What To Choose

[smithers1o1]

I've designed the hull and basic dimentions of my boat, but i cant choose what horsepower i need (im going to use an outboard) for a boat that is ~15 feet long, 4 feet wide at widest, and with a hull depth of 2 feet. There will be 2 1x1 outriggers 2' out on either side, for better stability, and that will convert my boat to a trimaran. I am planning to use plywood sheeting, and i would very much like help with what to choose. if anyone has any suggestions that could help me improve on my design, or suggest on what parts to use, i would very much appreciate it. if you have any suggestions, send them to:
smithers1o1.woodshop@gmail.com
TY


Here are the designes u have been requesting
15623

15624

[USCGRET/E8]

Check this site out:http://www.glen-l.com/

[smithers1o1]

thank you for ur suggiestion, and i am happy to say to you that it is the very site that convinced me to use plywood.TY!

[timgoz]

Hull depth? You talking draft or draft & freeboard. Sounds way to deep for a 15' boat if you are speaking of draft.

Tim

[BarendGrobler]

Use the biggest engine can find...always!

[messabout]

15 feet LOA x 48" beam is a small boat by most standards. Two feet deep is too deep if you are talking about the vertical distance from the sheer to the bottom when measured amidships. Bow height would normally be 2 feet or more. Transom height about 18". If you will use a short shaft outboard there will need to be a cutout section of the transom that is about 15" above the bottom.

I strongly disagree with Barend about the size of the engine. The biggest engine you can find will exceed 300HP. A boat this size boat will run 25+/- MPH with a 9.9 HP engine. More than 20 HP will be foolhardy. Consider the weight of the engine when you select one. Your boat at 4 foot beam is a skinny one. It will NOT take kindly to a heavy motor. A typical 9.9 weighs about 75 pounds if two stroke and near 100 pounds if 4 stroke. That is nearing the practical limit for a narrow boat like this.

[timgoz]

Messabout is 100% right concerning engine size.

Telling someone "to use the largest engine you can find, always" is not only blatantly wrong, but could lead a novice into serious trouble.

BG, please no what you are talking about before giving "advice" to others.

Take care.

Tim

[Gilbert]

I would strongly recommend a long shaft motor.
I would further recommend that you have 48 inch beam on the bottom and more at the sheer, that is to say, have some flare in the sides. If you do this, using two foot wide material for the sides is ok because as the sides tip out the actual depth of the boat becomes less than the two feet you mentioned. Also the maximum beam, at the sheer will approach 6 feet.
A 15 horsepower motor should be ideal and ten would work pretty well.
You don't want any rocker in the back 60% of the bottom if you want the boat to plane. If you don't want to plane, a much smaller motor will do.

[tom28571]

I would strongly recommend a long shaft motor.


Just wondering why you would recommend a long shaft for this small boat. We really don't know enough about this proposed boat to give any detailed advice.

[Gilbert]

I have two good reasons for recommending a long shaft.
If the sides of the boat are going to be anywhere near two feet high it just does not make sense to cut the transom down to 15 inches.
Also, statistics for small boat sinkings or swampings show that a large percentage of them happen when there is motor trouble in nasty weather. A passenger (usually THE passenger) goes aft to 'help' with the motor problem, the stern sinks lower in the water, the bow goes up, catches more wind and weathercocks the boat so the lowest point of the boat's freeboard is presented to the waves and OOPS! Five more inches of freeboard just might prevent such an accident.

[acp1934]

A 2' sheerline on a 15' boat will probably not look too good. If you are doing the design yourself,make a scale drawing [approximating the waterline] and look at what you've got.I would also go with Gilbert's suggestion about a 48" bottom width, increasing to the sheerline. I recently built a skiff about this size, although an inboard, and agree 10 HP should be plenty. Don't forget flotation chambers.Good luck with your project!

[smithers1o1]

thank you for your suggestion. according to all the mail i have recived, i have decided to use a 50lb 12hp 2stroke yamaha outboard. TY

[Gilbert]

You may find this link interesting.

http://www.smallboatforum.com/7_sba/7sbs.htm

When this page comes up you can click on "A Practical Outboard Skiff" an article I wrote about a skiff my father designed and built in about 1960.
Thom V is the moderator of the Small Boat Forum, and built a skiff from this article.
The beam on the bottom of this skiff is about 54 inches.
I will try to post some pictures on my next post.

[Gilbert]

It looks like the pictures will be attached this time.

[smithers1o1]

im sorry but i can not acces your web page, its blocked for some reason

[marshmat]

Smithers,

A few more ideas here, along the lines of some already offered.

The sketches you originally posted suggest the hull shape in the bow regions is somewhat faceted, much like an F-117 Nighthawk stealth fighter. (If I'm reading your sketches wrong, please correct me.) While this will hide you nicely from Soviet missile radar, any sudden changes in curvature (especially sharp edges) perpendicular to the water flow will really screw up the water flow under the boat. (Note that longitudinal sharp edges, ie. chines, are OK though.) Plywood bends quite well, and when bent makes nice fair curves- use this trait to your advantage and make all sharp kinks in the top (plan) and side (profile) views into smooth curves. You'll also find that it'll be faster and more efficient with a lower deadrise (ie, a shallower V-shape to the bottom); very sharp V-hulls are really only good in rough water, which a boat like this won't tend to see much of. If you want to do a slightly more formal 3D model, try Freeship/Delftship, it's free, easy to learn, and can calculate basic hydrostatic data as well as generating the cutting templates for the plywood panels.

As for power. Being in the US you'll need to check with the local coast guard about how to get a single-vessel capacity plate (in Canada you take some measurements of the boat, mail in a form and some photos, and the government does all the math for you). This plate will list the maximum safe power and crew/gear load the boat can take, based on a set of very well tested formulae that have proven reliable over time. My guess is that if you take your general concept and smooth out the lines, it'll go quite nicely with 10-15 hp and two crew.

[Gilbert]

Sorry about the link I posted.
I should have tried it first. It is the correct address, but somehow the browser doesn't find it.
I now see your drawing and realize you have been talking about something completely different that what I thought you were talking about. However, you may still be interested in the skiff article, so; type in small boat forum in your browser and when the site comes up click on Small Boat Adventures and the article A Practical Outboard Skiff will be there to click on.
I deserve at least 50 lashes with a wet noodle for the non-performing link.