What hp longshaft back up auxiliary outboard for a 23ft 2.5 tonne planing hull motor?

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Davey boy

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Hello floaters ,could anybody help with the above please?
Family of 4 using the boat on river crouch Essex and Thames estuary
Thank you
David
 
Mmmmpfff. Raggies might refer to us as floaters and reach for the handle but.....

To answer your question the biggest you could realistically use. That would be a 10hp 2 stroke before it becomes too heavy. How good this will be with a high torque prop on depends on current, wind, and your boat profile.
 
Davey boy said:
Thank you , I was thinking about 6hp .. just as a back up and also on a tender has and when needed
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Problem with that is a 6hp for an auxiliary does not make a good motor for a tender. First you will almost certainly need a long shaft whereas a dinghy uses a short shaft and 6hp is too big and heavy for many of the common sized tenders.
 
BruceK said:
2 or 4 stroke?
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Mine is a four stroke (if it was me you were asking). Yamaha F6CMHL with the lowest pitch propellor available.

I've not used it in bad weather or an emergency but it seems to cope quite well against a moderate tide.
 
I had a 23ft power boat with an inboard diesel, hanging on the transom was a 9.9 Evinrude 2 stroke.
Even with the 9.9 Flat out I was only able to do 3 knots. I put the slow speed down to the large 4 blade propeller underneath the boat causing drag.
So it depends on your main engine configuration
 
TheOrs said:
Mine is a four stroke (if it was me you were asking). Yamaha F6CMHL with the lowest pitch propellor available.

I've not used it in bad weather or an emergency but it seems to cope quite well against a moderate tide.
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Then 6hp re weight wise is about as big as you can realistically go
 
We're splitting hairs here. The OP wanted a recommendation on hp for an auxiliary motor. The obvious answer is : as much hp as weight will allow to cover the most conditions he might experience. That is a ~10hp 2 stroke. Sure he can go less. I imagine even a 1hp will push his boat in the most absolute ideal conditions. Not fast granted, but it will push it, eventually.
 
I'm not sure it's quite that cut and dried. Since the auxiliary engine is not going to plane the boat, arguably the smallest engine that will push it at displacement speeds (5 knots for a 23ft boat?) against a bit of headwind is perfectly sufficient. Bear in mind that auxiliary brackets are not always the strongest, so the torque of an overly powerful engine for the job trying to push the boat faster than it will go could cause issues.

Also a smaller more economical engine is going to achieve more range for a given (and probably limited, unless the OP has a way of tapping into the boat's main tank) fuel supply. Add in the weight saving of a smaller motor making it easier to deploy if not permanently fitted (and reducing bracket load if it is - bearing in mind small planing boats tend to bounce around).

I'd have thought that for a Falcon 23, a 6hp engine ideally centrally mounted on the transom rather than offset, would be sensible.
 
ari said:
I'm not sure it's quite that cut and dried. Since the auxiliary engine is not going to plane the boat, arguably the smallest engine that will push it at displacement speeds (5 knots for a 23ft boat?) against a bit of headwind is perfectly sufficient. Bear in mind that auxiliary brackets are not always the strongest, so the torque of an overly powerful engine for the job trying to push the boat faster than it will go could cause issues.

Also a smaller more economical engine is going to achieve more range for a given (and probably limited, unless the OP has a way of tapping into the boat's main tank) fuel supply. Add in the weight saving of a smaller motor making it easier to deploy if not permanently fitted (and reducing bracket load if it is - bearing in mind small planing boats tend to bounce around).

I'd have thought that for a Falcon 23, a 6hp engine ideally centrally mounted on the transom rather than offset, would be sensible.
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Agreed to a point. A 6hp 4 stroke is going to weigh the same if not more that a 10hp 2 stroke, so why given the choice would you go less?
You also want some manoeuvrability and for that you need a bit of torque, the presumption here that you are going to get that with a bigger hp / cm3 engine.
You also dont want to be using all the hp all the time straining the motor. Going to max hp to start momentum then throttling off .
Your argument that the smallest motor possible would be very limiting imo. Here is a vid taken many moons ago of me pushing a 1 ton boat against a headwind and mild current only being able to make 2knts and this is with a 8hp motor that's such a wonderchild I still have it. Would it push a 2.5 ton Falcon which has much more windage in the same conditions, I think there is room for more than a little doubt.

 
9.9 hi-thrust would be as good as anything else
Needs to be a big foot/sailpower/high-thrust model not a standard engine. Will work as an engine for the tender but won't be ideal.
 
A good 2 stroke 8hp would see you ok....but the most important thing about this is the set up. The drive and prop MUST be in clean water below the line of the hull. Anything else and it will just pull water from behind the transom and cavitate. Also likely come out the water in a bit of chop (always the case when you break down, haha)
It’s a fine balance between weight on your bracket and transom and power. Also if you are choosing to leave it on the boat or not. Above 8hp they start to get heavy. For me a 4 stroke is out the question. Give me a yamaha 8hp smoker any day. They weigh the same as a 4stroke 4hp !
Comments about a 10hp....mostly the case that the 10 is the same engine block and weight as the 15.
I would Defo be with the 8hp.
 
I had in mind the 9.9, the rest of your comments are imo a bit fanciful. Have you done this? As in actually done it? :D The prop does not need to be in clean water at displacement speeds. It makes next to no difference. It does need to be positioned so the prop and cavitation plate are at their correct depth. But we're talking couple inches difference here, even between long and short shaft and all of this will make not one jot of difference if your boat is pitching enough for the prop to surface.
I have a old 8hp you extol. I agree it's a great motor. I already said one would be daydreaming if they thought that would be much help in anything but benign conditions for the Ops boat. I doubt a 10 will be all that much better to be honest, just it's the biggest he will reasonably get on. And the reason I say this is none have the torque to be able to effectively steer. They can push, but slowly gather momentum. Any wind etc that drives the boat beam on and you're in a losing battle to go anywhere else. My 8hp could just cope with a 1 ton boat. Just above marginal. Would it push 2.5 ton. Sure, in a mill pond. Its even pulled a 10 ton boat when on the tender, very very slowly, like 10 feet in a minute. Sailboats of a similar size can get away with the smaller engines thanks to keel and more importantly rudder with more efficient hulls. Planing boats can't.
 
BruceK said:
I had in mind the 9.9, the rest of your comments are imo a bit fanciful. Have you done this? As in actually done it? :D The prop does not need to be in clean water at displacement speeds. It makes next to no difference. It does need to be positioned so the prop and cavitation plate are at their correct depth. But we're talking couple inches difference here, even between long and short shaft and all of this will make not one jot of difference if your boat is pitching enough for the prop to surface.
I have a old 8hp you extol. I agree it's a great motor. I already said one would be daydreaming if they thought that would be much help in anything but benign conditions for the Ops boat. I doubt a 10 will be all that much better to be honest, just it's the biggest he will reasonably get on. And the reason I say this is none have the torque to be able to effectively steer. They can push, but slowly gather momentum. Any wind etc that drives the boat beam on and you're in a losing battle to go anywhere else. My 8hp could just cope with a 1 ton boat. Just above marginal. Would it push 2.5 ton. Sure, in a mill pond. Its even pulled a 10 ton boat when on the tender, very very slowly, like 10 feet in a minute. Sailboats of a similar size can get away with the smaller engines thanks to keel and more importantly rudder with more efficient hulls. Planing boats can't.
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All of the above is sure sign that the prop is not in clean water ! It needs to be or next to useless. There is no doubt about this. Been there.
 
Remember a prop pulls the boat through the water. Archimedes and all that. If it’s just sitting behind the transom, it just pulls a whirl pool and air from the surface at anything over quarter throttle. Tried and tested here.
 
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