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Old 07 October 2005, 17:00   #21
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on the river a heck of a lot of fairly sizable tubs use 9.9 hp to zip along so that would probably be ample depending on how fast you think you would go and how fast the waters are where you go, i would suggest 5 hp would be the minimum and 15 would be excellent. i dont have any experience of plodding around the sea in a big boat with a very small engine so can only go by what i see regular on the river

but the sentiment seems to be forget the 2.5
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Old 07 October 2005, 18:29   #22
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Power calculations

The theoretical horse power requirement of a displacement vessel (as presumably you canít get on the plane with your aux) travelling at hull speed is:

Hull speed [in knots] = 2.43 x √(Water Line Length [in m])

Horsepower = Displacement [in lbs] / ( 1502 / (Hull speed)^2 )

This assumes 100% efficiency in flat calm water. In reality the prop will slip a lot, and will probably only give about 40-50% efficiency at displacement speeds. So double the answer! You can similarly calculate the likely maximum speed an aux will provide by putting in the HP and displacement of your boat. Again bear in mind that prop slip will effectively mean you loose ~ 40-50% of the power, and that you are unlikely to exceed the hull speed.

As other people have stated prop size is critical.

Theoretical Boat Speed [in knots] = (RPM * 60 * Pitch [in inches]) / 72913.2

Again this assumes that the prop is 100% efficient (no slip), so you should half this number.

If the theoretical boat speed is higher than the hull speed then there will be power from the auxiliary which will effectively be of little use.
For the purposes of illustration (as I donít have actual data for your boat):

Water line length ~ 6 m. So hull speed = 5.95 knots.
Displacement ~ 1200 lbs, so HP to achieve hull speed = 1.88 HP Ė so would really need about 3.5-4 HP engine to achieve this. With a 2 HP engine you will probably get a maximum speed of around 3-4 knots at FOT on a calm day. If it has a prop pitch of 4.75Ē (as my wee 2 HP Honda does) then at 5000 RPM you get a theoretical speed of about 10 knots (even after accounting for prop slip) and so you will not be able to use full revs effectively.

HTH

Neil
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Old 07 October 2005, 18:47   #23
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So you reckon on getting about 10knots on a 6 metre rib using 4hp?




My 6.5 used a 30hp wing engine and it was not enough to make it plane. It may just have done it down the face of a wave with the help of someone in the bow but that's hardly reliable.

If you are out in following waves, you need enough speed to be able to keep control. A few hp just won't do it.
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Old 07 October 2005, 18:55   #24
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Quote:
Originally Posted by Polwart
The theoretical horse power requirement of a displacement vessel (as presumably you canít get on the plane with your aux) travelling at hull speed is:

Hull speed [in knots] = 2.43 x √(Water Line Length [in m])

Horsepower = Displacement [in lbs] / ( 1502 / (Hull speed)^2 )

This assumes 100% efficiency in flat calm water. In reality the prop will slip a lot, and will probably only give about 40-50% efficiency at displacement speeds. So double the answer! You can similarly calculate the likely maximum speed an aux will provide by putting in the HP and displacement of your boat. Again bear in mind that prop slip will effectively mean you loose ~ 40-50% of the power, and that you are unlikely to exceed the hull speed.

As other people have stated prop size is critical.

Theoretical Boat Speed [in knots] = (RPM * 60 * Pitch [in inches]) / 72913.2

Again this assumes that the prop is 100% efficient (no slip), so you should half this number.

If the theoretical boat speed is higher than the hull speed then there will be power from the auxiliary which will effectively be of little use.
For the purposes of illustration (as I donít have actual data for your boat):

Water line length ~ 6 m. So hull speed = 5.95 knots.
Displacement ~ 1200 lbs, so HP to achieve hull speed = 1.88 HP Ė so would really need about 3.5-4 HP engine to achieve this. With a 2 HP engine you will probably get a maximum speed of around 3-4 knots at FOT on a calm day. If it has a prop pitch of 4.75Ē (as my wee 2 HP Honda does) then at 5000 RPM you get a theoretical speed of about 10 knots (even after accounting for prop slip) and so you will not be able to use full revs effectively.

HTH

Neil
i dont think a 3.5 to 4 would give 10 knots but i may be wrong

i would have thought that the shape of the hull would impact the speed that could be gained from the boat before it hits that displacement wall so to speak, is there any theory around this ?

some great statistics and maths though, well impressed

no doubt gArf will have something to say on this
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Old 07 October 2005, 19:33   #25
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No it won't get to 10 knots.

Sorry obviously didn't explain myself clearly...

...the formula that gives 10 knots is the theoretical speed with no slip from the prop (and takes no account of either boat length, displacement or HP).

The point is that if the prop is trying to move forward at 10 knots and the boat its refusing to exceed hull speed (as there is insufficient power to get on the plane) then the prop just slips more. So there is little point is using a 4.75" pitch prop at greater that 1/2 throttle on a boat of this size with such a small engine. Accordingly to get the full power/throttle out of the engine you need a smaller pitch prop OR an engine that will get the boat on the plane.

Hope that makes more sense.

NEIL
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Old 07 October 2005, 19:38   #26
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Error in the above formula

NOTE

There is a ^ missing from the formula above. It should have been:

Horsepower = Displacement [in lbs] / ( 150^2 / (Hull speed)^2 )

Sorry,

NEIL
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Old 07 October 2005, 23:20   #27
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A lot depends on gearing - torque - size of prop etc. A bloke I know has a warrior 17' - had a 5hp aux which wouldn't move him against a strong tide - he borrowed his dad's old seagull and it had much more thrust - was only something like a 2 or 4hp.
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Old 08 October 2005, 02:36   #28
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Thanks for all the advice.

A few things are clear:
  • The 2.5 won't cut the mustard - Does anyone want to buy it?
  • My hull speed is around 6 knots, to exceed this would require a lot more power to get over the hump. I did not plan on the aux being able to get me on the plane, just out of immediate danger
  • Seastart is an option - I'll take a look at their website. In the meantime, what is their customer service and geographical coverage like?
  • A high thrust prop, like a sail drive will help at displacement speeds
Therefore, I'll probably go for around a 5 or 6 hp. If I remember correctly all 6 hps are twin cylinder and, therefore, heavier to handle. If so, this might swing me towards the 5 with a sail drive prop.

Thanks for your advice.

Rich

PS Anyone fancy a trip to Alderney next year?
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Old 08 October 2005, 02:45   #29
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Just reviewed Seastart's website. Looks good, but I can see the following few areas of concern:
  • Coverage is limited to within 3 miles off shore along my primarly cruising area
  • I pay labour costs after the first hour. It assume that this is from call out, not arrival as the web site is silent on this point
The cost looks reasonable at £130. However, three or four years of membership would pay for an aux.

Rich
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Old 08 October 2005, 07:10   #30
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Quote:
Originally Posted by rbw156
...I did not plan on the aux being able to get me on the plane, just out of immediate danger...
So, you're out in a F4-5, 10 miles from shore your motor stops and wont restart. You wont be able to make way against the sea and wind with 5hp. Your only options are a following sea or a beam sea. At one end of the beam sea is the shore and rocks, at the other end you are driving offshore. It's gotta be a following sea but you've only got 5hp flat out and each wave is going to try to broach you. Given that you are going to have to drive diagonally to get back to safety you're likely to have a journey of around 15mls. Even if you had the motor flat out and managed your 5knots for the whole return journey, that's going to be around three hours fighting the following sea.

Rather you than me. Be realistic about the possible senario.
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