economical coastal cruiser

Rick that boat might be okay in the canals but it would knock the livin snot out of you in Lake Erie, The Chesepeake Bay or the Albermarle Sound on a typical afternoon.

 

A valid point, and it could be applied to any boat intended for relatively calm conditions.

I've been working on something ( http://www.boatdesign.net/forums/projects-proposals/trailer-cruiser-revisited-trimaran-27032-12.html ) that's perhaps not quite as fuel efficient as some of the boats mentioned here, but that should be capable of handling awkward/heavy loads as well as taking on relatively rough conditions.

At 2.5 tonnes displacement, it shows 10.9 kW indicated for 12 knots (once propeller efficiency, etc. are considered, figure about 24 hp at 12 knots). With a gas outboard, that's about 6-7 nautical miles per gallon. But she's shaped to max out at 20 knots, and has unusually large outriggers to suit certain service requirements we have. With a bit of massaging, slightly lighter construction, and an inboard diesel with a big, low-speed prop, such a boat could probably get close to 10-12 miles/gallon.

Just a thought....

 

I expect it may be useful to give an example of what a large diameter high aspect prop looks like. I happen to have one laying about so I photographed it - refer attached.

This prop is 800X400 and is made of nylon. It is good for a maximum thrust around 500N. It flexes beyond that load. It would need to be quite a lot stronger to take the 3kN bollard pull capable with such a prop connected to a 20kW diesel. That can be achieved by making the blade chord larger and/or using higher strength materials.

The speed being considered on this thread is a region where cavitation can be avoided even when using quite high lift, thick blades. Standard marine props are designed for higher speed and considerably higher disc loading pressure - therefore lower efficiency.

One of the problems with the current state of marine propulsion is that all the moneyed effort is focused on the high speed end. You will not find anyone making props to a design that works efficiently in the application being considered here. The simple 12kt/12nmpg equation means the prop needs to handle 16kW at 6.2m/s. There is a JavaProp screen image attached that shows the efficiency of a 600mm prop under these conditions. JavaProp is a reasonably simple tool for analysing propellers where cavitation does not occur:
http://www.mh-aerotools.de/airfoils/javaprop.htm

I worked this out about 5 years ago and have been making my own props since. About 4 years ago I was pointed to JavaProp as it was doing much the same as the model I had developed from first principles. Where I can, I use off-the-shelf props but they are usually from model airplanes like the one pictured.

The main reason for the image is to show I am not considering the use of a tonne or so of bronze. It could be a low BAR sail-drive prop proves to be a good choice. That is the nearest you will find in the marine industry I expect.

Rick W

 

That's the whole point Matt... this is a boat intended for coastal cruising... not the local dam...

There's a whole bunch of stuff here that deserves comment. Sadly I don't have the time just at the moment. But I will offer just one thought...

Just how much study does it involve to see that a wave with an amplitude of 400mm (trough to crest) will strike that big flat plate?

It matters little what sort of multihull I'm talking about. The simple fact is that with insufficient bridge-deck clearance, waves will cause an intolerable motion and racket. 200mm is IMHO insufficient. I would consider 600mm to be an absolute minimum and as much as 900mm to be desirable

 

This is too simplistic. A boat when in motion creates divergent waves that produce a trough along the side of the hull that influences the height of the wind waves in proximity to the hull. The bridge is sitting above the trough. It is not like a catamaran where the hulls are usually separated by much more than 400mm. The rule of thumb there is a clearance angle of 45 degrees to the bridge deck. That is what the shape I presented above achieves.

I know from my own experience with a 100mm high clearance that I can operate in waves over 300mm before I start clipping them. And this is with a tiny hull that has miniscule waves - an order of magnitude smaller than what a 12m boat at 12kts with a 500mm deep entry will produce.

The attached shows the divergent wave from a slender hull with high T/B ratio - similar to the 12m shape presented. Displacement is a miserly 100kg though. You can get an idea of the divergent wave pattern at different speeds. The height is also apparent at the slower speeds. 10kph on this 3.6m hull will be roughly equivalent to 12kts for a 12m hull. The angle will be the same but the depth proportionally larger.

I would like some suitable means of analysing the situation short of building a hull as it is certainly and area that needs attention to determine one of the competing constraints for the bridge height. However the method of analysis needs to be considerably more scientific than the simplistic approach you are offering.

Rick W

 

:?: :?: :?:

I am getting used to have no price tag. Generally, buying people do not expect the same from a 20 000$ and a 100 000$ boat. But I may be wrong.

What rules to comply with ? I do not know any professionnal NA what would dare to sign and publish (even for free) a plan non compliant with his country rules (ISO or ABS/ABYC/USCG or ..) even for homebuilders that have not to show formal compliance. Liabilities are way too big.

Payload, sea state (or ISO design category) ? I have noticed in this thread a plan with limited side hull draft and bridgedeck clearance. Is this for loading as minimal operating conditions, or fully loaded ?

Overloading (which leisure boat will never be in this case ) tolerance, overpowering tolerance, life expectancy ?? i.e. design safety margins.

 

You are looking at a single number and not a combination of numerous factors. Since you are discussing outboards, I can do this and have done pretty close to your numbers with an outboard in complete comfort without being squeezed into a shoe box. The important number is what want to do in the course of a day, cover ground or spend your time sitting at a helm. How you value your time and your sole intent when boating sometimes places a fuel consumption number at the bottom of the lists in priorities.

The draft is truely the most important thing when considering a power plant for some people, me included and weighs in the decision making process.

There is another little discussed number and thats slippage at dedicated speeds with fuel burns in power boats which can be both looked at in power boat, trawler and motor sailor hull designs.

 

For the purposes of this thread, I brought up that example only to suggest a possible way to approach the OP's desire for 12 nmpg @ 12 kt in something that can handle significant sea states and still carry some accommodations. There are plenty more design parameters that can and should be discussed. I don't design based around a single number and I don't know any successful designers who do.

Agreed. This won't be the case for everybody, of course- we each have different priorities, and for many boats the cost of fuel is not a huge factor. For others, it is a major expense (fuel is the single biggest component of my current boat's budget).

A valid point, and one that our OP will have to consider, since the low-rpm, large-diameter props that tend to improve the efficiency of the drivetrain tend to be in conflict with a desire for shallow draught.

As in, propeller slip? That's a parameter used in some prop sizing calculations, but I'm not sure what use it is on its own....

 

Well if you are using a planing hull versus a displacement hull and use an inboard engine versus an outboard engine, you restrict one component in the uses and desire for a coastal cruiser, a very important component in choosing a particular design. As far as fuel costs and burn, if you are burning 2 gallons an hour at 20 mph, versus 1 gallon an hour at ten knots, this becomes apparent that you truely have not saved any money as the big criteria that you state. You also cannot under any circumstances push some of the hull designs with even the modest increased speeds without wasting fuel for an additional one to two mph in speed.

If you are intent on the highend thrust at low speeds and look at the single number as in the fuel burn, then you also loose the advantage of the top end speeds oe even mid range depending on the planing angles of your boat when needed or want it and the shallow draft advantage too if you go inboard. Sure in the days gone by, inboards were more dependable.

But you also loose interior space inside and may create some launching issues too, not withstanding loading and unloading at boat ramps. Under most situations inboards for what you think you may gain looses in so many other areas if you are directing your search to trailable boats and designs. The trailable aspect is the key component and must be the very first criteria in the creation of a new boat build.

As a side note, and for idle time reading, I am not sure if anyone here is familiar with a Glen L Cabin Skiff that was stretched and has about 27,000 nautical miles under his bottom. He decided to change out his outboard even though his reliable mechanic told him that it was not needed. Enjoy..

http://egyptian.net/~raymacke/

 

A prop of that style, perhaps forged from bronze or stainless steel, could be a good choice for a very high efficiency displacement hull. My only concerns would be the availability of suitably steep reduction gearboxes, and the draught issue (may or may not be a concern for our OP).

A valid point; however, we should probably take note of why "standard" props are designed with higher blade loading, higher DAR and tend to have lower efficiency. A large diameter, slim bladed prop rapidly becomes far too large in diameter (both for draught and for the structural stiffness of the blades) as we start looking at tens or hundreds of horsepower. "Standard" props do tend to be pretty well optimized for the constraints they face; they just aren't well suited to the rare situation where you have low power and no draught restriction.

2 gph at 20 knots is clearly the same fuel per distance as 1 gph at 10 knots, yes. Could you clarify how this fact relates to our (admittedly arbitrary) cruise target of 1 gph at 12 knots?

Below about 100 hp (and closer to 150 hp for lighter, planing hulls) I tend to prefer outboards, as do many folks (but not everyone). You'd have to use the boat a LOT for the fuel savings from an inboard diesel to outweigh the cost advantages, easy installation and beachability of a small to midsize outboard.

 

With apologies to Rick, I know that the large diameter props he is discussing have no application that I can think of for a practical trailerable coastal cruiser. All the issues that have already been mentioned will rule it out of any of the coastal cruisers that I am interested in. On the max draft that I would consider for this use with an inboard, I've been able to fit in only a 20" diameter prop without going to a tunnel. Even this boat is much more restricted in launching and beaching than the outboard powered ones. Even poor launch ramps can be hard to find in many places and good ramps are even more scarce. Secure parking for the tow vehicle and trailer can also be difficult to find. Nevertheless, the ability to tow your boat to distant cruising grounds far outweighs these problems.

I wonder how many who post here have actual experience with these issues?

I would be interested if some of that energy was spent in designing an "optimized" propeller for existing outboards. I have a Yamaha T50 that accepts a max diameter of 14". This engine does sport a relatively high gear ratio and is aimed at the lower speed, high thrust market. Is it ideal for the speed range from 10 to 20kts? I don't know and Yamaha will not furnish any details.

I am a bit familiar with the effects of low bridge decks on Cats and can say that high is better. Driving or riding on a cat with a low bridgedeck in any significant chop is one of the most uncomfortable situations situations and an example of poor design. One 44' under construction locally has a clearance of 2' 9" and is projected to be capable of Atlantic crossing.

 

Tom, just a suggestion about leaving you trailer and truck in the parking lot for a couple of days. Locking hub nuts and wheel locks will slow down thieves but the best I've seen on stealing a truck is find the fuel line and where it turns upward either inside the dash or in the engine bay, cut it in half and put on a manual shut off valve(like the one going to your refrigerator ice maker).
They will only get 100 yards or so and will abandon the theft.

 

Well its important if you are attempting to design any cruiser to maximize its uses , trailering is at the top of the lists in this discussion when it comes to even the bottom shape. But as a rule an inboard almost always weighs more than an outboard and drafts more and even costs more even if you boat bottom is a shallow draft planing hull. You can purchase a lot of fuel for five grand by comparison to an average inboard setup versus an outboard hull.

When you go to a displacement or semi displacement bottom, you complicate your launching, trailering and restricts for sure the cruising areas of all coastal regions in 2/3rds of the east coast and in the gulf coast region. You have defeated the intended purpose if your intention is fuel efficency if you attempt to use an offshore design or semi displacement primarily inshore. Even with a planing hull and inboard, the draft is just too prohibitive to do much unless you are a marina queen.

So I go back to the idea and notion that pound for pound, dollar for dollar, the outboard on a planing hull with some interior comfort for all weather use is the only way to go especially if you are looking at fuel burns and liteweight builds which assists you in towable hulls. When you do this, you have the advantage for low fuel use and the potential speed when needed which is not much past the intended stated burn.

 

Mike, You and I have been all through this kind of decision tree and through experience have arrived at the same conclusion. I hope you will be able to weather the current crisis and that, at the end of it, the boat is still yours and Linda's for your cruises as you intended.

 

Here is Phil Bolger and Susanne Altenburger's take on all this. Wading through Susanne's verbose prose is not for the faint of heart, but there is a great deal of information there.....I wish Phil was still around to edit.