runabout hull design

[spear]

Hello to all.can anyone give me any info on the shelter island runabout? Ive decided to build this boat at 8 -8.5 meters length.Im thinking about wood construction or aluminum.I,m a professional woodworker but the idea of less maintenance with the aluminum sounds better. I am a good welder with steel but have never welded aluminum and thats the thing thats got me a little reluctant in using it.Plus with alloy it would be lighter than wood wouldnt it?this particular boat has twin v8 engines.Im thinking of using a smaller diesel motor for economy.as Im not interested in speed so much.would this play any role in what type of hull design I should use? if anyone out there can give some assistance to this first timer i would greatly appreciate it. thanks.

[Jeff]

First, welcome to our forums!

When you say you are going to build a 'shelter island runabout' at 8-8.5 meters, you are referring to this boat specifically or just the general appearance/style?

Will you do the design work yourself, or will you be looking for existing plans? Keep in mind that if you copy the style too closely (however this comes to be defined in the courts), you can get into trouble, for example http://www.soundingsonline.com/archive.story/1648.html
Granted this isn't directly applicable to a one-off project, but I'm still uncomfortable saying you're going to build someone else's design unless you have permission - better to refer to styles or a boat similar to but different than...

In any case, welcome to our forums. It's been a long day for me already, so hopefully the other members of our forums will jump in and provide better answers to your questions.

[Polarity]

What a great looking boat!

[spear]

thanks for the warm greeting and the legal tips.what I meant is ,Im planning on building a SOMETHING like this style boat and then do he deck and interior to suit my needs .I would like to use traceable plans if I choose to make it in wood.I saw something of chesapeake bay pocket cruisers which looked like they had pretty much the same hull design. or something close to the above pic .If anyone has any info on where I can get traceable plans on something like this style boat I would greatly appreciate it .and thanks again for the welcome.

[Portager]

The Shelter Island Runabout http://therunabout.com/ is a gorgeous boat, but is appears to be a production boat. It is 35 feet long not 8-8.5 meters. I do not think plans are available but you could contact the builder and find out. In addition, it is built using Vinylester resin infused Kevlar/E-glass composite with core.

Scaling a design it not straight forward as one might think and converting from composite to wood or aluminum isn't easy either. Doing all this for a design that you can't get the plans for is essentially a new design and requires the services of a Naval Architect.

On aluminum versus wood. You are correct that welding aluminum is more difficult that steel, but it is mainly a matter of using better cleaning practices and welding under inert gasses. Aluminum can be worked with wood working tools such as circular saws, rasps, ... Aluminum should be lighter that wood of the same strength, but only if the boat is designed for aluminum. Aluminum requires better vibration isolation and insulation. Sound and heat transfer very well through aluminum.

Wood boats (pure wood without epoxy encapsulation) are known to require more maintenance and therefore have significantly lower resale value. The main problem with epoxy encapsulated wood boats is sooner or later the epoxy will crack and let the water inside and then it starts rotting from the inside out. Maintenance requirements are lower but resale is about the same pure wood (this is just my opinion).

Gas (petrol) versus diesel

I am generally consider a diesel advocate so you might want to take this with a grain of salt. I prefer diesel mainly for safety. 22 years as a rocket propulsion engineer has made me conservative towards safety. You rarely get a second screw up in rocket propulsion. The safety difference between gas (petrol) and diesel is gasoline will evaporate in a standard atmosphere whereas diesel will not. These gasoline vapors are slightly heavier than air so they will collect in the bilge but since they are not a lot heavier than air they will mix with air. If the gasoline to air mixture ratio is between 1.4% and 7.6% then the speed of the flame front will be supersonic which means that combustion will be explosive. In general I try and avoid the E word. To reduce the risk of explosions gasoline powered boats should have explosion proof blowers (explosion proof just means that they do not produce sparks which would be an ignition source. Some builders have installed explosion proof blowers with non explosion proof activation circuits, such as non-explosion proof relays. Some owners do not realize that when the bearings in explosion proof motors wear out they can provide an ignition, but I digress) to evacuate the gasoline vapors from the bilge before starting the engines.

In general gas engines tend to have a higher power to weight ratio and lower initial or acquisition cost than diesel. Diesels have lower fuel consumption in gallons per horse power/hour, better longevity and reliability (assuming good design and maintenance). People who are interested in high speed and/or low acquisition cost generally select gasoline engines. People interested in safety, long range, operating cost, reliability and resale value tend to opt for diesel.

Let me expound on the fuel economy advantage because this is commonly misunderstood. In general diesels have a slightly lower brake specific fuel consumption (BSFC) than gasoline (some high efficiency gasoline engines are better than some less efficient diesels). Since BSFC is a measure of fuel burned in pounds per unit of horsepower per hour (lb/HP/hr) and we buy fuel by unit volume we need to compare the volumetric specific fuel consumption. The density of diesel is 7.1 lb/gal and gasoline is ~6 lbs/gal (it varies from 5.9 to 6.2 but generally is about 6), so if BSFC were the same the volumetric fuel consumption of the diesel would be 18.3% lower than gasoline. Given the BSFC advantage diesels tend to be 25% to 33% better than gasoline.

If you want to build a boat with the looks of the Shelter Island Runabout and unless you consider yourself a qualified Naval Architect, I recommend you try and find an existing design that you can buy the plans for or contract with a Naval Architect to design it for you. If you are interest, I am sure this forum can recommend a list of candidates designs or Naval Architects.

Cheers;
Mike Schooley

[spear]

thanks for your time . Ive got a better picture of things now.Just curious as to your quote on wood epoxy "sooner or later it cracks". in salt water conditions what would the average life span of wood epoxy be?I have just about excluded aluminum for lack of experience with the material and thinking of either wood or steel now. there´s a Bruce Roberts design waverunner designed for wood/fiberglass or steel construction which looks like it suits my needs .if there are any opinions on steel construction ,I would be glad to hear them. thanks again.

[Portager]

Originally posted by spear
Just curious as to your quote on wood epoxy "sooner or later it cracks". in salt water conditions what would the average life span of wood epoxy be?"

All I could do is guess. It depends on the loading of the boat, how hard it is used, the quality/type of construction and if it is kept in the water or out. I'd guess 10 to 20 years.

Look at some used boats at www.yachtworld.com and compare the resale value of wood versus fiberglass and steel boats. While your at it compare the price of production boats versus home built or custom boats.

I have just about excluded aluminum for lack of experience with the material and thinking of either wood or steel now. there´s a Bruce Roberts design waverunner designed for wood/fiberglass or steel construction which looks like it suits my needs .if there are any opinions on steel construction ,I would be glad to hear them. thanks again.

Steel is generally much heavier than fiberglass in this size range due to the minimum gage required to prevent buckling. In the 8 meter size range the deck house will probably be made of wood or fiberglass to reduce weight and lower the center of gravity.

I assume that you are looking at Bruce Roberts Waverunner 28. I notice that he calls the fiberglass version a planing hull but the steel version is called semi-displacement. I also notice that the fiberglass version http://www.bruceroberts.com/public/H...escription.htm is listed at 8,000 lbs, draft from 1' 8", engine is 150 to 300 HP and speed is 15 to 30 knots, but the steel version http://www.bruceroberts.com/public/H...escription.htm doesn't list weight, draft 3', engine is 20 to 150 HP and speed isn't listed. This tells me that the steel hull version will be much heavier and slower. If you want to go fast I'd recommend you go with the fiberglass hull. If you are satisfied with displacement speed, i.e. 7 knots, want a more rugged boat that can handle rougher sea states and provide a more comfortable ride, then go with the steel hull.

Cheers;
Mike Schooley