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Working Out The Speed Of Your Boat... (Based On Hp And Length)


Doodlebug

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Hi all,

 

As mentioned before we are getting the boat stretched. From 23ft to 33.

 

We can not afford to get the engine upgraded for a good few months and so are going to make do with what we currently have (a 10hp honda outboard).

 

Its a good little engine and does its job well but I am aware of the fact we will go slower once stretched. Thats fine by me but I need to calculate the number of days it will take to get from Rugby to milton keynes. Canal planner say 3 days but we will obviously be going slower. So has anyone got any idea how I should try to work this out?

 

Thanks!

 

Doodlebug

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well unless at the present you have to run at full throttle surely there will be capacity to increase the revs to compensate a friend runs a 45 footer with a 850cc vetus .

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Its a good little engine and does its job well but I am aware of the fact we will go slower once stretched.

 

Yes and no, you may take a little longer to get up to speed but once there, I doubt you will notice the difference.

 

It will also take longer to slow, so earlier anticipation of hazards will be required. wink.png

The engine will be working a little harder but just be aware of cooling etc. and all will be well.

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My thoughts - but may be wrong, and these are based on approximations - power to fuel consumption being a straight line etc.

My engine is 43hp and according to spec sheet will use about 8.5l per hour at rated power.

When cruising at 4mph it uses about 1.5l per hour so based on fuel consumption ratio the engine is probably about 8hp of power output.

So your outboard at close to full power might get you to 4mph, however it would be kinder on the engine to drop it down to 3mph. To consider this another way, how much throttle gives you how much speed at the moment?

 

But one thing to remember as said previously, you have no 'reserve' power for fast stops and starts, turns etc. and the boat will be a fair bit heavier.

 

Go back a few years and boats managed just fine with one horse power :-))

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Years ago I had a cruiser with a seriously underpowered outboard. I found that progress was (or at least seemed to be) noticeably faster if I watched where I was going and kept a straight course. Progress was a lot slower if I let my attention and my course wander.

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At 33' your hull limiting speed will be higher, even in a narrow canal, than it was at 23'. Any increase in draft will decrease your maximum speed, without producing a breaking wash.

 

Two or three Hp will still push you along at the maximum 3mph attainable on most of our canals.

 

Alan

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When I had one of my barges shortened from 33 metres to 23 metres, she lost 1.5 km/h of speed.

 

A friend who had his Rhine barge lenghtened 15 metres gained about 1 km/h of speed without changing his engine or prop.

 

Don't know if it will be the same with an outboard, but it's likely to be so, will only take longer to get up to speed and to get to stop because of the added weight.

 

Peter.

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Ok thanks - I was working on the basis that one horse used to be enough!

 

I dont know how it could end up going faster - my basic physics knowledge says that is impossible? But as long as you think it will be ok I will just add another day on to the cruse time to allow for that.

 

Thanks again

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Years ago I had a cruiser with a seriously underpowered outboard. I found that progress was (or at least seemed to be) noticeably faster if I watched where I was going and kept a straight course. Progress was a lot slower if I let my attention and my course wander.

 

That works on our boat too. I do try to keep my tiller movements small and make a nice smooth wake from the prop.

 

Richard

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Chris Deuchar in this particular case, it is the same thing though

 

It's very satisfying to look back and see a smooth prop wake following you right around a bend

 

Richard

The best book ever written on boating techniques, some on here could do with reading it.

I have a rare unsigned first edition ;)

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I think you are going to really struggle with a 10hp o/b pushing a 33 foot boat.

 

The limit for "portable" outboards tends to be around 25 foot and three tons, any longer/heavier and you run into problems with starting, stopping, and handling.

 

It's not about horsepower, the little honda develops its power by turning a tiny prop really fast ie there isn't much torque to swing a big prop which is what you need for a heavy steel boat.

 

If you really have no alternative for the moment see if you can fit a "high thrust" prop to your motor, it's basically a wider prop with less pitch and gives you a bit more control. I know you can get one for the Honda 15 not sure about the 10.

 

 

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