Mini wooden race boat

[Majstersztyk]

Hello

I have 2x briggs 5hp vertical motors
I have found 6 nice outdoor 3/8 plywood planks,
I have a full access to woodshop at work with lots of strips of hardwood and have fiberglass and epoxy for reinforcement.

inspired by: http://www.vintageprojects.com/boats/little-boat.html
...and want to build "mini" speed boat with inboard motor using outboards lower end going through floor.

Question....how about i stuff the boat with 2 of these motors side by side (double it)

What are the advantages to running 2 inboard motors instead of 1.

...tourque...(or what its called in marine terms), or better acceleration top end speed or both?

I'm sure if i race it wherever in the hood or querilla, i have better chances of making it home with 2 inboards.


Ps: those briggs can be port and polish up to 10-15/hp quick and cheap, retaining durability.
I am new to motorboat subject- please advice.

[Majstersztyk]

opps. 83 views no replies.
worthless effort
moderator please delete the post
or move to motorized (or more appropriate) section of the forum.

...below pic of progress

[PAR]

This isn't a worthless post, though you do need to consider the realities of engineering and boating 101. Out of the box is quaint and refreshing, but often not reasonable, which is likely why you've received zip in replies.

Why? Well, you haven't considered the conversion of the Briggs engines, nor the twin engine/drive considerations, plus all the other things decided on with this sort of build. As a result most have skipped over the post to look for other, less hair brained posts to respond to.

This isn't a personal dig at you, as we see this sort of thing all the time. It is a clue as to how far off base you might be though. Again it's not directed at you personally, but many "interesting" ideas get posted, most have been handled in previous threads. Maybe you should evaluate the concepts and ideas behind your "idea" and consider the ramifications of your concepts.

[Majstersztyk]

thanx for dialogue par

I am not in a situation to question any responses, since i am a total nooob, anything from vocabulary of the subject, to boat design is a unknown for me, so thanx for sticking here and i can try make this tread more worthy of existence

Now i will go into more details of this design and build.
Please comment.

1. In reference to motor, conversion it shown in the first post in attached document, on the perspective drafting of the "little fellow"

It utilizes bottom end of 3.5- 5hp outboard motor, installed inboard below the floor, and motor attached directly to the top of "bottom end" (middle of outboard is eliminated).

Bottom ends are fairly available online, I might fabricate the motor bottom, end connection myself (or ask machine shop if decided to to w/ double motor). This in low h/p application balancing might not be crucial... will see.

2. In reference to the frame i could not find any zip files, so did transfer the plan (bottom of page 3 of attachment)as shown onto full scale 1:1 gray paper. I have retraced all the sections of the frame as the proportions follow, and soon i will be trying to start bending frame- which hopefully will be not too tricky. On another hand i am an industrial designer-model maker by trade and build race cars after work (72 fully ported 1641 vw beetle and 78 bridgeport/widebody rx7), so i think this little project is doable and since i have majority of resources on hand (wood, motor, epoxies, pulleys and years of gathering mechanical junk) it will be pocked friendly.

I will be trying to build this boat fairly quick, and will worry about motor application after, therefore i have created this documentation to give me some more technical and designing help.

Thanx for viewing.
M

[gonzo]

If you are going to hot rod the engines to 15HP the 3.5 lower units won't handle the power. The little plastic propellers with the sideways pin will break. The gears and shaft won't last long either.

[Majstersztyk]

Thats the dillema.

In that situation I would not go much more than stock 5h/p...I would keep original cam and piston.

... just polish it to so it breaths crisp and puts out little higher rpm.

Gonzo you have mentioned about plastic propeller breakage, how different would be 15h/p from 5h/p spectrum lower ends of outboards . I am interested in strength, but also size-to minimize traction (drag of water). Thanx for hint

Thats why i though about putting 2 smaller motors, but i dont know of advantages that it would give in water, vs larger but also heavier unit.

Does marine application have the same weight-h/p relation as in cars or motorcycles?

[gonzo]

The 3.5 has a little sideways pin. The 15 has a proper spline and a metal propeller. A too small propeller will cavitate and stall. Kind of like putting skinny tires with a lot of power. You'll spin them in place and get nowhere.

[Majstersztyk]

Great set of powerful clues
Gonzo- in reference to your last post "The 3.5 has a little sideways pin. The 15 has a proper spline and a metal propeller."

Ill take look at this detail difference using my favorite resarch engine for pictures of mechanical aspects- ebay
Thanx

[gonzo]

If you are looking at used legs, there will be little or no difference in price within those sizes.

[Majstersztyk]

yes. that so called leg is probably one thing that i am missing at this point,

gonzo...based on your personal opinion and the size of the boat that i am doing (8").....and i know this project is not a norm...

would you say 2x 5h/p
or 1x bulky 15h/p
i am trying to find out life difference between application of 2 motors vs larger 1

[gonzo]

One engine is always faster because there is less drag and mechanical friction. Also, it is lighter. Check out 15HP formula boats and you'll see the speeds they get.

[gonzo]

http://www.google.co.uk/search?q=15+hp+racing+boats&sourceid=ie7&rls=com.microsoft:en-GB:{referrer:source}&ie=UTF-8&oe=&redir_esc=&ei=3n9ITf_PHMOD5AbNlNigBQ#q=15+hp+racing+boats&hl=en&rls=com.microsoft:en-GB:%7Breferrer:source%7D&prmd=ivns&source=univ&tbs=vid:1&tbo=u&ei=3n9ITcH6OdmW4gan7uXfBQ&sa=X&oi=video_result_group&ct=title&resnum=7&ved=0CFsQqwQwBg&fp=b9b7533918968408

[Majstersztyk]

yeah..
http://www.youtube.com/watch?v=SfGRKgjI0pM

video shows bunch of this mini racers
I even think i see a design (yellow boat on begining) which is similar
to minimax/minimost set up.
I was inspired by that design especially when i found those planks of 3/8 outdoor plywood
but i'll go to ocean shore line with some waves, so i eliminated flat design,

thanx for hints
M

[tunnels]

Growing up i had a bunch of pictures of those boats , The were using out board motors with out the mid sectionMotor inside and the lower part outside . Lots changed but the idea was great . Just strap a outboard on the back !!!

[PAR]

I too raced the class hydros as a kid. I was a three pointer once I got some experience and up 'till not many years ago I still had one of those old three pointers. I used to go fishing in it and people thought I was nuts, fishing rods sticking up in the air and all.

The gas, garden tractor engine over an outboard's lower leg isn't a new idea. I've done it for special applications, usually "inventing" a home made sail drive on the cheap. The basic issues are many, unless you live in a place where the air temperature is always cold, cooling will drive you nuts. You'll need substantial duct work for an air cooled engine, because without the duct work, you have to hear the damn thing make all it's racket. Noise is another issue, so most stick it in a well insulated box (hence the ducting). The exhaust is another issue. If dry, you got to deal with the pipe from hell. If wet, you have to install a pump and fab up a manifold.

Frankly, considering a few hundred bucks will get you a fully operational outboard, which none of these issues, it's a bit silly to consider putting a lawn mower engine on an outboard leg, even with a V drive and straight shaft (why for God sake).

Two shafts will have a lot more drag then one and speeds will suffer for them. There's no advantage to twin shafts in small craft. In large craft, where maneuverability can be a big issue, twins can shine, but in the boats you're thinking about, you can stick you hand in the water and turn quite well, if the rudder takes a dump on you.

Lastly, straight shafts truelove suck when it comes to maneuverability. A rudder requires a good bit of flow over it, before it slowly responds to a helm input. This is fine at speed, but steering is usually more critical when you're attempting to avoid things (like concrete docks and other boats) while traveling a low and very low speeds. This is when the flow over the rudder issue lifts it's ugly head and in spite of having the helm hard over, the boat doesn't do squat and plows into what you're attempting avoid.

Why the aversion to an outboard? You can rebuild a broken one for the money and effort you'll put into engineering a weird V drive, lower leg, garden tractor motor setup. With the outboard, you'll have unmatched maneuverability and it'll be reliable too.