specifications

M

mudslinging79

Member
Joined
Feb 23, 2009
Messages
69
Location
salem oregon
looking at building a boat, gbot licenseing info, looking for the formula on how to determin capacity/motor size, if anyone has the formula the law uses, dont want to build too small for 3 people, 18-25 horse minimum...
 
J

joesnuffy

Active member
Joined
Oct 1, 2008
Messages
645
Location
Baker City, Oregon
I don't know the formula but this is the rule i go by: The bigger the boat, the bigger the engine.

I hope this helps :p lol
 
M

mudslinging79

Member
Joined
Feb 23, 2009
Messages
69
Location
salem oregon
lmao, true, but i dont need a 16 foot monster, looking bout a 10-12 foot johnboat.
 
M

mudslinging79

Member
Joined
Feb 23, 2009
Messages
69
Location
salem oregon
we talking bout the same thing? last time i was googleing i forgot to lock the door...
 
T

Thuggin4Life

Well-known member
Joined
Apr 10, 2009
Messages
3,778
Location
Springfield, Oregon
I have see nthe formula before and its only for weight. I think just about all 12' boats are rated for 3 people at so much weight and just so much weight. seems like with the motor gear and 3 adults you are always slightly over the weight. after talking to several cops as long as the boat doesn't look overloaded and unsafe it will never be an issue. now if you put five people in it and someone gets hurt it will deffinantly come back on you but if its just threee guys in a boat that has three seats and you are slightly overloaded don't worry. As for the motor not sure how they determine that. But on a small boat like you are thinking I doubt legally you can go as big as you want. my boat is rated for a 7.5 and I doubled it to make it satisfactory. Built a very strong transom and try to derect some of the energy up the sides of the boat and as long as the motor isn't too heavy you should be fine. I bet on a little john boat a 15 would be plenty. My buddy has a 14' lon boat with a 9.9 and the boats heavier than my 12' and with the same amount of weight and a 9.9 on my v hull boat he went noticably faster than me. Also most 8, 9.9, 15 horse motors are basically the same motor and don't varry to much in size and weight. you don't want to go with to much power or your boat will be to squirrely. like if i put a 25 horse on my and had a heavier guy in the bow it still would be way to much motor for my little boat and could be dangerous. But back to the weight formula if i remember it correctly if you make a 12' boat all you would have to do is make it wider and deeper to get a heavier capacitty rating. My boat is 5'3" at is widest and has a "flat hull toward the back so its really stable. Maybe make your john boat 12-13' long mines 12'3" and is a 12 on the title and had a 11'9" that also was a 12' so maybe look into that as well. but make the transom instead of 15" for a short shaft go with however is meant for a long shaft giving you a deeper boat wth more freeboard and in turn giving you a highr capacity rating. I think a 12'3" deep wide jon boat built solid could easy hold three adults and be very stable and even since its wide you could look into making some storage. i have been in jon boats with wood floors and storage and they are way better than the basic aluminum ones I have been in. Also a well built sturdy deep wide jon with a renforced transom might be able to handle the bigger motor you had in mind. Also you could dropp cut the transom to give you the deeper sides and still fit a short shaft on it. And 2 strokes wiegh less than 4 strokes buy quite a bit so that could be the way to go to get a bigger motor in a smaller package reducing weight in the boat and on the transom. Hope this helps a bit just my ideas.
 
M

mudslinging79

Member
Joined
Feb 23, 2009
Messages
69
Location
salem oregon
hmmm, well my 12 foot zodiak flies with me and the 18, may just do a 12 foot johnboat and use the 18 on that also. just dont want to be underpowered.
 
T

Thuggin4Life

Well-known member
Joined
Apr 10, 2009
Messages
3,778
Location
Springfield, Oregon
A 12' jon will never be underpowered with a 18 on it. Keep us posted as you progress on your project and be sure to include pics. Good luck.
 
T

Thuggin4Life

Well-known member
Joined
Apr 10, 2009
Messages
3,778
Location
Springfield, Oregon
Here is the formula I was thining of turns out its jut for people capacities (Boat Length In Feet X Boat Width In Feet) divided by 20 = Number of Persons and this is based of a 150lb average for a person. so if it fits 3 you could substite on for to small kids and have 4 people legally but as i said before going deeper and wideer wil give you more displacemant equalling more weight capacity.
 
T

Thuggin4Life

Well-known member
Joined
Apr 10, 2009
Messages
3,778
Location
Springfield, Oregon
Oh I thought you were going to be making one out of aluminum. Looks like a good boat for ponds kinda small to do any crabbing and could do some trolling but be carefull if the wind pics up. If it were me I would expand on the plans making it wider a lot wider so it will be more stable. also I would try to make it deeper and like you said maybe shorten it up a bit. Maybe ad some extra braces for a motor and if you go wider and deeper I wouldn't put in any seats until the whole shell was completed then you could configure the inside anway you want with the benches and maybe add some compartments. as that boat stands I would imagine an 18 horse being too much motor for that boat but looks good but seems its more of a paddle boat with the option to put a small outboard on it. Good luck.
 
M

mudslinging79

Member
Joined
Feb 23, 2009
Messages
69
Location
salem oregon
yea, saw that, figured wouldnt be too hard to make one, ill get the shell dimensions from a mas produced one, and make one like i want it.
 
S

stream2.5

Member
Joined
Jan 26, 2010
Messages
117
Location
NE P-Town
max hp is 2X( hull length in feet X beam in feet) -90 so to get 25 hp rating in 12 ft of boat one would need a 5 ft beam


so for example... 12 ft boat 5 ft wide... =60X2 =120 -90 equals30 hp.....maximum rated.... most mfg drop this number around 10 percent for overloading and safety and cheeper liability insurance..... my little go fast was rated at 175 hp..... its 7.5' wide and 17.5 ft long....

so 17.5X7.5 X2 =262.5 - 90 is 172.5 so heres where things get tricky.... numbers are rounded to the nearest higher 5 hp.... thus the 175 rating..... but the USCG is more concerned with weight than actual hp so thus the reason i can run a 245 or 260 or 280 or even a 2.5L 300 hp motor on it because those engines are all the same weight as a production 175 and so thus they still meet the level floatation requirements set by the USCG and as such are not over powered according to the posted capacites plate....

my alumaweld is actually derated based on that formula..

20X8X2 is 320-90 is 230..... it was derated to 145 prop/200 jet.... because alumaweld didn't want the liability payment full rating would cause... which is a waste being the boat isn't even goverened by that standard..
 
Last edited:
Top Bottom