What power auxiliary outboard for 6.5m?

[Hugh Jardon]

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

[Poly]

The theoretical horse power requirement of a displacement vessel (as presumably you can’t 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 don’t 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 – 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 4.75” (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

[jwalker]

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.

[Hugh Jardon]

Originally Posted by Polwart

The theoretical horse power requirement of a displacement vessel (as presumably you can’t 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 don’t 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 – 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 4.75” (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

[Poly]

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

[Poly]

NOTE

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

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

Sorry,

NEIL

[codprawn]

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.

[rbw156]

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?

[rbw156]

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

[jwalker]

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.

[The Jackeens]

Go Teach!!!

missus

[rbw156]

Originally Posted by jwalker

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.

Sure, I'm only too well aware of the dangers of a lee shore and the challenge of powering a boat through big waves. Indeed, a purist would argue that I should have specified the RIB with twin engine, twin everything, just in case. However, my budget does not run to this . I've scanned the advice so far ... I'd be happy to receive your opinion on what size engine to mount as an aux ... 6, 8, 9.9, 15, etc.

Rich

[jwalker]

As big as your boat and pocket can stand. Also, rig a tie bar from the auxiliary motor to the centre of the transom which can be removed from its fixing and connected to the steering arm on the main motor. You can then steer from your normal seating position even if you have to slightly raise the main engine to decrease the drag from the prop. It also serves to fix the aux. and prevent it swinging when not in use.

I reckon you'll need as much power as possible for good steerage if the going gets tough.

[jwalker]

Originally Posted by The Jackeens

Go Teach!!!

missus

Yeh, I know. It felt like it as I was typing.

[Hugh Jardon]

but in reality do you go out when it is rough and noone else is around, you have a radio and mobile i suspect so if you were in dia circumstances you could radio for help, but in most scenarios or less extreme scenarios you could plod back with a 5 i would suspect

for me i would be very happy with a 5 as is much better than nothing, and i s light etc and can double up as an engine for a little sib for kids etc to get some use from it

but the ultimate question is ......what price safety and piece of mind...

[brucehawsker]

I have a 6hp 4 stoke Suzuki on my 6.5M Solent. Quite usable - yes, Jackeen are right, 6 knots is about the WOT speed. Use the big engine as a rudder and keep the aux locked in centre. Have not had to use her in anger, but have pootled round dock and river and had no problems mooring etc. Anything uder 6 would scare me - and I do not look forward to having to use the 6 for real. But better than nothing. If my big engine did pack up in anything but really good conditions or I was any distance from harbour, I would advise the Coastguard of the problem as a matter of course. Suppose I expect any problems on my big beast to occur somewhere from Berthon to the mouth of the Lymington River, and the aux would allow me to limp back....

[The Jackeens]

Hi Bruce,

From memory I think you keep your aux reversed on the bracket?
If that is so, have you ever attempted to get it going it at sea just to see what it would be like if needed in an emergency?

(vg advice as usual JW. Have an apple Have to say tho' I'm a big fan of Seastart.)

missus

[rbw156]

Originally Posted by jwalker

Also, rig a tie bar from the auxiliary motor to the centre of the transom which can be removed from its fixing and connected to the steering arm on the main motor. You can then steer from your normal seating position even if you have to slightly raise the main engine to decrease the drag from the prop. It also serves to fix the aux. and prevent it swinging when not in use.

This sounds like great advice. I like this idea a lot. Have you any pictures &/or diagrams?

Originally Posted by Chris Hartley

but in reality do you go out when it is rough and noone else is around, you have a radio and mobile i suspect so if you were in dia circumstances you could radio for help, but in most scenarios or less extreme scenarios you could plod back with a 5 i would suspect

I have DSC VHF, mobile and flares as my principle ways of sending a distress signal.

If a 6 hp will achieve my hull speed of 6 knots. I know that additional speed comes with disproportionately more power as the boat transitions onto the plane. What speed might I expect with a 9.9 hp?

I would not go out in bad conditions, however, a few years ago sailed from Cowes to Keyhaven in my 19' sailing yacht. We left Cowes in a F5. Within an hour it was a F8 (confirmed by Coastguard). The wind against tide conditions made it challenging at the least. Therefore, while I don't intend to go out in bad conditions, it is easy to be caught out .

[jwalker]

Originally Posted by rbw156

....This sounds like great advice. I like this idea a lot. Have you any pictures &/or diagrams?....

Nah, I've no pictures but I can explain it.

I used 3 rose joints of the type used as a standard steering fitment on an outboard. The steering arm on the main motor had a spare threaded hole and one rose joint was greased well and bolted onto it. If there is only one hole in the steering arm, you could easily use a long bolt and fit the new joint on top of the original steering draglink. This joint is to be left here and, hopefully, never used so it can be pivoted out of the way.

If your auxiliary doesn't have one, you'll need to get/make a bracket to work as a steering arm on it.

You'll also need a bracket to mount near the centre of the transom to which one of the other rose joints is bolted. I had used a length if angle across the transom to spread the load of the outboard and to prevent theft using a chainsaw. I bolted the rosejoint to this.

I made a tie bar from a length of Ø20 aluminium alloy with a stub of M10 thread screwed firmly into each end. The rose joints used an M10 thread.

The third rose joint was screwed onto one end of this tie bar and a locknut was used to allow some adjustment in length at that end.

The other thread of the tie bar was greased and screwed into the joint on the central bracket but it was not completely tightened up so it would allow a bit of rotation of the bar.

A bolt and wingnut then connected the tiebar to the auxiliary motor.

The auxiliary can now be tilted and lowered whilst attached to the bar. I do remember having to put a slight bend into the bar to clear an obstruction. I think it was if the main motor was tilted and also over on full lock it contacted the tie bar. It was an easy fix.

How to use: Spin off the wingnut and remove the bolt. Spin out the tie bar from the joint on the central bracket. Remember it wasn't tightened up so no tools are necessary. Spin the tiebar into the joint on the main motor's steering arm and then reconnect the bolt and wingnut. Voila.

As always, it sounds a bit complicated when written but it's a straightforward job.

The auxiliary engine was not allowed to be supported on the tilt pin when underway. There was a pivoting bracket on the outside of the transom to support it in the tilted position. I've no pics of that either. The engine clipped to the support using the its normal catch so it was only necessary to press the release lever and the bracket fell into the down position and the engine was then lowered. So no mucking about required.

----------------------------------------------


I only once had to use the auxiliary and I was pleased it was 30hp. I suppose it was a bit unusual in that one bank of cylinders on a Merc 200 stopped firing and the engine didn't have enough power to get the boat planing. I was faced with a 20 mile trip home at a few knots, it would be dark in a short while and I'd also miss the tide on the slip by a long way when I got there. Bummer. However, the auxiliary was connected as described above and both engines were used. The boat easily planed and we travelled at close to 30mph for the whole journey. Still missed the slip though and it was just dark as we arrived back at the harbour but a success, none the less.

[rbw156]

Fantastic, thanks for this detailed description. The only bit I'm like to clarify is this section:

Originally Posted by jwalker

The auxiliary engine was not allowed to be supported on the tilt pin when underway.

I assume that you fixed the aux to a pivoted drop plate and that while the aux was in the up position that you also had the o/b tilted up. Is this the case?

Thanks, once again for your advice.