Planing Hull at Disp Speeds

...speaking of different paths to get to the same destination!

I applaud both! I do everything I can to try and improve efficiency but will never reach your bars.

 

To me the issue of making a displacement boat efficent is the problem that the prop curve is almost the opposite of the engines torque curve.

Peak torque is peak Volumetric efficency so usually best BSFC

As pointed out above the old Ford Lehman in a GB42 has 3 times the power required to cruise and you have 2 of them?

On paper it looks like an electric drive ( or 2 where you need 2 props to get the blade area)with 3 or 4 small gen sets would be the best on gph/gpm

and of course CPP helps to get that miss matched prop curve to be closer the torque curve.

 

Wow ... you guys are rompin ahead fast. It's strange to see my Willy runnin around loose. Assumptions have been made about the Willard. The W30 hull is not optimized for efficiency. The keel is choped off square 4 or 5" wide just ahead of the propeller. She's 10.5 x 30' ... not very narrow (see pic). She carries 2 tons of ballast (concrete & steel punchings midships and lead aft). She has a 2.57-1 gear and an 18" wheel. There's plenty of room for a larger propeller between the shoe and hull. The aft end of the hull turns up rather abruptly at the stern. If she was optimized relative to the above she'd burn less fuel (perhaps half as much) and require a repower to avoid underloading the new engine and to optimize the engine speed. The new W30E (E for efficiency) would have very different rolling characteristics (probably undesirable), have a much higher CG, would have an extremely narrow cabin or no side decks, be more pitch sensitive to propeller loading and in the general scheme of things be considerably less seaworthy. I'd like to change the keel, the Prop/gear and extend the stern a bit but I'll be lucky to just keep up w the maint so I can continue cruising. I'd really like a 32' nordic Tug but I'd only go 2 knots faster and consume 3 or 4 times as much fuel. Here's my Willy long & low and short & fat.
Tolly .. Wabasto and soon .. a small Sigmar diesel stove.
Will, we don't shoot tourists up here ..they feed us.

Easy

 

Sigmar makes good stuff. They were a breakaway from Dickensen I believe.

I had an Espar, very nice when it worked, tempermental, the price of parts made it virtually unservicible for mere mortals. Forced air and a thermostat is nice but not that nice. I yanked it out and replaced it with an old Dickensen Arctic with a gravity tank. I love that. I like cruising in the winter almost better than in the summer.

Your boat looks like a snug comfortable cruiser. I admire your fuel burn! Speed is often overrated. Most of the time it's not the destination so much as the journey, out enjoying the water and listening to the burble of the motor.

 

The best boat is the one you already own........the very best boat is the one you just sold........

To go back to the numbers for a sec....

I don't know the exact weight of Easy's Willard, published displacement is 17,000 pounds. At 6.15 knots (S/L 1.17) and .85gph she is using about 17.6 HP and requires almost the exact same HP/LB as the Camano 41 when running at that same speed/length ratio. Thus the Willard at 6.15 and the Camano at 7.1 knots have identical transport efficiency.

At S/L 1.82 Will's boat and the GB41 show very similar HP/lb requirements.

 

Camano Efficiency

TAD, I love your boats and I like your website and I look fwd to what you have to say on D.net but I just don't believe that a 41' Camano can driven that easily. I'd like to have a 32' Nordic Tug w the stern up and out of the water in true FD fashion ... and power it w a 54hp Isuzu. I liked the Camano, especially the house .. then I saw one out of the water. It was like a big aluminum skiff made out of plastic w a wide, flat, submerged transom draging through the water. I just can't imagine it making 7 knots on 18hp. But then most people can't imagine my Willard going 7 knots w 18hp either. Just tell me it isn't so TAD.

Easy

 

Easy.......

It ain't so.......

The 32,000 pound Camano will not go 7 knots using the same power as your 17,000 Willard.

The larger and heavier Camano requires 37.5HP to go 7 knots, I have no figures for what your boat requires to achieve this speed.

But.....at a speed/length ratio of 1.1......one horsepower moves the same amount of either Camano or Willard, (about 900 pounds). This just illustrates that, at this speed/length (only!), hull form resistance is about equal for either boat, (again pound for pound).

 

TAD,
Well TAD, I'm not even sure my Willard will go 7 knots .. It probably does .. but just barely. That's all the power I have ..37hp. That's probably WOT in the pictures. It's too short. If my Willy was streched to 41' (same everything else) would it go faster? Couldn't do that though. At 41' the boat would be heaver .. then we'd be talking different power per ton. I'm guessing you'll say yes though. I'm guessing the Willard is over powered at S/L ratio of 1.34 and should'nt be run that fast. Also I'm guessing your hp estimates for the Camono are based on the 1.8 gph that you got from where?

Easy

 

This is an interesting topic for me. I would like to switch from sail to power but can't afford to have a custom boat built. So I have to look at what's available on the used market. We all know the problems with the typical powerboat being too wide and fat with huge engines. Personally I don't like the look of double enders and not having much experience with powerboats, I have no way to evaluate how much roll a displacement powerboat with round bilges would have. If it is comparable to the roll a sailboat has no big deal.

In my travels on the internet I have come across the 40' navy utility built by Willard. Most have a layout to accommodate lots of passengers and a few that have been converted to yachts. there have been comments that these boats tend to roll. But that means nothing to me. The basic spec's are 40' LOA 39' LWL 12' Beam and I have seen displacement quoted from 18000 lbs to 25000 lbs. Engine is usually a detroit diesel with 160 hp so they can travel at up to 15 or more knots. Attached are photo's of one I recently found on yachtworld. So what can you folks tell me about this boat. Would it be fuel efficient at 8 knots, 6 knots or 10 knots? How fast might it go burning 1 gph? I would like to get the same fuel efficiency as my sailboat which is 1 gph at 6 knots. I would think that without having to drag a big keel and sailing rig around would allow the powerboat to go slightly faster with the same fuel burn. What about re-powering with a 50 hp to 75 hp engine. I don't want to travel any faster than 10 knots and an 8 knot cruise speed would be just fine.
Am I missing something here?

 

I'm arriving a bit late to this interesting discussion.
Easy, you should google for Van Oossanen's paper "Motor Yacht Hull Form Design for the Displacement to Semi-Displacement Speed Range". At the page 632 (don't worry, the paper starts with the page 629 ) there is a graph which shows a linearily increasing correlation between the immersed transom area and the resistance (expressed through the Telfer coefficient, explained in the paper). The graph is a result of a significant number of towing tank tests performed by Wolfson Unit on a number of different displacement and planing hulls.
The graph shows that in the displacement cruising speeds range (Fn,L = 0.35), a planing type hull will have a resistance about 2.2-2.5 times higher than a displacement hull of same length and displacement. Which is, If I understand it well, what you've been seeking for in your initial post.
I'm typing this from my magical mobile phone, so I can't attach the pdf of the paper, but it is easy to find through Google.
Cheers!

 

That doesn't mean that it will use 2.2 - 2.5 times the amount of power go a given speed though does it?

 

According to the graph from the said paper, for two hulls running at the same displacement and S/L ratio the resistance will be proportional to relative Ct (Telfer coefficient) values, which is a function of wet transom area. But the power will depend on the speed, which will again depend on the S/L ratio (for a given length).

This is the link to the paper: http://www.oossanen.nl/download/perryvanoossanen_-_motor_yacht_hull_form_design.pdf
The graph is at the page 632.

However, there is a significant scatter of the data, on average around 20%, and it has to be taken with due caution. Also because no details are given about geometry of tested hulls and no clues about validity range of hull parameters.

 

I would be very leery of data trying to sell something, this may be a case of comparing apples with apples that do not exist.

As no clearly defined data is included one assumes these data points come from test model families (perhaps?). The reason for creating test model families is to define the edges of what's workable, but often much of the family is made up of hull forms one would never propose in reality.

This is perhaps the case here.....data is being presented in a way that accentuates a difference that would not be so apparent IRL. It would not be so apparent because other factors would change as well as the wetted transom area. LCB and total wetted surface will change as well as the entire configuration of the entry........

The various factors that make a successful design are deeply interrelated, researchers spend lifetimes trying to isolate one factor and it's influence.....but that's not easy........

 

daiquiri,
Just scanned the link and it looks really interesting and a bit over my head but I'm sure I'll get much out of it anyway. Thanks very much.

Easy Rider

 

I'm sorry - I don't get it. I was under the impression that (within reason) the greatest influence on the power required at displacement speeds is weight. But here you say that for a given displacement and speed, a planing hull will require 2.5 times as much power as a displacement hull....

If we just use the two examples in this thread... Easy's 40' Willard and Will Allison's 27ft (WL) Graphite.
Will has posted the fuel consumption for his boat and at 7.1 knots the engine is operating at 1500rpm and using 5 LPH. I took a look at the Yanmar website and according to the prop curve, at 1500rpm, the engine is producing approximately 20hp.
On that basis the (much longer) displacement hull Willard ought to require just 8hp to travel at the same speed, yet Easy says it needs the full 37hp that he has on offer.
What's more, Will's boat would be operating at just over theoretical hull speed whereas the Willard would be operating at just under it....

I realise that I'm not sticking to the rules here - they are different WL length, different displacement and thus operating at different SL's.... but that should make matter's worse for Will's boat, not better....

So, either the notion that a planing hull requires as much as 2.5 times the power to operate at displacement speeds as a displacement boat is wrong, Will's managed to craft a hull that defies all the rules, or I'm missing something.....