# what motor to use? 16 hp needed



## Frank (Dec 6, 2008)

Do a search as some vendors offer kits I believe. A friend has converted 2 boats, one using a"regular" motor with coupling hooked to the prop shaft. The other is smaller and placed a"pancake" motor on top of the lower unit of an old outboard motor. You can probably use lead acid batteries at low voltage. Both are viable. With an inboard I'd be thinking of a golf cart sized system. EBay?


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## captain jack (Jan 12, 2014)

Frank said:


> Do a search as some vendors offer kits I believe. A friend has converted 2 boats, one using a"regular" motor with coupling hooked to the prop shaft. The other is smaller and placed a"pancake" motor on top of the lower unit of an old outboard motor. You can probably use lead acid batteries at low voltage. Both are viable. With an inboard I'd be thinking of a golf cart sized system. EBay?


i'm trying to avoid commercially available kits and units. they are all fairly expensive...at least the ones i have found. i want to do a complete DIY set up. i think it could be done for drastically less than buying a premade set up. 

i had been thinking about a golf cart motor. i also just happened to remember that my father had a number of electric motors, of various sizes, that he had collected to build lathes and other equipment. he never got to use most of them. i will have to see what he had.


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## boatclassic (May 15, 2009)

Try posting on [email protected]. They are pretty helpful


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## captain jack (Jan 12, 2014)

boatclassic said:


> Try posting on [email protected]. They are pretty helpful


thanks. i will try that, too.


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## Duncan (Dec 8, 2008)

Hi

Boats and power are funny - you will find that it will only take a small amount of power to reach your "hull speed" - if you try to go faster you will use a lot of power for a tiny increase in speed

Motors
Golf cart or forklift motors will do fine, most can be run at higher voltages than the nameplate,
Read the thread on using an old fork lift motor

For your purposes if you can find an old golf cart or forklift think about taking the motor and the controller - and any other useful bits like contactors and cable


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## captain jack (Jan 12, 2014)

Duncan said:


> Hi
> 
> Boats and power are funny - you will find that it will only take a small amount of power to reach your "hull speed" - if you try to go faster you will use a lot of power for a tiny increase in speed
> 
> ...


thanks. i will read that thread.


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## Karter2 (Nov 17, 2011)

How big is this boat ( length width, weight) ?

how much "Run time" do you need under power ? ...that will help determine the battery requirements.
As Duncan said, you will probably need little power to drive it to its "hull speed" ( 4- 6 knts), i would be very surprised if its more than 8-10 hp.
You will need to think carefully about your choice of prop also to get the best efficiency from the power train.


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## PStechPaul (May 1, 2012)

You might get by with a 3-5 HP electric motor. Gas engines are rated peak HP at a specific RPM and torque. An electric motor will produce fairly flat torque for near zero to maximum RPM, and can be driven at least 2x rated power for a minute or so, especially if you can provide extra cooling (pump some water through some copper or stainless tubing wrapped around the motor. Should be fine in fresh water, maybe not so much for brine.

A 3 HP treadmill motor might do the job, and can be obtained for less than $100. They usually run on 90 or 180 VDC, but it is possible to boost 12, 24, or 48 VDC to 140 VDC using an automotive inverter. But for marine use higher voltages may be hazardous. Even 12 VDC battery cables in salt water can cramp your muscles and make it hard to swim. An old party trick was to put a solver dollar on the bottom of a fish tank with electrodes and tell people they can keep the coin if they can reach in and grab it.


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## captain jack (Jan 12, 2014)

Karter2 said:


> How big is this boat ( length width, weight) ?
> 
> how much "Run time" do you need under power ? ...that will help determine the battery requirements.
> As Duncan said, you will probably need little power to drive it to its "hull speed" ( 4- 6 knts), i would be very surprised if its more than 8-10 hp.
> You will need to think carefully about your choice of prop also to get the best efficiency from the power train.


27.34' length over all and 22.33' length at the water line ( this is the one that matters, for this ). the beam ( widthy ) is 9'. she displaces ( weighs ) 5400 pounds. like i said, the original motor was a 7 hp two stroke but it was recommended i go for more, between 10 and 12, at least.

as far as run time goes, i have been sailing for 18 years and never had a motor. i have, actually, never used a boat under power, in my life. as a kid, growing up on the monocacy river, we never used a motor. we poled flat bottom boats. poling works even when you can't touch bottom. my grandfather always had an outboard, in case of emergency, but we never used it.

i have been considering going motorless and making a chinese sculling oar, a yuloh, for tight spots in the marina. but i think a motor might be a good safety back up, not that i won't, also, make that yuloh. never trust machines 100%. they break down when it's the worst time.

i am probably not going to use it much more than getting into and out of my slip. it's tight. i will sail out and in as much as possible but if i think it's going to risk my( or someone else's ) boat, i will either power out or scull out.

i suppose if i get caught in a dead calm and, either the boat is in danger or i am pressed for time, i might need to use it for a short spell. but i'm not the kind of sailer that uses the iron sail everytime it takes a little skill to sail.

if i had 2 to 3 hours run time, i think that would be more than i will ever need between charging. i have heard that some people hook an alternator to their electric motor, so that's a thought.

the hull speed on this boat is around 6.5 kt. of course, around my slip, i won't be even getting close to hull speed. there, it will be more of short bursts.

as far as a prop, i have one. the original is still there. it's a folding prop. i just need to hook up to the shaft.

you might be right about not needing more than 10hp but, i wouldn't go lower. rpms need to be lower than for the standard power boat. i think 1000 to 1700 rpm would be a good range.

i have a question for you guys. i see some electric trolling motors for sale, from time to time, at a good price. they are always rated in pounds of thrust and not hp. i am sure there is a way to find the hp, though. there must be. so, i was wondering if one of those motors could be cannibalized to make an electric inboard? they seem much less expensive that the golf cart and forklift motors i have seen on ebay ( using ebay for a price base ).



just looked on west marine's site. they recommend 120 pounds of thrust for 27'. however, since the length on the water is actually less that 23', i think that i should use that length dimension. that would mean 70 to 80 pounds of thrust. but, would such a motor turn my existing prop and shaft or only it's own? i don't know what the power ratings are for those motors. if they listed amps, i could figure the actual hp. that might give me a better idea.


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## captain jack (Jan 12, 2014)

PStechPaul said:


> You might get by with a 3-5 HP electric motor. Gas engines are rated peak HP at a specific RPM and torque. An electric motor will produce fairly flat torque for near zero to maximum RPM, and can be driven at least 2x rated power for a minute or so, especially if you can provide extra cooling (pump some water through some copper or stainless tubing wrapped around the motor. Should be fine in fresh water, maybe not so much for brine.
> 
> A 3 HP treadmill motor might do the job, and can be obtained for less than $100. They usually run on 90 or 180 VDC, but it is possible to boost 12, 24, or 48 VDC to 140 VDC using an automotive inverter. But for marine use higher voltages may be hazardous. Even 12 VDC battery cables in salt water can cramp your muscles and make it hard to swim. An old party trick was to put a solver dollar on the bottom of a fish tank with electrodes and tell people they can keep the coin if they can reach in and grab it.


that's funny. sadistic but funny. 

i am not sure about 3 hp being enough. but, you are right, gas motors are rated at peak hp, which you don't use when cruising along. i don't know as much about electric motor power delivery. if torque is steady, throughout the rpm band, is hp also steady throughout the rpm band? if so, i would suppose that i could get away with 7 hp because that was the peak hp of the original motor.

i looked at the motors my father had. all AC motors and ranging from 1/2hp to 1 hp. i had hoped one might be bigger, but i hadn't considered that they would probably all be AC.

anyhow, $100 or under would be a great price. i have seen the trolling motors go for between $100 and $150, but i don't knw if one would be good to power an inboard set up.

i had never considered the motors from exercise machines. do they make treadmill motors any bigger than 3hp?

most electric sailboat motors run 24V or 48V systems. some make due with 12V but i believe that takes more batteries.


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## Frank (Dec 6, 2008)

In general torque will be constant until you reach a certain speed and power will rise from zero to some maximum (at the point where torque starts dropping off). I think you need to know your prop dimensions and understand how fast it has to turn to supply required thrust. 

As a data point, my in-laws boat is a 48' custom cutter (cold molded) that iirc displaces about 17 tons. She has a 144 volt nominal system that will cruise at 5 kt. while drawing something like 80A or 11kw. 

There must be someone out there who has a boat with similar dimensions to yours with specific experience. If all you need is something to get on or off a slip you won't need much.


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## kennybobby (Aug 10, 2012)

*You don't need 16hp*

For what you have described, you just need a little motor to help you get up the creek without a paddle. One of those trolling motors would likely work--they are cheap and easy to hook up and just give it a try. Or you could try using a hand-held electric drill chucked up to the shaft--at least it would give you some data about what it takes to move the boat. 1 hp is 746 watts, at 24 volts that draws 31 amps. Using 1200 rpm that would be 4.4 ft-lbs of torque. Run that into a 12" prop and it could be pushing 12 to 24 lbs thrust. How hard do you paddle with the oar?


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## captain jack (Jan 12, 2014)

Frank said:


> In general torque will be constant until you reach a certain speed and power will rise from zero to some maximum (at the point where torque starts dropping off). I think you need to know your prop dimensions and understand how fast it has to turn to supply required thrust.
> 
> As a data point, my in-laws boat is a 48' custom cutter (cold molded) that iirc displaces about 17 tons. She has a 144 volt nominal system that will cruise at 5 kt. while drawing something like 80A or 11kw.
> 
> There must be someone out there who has a boat with similar dimensions to yours with specific experience. If all you need is something to get on or off a slip you won't need much.


that is a little different, then, from internal combustion engines. with them, torque starts higher than hp but it isn't steady. they both rise, hp at a faster rate than torque, until they hit the point where they cross. then torque begins to drop and, after the maximum rpm where power is efficiently developed, hp begins to drop, too.

i imagine there must be someone with a similar sized vessel, as you suggest. it's a matter of finding tem. but, that woud be very helpful as a guide as to my power needs.

i agree. one wouldn't think i would need a huge motor to reach and maintain cruising speed. however, if you ask around on sailing forums, everyone urges you to go a little bigger than you think. i'd hate to set it all up and find i didn't have enough power, when i needed it.


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## captain jack (Jan 12, 2014)

*Re: You don't need 16hp*



kennybobby said:


> For what you have described, you just need a little motor to help you get up the creek without a paddle. One of those trolling motors would likely work--they are cheap and easy to hook up and just give it a try. Or you could try using a hand-held electric drill chucked up to the shaft--at least it would give you some data about what it takes to move the boat. 1 hp is 746 watts, at 24 volts that draws 31 amps. Using 1200 rpm that would be 4.4 ft-lbs of torque. Run that into a 12" prop and it could be pushing 12 to 24 lbs thrust. How hard do you paddle with the oar?


if i could find amp ratings for one of these trolling motors, it would be a big help. with amp and volt ratings, you can figure hp. they have no real shaft and they have their own, specific, prop. thrust ratings are based on that. they never give you hp ratings. turning my shaft and prop might not yield similar results because they may not have the hp to handle the extra load. but i do think one of the bigger ones might be a good motor source, for this. i will have to try to write one of the companies and see if they can give me amp ratings.

minn kota tells you that their props are designed for max acceleration, rather than max top speed, and that their motors will push a boat at a max speed of around 5kt. bigger motors will simply get you there faster. they also tell you how to estimate hp but then they fail to note the amps. i looked. they also do not recommend after market props because their motors are designed around the props they have. all of that makes me worry they might not have the hp that you normally think a vessel of a certain size, for which each motor is rated, should have.

$150 isn't a bad price for a motor; really good, in fact. however, no need to spend the money, to figure out if it'll work, if i can figure it out, by the numbers, without a needles expense.


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## captain jack (Jan 12, 2014)

another thought i had was jusing 'gear ratios' to give more torque to a smaller, high rpm, motor. since my rpms need to be lower, if i had a motor with high rpms, but not quite the hp rating i was looking for, i could use a small pully/sprocket on the motor and a larger one on the shaft running to my prop. instead of mounting the motor right on the shaft, i could mount it beside the shaft; connecting it through a belt of chain.

if you use a smaller sprocket on the drive pulley and a larger one on the driven pully, you reduce rpms and create more torque.


thoughts?


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## PStechPaul (May 1, 2012)

You might be able to figure HP from the prop thrust and speed. But first you would need to convert thrust to torque. If the prop is 12" diameter, then the radius is 6", and the point of contact with the water is probably about 2/3 out from center, or 4". The formula for power is:


```
P = T * RPM / 5252
```
So in your case, for 120 pounds of thrust and a prop speed of 1200 RPM: 


```
P = (4/12) * 120 * 1200 / 5252 = 9.1 HP
```
However, at lower hull speed you probably will have proportionally lower prop RPM at maximum thrust (too fast and you have turbulence and lots of bubbles and less transfer of force to the water), and at maximum speed you don't need as much force as when accelerating. Also some of the hull may lift out of the water at higher speeds so the drag may be less.

Most DC motors run at fairly high speed, 3000-5000 RPM, so for a given HP their torque is less. A speed reduction pulley would probably be needed to get the ideal prop speed and thrust. You might also be able to use the reduction drive in an electric drill, but it might not handle the power for continuous use, not for long, anyway. 

An electric winch might work, although maybe the RPM would be too low. You will also need some sort of controller, which might be free if you use an electric drill, and you might also be able to scrounge a motor and controller from a kid's "Power-wheels" vehicle, golf cart, or battery powered mower. Usually such things have dead batteries while the motor, controller, and transmission are still OK. Check the local dump, Craig's list, yard sales, and Freecycle.

I think you can get something up and running pretty cheaply, and then you can beef up and tweak as needed for the performance you want.

Good luck!


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## captain jack (Jan 12, 2014)

PStechPaul said:


> You might be able to figure HP from the prop thrust and speed. But first you would need to convert thrust to torque. If the prop is 12" diameter, then the radius is 6", and the point of contact with the water is probably about 2/3 out from center, or 4". The formula for power is:
> 
> 
> ```
> ...


i will have to see if i can find rpm ratings for the trolling motors. i will have to measure my own prop. of course, the problem, in this case, is the difference between HP at the motor and HP at the prop. if the HP, at the motor of a trolling motor, isn't enough to overcome the additional drag and weight of my long shaft, it will greatly reduce the HP at the prop is, once you subtract all the extra drag and spinning mass. that's actually my fear. i was hoping to calculate HP at the otor and not the prop, for that reason. i can't accurately calculate the affect of the added drag and spinning mass.

that's my biggest fear about the effectiveness of the trolling motors, for this use. they have a certain amount of thrust, but that's power at the prop. it takes in account for prop design, too. trolling motors have no real length of shaf. the shaft exits the motor and the prop is mounted immediately behind the motor. no added drag of a long shaft passing through the shaft 'log' and far less additional weight, outside of the prop. my shaft is long enough to add substantial extra spinning weight. the extra mass, to spin, will use up power as will the additional drag of the long shaft. 

when trying to figure out a gas or diesel motor to use, you calculate power at the motor. if i had the amps of the trolling motor, i could use the volts and amps to figure out the watts and convert that to the actual hp of the motor, before it has to spin the long shaft and the prop. if i calculate the power of a trolling motor, the way you suggest ( a good idea though it is ) it will not be an even comparison. the trolling motor Hp would be including prop design, which is probably not the same as my folding prop. that means it's not an actual calculation of the motor's HP. such an HP calculation would change if you changed the dimensions of the prop. and, since i have no idea of the thrust rating of the original two stroke motor ( it was rated in HP ) i can't do a similar calculation for it's HP. if i did have the original motor's thrust, i could come up with figures tgat i could accurately compare. however, without that, it's comparing apples to oranges. it would be hard to predict the actual performance, when hooked up to my shaft and prop. although i suppose it's better than no comparison, at all.

a reduction gear was kind of what i was talking about, although the same thing can be done in a better way, for this kind of application, with sprockets/pulleys and a chain/belt drive system. if i had a 3 hp motor, for example, that turned at twice the rpms that i needed, i could do a 2:1 set up, with the small sprocket/pulley on the motor. that would reduce rpms at the prop and increase torque.

torque is really the important thing, here. torque is the amount of work a motor can do and horsepower is how fast it can do it...to simplify things.

if i take a motorcycle motor ( i actually did this with a 600 Honda Shadow I had chopped ) that will do 120 MPH and i change the driven sprocket out for one that is bigger in diameter, i reduce the top speed but i create more torque at take off. so that, with the Shadow, my top speed dropped to 110 mph but i was able to walk away from an 883 sportster, drag racing off the line. ( of course, i did a lot of intake, carb, and exhaust modifications, too. get the most out of the motor you have ). i wasn't worried about top end. how often will i go over 110 mph? but acceleration is important, useable performance.

anyhow, looking at the motors that are available, outside of specifically marine motors, that may be my best bet...especially considering the cost of motors that have the right HP and RPM ratings.


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## Karter2 (Nov 17, 2011)

Forget the trolling motors, they are intended for dingy/row boats and have <2hp at max ( most are 1hp...12volt x 50amps = 600Watts), and also have their own specific prop designed to run at motor rpm. Whilst they have been used on 20' sail boats , they are as described "Trolling" motors, not intended to be relied on for powering against wind or currents.
Note: 1hp = 55-60lbs thrust (approx)
Have you read this .. http://bbs.trailersailor.com/forums/articles/index.cgi/noframes/read/23
as was said before, to do this cheap, inboard , ideally you need to start looking for those scrap 6" golf kart motors and motor controllers.


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## Frank (Dec 6, 2008)

You cannot change power with gearing: only torque.


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## captain jack (Jan 12, 2014)

Karter2 said:


> Forget the trolling motors, they are intended for dingy/row boats and have <2hp at max ( most are 1hp...12volt x 50amps = 600Watts), and also have their own specific prop designed to run at motor rpm. Whilst they have been used on 20' sail boats , they are as described "Trolling" motors, not intended to be relied on for powering against wind or currents.
> Note: 1hp = 55-60lbs thrust (approx)
> Have you read this .. http://bbs.trailersailor.com/forums/articles/index.cgi/noframes/read/23
> as was said before, to do this cheap, inboard , ideally you need to start looking for those scrap 6" golf kart motors and motor controllers.


yeah. after lots of research, i realize that it won't work with a trolling motor. if they rated them by hp, i'd have not have considered it.


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## captain jack (Jan 12, 2014)

hey. does anyone know anything about wheelchair motors?


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