have operating expenses too. If the rig has to drive the boat in all sorts of weather a selection of sails must be on hand. The sails and rigging are an expense. At 10 knots the rigging has a 3000 hour service life, the sails perhaps 1000 hours.

It would seem that not having a huge rig for light air power by using the engine for propulsion would save money.

Does motorsailing ever result in lower overall costs? Can the sails save enough fuel to justify the added expense?

It would seem that a motorsailer and modern power assisted sail handling systems would make an ideal passage maker, are these boats limited to displacement speeds?

If the goal was a nearly all weather 10 knot boat, for the lowest cost per mile, would it have sails or not?

How big would it be?

[Knut Sand]

Aaaannd here comes one of my favorites (linked to on many an occasion....):

http://www.setsail.com/dashew/do_paradigm.html

:D

Edit... But initially I would expect that sails would be cheaper, if you do not expect 100% performance/ stiffness out of them, accept to repair from time to time, replace them when _really_ worn out.... The boat linked to, have moderate engine installations, compared to the size of the boat, which again give less cost per seamile.

[lazeyjack]

sailing is not cheap, , yes when you have finished the high cost of the build it become so, gpm, but 200000 k of above deck gear, masts, sails rig, winches on a 55 ft boat buys a lot of fuel
here is a site
we have just finished new design same principal different use see the economy
http://www.boattest.com/oem/general-info.aspx?ID=476&lp_id=295#OverView
the red boat is the model, successful, that we based our idea on, the render is by bhnautika, an idea by myself and him
hope to go into production soon
Mirabella, economical for such a size
you take a Riviera, aust design, 55 foot, twox1015 shp cats, 390 litre per hour at rated hp, 5000l tanks, , in my view ought to be banned, using more than their fair share of worlds resource

[northerncat]

cant go past a 40 ft displaning powercat in my humble op, peter kerr is cruising around the whitsundayas in one of his new cats designed around this concept from all accounts he gets 20 knots flat out 14 on one motor and economical cruising at around 12 -14 knots
sean

[Tad]

There is a huge thread somewhere here on sail vs power and which costs more per mile. What the argument boils down to is, "what is the design criteria?"

Adding a modern high-tech aluminum or carbon spar, and dozens of expensive rigging and deck fittings, the structural reinforcements as required, plus a keel and rudder up to the job, will never pay off. Except when the cost of diesel goes to perhaps $100 a gallon! (wild guess)

But you can turn down a telephone pole to make a mast, and sew a lug sail out of plastic tarps, and the payback will start very quickly.

The most efficient 10 knot boat might be a pure power trimaran. The main hull will be perhaps 100' long and 10' wide, ocean crossing weight will be 65,000 pounds, in flat water with no wind she would make 10 knots with 58 HP. About 2.7 gph.

[TeddyDiver]

If the goal was a nearly all weather 10 knot boat, for the lowest cost per mile, would it have sails or not?

What's the hurry .. I mean if there's time limit's so why not to fly to destination and stay in a hotel..

Seriously.. 10 knot boats aren't motorsailers...

[the1much]

sails are only more economicle if the winds blowing.
Lazyz,,,i like that first pic.,, reminds me of the mail boats we had in maine. :)

[RHough]

There is a huge thread somewhere here on sail vs power and which costs more per mile. What the argument boils down to is, "what is the design criteria?"



I thought I had read that. I did a few searches and must have missed it. I don't like starting new threads if I can learn from existing ones. :(

To TeddyDiver,

What's the hurry? LOL!!!!

10 knots is pretty slow. If a passage is planned, there is a trade off in weight of stores and fuel. If the extra weight of provisions exceeds the weight of the fuel to get there sooner, you might as well drive the boat faster. ;)

Speed is also a safety factor. Weather routing is easier at higher speeds for two reasons, shorter passages do not require as long a range forecast and higher speed gives more escape options. 10 knots vs 8 knots is more than a 50% gain in area you can cover in 24 hours.

The idea that the costs of the rig and sails are never recovered wold seem to be based on economical cruise speed under power ...

The goal is 10 knots cruise. That is a S/L ratio of 1.5 on a 44.5 ft LWL.

If the fuel usage is based on that, is should be much higher than at best economy in displacement mode. Now the cost of the rig is used to offset the extra fuel required by the speed.

The total cost per mile has to include more than just direct operating costs.

Allow me to change the question.

The design brief is 3000 miles per year at 10 knots for the lowest total cost. Range is to be 3000 miles on trade winds routes (average wind 14-15)

The cost of the boat is factored in on a 10 or 15 year depreciation schedule.

10 knots at S/L = 1 means 100ft LWL

Will a 100ft boat burn less fuel at S/L = 1 than a 45 ft boat at S/L 1.5?

The 100 ft LWL boat might cost 11 times the 45 ft LWL boat.

At $5 per gallon how many miles will it take to make up the difference in cost?

Could a 45 ft LWL boat carry enough fuel to have 3000 mile range @ S/L 1.5?

Will the choice of trade winds routes with higher speed and more reliable wind make having a good reaching rig worthwhile?

If someone has already looked at this, I apologize ... just point me to the information. :)

[FAST FRED]

That SL of 1.5 on a sail hull will require tons of power , even if the boat is very skinny.

Simply make the boat longer , but NOT wider and higher , and plan on a SL of .9 under power for low fuel use.


A conventional old fashoned alluminum mast and SS rigging is well proven , and perhaps could be found used .

The Parachute kite folks can also make an argument , but I'm not sure how reliable it would be all night with the SS driving.

A 100 ft lwl boat does NOT have to cost 11 times a 50 ft boat, tripple maybe , but that's it IF its the same beam and hight.

FF

[RHough]

That SL of 1.5 on a sail hull will require tons of power , even if the boat is very skinny.

Simply make the boat longer , but NOT wider and higher , and plan on a SL of .9 under power for low fuel use.


A conventional old fashoned alluminum mast and SS rigging is well proven , and perhaps could be found used .

The Parachute kite folks can also make an argument , but I'm not sure how reliable it would be all night with the SS driving.

A 100 ft lwl boat does NOT have to cost 11 times a 50 ft boat, tripple maybe , but that's it IF its the same beam and hight.

FF

Before I crunch numbers to check ...

Dave Gerr has a revised "Hull Speed" formula:

The formula is: V = 1.24*L^1.433 / D^0.311

V= Speed Knots
L= LWL feet
D= Displacement pounds

His formula gives S/L = 1.34 at a D/L of about 350

For D/L below 350 the Gerr formula gives a higher number for displacement limited speed.

.9 S/L is 10 knots on a 69 ft LWL
1.34 = 11.1
As it happens 10 knots is 90% of "hull speed"

At D/L = 100 Gerr predicts 16.4 as the limit. 90% of that is 14.7 or S/L 1.32 ...

Any idea how the D/L effects the fuel curve? Gerr's formula suggests that the near linear part of the drag curve extends past S/L = 1.34 for lighter hulls.

Just so we all understand my scale for economy ... the boat I take delivery on in December gets almost 1 MPG at cruise speed of 29 knots, and at 30,000 lbs and 850 hp, she is considered an economical boat. :(

I suspect that no hull designed to run S/L 1.5 as "economy cruise" will sail worth a damn. And no sailing hull will be effecient under power above S/L = 1

Just kicking ideas around ...

Mac65 with a 200 hp engine ... looks close? Cabo to Hawaii at 10 knots for 600 gallons?

[TeddyDiver]

What's the hurry? LOL!!!!

10 knots is pretty slow. If a passage is planned, there is a trade off in weight of stores and fuel. If the extra weight of provisions exceeds the weight of the fuel to get there sooner, you might as well drive the boat faster. ;)

Speed is also a safety factor. Weather routing is easier at higher speeds for two reasons, shorter passages do not require as long a range forecast and higher speed gives more escape options. 10 knots vs 8 knots is more than a 50% gain in area you can cover in 24 hours.

The idea that the costs of the rig and sails are never recovered wold seem to be based on economical cruise speed under power ...

:D Well it's so slow in the world of aeroplanes that no harm done if it's halfed down once more.. I'm counting on continous 5 knots with my ms (hull speed 7.5) just becouse you can't go straight and there's allways something to see. (taking a dive with dolphins..) More than a safety factor I consider speed more an uncomforty factor and believe having the boat and it's crew being better prepared against the weather when running away isn't the main strategy.
And maybe the most important reason.. Building a boat alone takes a lot of time and with limited funds ones gotto accept certian limitations, like moderate speed:cool:

[Tad]

Adding length is the cheapest speed you can buy.

The 45' boat will have to weigh 30,000 pounds, be 11' wide, and will require at least 90hp at the prop (in flat water, no wind, or current) to manage 10 knots. That's about 4.4gph, or a minimum of 1320 usg for the 3000 mile voyage.

Make the hull 60' long, S/L of 1.34 at 10 knots. Weight can go up to 40,000 and beam drop to 10', with required power at 76hp for 10 knots. That's about 3.7gph, or 1100 usg for the voyage.

Say the sail rig knocked 20% off the fuel consumption, about 220 gallons less (per year) for the 60' boat. At $5/gal that's $1100 per year or $11,000 over 10 years. The addition of a sail rig may or may not cost more than $11,000.

[Tad]

I suspect that no hull designed to run S/L 1.5 as "economy cruise" will sail worth a damn. And no sailing hull will be effecient under power above S/L = 1



Both statements above make no sense. Your suspicions are incorrect. There are far too many variables involved to make such blanket declarations.

Certainly many (most?) sailing hulls can be powered to S/L 1.34 at better than 1mpg (your scale).

How does one evaluate "worth a damn" as a measure of performance. TP 52's and VOR 70 hulls are optimized for far higher speeds than S/L 1.5. I would suggest these hulls would make great performance motorsailers, but of course there are some other considerations.

[RHough]

Both statements above make no sense. Your suspicions are incorrect. There are far too many variables involved to make such blanket declarations.

Certainly many (most?) sailing hulls can be powered to S/L 1.34 at better than 1mpg (your scale).

How does one evaluate "worth a damn" as a measure of performance. TP 52's and VOR 70 hulls are optimized for far higher speeds than S/L 1.5. I would suggest these hulls would make great performance motorsailers, but of course there are some other considerations.

Sorry ... yes ... the statement was not qualified and I should have either shut up or thought it out ... :(

[FAST FRED]

"The addition of a sail rig may or may not cost more than $11,000."

By NOT requiring a fully found sailing vessel, giving up the ability to "beat off " the proverbial lee shore in a gale , sailing gear becomes CHEAP.

The mast you require are found holding lamps by the side of the road , or if you need really tall ones at the airport. Since there heavy wall aluminum far less rigging would be required .

The old square sail was quite efficient ( Nelsons tacks were 90deg) especially if simply use for reaching and running.

To simply cut fuel use and do away with a hugely expensive and complex stability system, a square sail rig would do fine.

So long as its considered the secondary propulsion , although it would be a fine get home.

Biggest added expense would be for a controllable pitch propeller ,CPP , at the newbuild stage to actually take advantage of the sails.That might be $10.000 but would help 100% of the time under power OR sail.

FF

[RHough]

"The addition of a sail rig may or may not cost more than $11,000."

By NOT requiring a fully found sailing vessel, giving up the ability to "beat off " the proverbial lee shore in a gale , sailing gear becomes CHEAP.

The mast you require are found holding lamps by the side of the road , or if you need really tall ones at the airport. Since there heavy wall aluminum far less rigging would be required .

Biggest added expense would be for a controllable pitch propeller ,CPP , at the newbuild stage to actually take advantage of the sails.That might be $10.000 but would help 100% of the time under power OR sail.

FF

Good points.

I question the propeller. I don't see a need for variable pitch. Feathering/folding yes, but not controllable as in constant shaft RPM. What benefit do you see that I don't?

[brian eiland]

...The most efficient 10 knot boat might be a pure power trimaran. The main hull will be perhaps 100' long and 10' wide, ocean crossing weight will be 65,000 pounds, in flat water with no wind she would make 10 knots with 58 HP. About 2.7 gph.

The Parachute kite folks can also make an argument , but I'm not sure how reliable it would be all night with the SS driving.

How about an 'economy model' of this kite-assisted power trimaran?
New Age Trawler/Motorsailer; Kite assisted PowerYacht
http://boatdesign.net/forums/showthread.php?t=20319

Back to those design concept drawings by Humphrey’s (attached PDF). I can fully imagine a scaled-down version of this tri-hull design, in the 65-70 foot range, with a wonderful rear ‘swim’ platform deck incorporating a sportfishing arrangement and/or a Scuba diving platform. Just inside could be a tender stowage as shown; or rather a complete diving & fish tackle facility. The tenders could then stow up on rear deck, or one forward, one aft.

The power would be a single main engine sized to develop the vessel’s desired top speed, and it could transmit this power by conventional shaft/prop arrangement, or with a azimuthing Volvo IPS dual prop unit, or via a retractable azimuthing Rim-Drive prop unit, or a simple chain/belt drive unit....

[TeddyDiver]

I don't see a need for variable pitch. Feathering/folding yes, but not controllable as in constant shaft RPM. What benefit do you see that I don't?
The point I see in this you can get the just that constant shaft RPM assisted with the sails and the max fuel effiency.
Or when desirable the max speed like running away Gustav;)

[FAST FRED]

"Feathering/folding yes, but not controllable as in constant shaft RPM. What benefit do you see that I don't?"

ALL diesels like to run heavily loaded for longest life and best efficiency, regardless of the RPM chosen.

With a CPP and sail assist you can set advance speed , observe the EGT and optimise the load easily by cranking the pitch.

Weather the sail is making 90% of the propulsion or none, the engine can be run loaded , its best condition.

This works for all engine speeds , so when powering into head seas , that too can be done at highest efficiency.

The difference between a well loaded engine and a poorly loaded unit making the same HP cam be a 60% or greater increase in fuel consumption.

FF

[fcfc]

The engineering, manufacturing and material costs of a super efficient boat would cost MUCH MUCH more $$$$ than you ever spend in fuel bills in a normally efficient current boat :P

Current boats are already fully optimized for their intents. The manufactuer can earn some money building them, the customers can afford to buy one, use them, and get some enjoy using them.

If you build a technically super efficient boat, say like solar powered car, but bring the same reward to customers using them as solar cars are useful to day to day drivers, you will just loose a big bunch of money.

People do reduce their fuel bills and costs. Instead of buying a V8 gas powered ski boat, they just buy a PWC. Not an ultra slow wet narrow rowing skiff converted to power that could keep them water skiing.


You spoke of 3000 nm at 10 kts at lowest cost. OK. So the safety and comfort provided by ocean rowing skiff or minitransat is acceptable. No other constraints.

[brian eiland]

The engineering, manufacturing and material costs of a super efficient boat would cost MUCH MUCH more $$$$ than you ever spend in fuel bills in a normally efficient current boat :P

Current boats are already fully optimized for their intents. The manufacturer can earn some money building them, the customers can afford to buy one, use them, and get some enjoy using them.

If you build a technically super efficient boat, say like solar powered car, but bring the same reward to customers using them as solar cars are useful to day to day drivers, you will just loose a big bunch of money.
First off let me say that boats are not often good 'investments' as far as return on invested amount...but then again that is not necessarily why we buy or build new boats.

I will point out that there are probably quite a few multihull buyers over the past number years that may well end up making a profit on their boat investments or maybe breaking even...and on larger vessels as well. The price of building a new vessel has gone up so dramatically in the last few years that the price they paid for their present vessel may well allow for them to make a profit upon sale.

You might also take into account there are a 'limited' number of good used multihulls on the market and these are generally all priced below what a new one cost to build...and yet sometimes more than what the original buyer paid. So he has gotten some good enjoyment out of the vessel, and sold it for close to what he paid....maybe minus his operating and maintenance bills...still not a bad deal.

Then look at the huge price Tom Perkins has spent on a 'really different' vessel, Maltese Falcon (http://boatdesign.net/forums/showpost.php?p=222063&postcount=164), and he has just taken on a partner who likely paid for his share the entire investment that Perkins had made...not a bad deal.

I believe someone could build a nice version of the kite-assisted trimaran (http://boatdesign.net/forums/showthread.php?t=20319) I suggested....maybe even in epoxy-saturated wood construction...maybe over in low wage market such as Philippines, Thailand etc,...and sell for a profit down the road when fuel prices will most certainly be MUCH higher. ...and in the meantime have a lot of fun with the project

In fact I venture to say I could sell one tomorrow, IF it were already built. Buyers in the market now (at least those that really have the money NOW) want their vessel 'yesterday', not neccesarily a year or two from now.

[RHough]

The engineering, manufacturing and material costs of a super efficient boat would cost MUCH MUCH more $$$$ than you ever spend in fuel bills in a normally efficient current boat :P

You spoke of 3000 nm at 10 kts at lowest cost. OK. So the safety and comfort provided by ocean rowing skiff or minitransat is acceptable. No other constraints.

You are right. I didn't state all the constraints that I thought were implied. Passage making to me implies relaxed, comfortable cruising. ;)

Cruising means different things to different people. Most people would not describe 12 days driving a mini to a 10 knot average as cruising. :P

[fcfc]

If you add comfort in the game, there is no issue.

The nordhavn 72 http://www.nordhavn.com/72/overview.php4 is probably the best selling 3000nm @ 10 kts boat. It is built since 2004, and builder claims to build 4 a year. Over 10 sold would be a fair claim.

On the opposite, the FPB 83 http://www.setsail.com/dashew/do_PARADIGM.html , while technically a much better performer (6300 nm @ 11.3 kts), has NOT sold a single sistership, despites its press coverage.

Both boats are in the 4 Millions $ ballpark. (Prices may be some years ago)

So, although ordinary people, absolutely unable to buy a FPB 83, may find this boat very comfortable, the ones who really have the money must find it lacks MANDATORY features a 4 M$ boat MUST have (such as crew quarters I guess, I do not really know, I am very very very far from having 4M$ to spend in a leisure boat), and find the fuel burn issue much less important. Otherwise, that boat would have sales.

Now, if you still want the minimum boat 3000 nm @ 10 kts, with comfort more or less on par with the price, that should be something between FPB 64 http://www.setsail.com/dashew/FPB64_intro.html 4700 nm @ 10 kts and passagemaker lite 56 http://www.passagemakerlite.com/designs/passagemakerlite-56 1700 nm @ 10 kts. I would say at least one million $ for a professionally built with expected equipment.

NB both minimum boats sell less than the bigger N72. And will probably sell much much less than the N52 / N55 despite smaller nordhavn products have lower performances for a similar price range.

Perhaps, you can save a few more gallons going lighter, http://www.nigelirens.com/FRAMEMolly.htm http://www.tuco.dk/admin/bib_img/_mgl0949120608154717.jpg . Note this boat is not intended to do ocean passage. You will certainly severely reduce comfort going light. But not that much the price because light building technics like Corecell sandwich with infused epoxy are not cheap building methods.

It is about less than half the weigth of the FPB 64 but same length. So half the fuel burn for the same speed is not too far. So 1700 gal instead of 3400 gal. And reduce range from 4700 to 3000 nm, so 1100 gal would fit. That should make around 3.5 tons of fuel in a 15 tons hull. Doable, but expect to put ALL your passagemaking gear in a small bag. A passagemaking version based on hull lines and displacement will probably match 3000 nm @ 10 kts with avery good nmpg, but will be a boat where the comfort available onboard will not match the building cost (or where the ones accepting the comfort will be unable to pay the price) , and hence will not sell. Like the FPB 83.

[Tad]

Fc...

All reasonable points, comfort is important to those going offshore for pleasure.

Dashew mentions a half-load displacement for the FPB64 of 75,000 pounds, Irens published a full load displacement for Molly Ban of 36,700 pounds.

Molly is designed for coastal cruising (short term) and carries only 1000L of fuel. Water tankage is not mentioned. To go offshore in Molly add 5000+ liters of fuel, substantial fresh water, ocean going structural modifications, ocean cruising gear (anchors and chain, etc), stabilizing gear, and owner's items for long term living aboard. This will increase weight considerably. I doubt a 64' ocean cruiser could be ready for sea at less than 50,000 pounds. Of course this increase means more fuel, and so it goes.

[sabahcat]

There is a huge thread somewhere here on sail vs power and which costs more per mile.


http://www.boatdesign.net/forums/boat-design/cruising-costs-maintenance-price-boat-sailboats-versus-motorboats-11479.html?highlight=power+vs+sail

[alangluyas]

Do you have the source of Dave Gerrs "revised" formula? I have been aware for a long time that the SL of 1.34 is a theoretical figure and that is very conservative for long,skinny or light boats but I would be interested in seeing whre or how he came up with this calculation.

Cheers

Al

[Guest625101138]

Do you have the source of Dave Gerrs "revised" formula? I have been aware for a long time that the SL of 1.34 is a theoretical figure and that is very conservative for long,skinny or light boats but I would be interested in seeing whre or how he came up with this calculation.

Cheers

Al

Hull speed is a defined parameter solely based on the waterline length of a hull and the gravitational constant. It is the celerity of the lateral wavetrain of the hull with wavelength equal to the hull waterline length. The formula for celerity is:
V=sqrt(g x Wavelength / (2 PI)) (g is gravitational constant 9.8m/s/s)

If you use British units for length and speed then the constant is 1.34.

Rather than use some less than ideal concoction of this basic relationship there is very accurate software available for predicting hull drag for slender hulls. It can be downloaded free from here:
http://www.cyberiad.net/michlet.htm

There are many users of the software on this forum able to help you get started. Once mastered (read the manual) it is very simple, fast and accurate.

The notion of hull speed is not really relevant to slender hulls.

Rick W

[alangluyas]

Hi Rick

Thanks for that and I understand about the relationship between gravity, density of water and the length of the vessel, but I am sure that Dave Gerr also understands this and he must be considering other factors if he has published another method of calculation, as posted by RHough.

I would like to some support for the idea before I use it in anger.

I also realise that slender hull forms are not really subject to the SL 1.34 paradigm, but that leaves a huge area of ignorance between "normal" hull forms and "slender" hulls. When I read of Gerrs revised formula, it occured to me that he may have done some work in this area and found an algorithm that applies.

I have done a couple of designs that exploit slender hulls efficiently ( I built a 28 foot alloy diesel cruiser to work at SL's of 2.6 to 2.8 a few years back) and
I am very interested in developments in this area.

We have far too many pleasure boats with beam/length ratios of nearly 2:1 that seem to need 100 SHP per tonne to just go fishing at Rottnest.


Cheers

Al

[marshmat]

If you add comfort in the game, there is no issue.

The nordhavn 72 http://www.nordhavn.com/72/overview.php4 is probably the best selling 3000nm @ 10 kts boat. It is built since 2004, and builder claims to build 4 a year. Over 10 sold would be a fair claim.

On the opposite, the FPB 83 http://www.setsail.com/dashew/do_PARADIGM.html , while technically a much better performer (6300 nm @ 11.3 kts), has NOT sold a single sistership, despites its press coverage.

Both boats are in the 4 Millions $ ballpark. (Prices may be some years ago)

So, although ordinary people, absolutely unable to buy a FPB 83, may find this boat very comfortable, the ones who really have the money must find it lacks MANDATORY features a 4 M$ boat MUST have (such as crew quarters I guess, I do not really know, I am very very very far from having 4M$ to spend in a leisure boat), and find the fuel burn issue much less important. Otherwise, that boat would have sales.

Now, if you still want the minimum boat 3000 nm @ 10 kts, with comfort more or less on par with the price, that should be something between FPB 64 http://www.setsail.com/dashew/FPB64_intro.html 4700 nm @ 10 kts and passagemaker lite 56 http://www.passagemakerlite.com/designs/passagemakerlite-56 1700 nm @ 10 kts. I would say at least one million $ for a professionally built with expected equipment.


Good points. And while the FPB 83 hasn't sold any sisterships, the FPB 64 are apparently selling like hotcakes- last I heard, they've started joinerwork on the first one, the second hull is well underway, and there's a couple of years on the waiting list to get one of the later hulls.

Part of that may relate to the issue that, even though the 83 is a four-point-something million dollar boat, it really doesn't have any more space than a typical 55-60' trawler. It's a sea boat, and from what I've heard, a good one- but it seems the people who have that kind of money to toss around just aren't biting on a boat designed for a couple to live on and handle by themselves. Hence the cheaper, single-diesel 64. (Still worth an order of magnitude more than the average house, though.... so no-go here!)

As far as efficient cruising at 10 knots- well, for sheer cost efficiency, I think you'd have a hard time beating a Wharram cat. Maybe give it saildrives instead of the deck-mounted outboard, and you'd have a half-decent motorsailer. They have their quirks and their detractors, but there's a few thousand of them out there, and you can have one ready to sail for less than the cost of the CF mast and mainsail for a modern racer/cruiser type, and about the same as what the engine and driveline alone of the FPB 64 costs.

[Guest625101138]

Hi Rick

Thanks for that and I understand about the relationship between gravity, density of water and the length of the vessel, but I am sure that Dave Gerr also understands this and he must be considering other factors if he has published another method of calculation, as posted by RHough.

I would like to some support for the idea before I use it in anger.

I also realise that slender hull forms are not really subject to the SL 1.34 paradigm, but that leaves a huge area of ignorance between "normal" hull forms and "slender" hulls. When I read of Gerrs revised formula, it occured to me that he may have done some work in this area and found an algorithm that applies.

I have done a couple of designs that exploit slender hulls efficiently ( I built a 28 foot alloy diesel cruiser to work at SL's of 2.6 to 2.8 a few years back) and
I am very interested in developments in this area.

We have far too many pleasure boats with beam/length ratios of nearly 2:1 that seem to need 100 SHP per tonne to just go fishing at Rottnest.


Cheers

Al

If you are interested in design of slender hulls then you should take the time to learn Michlet and its hull optimising component Godzilla. It takes all the guesswork out of the hull design and produces reliable results based on analytical solution of the wave drag.

I can tell you from my experience that formulas based on empirical analysis of data sets are ordinary by comparison with Michlet.

Rick W