Which engine should I use??

[Mr.Design]

Hey all,

I'm currently studying Engineering, and my major project is to design a boat. I just have a couple of questions for you guys I'm hoping you can answer as I don't know alot about boats.

Firstly, I want to know how much Horsepower motors do i need on a 20 feetfishing Boat for the following specs:

* carrying capacity - 8 adults
* overnight sleeping capacity - 2 adults
* speed -5knots (Petrol) or 25 Knots (Diesel)

I also need any schematic wiring Diagrams to connect the electrics on the boat (such as Navigation lights Rear stern lights, decklights etc.)

I appreciate your support!!

[TerryKing]

http://www.boatdesign.net/wiki/Elect...ysical_Layouts
has some diagrams...

For the engine, you better do some reading on this, see some spreadsheets on this system, and Do Some Engineering....

[ROO]

Normal you want the vessel to cruise at a nominated speed regardless of its power source, so work out your top speed, if its 25kn fine, forget about diesel in such a small vessel (heavy), for simplicity sack stick to an outboard, as installation is so simple. Unless your talking about an outboard diesel. Go down to your local outboard outlet or give them a ring, tell them lenght of vessel, displacement (including weight of 8 people and gear) and ask them what outboard would you need to plane at 25 kns. ROO

[Mr.Design]

Thanks for the help guys i really appreciate it!!

more wiring diagrms would be apprecited

Cheers
Mr.Design

[Frosty]

Originally Posted by ROO

) and ask them what outboard would you need to plane at 25 kns. ROO

20 foot with 8 people and gear fuel etc etc?---mmmm well sir the biggest we have is 250HP -that might just do it!!

[ROO]

Dear Jack Frost,
your reply doesnt help anybody, most dearers are very helpfull, they rely on "word of month" as its the cheapeat form of advertising.
I was just down at my local Mercury dearer inquiring about outboards for a yachts up to 6.5 tonnes and he recommended a 20hp, it was called a Bigfoot, because the the gearing was 30% stronger than the conventional outboards and the prop was larger for because the speed was never intended to exceed the hull speed of about 7 to 8 knots. I also inquired about electric start and electric tilt witch he included, so not only did I get his advice but a brochure and a price list to cover all that I needed. And why stop at one 250hp why not not stick on 2, yes Im being sarcartic as well, did it help anybody I dont think so? Mike K

[Frosty]

250HP X gear ratio ( say 1.85) = 462.5 SHP divided by wieght, at a guess 20 foot boat +8 people +fuel + tackle etc etc= what say 4.5 tons =102

Now divide that by a K which is the efficiency of the drive

Usuall 3-4 lets say 3.5

102 divided by3.5=29 MPH divided again by 1.1 for knots=26.4KTS

Just made it.

Not bieng sarcastic its the way I talk - Hope youlle get used to it.

Fuel consumption of a 2 stroke is 10 gallons per 100hp used

250Hp = 25 gallons of petrol per hour.

At the risk of being sarcastic that why they make diesels for boats.

Hope this was more helpfull!!

[ROO]

Now that Jack Frost has thawed out and given you some very informative information, a smaller four stroke outboard would fulfill your 5knots, and simplify your design
speed -5knots (Petrol)
Good luck with your assignment

[Poida]

Now we have the engineering out of the way, what about safety?

The safety recommendations where I live is 1 person per 1 metre of boat length. 20ft = about 6 metres = 6 people.

The more weight in the boat = more displacement = lower in the water = swamped by wave.

I have a 6 metre boat and usually have no more than 4 for comfort.

And I also have a lack of friends but so does Jack so I'm not alone.

Poida

[ted655]

Sometimes Roo, peoples stated requirements do not equall their design.
Yours is such. Too many people & supplys/gear even BEFORE you add any cargo or run into any bad weather.
You aren't close to wireing or engines yet. You can get help here but when things don't add up, we say so. Some of us also have a bit of good natured fun on here.
1. Decide the use & range of the boat. 2. Be realistic. 3. pick a hull type or boat style that is SAFE, seaworthy & suited to the job. 4. Chill out. Then.... we will help with your homework.

[ROO]

Ted655,
a lot of your points are valid, but I believe that Mr Design is at the prelimary stage at the moment, just gathering peoples idears. I was looking at it from a design point of view. Its a lot easier to design for an outboard than inboard, for example an outboard you dont even need a rudder, the outboard can steer the vessel. You dont have to worry about seacocks, shaft, shaft tubing, no through hull fittings, no engine bed etc etc just a sturdy transom. And it doesn't encroach apon your cockpit or accomodation area. Its pretty much clamp it on and off you go. As far as other engineering, stability and seaworthiness that's up to Mr Design to work out. Remember its an assignment, Im just trying to make it simple for the guy.

[ROO]

Hey Poida,
yea mate I raised my eyebrows when I read 8 people in a 20 foot boat. I was thinking that maybe Mr Designs teacher did that on purpose.
All I can see is Mr Design taking his little boat to sea, and sitting next him is The Skipper, Gillian, Ginger, Maryanne, The Professer, Thurston Howl the 3rd and his wife, on that fatefull day. Mate, I have a shore cure for loneiness, have a couple of kids and you will never be lonely again, then again you will never know peace either. LOL

[Frosty]

Mr Designs tutor needs tutoring.

These are quite alarming if not amusing specifications.

A boat --a planing boat of 20 feet that needs sleeping accomodation for 2.

Well at 20 feet LOA,-- a V berth would be more or less at water line and would require 6 feet minimum. say water line 17 feet less 6=11 feet left.


Now during the day 8 people will need space to fish in 11 feet. we still have not a toilet or basin, lets assume these would be esential, if they are not them why the sleeping accomodation.

Toliet one side basin the other say 3 feet, we are now down to 8 feet!!!!

The engine is what? mounted on the transom, so a slosh well is needed,--say min 1 foot (small slosh well).

Now we are down to 7 feet. We havnt got fuel in yet or water or a live bait tank,holding tank, batteries etc etc.

8 people in 7 foot fishing? With fishing rods?????? hey we havnt got a steering position yet?

Mr Design tell your tutor that his design is--- well you can think of some thing, and does not deserve the time thinking about . You will probably get an A.

[ROO]

Hey Jack
Can you imagine the 20' SS Minnow (aka Mr Design) at sea, and one of the fishermen hauls in a 3' shark, where would you run, everyone would be jumping into the sea, the only one left on board would be the shark. I've solved the sleeping problem and the toilet (head) arrangement, sleep standing up, and they will just have to crap in their pants.

[mvd535]

Hi Guys ¡¡¡,

Sorry for change the matter, but I am new in this forums and I don´t know the exact place to go into ask for help.
If anybody here can guide me I will appreciate very much.

I am 54 years old Chilean naval architect but I never was involved in stern bulbs. I recently was asked to give my opinion on a particular design which I am attaching here (PDF).

Please if anyone would like to get into this matter he will be very welcome.

(Sorry about my english, I will try to do my best)

Kind regards,
MVD535