Displacement Glider, PowerKeel, etc?

[yacht371]

Hull design for fuel efficiency is highly speed dependent. I have been reading the posts on this thread, and it seems this is an area that needs to be addressed. Speed/Length ratio is defined as (speed in knots)/(square root of LWL in feet).

S/L Hull characteristics for best resistance
<1 Round bilge, wide beam, lowest wetted surface (sail boat, Nordhavn 46), low prismatic coefficient (.55), transom clear of water, or double ender.
1.0 to 1.3 Similar shape, narrower but higher prismatic (.6), transom close to waterline
1.3 to 1.6 Immersed transom, straighter buttocks, prismatic .7, possible PowerKeel or very narrow hull(s)
1.6 to 3.0 DG or Flowmocean hull, other semi-planing hull, dipsplacement catamaran
>3 True planing hull, vee or deep vee, planing catamaran

This is somewhat of a simplification as there are seemingly infinite variations within these types, but the trend is clear. You have too decide the cruising speed first.

The latest Nordhavns are not as fuel efficient as their first model, the 46. This seems to be because they discovered, capabilities notwithstanding, most owners are NOT going around the world, and want better accommodations for the length.

Sea state is another important factor. It is relatively easy to optimize a hull for efficiency in smooth water, but a stiff headwind and a steep chop will stop some boats in their tracks. Even boats which slice smoothly through a 3' chop may not do well in 6'. Many power cats have this failing. Very smooth in semi protected waters, until the waves get big enough to hit the bridge deck, then BANG!

So let us set parameters befor designing the boat:
1. How fast (determines shape parameters)
2. How far (determines displacement and fuel load)
4. What amenities are essential
5. How big (length and beam)
6. Calculate power needed
7. Estimate cost
8. Go back and remove some speed/amenities/size and repeat cycle

That is the design spiral...

Custom boats:
The average custom boat loses half its value when launched. This is mainly because they usually cost twice as much as the equivalent production boat when new, but don't have "brand recognition" when sold used. Home builds cost less, but are even harder to sell used at a decent price.

If you are looking for the best (inshore cruising) bang per buck, buy a single gas powered Bayliner, fairly new but at least one year old, use it until you fear it may give you trouble, and trade it in. For more serious offshore cruising, Nordhavn and Selene are amazing value and have high resale. I would still advocate buying used.

Those who go for better quality production boats, custom boats, and home builds, do so for other reasons. Pride of ownership, satisfaction in having created something, prestige (by the way, the non boater is likely to be just as impressed by the Bayliner!), or just possibly safety.

Gas engines, although they use more fuel, cost so much less to buy, and are much cheaper to replace or rebuild, so you will never pay for a diesel in fuel savings unless you run a ferry.

Gee, I hope this doesn't kill the thread

Grahame Shannon

[Vega]

Nah, you just put some reality in it

Average motorboater does very few miles a year in their boats. Costs of fuel are irrelevant regarding initial cost and maintenance costs (marina, antifouling etc). They do few miles but certainly like to brag about the power of the engines and the speed of the boat and occasionally they like to impress some friends with it.

I am not saying that this thread is not interesting, just that if motorboat market is going in one direction (speed and interior comfort) it is not because boat designers are fools, but because clients want just that.

The economical motorboat cruiser will always be a marginal market, even if an interesting one and in this market I believe that it is possible that we will see some improvements and perhaps they will manage to reach production stage. But let’s not forget that one of the most promising boats that has reached production, the Range boat, designed by Nigel Irens and made by Seatec is not made any more by them, or at least the guys from Seatec don’t answer (for several months) any emails regarding interest in their boats and that is not a good sign to the success of that enterprise, neither of the interest those boats raise in the global motorboat market.

[Stephen Ditmore]

fcfc:
Just took a look at that Savitsky paper. Very relevant to the semi-displacement discussion.

All:
While narrow is good in the semi-displacement speed range*, it must go hand in hand with low center of gravity or your monohull will be unstable. How do you quantify this relationship? See How wide should it be?...a method proposed

*exception: for sailboats, wide & light may beat narrow & heavy, performance-wise.

Have you all seen Rick Loheed's boat? http://www.boatdesign.net/gallery/sh...php?photo=4877

[fcfc]

And hullform is not all.

For efficiency of powerboats, you must also consider efficiency of the propulsion system.

And then you have nighmare if you look for small powerboats.

Such engines as http://www.marinedieseldirect.com/re...D2_75_75hp.pdf or http://www.yanmarmarine.com/products...E_TechData.pdf , will lag efficiency behind engines like http://www.fordpowerproducts.com/For...let_4_2003.pdf (the transit 75 PS). This latter engine is also rugged, since it is for commercial / ligth trucks, not cars. At 30 kW, both marine engines are burning around 9.0 9.5 l/h. That translate to a bit over 255 g/kW.h for BSFC. (9.2 l/h * 842 g/l / 30 kW). Just draw a line on the ford map at this level.

I agree what is missing is the power loss in the marine redrive. But even at 245 g/kW.h it is high. (4% loss in the transmission = at 30 kW, makes 1.2 kW of heat dissipation and vibration in the gearbox = your oil is boiling ...).

Next for marine gearbox. It is known for a long time, that the slower the prop, the better the efficiency. But you will have hard time to find a small gearbox with a reduction ratio better than 1:3. That means the prop will likely be over 1100-1200 rpm.

For last, if you find that dream gearbox like ZF45 (the only rather small that can do 1:3.74 , but still overkill for 70 hp ), you wont find a prop. Now, you will need a big diameter prop with thin and narrow blades since your prop will turn slow with a small power (BAR around 0.4). That prop do not exist on the market. It must be custom made (like the Whio).

I do not even speak of contrarotative props for sub 100 hp or sub 15 kts boats.

Seems that technical solutions for better efficiency are known and proven, much better than new hullform. But as Vega said, customers are not interested to put money in them.

[brian eiland]

Originally Posted by Stephen Ditmore

...This 25' launch represents my first attempt to design a better semi-displacement monohull....The idea is that a narrow forward hull is followed by a perfectly flat aft portion, so the wave being pushed up in front of the flat aft sections cancels the trough behind the bow wave....

Stephen, are you familiar with some older work done by Peter Payne and referred to as SeaKnife?? Your description above brought it to my mind. His patent started out, "a boat capable of cutting through rough waves at high speed with astonishing stability has a hull provided with a flat planing surface, which in plan is the shape of a thin wedge or delta. The sides of the boat rise upwardly and outwardly with a simple concave curvature from the two edges leading from a knife edge bow at the point of the wedge. The slender wedge shape moving through the water at high speeds develops continuous spray sheets up its sides, which are intercepted by the outwardly curving portions of the hull sides. Spray rails or deflectors may also be utilized to intercept the spray sheet, such deflectors being inclined at a small angle to the bottom of the planing surface. The knife edge bow rises upwardly and forwardly with a concave curvature.... . I posted some info and references both here and on another forum:

Peter Payne, "Design of High Speed Boats, Planning"
http://boatdesign.net/forums/showpost.php?p=81761&postcount=8

Original Patent, SeaKnife
http://www.yachtforums.com/forums/16028-post48.html

Constant vs Variable Deadrise
http://boatdesign.net/forums/showthread.php?t=6668


(there are a few other reference sites on those I listed above leading to other related sites)

[brian eiland]

Originally Posted by Stephen Ditmore

...Have you all seen Rick Loheed's boat? http://www.boatdesign.net/gallery/sh...php?photo=4877

This brought up another subject I've participated in:

Trimarans & the BladeRunner
http://www.yachtforums.com/forums/general-yachting-discussion/2701-trimarans-bladerunner.html
...lots of discussions

[brian eiland]

Originally Posted by mark424x

...excerpt
(6) Grahame Shannon's "powerkeel". (http://www.aviadesign.com/powerkeel/index.htm)

I noted here, "I went to the Shannon website, and as I looked thru some of their references I noted a photo of the vessel at speed in the Yachting '05 issue. It sure appears to me to be throwing off considerable water in its midsection??

http://www.yachtforums.com/forums/15988-post38.html

[Stephen Ditmore]

I'm looking for a quick, easy formula to plug into a spreadsheet for semi-displacement horesepower required. Not a graph... a formula. I'll research & refine it later (perhaps from fcfc's Savitsky paper). Something in the form

shaft horesepower = (C * Displ * V^2) / length^n

would be nice if someone could tell me what C and n are.

If you happen to have a copy of Beebe's Voyaging Under Power, it's my recollection that it has a good general purpose resistance curve that agrees with the horsepower slide rule that Caterpillar used to give out. If someone were to fit a formula to that, it might do nicely for the moment.

Thanks,
Stephen

reference: Hull speed

[Guillermo]

Stephen,
I do not know of a single formula without coefficients to be taken from a table or graph, but one of the the simplest that may be used is Skene's formula:
BHP = D*V^2*2881/C^2 (D in Tons and V in knots)
C varies between 180 and 185 for recreational boats.
It tends to give low values, in my opinion.

I've found that using:
BHP = 0.1512*D*V^2
gives more accurate results, in my opinion, for Lwl around 28'

We could also make Lwl to come into equation as:
BHP = 0.09*D*V*L^0.25
This gives almost the same results as before for Lwl between 25' and 28'

As Skene's formula is not conceived specifically for semi-planning conditions, but planning, results have to be taken with care, although it serves as a first approximation.

I'm working on the derivation of a more complex and (I hope!) accurate formula seeming to be valid for the 1.8 to 4.1 S/L ratio range, that includes Lwl and avoids the use of variable coefficients. I'm almost there, I think. If I succeed, I'll post it here when done.
Cheers.

[fcfc]

What my spreadsheet contains is:
HP = D*((SPEED/(11,963 * LWL ^0,5))^3)
SPEED = (LWL^0,5) * 1,18 * ((HP*1000/D)^0,33)
Units kts, ft, lbs, hp.

For what's worth.

[Guillermo]

fcfc:
I've checked your formula against the corrected Skene's in my post, for a theoretical boat as follows:
D = 6000 pounds
L = 25 foot
I find it gives similar values for 15 knots, around 92 HP, but lower values under it and higher values over it. Using Crouch-Werbach's, Barnaby's and Philips-Birt's I get around 72 BHP.
It seems quite difficult to get correct values using just a formula without the help of tables or diagrams for the various coefficients.

I'm getting mad trying to arrive to a better formula. The one I'm working on calculates frictional resistance and residual resistance, to derive the HP, but it is giving what seem to be too high values (Around 120 HP for the same boat). Crouch-Werbach's, Barnaby's and Philips-Birt's formulas (they depend on coefficients to be taken from tables or graphs), give consistent results, always quite close among them. I'll try to adjust my formula to these, and see if I get something. Not an easy task....

Somebody else has better info?

[Guillermo]

If somebody wants to check it, here what the formula could be, after adjusting it:

SHP = 0.016*(0.061*D*V^2/L^0.5 + 0.01*V^2.825*D^0.667*(36.38+1.636*L/D^0.333))

D in pounds, V in knots and L (Lwl) in feet.

It is supposed to work for S/L ratios between 1.8 and 4.2

I'd appreciate whatever feedback.

[Stephen Ditmore]

I went ahead and looked to the Savitsky paper for an answer. Attached is my first shot at fitting an equation to the curve in Savitsky's Figure 4. It only works up to a speed/length ratio of 3.0 in its current form, and I haven't checked the lower (displacement) end carefully (yet).

In translating total resistance to horsepower I use a lower "Overall Propulsive Coefficient" than Savitsky.

[Guillermo]

Stephen,
Your formula gives 55 HP for Lwl = 25 and D = 2.68 LT , while mine gives 71.55 for the same thing. fcfc's one gives 94.62 HP
Your formula is in line with Skene's (54 HP) but I think, as said before, this is a low figure. Should be nice to check them against real boats performances.

To easy checking, I attach an spreadsheet with the calculations.

[Stephen Ditmore]

What speed are you using, Guillermo? At that length mine goes wacko at anything above 15 knots.
-- Oops, I see in the previous post 15 knots is what you're using.
It may be the Savitsky curve is for slender ships, and needs a correction based on beam to be applicable to smaller boats.

Something's getting lost in translation.... I can't get either of your formulae to work in my spreadsheet (see attached).