# how many lbs of thrust is equal to 1 hp?

Discussion in 'Outboards' started by deermaster, Nov 15, 2008.

1. Joined: Apr 2004
Posts: 892
Likes: 53, Points: 28, Legacy Rep: 422
Location: Finland

### JoakimSenior Member

I have a VPP that accounts for windage and resistance in waves, but it is made for sailing performance prediction up to 20 knots. There shouldn't be a problem with windage, but my model for calculating wave height and resistance in waves may not work at 60 knots.

However for my previous boat (about 3 tn, 27 feet):

Calm: 6.4 knots with 9 hp Yanmar 1GM10 (VPP predicts 6.4 knots already with 7 hp)

25 knots headwind: 5.5-6 knots (VPP predicts 5.7 knots)

60 knots headwind: no experience (VPP predicts 2.3 knots, but does not take into account much reduced propulsion efficiency. In reality you would go backwards)

5.7 knots was a good motoring speed for that boat. To reach that at 60 knots headwind the VPP predicts a 56 hp motor.

According to my VPP the drag at 5.7 is in calm 440 N, 25 knots 1130 N (windage 410 N, waves 280 N) and 60 knots 6900 N (windage 2100 N, waves 4400 N).

Assuming flat water you could make 5.7 knots in 60 knots head wind with 20 hp, but that is really no a good assumption.

2. Joined: Sep 2012
Posts: 28
Likes: 1, Points: 0, Legacy Rep: 25
Location: chico ca

### larry sellersJunior Member

Many Thanks! My background is in industrial construction - building steam and gas turbine power plants, canneries and such, though I have messed about in sail and power since, I guess, 1955. Retired now, and planning to "build" an old Mac hull into an overnight cruiser for California-Oregon coastal sport-fishing and, well, messing about. I am (ahem) somewhat out of my depth in the subject to hand, and I am very grateful for your kindness in running those numbers. Your more precise analysis aligns pretty closely with my nomographic and algebraically (from my 1940 copy of Marks, derived estimate - graph paper and all. The engine I expect to use is a "marinized" 2 cylinder Wisconsin. It can make 18 hp. Fuel consumption with this engine is about 2/3 (0.66) gallon per hour at minimum safe rpm/load - not great but good 'nuf. I knew that the violent fluids would make the 60 kt results more or less useless, but had no method for taking non-calm and turbulent water/wind into account, so especial thanks. The anticipated use area between Astoria and Ft Bragg tends to have onshore afternoon winds that kick up fast (so I understand anyway) and there is always a risk that "emergency power" will be essential to stay off the beach... Again, many thanks. I'll put a copy of your VPP (what is VPP? I assume it's a 'puter program to model) results in the folder for the boat. Best!

3. Joined: Aug 2010
Posts: 709
Likes: 25, Points: 28, Legacy Rep: 226
Location: Springfield, Mo.

### WavewackerSenior Member

Larry, any pics of your terminal cruiser, strawler, terminal trawler. Did you do interior mods, cut the roof out for your pilot house? I know, off topic, sortta, you might chime in on the sail boat to motor/terminal trawler threads.

4. Joined: Sep 2012
Posts: 28
Likes: 1, Points: 0, Legacy Rep: 25
Location: chico ca

### larry sellersJunior Member

In planning stage. Will post as project gets to the photogenic phase... Thanks. Plan calls for center enclosed pilot house with ducted engine (it's air-cooled) essentially on the center of lift of the original swing keel. S.K. replaced with a longitudinal fabricated steel keel enclosing prop and shaft and ending with kitchen rudder - a big welding and bedding job for one man, something about 13 feet long. Yes, much sawzall work topsides... The anticipated demo of the original cockpit is to remove the seats and cockpit deck and replace with bedded aluminum sheet (probably 10 gauge) assembly that drops in an bolts to the tapered fiberglass "seat backs" - a tricky fab job there too, 'cause it's gotta fit. Then the step-down into the Mac. I'll discard the poptop and fab the house over - I expect, that's ytbd (yet to be determined) I am naturally very concerned about the coupling of large metal assemblies to fiberglass, but my experience has been that sea-k-flex gradually bonds so well that the two materials get along pretty well. It's my "big project". I built a Harley "magnum" 45 (it was 55") stuffed into a hand made frame, I've built a house, and now for the boat project. (Then I can die happy...) Thanks again for you interest.

5. Joined: Nov 2015
Posts: 5
Likes: 0, Points: 0, Legacy Rep: 10

### johnmcneelyJunior Member

Trying to decide if electric outboard vs gas engine on pontoon boat

6. Joined: Oct 2010
Posts: 10,401
Likes: 1,036, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

You have 2hp Vs 10 hp, seems to me if you encounter headwinds you might be going nowhere fast with the little one.

7. Joined: Nov 2003
Posts: 19,133
Likes: 496, Points: 93, Legacy Rep: 3967
Location: Eustis, FL

### PARYacht Designer/Builder

Welcome to the forum.

A less than one HP electric motor will have a hell of a time pushing a 3,100 pound tritoon around, that's for sure. Maybe 2 MPH for a few hours, assuming no wind or currents. You should look into the Torqeedo outboard line. These are more costly than the dainty Mini Kota, but at least you'll have some punch.

Speaking of punch, the 9.9 should be a "high thrust" version of the Suzuki, so you're not beating the water to a froth behind you boat, as it tries it's best to move it. Lastly, why do you need power tilt and trim on a 9.9 outboard?

8. Joined: Nov 2015
Posts: 5
Likes: 0, Points: 0, Legacy Rep: 10

### johnmcneelyJunior Member

Trying to decide if electric outboard vs gas engine on pontoon boat

Thanks for the replies. The high thrust option is the plan on the Suzuki. As for the power tilt and trim, it comes with the motor and I am not as nimble as I was in my 20s, and the 9.9 weighs a hundred or so pounds. I can't see myself manualy raising and lowering the motor while waist deep in Colorado's frigid waters (smile). I have considered the 4-hp Torqueedo, but it is a bit pricey and the 15 minutes you get at wide-open throttle doesn't seem like much for the expense. Some user forums report the Torqueedo is a delicate apparatus and that the manufacturer is not sympathetic. I may eventually have both the 2-hp Minn-Kota AND the Suzuki as funds permit. Good points about wind and waves. At the first sign of those building, I should probably head for the ramp. Pontoon boats are probably particularly susceptible to the effects of wind, waves and current for that matter given their bulk.~John

9. Joined: Apr 2004
Posts: 892
Likes: 53, Points: 28, Legacy Rep: 422
Location: Finland

### JoakimSenior Member

The E-Drive is a 2 HP model drawing 40 A at 48 V. Why would it only push 2 MPH? It seems to cost about the same as Torqeedo C2 (2 kW) model, which is probably much better.

Here are some test results. Note that Torqeedo takes clearly less power at the same speed and 1 kW Torqeedo is almost as fast as E-Drive taking almost 2 kW. http://demandware.edgesuite.net/aaud_prd/on/demandware.static/-/Library-Sites-TQShared/default/dwff4ab1fe/about-torqeedo/press-comments/press-comments-fr/fluvial_04_2012.pdf

This is the boat "B" they used in the test http://www.rigiflex.fr/16cap-CAP_4700

10. Joined: Nov 2015
Posts: 5
Likes: 0, Points: 0, Legacy Rep: 10

### johnmcneelyJunior Member

Trying to decide if electric outboard vs gas engine on pontoon boat

My understanding is the EDrive 2-hp will push a boat 5mph but only for 2 hours at wide-open throttle. My concern about Torqueedo is a reported lack of ruggedness and little support from the company. I've heard much better about Minn-kota.

11. Joined: Nov 2015
Posts: 5
Likes: 0, Points: 0, Legacy Rep: 10

### johnmcneelyJunior Member

One other thing, Minn-Kota tells me that only their primary propulsion motors are designed for extented running time at full throttle. And that all other of their motors could overheat. They have 3 models: the 2-hp EDrive, a 1-hp EO (Electric Outboard) and a 1/2 hp model. I had been unaware of this ~John

12. Joined: Sep 2006
Posts: 2,043
Likes: 120, Points: 63, Legacy Rep: 1818
Location: Cape Town South Africa

### Manie BSenior Member

I will be running my own tests soon and will post the results for you guys
24v 86lbs thrust vs 5hp fourstroke
should be within the next month, waiting for shipment

13. Joined: Nov 2015
Posts: 5
Likes: 0, Points: 0, Legacy Rep: 10

### johnmcneelyJunior Member

Trying to decide if electric outboard vs gas engine on pontoon boat

Great Manie, that should help.

14. Joined: Dec 2005
Posts: 2,873
Likes: 532, Points: 113, Legacy Rep: 506
Location: Monroe WA

### ondarvrSenior Member

Without doing all the math I can only tell you from my experience with many electric trolling motors and small gas motors.

Even the smallest gas outboards will move your craft faster and better than any of the electrics will, there is no comparision between the two. So when you step up to a 9.9 and compare it to a 65# electric it's not going to even be close.

Now I do have customers that put other types of electric motors in very efficient hulls and they do OK until the batteries go dead.

15. Joined: Apr 2004
Posts: 892
Likes: 53, Points: 28, Legacy Rep: 422
Location: Finland

### JoakimSenior Member

These comparisons have been done many times. Here is one: http://www.torqeedo.fi/testit/to_vi_bataegare_1208_svenska_se_.pdf

The top speeds on a 4.4 m fishing boat were Torqeedo 801 4.5 knots, Torqeedo C2 5.3 knots, Mercury 2.5 5.5 knots and Mercury 5 6.1 knots.

Torqeedo 801 has been measured to have 31 kg (70 lbf) and C2 62 kg (136 lbf) bollard pull. Torqeedo markets them as "equal" to 2 hp and 5-6 hp outboards. Yes they have equal bollard pull, but nowhere near equal power and thus speed.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.