Engine size

[boatman200]

I have a 21 ft marshan speed boat and intends a spare little outboar incase the engine goes wrong. We have been looking at engines for it and we were wondering, what is the smallest engine that would manage to push it out of trouble?

[Overdraught]

One question thatwould need answering first is; what is the fastest tidal stream which you normally encounter? Spring tides where i launch in an estuary run at about 6 knots mid-tide: too small an auxiliary here could still see you going backwards over the ground except near the shoreline!
I am sure others will be along with other answers soon.

Overdraught

[jokaboat]

I would think a 4 HP unit would get you out of trouble but I agree, if you want a real get you home motor you may want more if you expect strong tides. Depend if you want something to get you to somewhere that you can stick the anchor down.

[Noddy]

If you go above 6hp you are moving into twin cylinders with most manufacturers, hardly ancillary territory. It is worth noting though that most 6hp engines are the same block as their 4hp counterparts. Still unlikely to give you 6kts though, but how much do you really need to get to a safe anchorage and wait for the tide to be in your favour?
Is there any way you could try an engine? Maybe a supplier could lend you a demo. You might get away with 2.5 hp - a much easier lump to manhandle.
I, like many sports boat owners, have no where to fit an ancillary; this used to bother me until a friend, who is a lifeboatman, pointed out that they would be more than happy to tow me in if I got into trouble. This does mean that I consider that it would be irresponsible to put to sea without a well maintained boat though.

[Ed]

I had a 2.3 on my Fletcher 165. It moved it at an impressive speed of 2.5mph

[markanddawn]

Presuming you will not want to leave the auxiliary motor attached when not required, you will want a light unit and consider whether you want to carry two different types of fuel (2 stroke various mixes or 4 stroke), it may be best to match the fuel requirement to your main engine. I would go for the largest motor that you can physically handle when lowering onto the bracket (when in perhaps a less than ideal situation) and one with either a built in tank, or one that can run off your existing tank. Most importantly, carry a good well suited anchor, life jacket, flares and VHF. The RNLI are more than happy to provide a tow (talking from experience) but ultimately my anchor saved my bacon until help arrived. Whilst an anchor can't get you home, it can prevent a situation worsening, buying you valuable time to fix a fault or call for help. As for size, it really depends on the tides in your area but would suggest any engine is better than none and while it might not push against a tide or headwind, may just help you steer nearer to shore or avoid an underwater obstruction.

[Torque of the Devil]

I had a 2hp Yamaha on my Dory 13 for when (and if) the 40hp packed up. That too moved it at an impressive speed of circa 2.5mph

[Ed]

Its time to organise a backup engine race!!!

[Torque of the Devil]

Happy to do that but I will have to stand on the swim platform and dangle an outboard in the water. There is no way I am drilling a transom plate for an auxiliary outboard on my Regal! Sea Start are my auxiliary unit

[Ed]

haha, have you ever tried holding onto even a small power engine.. Its impossible

[Torque of the Devil]

It sounds impossible but I'd like to try- although I'm a bit scared of props now after getting struck by one when I was 16. Shattered my ankle, lost 2 pints of blood, and severed my achilles tendon. All in all it was quite a nasty experience. Not my fault though!

[Matt13]

It sounds impossible but I'd like to try- although I'm a bit scared of props now after getting struck by one when I was 16. Shattered my ankle, lost 2 pints of blood, and severed my achilles tendon. All in all it was quite a nasty experience. Not my fault though!

Blimey that sounds terrible.

[Torque of the Devil]

I won't bore you with the gory details here- but I did write about it in a readers letter to SB&R a couple of years ago. It won me a VHF radio!

[Matt13]

I won't bore you with the gory details here- but I did write about it in a readers letter to SB&R a couple of years ago. It won me a VHF radio!

Oh well, every cloud..

[Ed]

I won't bore you with the gory details here- but I did write about it in a readers letter to SB&R a couple of years ago. It won me a VHF radio!

Did it ever arrive lol...

Sorry couldn't resist...