Boat weight to HP ratio ?

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JJB

JJB

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Is there a way to figure horsepower needed for a boat by it's hp to weight ratio ?
 
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stevens

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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.go-fast.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.
 
tashasdaddy

tashasdaddy

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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=191283
 
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zibzer

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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 90-100 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.
 
txmntman

txmntman

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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 flat-bottomed 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 v-hull, 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....
 
woodee148

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Re: Boat weight to HP ratio ?

Modified Vee
4000lbs (empty)
260 hp

thanks !
 
JJB

JJB

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Re: Boat weight to HP ratio ?

my two factors are 3700 lbs loaded and a modify deep v hull what would be mim plane hp ?
 
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BigNiner19

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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 16-17 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
 
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zibzer

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Re: Boat weight to HP ratio ?

BigNiner19 said:
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 16-17 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
Click to expand...

according to Popular Mecahnics:

1600+200lbs Load+60HP= 30mph.

add another 200lb passenger and
=28mph

1600 sounds about right to me!
 
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zibzer

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Re: Boat weight to HP ratio ?

Any gurus want to tell me if the P.M. book is telling the truth or not?
 
Mkos1980

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Re: Boat weight to HP ratio ?

3550lbs
270hp
________
55.1 Gps
 
oops!

oops!

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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 re-phrase the question

cheers
oops
 
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wca_tim

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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....
 
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zibzer

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Re: Boat weight to HP ratio ?

Mkos1980 said:
3550lbs
270hp
________
55.1 Gps
Click to expand...

sorry, your off the charts. P.M. says after 40mph its hard to dictate what a given boat and weight will get. so on that note. your boat will get something over 40mph
 
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wca_tim

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Re: Boat weight to HP ratio ?

Oh... and 2500 ponds, 220 horsepower, 58-60.

That calculator based on weight has me doin like 70... maybe behind the truck headed to the water... but not with this engine!
 
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Ekorib

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zibzer said:
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 90-100 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.
Click to expand...
Hi, I realise very old post!! Can you still help?
A 9.6m rib
Elec outboards and a lot of batteries
Planing hull alumninium
1800kg boat and engines and 1500kg of batteries. Total probably 3500kg = 7700lbs
Engine equiv is 140hp.
I realise very low and not expecting high speed.
It is actually 2 elec equiv 40hp each, 80hp
And 60hp petrol in middle

So can run as hybrid to help get on the plane then switch off the petrol. That's my thinking.
Really appreciate any help to guesstimate it all.
It's a boat I am building with some development engines.
 
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