economical coastal cruiser

A week of travelling has started again, so I've not had time to read all new posts thoroughly. I'lö do that.

I'm not trying to represent an unknown client. I'm inserting my very own requirements and the needs I have. I don't expect they will be the same as for the average client, but at least they are applicable for one person. I took the chance to push a very interesting thread in the direction that was of most interest to me.

In my opinion the requirement on fuel efficiency doesn't have to be met with 20 knots headwind. The average wind in Sweden is 6 knots, and the direction relative to the boat could be anything. I suggest the 12/12 requirement should be met without headwind and at a relatively calm sea state.

I note that different people have different needs to trailer the boat. For me personnally it's not very important, and I'm fairly sure I can borrow the equipment for the short hauls I would need. "Barelly trailerable" best discribes my need.

Erik

 

Interesting, I'll have to study this a bit. I'm still unclear as to just exactly how to do that little trick. Looks like I may learn something valuable today.
If you ever get your pics back I'd love a look at them.

 

See if you can figure it out from these two shots that I have also for the existing hatch. I create the raised trim and angle the edge down. I then fit the initial outside moulding or frame and glue up. I then tape all the parts and sit back in place and mix glue up and fill the void. If you look closely you can see the cured glue before you install the top in the case of a hatch. If you also look at the way I deal with the crown, I also tape the bottom of the frame and the outside of the frame and tape off
the decking and place glue between the hatch frame and decking and let cure. This also makes a flat seal and deals with the crowned decks around the raised trim. I do not have to custom fit the frame either. You can go as high as you wish. Then you just paint everything out.

 

Oyster, that is a very nice looking boat to to my eye!

 

Thanks Oyster,
I'll be studying these. David's right about your boat.

 

Getting back to the "feasibility study." And I think that's the right term for this effort.

Studies so far (most of the useful data from Rick), and the results:

1) 12/12 is possible, but only with "very light" displacement. It looks like 3t is too heavy to be possible. 2t looks more likely. 1t is demonstrably possible with actual boats (that are "too short").

2) Length looks to be 12m or 40 feet. Although the investigation actually suggests 14m or 46 feet is a more appropriate size for efficiency, other concerns (trailering, moorage, handing, maybe even cost) suggests trying a shorter length over a longer one. And there is that woodenboats competition... ;-)

3) Easily trailerable, meaning approx maximum limits of 2.5 m (8.5 feet) wide and 12m (40 ft) long. Lighter is better for sure. Smaller in all dimensions makes trailering easier.

4) Air resistance study indicates that at this speed and power to meet 12/12 its pretty much got to be a conventional monohull, not stabilized monohull. This wasn't so clear, so let me clarify: At some point we need to put stuff inside, so volume will matter. A stabilized monohull has far more windage for the same volume. Since wind drag is as important as water drag, better to reduce wind drag for a small increase in water drag.

5) Initial stability and water drag power study indicates 2.5m beam is adequate for stability, again supporting the conventional monohull selection.

6) Assume "large" diameter prop is OK, because it might be attached to an outboard, or might be oriented horizontally when on the trailer. Still, "coastal cruising" infers a draft restriction, but not such a firm number as the beam and length due to trailering. Nevertheless, the much larger than "normal" prop shows dramatic efficiency contribution at this very low power 12/12 regime.

Sound correct?

Rick, what's next in your design spiral? I think its nailing the drive train. To get 12/12, that's two different solutions, one for outboard, and one for diesel. At least we've narrowed the power requirement. For outboards, probably need to use one of the "bigfoot" engines to get a tiny bit more reduction. Still not much, and still the 10hp/gallon/hour issue.

Is it possible to reach 12/12 with a bigfoot outboard? How light would we have to go?

 

Oh, one more thing. Tad, I think your sketches are beautiful, but I agree with your earlier statement that even 4t won't get the efficiency target, and 18t is way too much. For this feasibility study.

 

I think this has turned into an exercise in dreaming instead of really doing something. If not then I would suggest that those dreamers get together and make the boat. I would suggest in Oz to do it in plywood/epoxy/basaltic fiber for cost and weight.

The prop, who knows how you would make a 2 blade prop three feet in diameter that would be able to be turned by a low hp engine and have the pitch to be able to move the boat forwards at 12 knots.

In the end you will probably save $10 in fuel on a weekend jaunt.

 

Erik
To take the first iteration to something that you might use to go forward -

Reducing the windage and wave requirements make the key objectives feasible.

Attached shows one quick option of what it could look like. There are still many aspects to work through of course.

The frontal area of this is about 6.5sq.m and the Cd will be better than 0.3. With no ambient wind the windage underway drops to 180W. So the power drops to 8.5kW. This translates to 1370N thrust.

A 500mm 4-bladed prop will require 10.8kW to produce this thrust. There will be transmission losses and house loads to cater for as well while underway. If you look at engine fuel maps I believe you will find a little diesel that will produce this power with less than 1USg/hr.

So it is feasible within the current weight constraint. There is 44sq.m in the hull and wing plates. The stabilisers, sides, deck and cabin have 72sq.m. You would need to allow for bulkheads as well.

One potential issue with this concept is the width of the hull. It tapers from 700mm wide down to 600mm wide. I expect it can be widened without increasing power demand but this requires another iteration. There are many other aspects that need to be set out before moving on.

Rick W

 

I should add that the estimate I made for added power due to waves was somewhat pessimistic. I applied it to to the combined calm water drag and windage at 20kts. The 20% allowance should only apply to the hull drag not the wind drag as well.

So both windage and wave drag allowance, quite a few posts back, were pessimistic. With the reduced area, slicker shape and correct wave allowance it would now be close to meeting the original requirements. The prop diameter would be more like 600mm than 500mm though to avoid throwing too much power into turbulence if the wind/wave requirement was included.

Rick W

 

It's a little unclear as some of your model's appear to be in conflict with one another, but I assume that the main hull is somewhat shorter than the main 'body' of the hull.
I expect you are planning on locating the prop (which I see you have 'optimised' down to a less than optimum diameter of 500-600mm) aft of the central hull in order to keep draft to a more acceptable level? In which case, have you considered the problem of locating the prop so close to the surface without the addition of any kind of (drag-inducing) anti-ventilation plate? The closer a prop is to the surface, the more prone it is...

Also, on a simple, practical note, you show a bridge-deck clearance in the order of 150mm. From personal experience, I can attest to just how unpleasant it is to spend time aboard a boat with such a setup. Every passing wave causes the boat to shudder - both underway and at rest. I think you need to seriously rethink that bit.

 

Rick, I seem to remember a video of a human powered boat that used a surfacing propeller. How did that work? Is there a reason a boat in this speed regime could not use a surfacing propellor?

Seems like an easy way to get a much larger prop without also taking the hit for shaft and strut drag, or non-horizontal thrust.

 

David
A surface prop is limited to about 70% efficiency. There have been none used with great success on HPBs as far as I know. I used one but never video it. It was close to hopeless but the design was not very good either.

Removing the wind/wave requirement gets down to a prop of 500mm before losses are significant compared with going any bigger. I don't think this size present any major issues for mounting and operating.

Rick W

 

Will
You have to draw the distinction between efficiency and optimum. The diameter of the most efficient prop is usually achieved when the diameter reaches the point where the appendages required to carry it create more drag and resulting power loss than the efficiency gain by increasing the diameter will achieve. I have attached the definition of efficiency as applied to physics.

There are many other elements in the design space when considering the optimum. My original aim of entering the discussion was to point out that the prop selection should be the result of optimisation rather than a simple compromise based on limited information.

As to the wave slapping I would need to know what hulls you have experience with to see if it is relevant here.

The wing or bridge clearance is 200mm. The width of the bridge is 400mm. Under way the hull will create a trough under the bridge. The stabilisers will ride on the crest of the wave. It would take a fair amount of study to determine what conditions will result in wave slap. Irrespective the bridge is a very narrow and stiff area compared with what the typical sailing cat has.

At anchor I expect anything bigger than about 500mm waves will cause slap. If you are tired enough you will sleep; some will find it soothing. Others will take time to get used to it or find another anchorage. I have slept in much worse conditions. My annoyance was the halyards slapping inside and outsidse of an aluminium mast.

In any case the sketch is a single solution as the result of a feasibility study. Anyone familiar with design knows that this is about 1 to 2% of the design process. I believe I have demonstrated it is feasible. We do not have anywhere near enough information to know if it will resemble the best solution. From what I can see everyone has their own view on what constraints should be applied and this is reasonable as the optimum will change depending on use. At least now there is some basic relationships between parameters like length, speed, stability, configurations, windage, waves, propeller diameter and power that will have similar relationships for other options that might be considered. I did not set out to design a boat. My object was to make a point about how prop selection is important to the solution.

Rick W

 

Oyster - its looking very nice, and seems to be a lot of boat for 8" draft. Thanks for some good ideas, sharing, and inspiration to build a dream.

I've got to do some work for a few days but will keep on eye on the thread and look forward to Erik and Ricks next stages. This feasibility study is broadly going where i want to be, but obviously still has a while to go before the epoxy gets mixed anywhere. I'll be a sponge for a while, soak up the good ideas and in due course come back. Some compromise of my 'boundary' specs in earlier posts aren't a show-stopper, and interesting potential solutions are coming up anyway.