what size outboard do I need

[Freebee]

I have a 24 foot fibre glass fishing boat weighing about 3 tonnes dry weight, I will be using it in the Solent mostly tidal estuary work-what size auxiliary outboard do I need

 

[Shaz77]

20hp and you will probably only run at half power as long as you are a displacement hull and keep the bottom clean. It also depends on max tidal flow. I am thinking 2knots but if it's more than that then you need to step up the power because a strong spring tide with a strong wind on the nose requires more power. Being 24ft you wil have a max hull speed of around
6 knots. You can get a big lump but unless you are towing nets it will be wasted.

 

[Chiara’s slave]

20hp and you will probably only run at half power as long as you are a displacement hull and keep the bottom clean. It also depends on max tidal flow. I am thinking 2knots but if it's more than that then you need to step up the power because a strong spring tide with a strong wind on the nose requires more power. Being 24ft you wil have a max hull speed of around
6 knots. You can get a big lump but unless you are towing nets it will be wasted.

Tidal flow is quite variable, depending where you are. Well over 4kn in a few places, and the whole of the west is quite rapid. But overall that 20hp matched to a suitable prop sounds right to me.

 

[jamie N]

I notice that you wrote "auxilliary outboard". I'd interpret that as being in addition to (say) the permanent inboard, that is used for getting the boat from A to B, and the outboard is simply for when you're at a location and 'pottering', which might change some suggestions?

 

[neil_s]

Have been thinking the same thoughts about my 28 foot sail boat - also about 3 tons displacement. Being a bit of a cheapskate, I have opted for a Seagull silver century plus long shaft. 5 hp, but has an 11 inch prop and 4:1 reduction gearbox. My calculation says this motor will give me 4 knots.

 

[Freebee]

main power is Perkins 4108m

 

[Shaz77]

In theory if you put a 2hp on the back on a calm day at slack water it will push you along but you need some grunt for when the weather kicks up and you are fighting a foul tide and want to get somewhere safe asap. A 5hp is alright for ok conditions but I would be looking at a 10-15hp for a 28 foot boat. In the past I have been underpowered in the wrong conditions and it's very stressy.

 

[Shaz77]

main power is Perkins 4108m

That's about 50hp. Push you along on tick over.

 

[Jim@sea]

I notice that you wrote "auxilliary outboard". I'd interpret that as being in addition to (say) the permanent inboard, that is used for getting the boat from A to B, and the outboard is simply for when you're at a location and 'pottering', which might change some suggestions?

I had a 24ft fishing boat, it had an inboard shaft drive 72hp diesel, and a 6 hp auxiliary stuck on the transom. I tried the outboard flat out and it only did 2,5 knots. If I still had that boat, I would thicken the transom and put a 25hp outboard on it, plus I would try and find a 2 stroke as they are lighter.

 

[Dellquay13]

There are loads of 15-25hp outboards out there, but look closely at the prop size, most will be propped for planing in much smaller lighter boats. It’s not the end of the world to have to change the prop size. Try to get one of the ‘Bigfoot’ type outboards with the oversized gearbox aimed at heavy slow boats.

 

[William_H]

I have gone for a Honda 2.3 aircooled for my 21ft sail boat. Does the job when becalmed indeed did recently tow another 22fter both at about 3 knots. I chose this motor for light weight and reliability with no water pump system. It lies in the bilges for months on end. Now mention of tides just means you may want a higher water speed but that will be mostly dictated by hull speed. When you really need more HP will be against a strong wind and wave.
So factors of light weight ability to easily stow and reliability to run when you need it may affect your choice of engine size. ol'will

 

[Chiara’s slave]

There are loads of 15-25hp outboards out there, but look closely at the prop size, most will be propped for planing in much smaller lighter boats. It’s not the end of the world to have to change the prop size. Try to get one of the ‘Bigfoot’ type outboards with the oversized gearbox aimed at heavy slow boats.

100% this. We are a 30ft 2 ton sail boat and have a 15. It has been re propped.

As mentioned, if it’s just a spare motor, maybe 8hp would do the trick, with a saildrive type prop. If you were pushing the tide, you’d have to do what us raggies do, and head for shallow water, or a back eddy. We can gain ground against the tide in a non engined 20ft classic, so it can be done.

 

[shanemax]

100% this. We are a 30ft 2 ton sail boat and have a 15. It has been re propped.

As mentioned, if it’s just a spare motor, maybe 8hp would do the trick, with a saildrive type prop. If you were pushing the tide, you’d have to do what us raggies do, and head for shallow water, or a back eddy. We can gain ground against the tide in a non engined 20ft classic, so it can be done.

Yes, head for shallow water, that always reduces the flow, as long as you are not a deep fin, most never leave deep water or the channel for obvious reasons.

 

[Chiara’s slave]

Yes, head for shallow water, that always reduces the flow, as long as you are not a deep fin, most never leave deep water or the channel for obvious reasons.

Only superyachts need to keep to the channel in the Solent. I would estimate that we spend 75% of our actual sailing time inside tye 5 metre contour, and a surprising amount of that inside the 1m contour. Given that the tidal rise is never zero here, it’s not as daft as it sounds.

 

[Stemar]

Only superyachts need to keep to the channel in the Solent. I would estimate that we spend 75% of our actual sailing time inside tye 5 metre contour, and a surprising amount of that inside the 1m contour. Given that the tidal rise is never zero here, it’s not as daft as it sounds.

Same. We draw 70 cm, so the rule is just keep out of the green bits

 

[billskip]

Seagull silver century plus long shaft. 5 hp, but has an 11 inch prop and 4:1 reduction gearbox. My calculation says this motor will give me 4 knots.

And make you deaf...

 

[Davy_S]

We had a silver century fitted onto the bathing platform on a 22ft cranchi Weekender, we knew if we needed it it would always start, we got 4 Knott's from it in the non tidal Ionian.

 

[shanemax]

Only superyachts need to keep to the channel in the Solent. I would estimate that we spend 75% of our actual sailing time inside tye 5 metre contour, and a surprising amount of that inside the 1m contour. Given that the tidal rise is never zero here, it’s not as daft as it sounds.

It is possible to get a negative low spring tide that drops below chart datum, I have seen it on a few occasions on tidal prediction charts plus you can get unexpected surges and also unexpected negative surges. These usually happen if it is high pressure with a strong unfriendly wind.