Is there a way to figure horsepower needed for a boat by it's hp to weight ratio ?
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Boat weight to HP ratio ?
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Re: Boat weight to HP ratio ?
Don't think there is. Hull shape (length, draft, beam, deadrise, chines) will influence as well, making one hull faster than another even though they may nominally weigh the same.
Typically, in boat speed calculators (like this one: http://www.gofast.com/boat_speed_predictions.htm ), they operate with different constants for different types of hulls.
In other words, you probably can't determine the optimal HP for a boat by its' weight alone.
Now, if you know the weight and the hull type (i.e. constant), you could use that same calculator in reverse to give you an indication of the power needed to move the boat at a given speed.

Re: Boat weight to HP ratio ?
all boats since 1972 should have a plate on it with the maximum hp listed on it. you want atleat 70% of that rating or higher. if the boat doesn't have the plate, or it is homemade, you have to go by the USCG guidelines in this post,
http://forums.iboats.com/showthread.php?t=191283FLORIDA GATORS
TEBOW Country
Please, NO PM's (Private Messages) regarding boat/engine problems. they will not be answered.
That is what these forums are for. Post your questions, in the appropriate Forum.
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Re: Boat weight to HP ratio ?
i have an old popular mechanics book (from 1983) that has a slide rule that allows you to pick 3 of the following 4 options and it gives you the last option.
Planning Hull / Deep Vee
HP
Boat Weight
Speed
The book states most boats with proper prop and properly tuned will receive 90100 percent of the rated HP. Occasionally the speed may be higher than the slide rule dictates.
heres an example
Planning Hull
100hp
2000lbs

36.5MPH
anybody post here 3 of the 4 options and Ill give you the 4th.
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Re: Boat weight to HP ratio ?
the Coast Guard guidelines is the best way to go.
That being said, another way to get the ESTIMATED MINIMUM HP required to plane a boat. This is assuming your weight is accurate, and you are dealing with a flatbottomed boat in still water. It is very simple. Take your boat weight and divide by 50. So, a 1000 lb boat (boat, motor, gear, passengers), flat bottom or still water would take 20HP to plane.
If your boat with all gear is heavier, it will take more HP to plane.
If your boat has a vhull, it will take more HP to plane.
If your boat is in rough water, it will take more HP to plane.
Note: this is only to plane, not run around at 30 MPH.
So, let's look at this. You have an aluminum boat that weighs 450 lbs.
You weigh 250 lbs. You carry 6 gal of gas (total weight approx 48 lbs). This is now a total of 750 lbs. You will need a MINIMUM of 15 hp to plane. So, you stick a 15 hp engine on it and you are happy.
You then invite your friend to fish with you on your new boat. He weighs 200 lbs. You now have 950 lbs, and your boat will not plane, and you are not happy.....
Use the Coast Guard guidelines and you find what is the MAXIMUM HP you can put on your boat......
Just something else to think about....
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Re: Boat weight to HP ratio ?
This is a great post...I'm currently trying to find the best option for a boat that will suit my fishing/boating needs. I just fully rebuilt and overhauled a 1991 Johnson 60HP (bored over .030). Im thinking this motor will now last a long time especially since I will take care of it, so I'm looking for a boat that this motor can push and plane out nicely. Im not to concerned with super speeds but I do want the boat to plane out nicely.
Right now the motor is on a 15' boat that is pretty light and only 5 feet wide...and honestly its probably to much power for this boat. Im hoping to find a nice "bay" style V shape up front to fish inshore and some offshore on calmer days as well. Hoping to find something 1617 feet and atleast 7 feet wide or so.
I guess the real question is...what might be the maximum weight that I could probably push with my motor and get up on plane nicely? My guess is probably around 1600lbs? maybe a little more or less?
Cory91' Johnson 60HP
"J60ELEIB"
Fully Rebuilt Powerhead March 2008
89' Yamaha 225 V6 Excel
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Re: Boat weight to HP ratio ?
Originally posted by BigNiner19 View PostThis is a great post...I'm currently trying to find the best option for a boat that will suit my fishing/boating needs. I just fully rebuilt and overhauled a 1991 Johnson 60HP (bored over .030). Im thinking this motor will now last a long time especially since I will take care of it, so I'm looking for a boat that this motor can push and plane out nicely. Im not to concerned with super speeds but I do want the boat to plane out nicely.
Right now the motor is on a 15' boat that is pretty light and only 5 feet wide...and honestly its probably to much power for this boat. Im hoping to find a nice "bay" style V shape up front to fish inshore and some offshore on calmer days as well. Hoping to find something 1617 feet and atleast 7 feet wide or so.
I guess the real question is...what might be the maximum weight that I could probably push with my motor and get up on plane nicely? My guess is probably around 1600lbs? maybe a little more or less?
Cory
1600+200lbs Load+60HP= 30mph.
add another 200lb passenger and
=28mph
1600 sounds about right to me!
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Re: Boat weight to HP ratio ?
im sorry.....i dont know what your trying to calculate here.....
do you want to know how much horespower your allowed?...hit td's coast gaurd link.....the answer is there
do you want to know if the pop mech's book will tell you how fast you can go?.......
sorry...it can only guess.....the hull dictates friction.....a hook will rob you 3 mph....(thats a lot).....a rock can gain you a few......so there is a ton of variables...it might be a perfect world guideline...but
you can also have bigger torque and hp...but low rpms...(diesel)...
so.... you might have 400 hp...but 2500 rpms......and rpms are what make it go.......ie....a 19 inch prop travels 19 inch thru the water with one full revolution......so there is a big difference between a 200 hp 2500 rpm diesel and a 200 hp out board that will rev at 5500 rpm
*****disclaimer...(to all the tech guys,....this is just a very basic laymans explanation,)
now,..... if you want to know how fast you might go with a certain hp....hit this link....
http://forums.iboats.com/showthread.php?t=225803h
that was posted by 45 auto......it cannot take into consideration hull designs...but it will give you a good base line......(btw...45 again..thanks...great work)
i hope that helps...if it doesnt..please rephrase the question
cheers
oopsThe Hull Extension Threadgreat info on all aspects on boat building with detailed information.
http://forums.iboats.com/showthread.php?t=234392
IN MEMORY OF Our friend SpinnerBait_Nut LESTER WRIGHT July 31, 1953  Nov 26, 2008 RIP
IN MEMORY OF Our friend Tashasdaddy Robert (bob) Griffis. October 27, 1948
November 29 2010 RIP
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Re: Boat weight to HP ratio ?
OOPS beat me to it... the info in that thread is dynamite... definately required reading....Just another moron with a computer and too much time on his hands...
typical rainy afternoon on the river... http://www.youtube.com/watch?v=8Izi3v75jTs
more recent, amateur camera operator for sure!... http://www.youtube.com/watch?v=EHoXsQc6644
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